Vol. 15 · No. 1 · January/February 2003

E. Ortega-Rivas, Mexico

Review and Research Trends in Food Processing

Abstract delegates from around the globe attending to present the most advanced developments in the area. With regards to powder The food industry is one of the largest commercial enterprises in and technology applied specifically to food materials, the world today representing important contributions of the there has not been as much activity as the one applied to inert gross national product of many countries. Numerous raw mate- materials. Some isolated efforts have been done to promote ex- rials and products in this industry are in powdered or particulate change of ideas among powder technologists with an interest in form, and their optimum handling and processing rely heavily in food and biological materials. For instance, sessions on food a deep knowledge of particle technology. Processing and de- have been included in the conference known as Food sign aspects of unit operations involving granular of par- Ingredients Europe in 1993 and in the 26th Annual Meeting of ticular relevance to the food industry are discussed. Theoretical the Fine Particle Society in 1995. considerations, operating principles, and applications of differ- ent techniques used in food powder processing are reviewed. The food industry should make more efficient use of its many 2 Characterisation of Powdered Food processes involving powders and in order to pro- Materials vide high quality products. In this sense, future competitiveness may be critically dependent on knowledge originated by re- Particle characterization, i.e. description of primary properties of search activities in particle technology applied to food materials. food powders in a particulate system, underlies all work in par- ticle technology. Primary particle properties such as particle 1 Introduction shape and particle density, together with the primary properties of a fluid (viscosity and density), and also with the concentration There is a relatively new branch of science and engineering and state of dispersion, govern the secondary properties such known as Powder Technology or Particle Technology which as settling velocity of , rehydration rate of powders, re- deals with the systematic study of particulate systems in a sistance of filter cakes, etc. It could be argued that it is simpler, broad sense. For the case of food products and materials, and more reliable, to measure the secondary properties directly some important applications of powder technology can be without reference to the primary ones. Direct measurement of mentioned. For example in wheat flour is an impor- secondary properties can be done in practice, but the ultimate tant factor in functionality of food products, attrition of instant aim is to predict them from the primary ones, as when deter- powdered foods reduces their reconstitutability, uneven powder mining pipe resistance to flow from known relationships, feeding flow in extrusion hoppers may affect the rheology of the paste, in data from primary properties of a given liquid (viscosity and and an appropriate characterisation of fluid-particle interactions density), as well as properties of a pipeline (roughness). As could optimise clarification of juices. European and Asian pro- many relationships in powder technology are rather complex fessional associations and societies have recognised the impor- and often not yet available in many areas, such as food pow- tance of powder technology for some time. Some of these as- ders, particle properties are mainly used for qualitative assess- sociations, such as the Institution of Chemical Engineers ment of the behaviour of suspensions and powders, for exam- (IChemE) and the Society of Chemical Industry (SCI) in the ple, as an equipment selection guide. Since a powder is United Kingdom include well-established research groups in the considered to be a dispersed two-phase system consisting of a topic. In the United States it was only until 1992 when a division, dispersed phase of particles of different sizes and a gas as known as the Particle Technology Forum, was formed within the the continuous phase, complete characterisation of powdered American Institute of Chemical Engineers (AIChE). In a more materials is dependant on the properties of a particle as an indi- global context, diverse associations from worldwide have or- vidual entity, the properties of the assembly of particles, and the ganised congresses on particle technology, which gather many interactions between those assemblies and a fluid.

2.1 Density Dr. Enrique Ortega-Rivas, Food and Chemical Engineering Programme, University of Chihuahua, Apartado Postal 1542-C, Chihuahua, Chih. 31161, Mexico The density of a particle is defined as its total mass divided by Tel.: +52 614 4241868; Fax: +52 614 4144492 its total volume. It is considered quite relevant for determining e-mail: [email protected] other particle properties such as bulk powder structure and par- Details about the author on page 68 ticle size, so it requires careful definition (1). Depending on how * Based on a paper presented at the World Congress on Particle Technology 4, the total volume is measured, different definitions of particle Sydney, Australia, July 21-25, 2002 density can be given: the true particle density, the apparent par-

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ticle density, and the effective (or aerodynamic) particle density. Since particles usually contain cracks, flaws, hollows, and closed pores, it follows that all these definitions may be clearly different. The true particle density represents the mass of the particle divided by its volume excluding open and closed pores, and is the density of the solid material of which the particle is made. For pure chemical substances, organic or inorganic, this is the density quoted in reference books of physical/chemical data. Since most inorganic materials consist of rigid particles, while most organic substances are normally soft, porous parti- cles, true density of many food powders would be considerably low than those of mineral and metallic powders. Typical non- metallic minerals would have true particle densities well over 2,000 kg/m3, while some metallic powders can present true densities of the order of 7,000 kg/m3. By contrast, most food particles have densities considerably lower of about 1,000- 1,500 kg/m3. Densities of some typical food commodities are given in Table 1. The apparent particle density is defined as the mass of a particle divided by its volume excluding only the open pores, and is measured by gas or liquid displacement methods such as liquid or air pycnometry. The effective particle density is referred as the mass of a particle divided by its Table 1: True densities of common volume including both food powders open and closed pores. In this case, the Powder Density (kg/m3) volume is within an aerodynamic envelope Glucose 1,560 as "seen" by a gas Sucrose 1,590 flowing past the parti- Starch 1,500 cle and, as such, this density is of primary Cellulose 1,270 – 1,610 importance in applica- (globular) ~1,400 tions involving flow Fat 900 – 950 round particles like in Salt 2,160 fluidisation, sedimen- tation, or flow through Citric acid 1,540 packed beds. Any of the three particle densities defined above should not be confused with bulk density of materials, which includes the voids between the particles in the volume measured. The differ- ent values of particle density can also be expressed in a dimen- sionless form, as relative density, or specific gravity, which is simply the ratio of the density of the particle to the density of Fig. 1: Descriptive diagram of density determination by liquid pycnometry: water. It is easy to determine the mass of particles accurately (a) description of pycnometer, (b) weighing, (c) filling to about 1/3 with but difficult to evaluate their volume because they have irregular powder, (d) adding liquid to almost full, (e) eliminating bubbles, (f) topping shapes and voids between them. The apparent particle density, and final weighing. or if the particles have no closed pores also the true density, can be measured by fluid displacement methods, i.e. pycnometry, which are in common use in industry. The displacement can be Fig. 2: Top-loading platform scale for density determination of irregularly shaped objects carried out using either a liquid or a gas, with the gas employed normally being air. Thus, the two known techniques to deter- mine true or apparent density, when applicable, are liquid pyc- nometry and air pycnometry. In liquid pycnometry, the particle density is calculated as the net weight of dry powder divided by the net volume of the powder, calculated by liquid displacement in a small bottle known as py- cnometer. Fig. 1 shows, diagrammatically, the steps involved in determining density by liquid pycnometry. When the density of larger irregular shaped solid objects, such as compressed or aggregated bulk powders is needed, a method available to eval- uate fruit or volumes might be used. As illustrated in Fig. 2, a beaker big enough to host the solid is partially filled with some kind of liquid, which will not dissolve the solid. The weight of the beaker with the liquid in it is recorded; the solid object is completely immersed and suspended at the same time, using a string, so that it does not touch either the sides or bottom of the beaker. The total weight of this arrangement is recorded again, and the volume of the solid can be calculated (2) to derive the density of the object.

19 Processing Food Powder Vol. 15 · No. 1 · January/February 2003

Air pycnometry can be performed in an instru- ment which usually consists of two cylinders and two pistons, one is a reference cylinder, which is always empty, and the other has a fa- cility for inserting a cup with the sample of the powder. With no sample present, the volume in each cylinder is the same so that, if the con- necting valve is closed and one of the pistons is moved, the change must be duplicated by an identical stroke in the other as to maintain the same pressure on each side of the differential pressure indicator. The method will measure the true particle density if the particles have no closed pores, or the apparent particle density if there are any closed pores, because the vol- ume measured normally excludes any open pores. Considering all the aspects involved in operation of air pycnometers, densities of most inorganic materials can be reliable determined using the standard procedure with normal air because their particles are normally hard and rigid. However, extreme care must be taken when dealing with food powders, which are mostly organic in origin and their particles may be soft and porous. A diagram showing the dif- ferent steps of density determination by air py- cnometry is presented in Fig. 3. Food powders have apparent densities in the range of 300 to 800 kg/m3. As previously men- tioned, the solid density of most food powders is about 1,400 kg/m3, so these values are an indication that food powders have high porosity which can be internal, external, or both. Bulk Fig. 3: Descriptive diagram of density determination by air pycnometry: (a) description of instrument, (b) filling of cup, (c) pistons displacement, (d) reading densities of some food powders are given in Table 2. There are many published theoretical and experimental studies of porosity as a function of the particle ing powders or models (e.g., steel shots and metal powders), size, distribution, and shape. Most of them pertain to free-flow- where porosity can be treated as primarily due to geometrical and statistical factors only (3, 4). Even though in these cases porosity can vary considerably, depending on factors such as Table 2. Approximate bulk densities of different food powders the concentration of fines, it is still evident that the exceedingly Powder Density (kg/m3) low density of food powders cannot be explained by geometri- cal considerations only. Most food powders are known to be Baby formula 400 cohesive and, therefore, an open bed structure supported by Cocoa 480 inter-particle forces is very likely to exist (5, 6, 7). Since the bulk density of food powders depends on the combined effect of in- Coffee (ground and roasted) 330 terrelated factors, such as the intensity of attractive inter-particle Coffee (instant) 470 forces, the particle size, and the number of contact points (8), it Coffee creamer 660 is clear that a change in any of the powder characteristics may result in a significant change in the powder bulk density. Fur- Corn meal 560 thermore, the magnitude of such change cannot always be an- Corn starch 340 ticipated. There is an intricate relationship between the factors Egg (whole) 680 affecting food powder bulk density, as well as surface activity Gelatin (ground) 680 and cohesion. Microcrystalline cellulose 610 2.2 Particle Size Distribution Milk 430 Oatmeal 510 Particle size distribution measurement is a common method in any physical, mechanical, or chemical process because it is di- Onion (powdered) 960 rectly related to material behaviour and/or physical properties of Salt (granulated) 950 products. Foods are frequently in the form of fine particles dur- Salt (powdered) 280 ing processing and marketing (9). The bulk density, compress- Soy protein (precipitated) 800 ibility, and flowability of a food powder are highly dependent on particle size and its distribution (10). Segregation will happen in (granulated) 480 a free-flowing powder mixture because of the differences in par- Sugar (powdered) 480 ticle sizes (11). Size distribution is also one of the factors affect- Wheat flour 800 ing the flowability of food powders (12). For quality control or system property description, the need to represent the particle Wheat (whole) 560 size distribution of food powders becomes paramount and Whey 520 proper descriptors in the analysis of the handling, processing, (active dry baker's) 820 and functionality of each food powder. There are many different

20 Vol. 15 · No. 1 · January/February 2003 Processing Food Powder

types of instruments available for measuring particle size distri- discharge rate. With regards to segregation, many materials bution but most of them would fall into four general methods: experience separation of fine and coarse particles (16) and sieving, microscope counting techniques, sedimentation, and such separation can seriously compromise the quality of the stream scanning. In particle size measurement two most impor- final product as well as the efficiency of the process. Flooding tant decisions have to be made before a technique is to be se- can be caused by the collapse of a rathole in a bin containing lected for the analysis; these are concerned with the two vari- fine powder, resulting in uncontrollable flow of material, loss of ables measured, the type of particle size and the occurrence of product and clouds of (17), among other problems. Per- such size. Pertaining particle size is important to bear in mind taining structural failure, each year over 1,000 silos, bins and that great care must be taken when making a selection of parti- hoppers fail in North America alone. Most of these failures cle size, as an equivalent diameter, in order to choose the most could have been prevented with proper and careful design, in relevant to the property or process which is to be controlled. which the loads imposed by the bulk solid being stored had The occurrence of amount of particle matter, which belongs to, been well considered. specified size classes might be classified or arranged by diverse The systematic approach for designing powder handling and criteria as to obtain tables or graphs. Particle size distribution processing plants started in the mid 1950s by the pioneering techniques have been reviewed in classical texts, such as the work of Jenike. His concept was to model bulk solids using the authoritative one written by Allen (13). principles of continuum mechanics. The resulting comprehen- sive theory (18) describing the flow of bulk solids has been ap- 3 Storage and Conveying of Food plied and perfected over the years, but is generally recognised Particulates worldwide as the only scientific guide to bulk solids flow. The procedures for the design of a bulk solids handling plant are well established and follow for basic steps: (a) determination of the 3.1 Silos, Bins and Hoppers strength and flow properties of the bulk solids for the worst likely flow conditions expected to occur in practice; (b) calculation of The food and related industries handle considerable amounts the bin, stockpile, feeder or chute geometry to give the desired of powders and particulate materials every year. Silos, bins and capacity to provide a flow pattern with acceptable characteris- hoppers used to store these materials vary in capacity from a tics, in order to ensure that discharge is reliable and predictable; few kilos to multi-ton-capacity vessels. Start-up delays and on- (c) estimation of the loadings on the bin and hopper walls and going inefficiencies are common in solids processing plants. An on the feeders and chutes under operating conditions; (d) de- important cause of these problems is the improper design of sign and detailing of the handling plant including the structure bulk solids handling equipment. A six-year study of 40 solids and equipment. processing plants in the U.S. and Canada (14) revealed that 80% of these plants experienced solids handling problems. The study also found that these plants were slow in smoothing 3.2 Conveyors operation, with an average start-up time for some plants aver- Materials handling, in the food and processing industries, is aging 18 months. Once start-up began, performance around concerned with movement of materials in different cases such 40 to 50% of design was commonly observed. The common as from supply point to store or process, between stages dur- flow problems in hoppers and silos can be summarised as fol- ing processes, or to packing and distribution. The movement of lows: (a) no flow, (b) segregation, (c) flooding, and (d) structural materials is a crucial activity which adds nothing to the value of failure. Lack of discharge in the no flow situation can be attrib- the product, but can represent an added cost if not managed uted to the formation of a stable arch over the outlet, or a sta- properly. For this reason, responsibility for material handling is ble cavity called a "rathole" (15). The ideal flow situation may be normally vested in specialist handling engineers, and many food considered when the hopper dimensions fall within the specifi- manufacturers adopt this procedure. For bulk particulate or cations of a mass-flow hopper, while the rathole condition powdered food materials, which fall within the scope of this ar- would lie within the funnel-flow design. Fig. 4 shows both silo ticle, a convenient classification of conveyors would comprise designs, although, in practice, any of them may need assis- the following types: belt, chain and screw, as well as pneumatic tance and/or a feeder adaptation to initiate flow and control equipment. In the literature all these forms of bulk material movement are known as conveyors and, thus, a proper catego- Fig. 4: Types of flow patterns in hoppers rization of the handling equipment for bulk particulate food solids would comprise the following groups: (a) belt conveyors; (b) chain conveyors: scraper conveyors, apron conveyors and bucket elevators; (c) screw conveyors; and (d) pneumatic con- veyors: dense phase systems and dilute phase systems. This classification groups the different types of conveying systems in virtue of their operating principle. For example, regardless of whether a bulk material is being moved horizontally, inclined, or vertically, chain conveying can perform the duty based on the same principle of fixing an element (a paddle or a bucket) to a system of chains externally powered. Dilute phase-pneumatic conveyors are widely used in transporting of food systems, due to their possibility of handling fragile and brittle materials with minimum risk of damage. Fig. 5 illustrates the main types of these sorts of conveyors. Bulk solids conveying represents one of those disciplines, which have been neglected in terms of scientific research and devel- opment, when compared with other operations in particle tech- nology directly related to processing, such as attrition or ag- glomeration. Conveying systems and supplies are normally provided by a large, capable manufacturing industry, which holds much of the engineering information in form of brochures, data sheets, and nomographs.

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portant in evaluating a product from comminu- tion. In an actual process, a given unit does not yield a uniform product, whether the feed is uni- formly sized or not. The product normally con- sists of a mixture of particles, which may con- tain a wide variety of sizes and even shapes. Some types of equipment are designed to con- trol the magnitude of the largest particles in their products, but the fine sizes are not under such control. In some machines the fines are minimised, but they cannot be totally elimi- nated. Some application examples of equip- ment used in size reduction of food materials are presented in Table 3, while some diagrams of equipment are illustrated in Fig. 6.

4.2 Agglomeration and Growth Size enlargement operations are used in the process industries with different aims such as improving handling and flowability, reducing dusting or material losses, producing structural useful forms, enhancing appearance, etc. Size enlargement operations are known by many names, including: compaction, granulation, tab letting, briquetting, pelletising, encapsulation, sintering and agglomeration. While some of these operations could be considered rather similar, e.g. tabletting and pelletising, some oth- ers are relevant to a specific type of industry, e.g. sintering in metallurgical processes. In the food industry, the term agglomeration is applied to the process that has the main objective of controlling porosity and density of materials in order to influence properties like dispersibility and solubility. In this case, the operation is also often referred as instantising, because rehydra- tion and reconstitution are important functional Fig. 5: Dilute phase pneumatic conveyors: (a) pressure system, (b) vacuum system, (c) combined sys- properties in food processes. On the other tem, (d) closed-loop system hand, when size enlargement is used with the objective of obtaining definite shapes, the food 4 Processing of Food Powders industry takes advantage of a process that may shape and cook at the same time, known as extrusion. In a more general con- Many unit operations involving particulate materials are of great relevance to the food industry. Some are traditional, like grain Table 3. Application examples of size reduction machines milling, while some others have been adapted from other indus- tries, like extrusion. Processes such as size reduction, size en- Fineness range: Crushing Hammer Attrition Tumbling largement, mixing, and separation, may be considered quite rolls mills mills mills strategic for the food industry, and will be briefly reviewed as fol- Coarse • lows. Intermediate • • • • 4.1 Comminution Fine and ultrafine • • • In many food processes is frequently required to reduce the size Chocolate • • of solid materials for different purposes. For example, size re- Cocoa • • duction may aid other processes such as expression and ex- Corn (wet) • traction, or may shorten heat treatments such as blanching and Dried fruits • cooking. Comminution is the generic term used for size reduc- tion and includes different operations such as crushing, grind- Dried milk • ing, milling, mincing, and dicing. Most of these terms are related Dried • to a particular application, e.g. milling of cereals, mincing of Grains • • beef, dicing of tubers, or grinding of spices. The reduction mechanism consists on deforming the food piece until it breaks Pepper • • or tears. Breaking of hard materials along cracks or defects in Pulses • their structures is achieved by applying diverse forces. The ob- Roasted nuts • jective of comminution is to produce small particles from larger Salt • • ones. Smaller particles are the desired product either because of their large surface or because of their shape, size, and num- Spices • ber. The energy efficiency of the operation can be related to the Starch (wet) • new surface formed by the reduction in size. The geometric Sugar • • characteristics of particles, both alone and in mixtures, are im-

22 Vol. 15 · No. 1 · January/February 2003 Processing Food Powder

Fig. 6: Size reduction equipment used for food materials: (a) crushing rolls, (b) hammer mill, (c) disc attrition mills text, however, instantising and extrusion of food processes are of a system. Since the components being mixed can exist in any the two common categories of agglomeration: tumble/growth of the three states of matter, a number of mixing possibilities and pressure agglomeration, and are referred as such in the lit- arise. The mixing cases involving a fluid, e.g. liquid-liquid and erature. Since rehydration and reconstitution are important solid-liquid, are most frequently encountered so they have been functional properties of many food powders, tumble/growth ag- extensively studied. Despite the importance of the mixing of par- glomeration is a relevant operation in food processing, which is ticulate materials in many processing areas, fundamental work attracting attention for development. Fig. 7 shows typical equip- of real value to either designers or users of solids mixing equip- ment for tumble/growth agglomeration of foods. ment is still relatively sparse. It is through studies in very specific fields, such as powder technology and multi-phase flow, that important advances in understanding of mixing of solids and pastes have been made. Mixing is more difficult to define and evaluate with powders and particulates than it is with fluids, but some quantitative measures of dry solids mixing may aid in eval- uating mixer performance. In actual practice, however, the proof of a mixer is in the properties of the mixed material it produces. A significant proportion of research efforts in the food industry are directed toward the development of new and novel mixing devices for food materials. These devices may be effective for many applications since they deliver a mixed product with the required blending characteristics. Due to the complex proper- ties of food systems, which can themselves, vary during mixing, it is extremely difficult to generalise or standardise mixing oper- ation for wider applications of mixing devices, either novel or tra- ditional. Developments in mathematical modelling of food-mix- ing processes are scarce and established procedures for process design and scale-up are lacking. As a result of this, it is virtually impossible to devise relationships between mixing and quality (19), especially for blending of food powders. Fig. 8 pre- sents some mixers used for blending of food materials.

4.4 Dry Separation Techniques Separation techniques are involved in a great number of pro- cessing industries and represent, in many cases, the everyday problem of a practicing engineer. In spite of this, the topic is nor- mally not covered efficiently and sufficiently in higher education Fig. 7: Equipment for tumble/growth agglomeration: (a) inclined rotating disc, curricula of some engineering programs, mainly because its (b) inclined rotating drum, (c) ribbon powder blender, (d) fluidized bed theoretical principles deal with a number of subjects ranging from physics principles to applied fluid mechanics. In recent years, separation techniques involving solids have been consid- 4.3 Mixing ered under the general interest of powder and particle technol- ogy, as many of these separations involve removal of discrete The unit operation in which two or more materials are inter- particles or droplets from a fluid stream. Separation techniques spersed in space with one another is one of the oldest and yet are defined as those operations that isolate specific ingredients one of the least understood of the unit operations of process of a mixture without a chemical reaction taking place. Several engineering. Mixing is used in the food industry with the main criteria have been used to classify or categorise separation objective of reducing differences in properties such as concen- techniques. One of such consists in grouping them according tration, colour, texture, taste, and so on, between different parts the phases involved, i.e. solid with liquid, solid with solid, liquid

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ily on a solid understanding of powder technology, there is a lot of potential to establish research programs and schemes to attend this demand. Many lines of research could be men- tioned as examples, but some urgent needs may be sum- marised as follows: With regards to comminution, an improve- ment of energy efficiency of the operation is necessary, as well as prediction of particle size and shape. Attrition is causing im- portant losses to different food industries, mainly cereal indus- try, so means of translating particle properties to single and multi particle breakage would be quite useful to manipulate process variables in order to prevent such an undesirable phe- nomenon. Size enlargement is widely applied in diverse food processes and some needs for research are better under- standing of melt granulation, development of combined oper- ations such as granulation and coating, effects of size and shape on rheology of extrudates, etc. Pertaining solids mixing, more reliable predicting methods of blending time and mixing indices are needed. Cyclones generally operate at low pres- sures with relatively low solid loadings, but they may be re- quired to function at higher solid loadings and system pres- sures; systematic data on how these variables affect cyclone operation are not available. It can be also stated that there is a dearth of information on modelling of drop formation and so- lidification in spray drying, as well as on the effects of operat- ing variables of fluidised bed drying in quality of the obtained products. Finally, fundamental research is needed on the gen- eral field of improving rehydration and reconstitution properties of diverse dehydrated foods.

References

[1] OKUYAMA, K, Y. AND KOSUAKA: Particle density; in: LINOKA, K., OTOH AND IGASHITANI Fig. 8: Mixers used for food powder blending: (a) tumbler mixers, (b) open rib- K. G K. H (eds): Powder Technology bon mixer, (c) vertical screw mixers Handbook; Marcel Dekker, New York (1991). with liquid, etc. Dry separation techniques constitute all those Fig. 8: Schematic diagram of a gas cyclone cases in which the particle to be isolated or segregated from a mixture is not wet. The most important dry separation tech- niques in processing industries have been reviewed by Beddow (20). In food processing, there are important applications of dry separation techniques, such as the removal of particles from dust-laden air in milling operations, the recovery of the dried product in spray dehydration, and the cleaning of grains prior to processing. Air cyclones are devices widely used in some of these applications. Fig. 9 shows a diagram of the operating principle of a cyclone.

5 Conclusions: Future Research Trends

Powder technology is significantly relevant to the world econ- omy, with a broad range of industries taking advantage of the rapidly growing knowledge in this discipline. Research within universities and similar institutions, coupled with the vested in- terest from the industrial community, has stimulated relevant results which have been applied to make more efficient a num- ber of processes. Periodical meetings and specialised publica- tions are spreading the most relevant and recent advances in diverse topics such as materials handling, particle formation, mixing, grinding and separation. As mentioned at the begin- ning of this article, there has been a rapid growth of adapting knowledge of particle technology worldwide in recent years. However, for the case of application to biological materials, specifically food powders and particulates, there is much to be done as very few research groups can be identified in this par- ticular field around the globe. Since many strategic food in- dustries, such as those based on grain processing, rely heav-

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[2] MA, L., D.C. DAVIS, L.G. OBALDO, AND G.V. BARBOSA-CANO- [11] BARBOSA-CANOVAS, G.V., J. MALAVE-LOPEZ, AND M. PELEG: VAS: Mass and spatial characterization of biological materi- Segregation in food powders; Biotechnology Progress 1 als; In: Engineering Properties of Foods and Other Biologi- (1985), pp. 140-146. cal Materials. Washington State University Publisher, [12] Peleg, M.: Flowability of food powders and methods for its Pullman, WA, USA (1997). evaluation - a review; Journal of Food Process Engineering [3] GRAY W.A.: The Packing of Solid Particles; Chapman and 1 (1977), pp. 303-328. Hall, London (1968). [13] Allen, T.: Particle Size Measurement; Chapman & Hall, Lon- [4] MCGEARY, R.K.: Mechanical packing of spherical particles; don (1981). In: H.H. HAUSNER, K.H. ROLL AND P.K. JOHNSON (EDS.): Vibra- tory Compacting; Plenum Press, New York (1967). [14] MERROW, E.W.: Estimating startup times for solids-process- ing plants; Chemical Engineering 89 (1988), pp. 89-92. [5] MOREYRA, R., AND M. PELEG: Effect of equilibrium water ac- tivity on the bulk properties of selected food powders; [15] MARINELLI, J. AND J.W. CARSON: Solve solids flow problems Journal of Food Science 46 (1981), pp. 1918-1922. in bins, hoppers and feeders; Chemical Engineering Progress 88 (1992), pp. 22-26. [6] SCOVILLE, E., AND M. PELEG: Evaluation of the effect of liquid bridges on the bulk properties of model powders; Journal [16] CARSON J.W., T.A. ROYAL AND D.J. GOODWILL: Understanding of Food Science 46 (1980), pp 174-177. and eliminating particle segregation problems; Bulk Solids Handling 6 (1986), pp 139-142. [7] DOBBS, A.J., M. PELEG, R.E. MUDGET, AND R. RUFNER: Some physical characteristics of active dry yeast; Powder Tech- [17] ROYAL, T.A. AND J.W. CARSON: How to avoid flooding in nology 32 (1982), pp 75-81. powder handling systems; Powder Handling and Process- ing Vol. 5 (1993), pp 63-67. [8] RUMPF, H.: The strength of granules and agglomerates; In: KNEPPER, W.A. (ed.): Agglomeration; Industrial Publishers, [18] JENIKE, A.W.: Storage and Flow of Solids; Bulletin No. 123, New York (1961). Utah Engineering Experiment Station, Salt lake City, UT, USA (1964). [9] SCHUBERT, H.: Food particle technology part 1: properties of particles and particulate food systems; Journal of Food En- [19] NIRANJAN, K.: An appraisal of the characteristics of food gineering 6 (1987), pp 1-32. mixing; in: SINGH, R.K. (ed.): Food Process Design and Eval- uation; Technomics, Lancaster, PA, USA (1995). [10] BARBOSA-CANOVAS, G.V., J. MALAVE-LOPEZ, AND M. PELEG: Density and compressibility of selected food powders mix- [20] BEDDOW, J.K.: Dry separation techniques. Chemical Engi- tures; Journal of Food Process Engineering 10 (1987), pp. neering Vol. 88 (1981), pp 70-84. 1-19. php

25

Current Awareness in Food Powder Processing

Enrique Ortega-Rivas

Food and Chemical Engineering Program University of Chihuahua Chihuahua, Chih., Mexico

ABSTRACT

The food industry is one of the largest commercial enterprises in the world today representing important contributions of the gross national product of many countries. Numerous raw materials and products in this industry are in powdered or particulate form, and their optimum handling and processing rely heavily in a deep knowledge of particle technology. Processing and design aspects of unit operations involving granular solids of particular relevance to the food industry are discussed. Operating principles, and applications of different techniques used in food powder processing are reviewed. The objective is to raise awareness, in the sense that the food industry should make more efficient use of its many proc- esses involving powders and particulates, since future competitiveness may be critically dependent on knowledge origi- nated by research activities in particle technology applied to food materials.

1 INTRODUCTION materials vary in capacity from a few kilos to multi- ton-capacity vessels. Start-up delays and ongoing There is a relatively new branch of science and en- inefficiencies are common in solids processing gineering known as Powder Technology or Particle plants. An important cause of these problems is the Technology, which deals with the systematic study improper design of bulk solids handling equipment. of particulate systems in a broad sense. For the A six-year study of 40 solids processing plants in the case of food products and materials, some impor- US and Canada [1] revealed that 80% of these tant applications of powder technology can be men- plants experienced solids handling problems. The tioned. For example, particle size in wheat flour is study also found that these plants were slow in an important factor in functionality of food products, smoothing operation, with an average start-up time attrition of instant powdered foods reduces their re- for some plants averaging 18 months. Once start- constitutability, uneven powder flow in extrusion up began, performance around 40 to 50% of design hoppers may affect the rheology of the paste, and was commonly observed. The common flow prob- an appropriate characterisation of fluid-particle in- lems in hoppers and silos can be summarised as teractions could optimise clarification of juices. absence of flow, segregation, flooding, and struc- European and Asian professional association and tural failure. Lack of discharge in the no flow situa- societies have recognised the importance of powder tion can be attributed to the formation of a stable technology for some time. Some of these associa- arch over the outlet, or a stable cavity called a “ra- tions, such as the Institution of Chemical Engineers thole” [2]. The ideal flow situation may be consid- (IChemE) and the Society of Chemical Industry ered when the hopper dimensions fall within the (SCI) in the United Kingdom include established specifications of a mass-flow hopper, while the ra- research groups in the topic. In the United States, it thole conditions would lie within the funnel-flow de- was only until 1992 when a division, known as the sign. Fig. 1 shows both silo designs, although in Particle Technology Forum, was formed within the practice any of them may need assistance and/or American Institute of Chemical Engineers (AIChE). feeder adaptation, to initiate flow and control dis- In a more global context, diverse associations from charge rate. With regards to segregation, many ma- worldwide have organised congresses on particle terials experience separation of fine and coarse par- technology, which gather many delegates from ticles [3] and such separation can seriously com- around the globe attending to present the most ad- promise the quality of the final product as well as the vanced developments in the area. With regards to efficiency of the process. Flooding can be caused powder and particle technology applied specifically by the collapse of a rathole in a bin containing fine to food materials, there has not been as much activ- powder, resulting in uncontrollable flow of material, ity as that in the field of inert materials. loss of product and clouds of dust [4], among other problems. Pertaining structural failure, each year 2 HANDLING SOLID FOOD SYSTEMS over 1,000 silos, bins and hoppers fail in North America alone. Most of these failures could have 2.1 Types of storage been prevented with proper and careful design, in which the loads imposed by the bulk solid being The food and related industries handle considerable stored had been well considered. Adequate design amounts of powders and particulate materials every of powder handling plants has been neglected, not year. Silos, bins and hoppers used to store these only in the food industry, but also in many others. D D not managed properly. For this reason, responsibil- ity for material handling is normally vested in spe- cialist handling engineers, and many food manufac- turers adopt this procedure. For bulk particulate or powdered food materials, which fall within the scope of this article, a convenient classification of convey- ors would comprise the following types: belt, chain and screw, as well as pneumatic equipment. In the literature all these forms of bulk material movement HD are known as conveyors and, thus, a proper catego- rization of the handling equipment for bulk particu- H H late food solids would comprise the following groups: (a) belt conveyors; (b) chain conveyors: scraper conveyors, apron conveyors and bucket elevators; (c) screw conveyors; and (d) pneumatic conveyors: dense phase systems and dilute phase Dead systems. This classification groups the different θ capacity types of conveying systems in virtue of their operat- θ ing principle. For example, regardless of whether a bulk material is being moved horizontally, inclined, or vertically, chain conveying can perform the duty B B based on the same principle of fixing an element (a (a) Mass-flow (b) Funnel-flow paddle or a bucket) to a system of chains externally powered.

Figure 1: Types of flow patterns in hoppers. Bulk solids conveying represents one of those disci- plines typically neglected in terms of scientific re- 2.2 Design of handling plant search and development, when compared with other operations in particle technology directly related to The systematic approach for designing powder han- processing, such as attrition or agglomeration. dling and processing plants started in the mid 1950s Conveying systems and supplies are normally pro- by the pioneering work of Jenike. His concept was vided by a large, capable manufacturing industry, to model bulk solids using the principles of contin- which holds much of the engineering information in uum mechanics. The resulting comprehensive the- form of brochures, data sheets, and nomographs. ory [5] describing the flow of bulk solids has been applied and perfected over the years, but is gener- 3 PROCESSING OF FOOD POWDERS ally recognised worldwide as the only scientific guide to bulk solids flow. The procedures for the Many unit operations involving particulate materials design of a bulk solids handling plant are well estab- are of great relevance to the food industry. Some lished and follow for basic steps: (a) determination are traditional, like grain milling, while some others of the strength and flow properties of the bulk solids have been adapted from other industries, like extru- for the worst likely flow conditions expected to occur sion. Processes such as size reduction, size in practice; (b) calculation of the bin, stockpile, enlargement, mixing, and separation, may be con- feeder or chute geometry to give the desired capac- sidered quite strategic for the food industry, and will ity to provide a flow pattern with acceptable charac- be briefly reviewed. teristics, in order to ensure that discharge is reliable and predictable; (c) estimation of the loadings on the 3.1 Size reduction bin and hopper walls and on the feeders and chutes under operating conditions; (d) design and detailing In many food processes is frequently required to of the handling plant including the structure and reduce the size of solid materials for different pur- equipment. poses. For example, size reduction may aid other processes such as expression and extraction, or 2.3 Conveying and conveyors may shorten heat treatments such as blanching and cooking. Comminution is the generic term used for Materials handling, in the food and processing in- size reduction and includes different operations dustries, is concerned with movement of materials in such as crushing, grinding, milling, mincing, and different cases such as from supply point to store or dicing. Most of these terms are related to a particu- process, between stages during processes, or to lar application, e.g. milling of cereals, mincing of packing and distribution. The movement of materi- beef, dicing of tubers, or grinding of spices. The als is a crucial activity that adds nothing to the value reduction mechanism consists on deforming the of the product, but can represent an added cost if food piece until it breaks or tears. Breaking of hard materials along cracks or defects in their structures unit operations of process engineering. Mixing is is achieved by applying diverse forces. The objec- used in the food industry with the main objective of tive of comminution is to produce small particles reducing differences in properties such as concen- from larger ones. Smaller particles are the desired tration, colour, texture, taste, and so on, between product either because of their large surface or be- different parts of a system. Since the components cause of their shape, size, and number. The energy being mixed can exist in any of the three states of efficiency of the operation can be related to the new matter, a number of mixing possibilities arise. The surface formed by the reduction in size. The geo- mixing cases involving a fluid, e.g. liquid-liquid and metric characteristics of particles, both alone and in solid-liquid, are most frequently encountered so they mixtures, are important in evaluating a product from have been extensively studied. Despite the impor- comminution. In an actual process, a given unit tance of the mixing of particulate materials in many does not yield a uniform product, whether the feed is processing areas, fundamental work of real value to uniformly sized or not. The product normally con- either designers or users of solids mixing equipment sists of a mixture of particles, which may contain a is still relatively sparse. It is through studies in very wide variety of sizes and even shapes. Some types specific fields, such as powder technology and of equipment are designed to control the magnitude multi-phase flow, that important advances in under- of the largest particles in their products, but the fine standing of mixing of solids and pastes have been sizes are not under such control. In some machines made. Mixing is more difficult to define and evalu- fines are minimised, but they cannot be totally elimi- ate with powders and particulates than it is with flu- nated. ids, but some quantitative measures of dry solids mixing may aid in evaluating mixer performance. In 3.2 Size enlargement actual practice, however, the proof of a mixer is in the properties of the mixed material it produces. A Size enlargement operations are used in the proc- significant proportion of research effort in the food ess industries with different aims such as improving industry is directed toward the development of new handling and flowability, reducing dusting or material and novel mixing devices for food materials. These losses, producing structural useful forms, enhancing devices may be effective for many applications appearance, etc. Size enlargement operations are since they deliver a mixed product with the required known by many names, including: compaction, blending characteristics. Due to the complex prop- granulation, tabletting, briquetting, pelletising, en- erties of food systems, which can themselves vary capsulation, sintering and agglomeration. While during mixing, it is extremely difficult to generalise or some of these operations could be considered standardise mixing operation for wider applications rather similar, e.g. tabletting and pelletising, some of mixing devices, either novel or traditional. Devel- others are relevant to a specific type of industry, e.g. opments in mathematical modelling of food-mixing sintering in metallurgical processes. In the food in- processes are scarce and established procedures dustry, the term agglomeration is applied to the for process design and scale-up are lacking. As a process with the main objective of controlling poros- result of this, it is virtually impossible to devise rela- ity and density of materials in order to influence tionships between mixing and quality [6], especially properties like dispersibility and solubility. In this for blending of food powders. Fig. 2 presents some case, the operation is also often referred as instan- mixers used for blending of food materials. tising, because rehydration and reconstitution are important functional properties in food processes. On the other hand, when size enlargement is used with the objective of obtaining definite shapes, the food industry takes advantage of a process, which may shape and cook at the same time, known as extrusion. In a more general context, however, in- stantising and extrusion of food processes are the Tumbler mixers two common categories of agglomeration: tum- ble/growth and pressure agglomeration, and are referred as such in the literature. Since reconstitu- tion is an important functional property of many food products, tumble/growth agglomeration is a relevant operation in food processing, which is attracting at- tention for further research and development.

3.3 Mixing

The unit operation in which two or more materials Vertical screw mixers are interspersed in space with one another is one of the oldest and yet one of the least understood of the Figure 2: Mixers used for food powder blending. 3.4 Dry separation techniques urgent needs may be summarised as concluding remarks. Separation techniques are involved in a great num- ber of processing industries and represent, in many With regards to comminution, an improvement of cases, the everyday problem of a practicing engi- energy efficiency of the operation is necessary, as neer. In spite of this, the topic is normally not cov- well as prediction of particle size and shape. Attri- ered efficiently and sufficiently in higher education tion is causing important losses to different food in- curricula of some engineering programs, mainly be- dustries, mainly cereal industry, so means of trans- cause its theoretical principles deal with a number of lating particle properties to single and multi particle subjects ranging from physics principles to applied breakage would be quite useful to manipulate proc- fluid mechanics. In recent years, separation tech- ess variables in order to prevent such an undesir- niques involving solids have been considered under able phenomenon. Size enlargement is widely ap- the general interest of powder and particle technol- plied in diverse food processes and some needs for ogy, as many of these separations involve removal research are better understanding of melt granula- of discrete particles or droplets from a fluid stream. tion, development of combined operations such as Separation techniques are defined as those opera- granulation and coating, effects of size and shape tions that isolate specific ingredients of a mixture on rheology of extrudates, etc. Pertaining solids without involving a chemical reaction. Several crite- mixing, more reliable predicting methods of blending ria have been used to classify or categorise separa- time and mixing indices are needed. Cyclones gen- tion techniques. One of such consists in grouping erally operate at low pressures with relatively low them according the phases involved, i.e. solid with solid loadings, but they may be required to function liquid, solid with solid, liquid with liquid, etc. Dry at higher solid loadings and system pressures; sys- separation techniques constitute all those cases in tematic data on how these variables affect cyclone which the particle to be isolated or segregated from operation are not available. It can be also stated a mixture is not wet. The most important dry sepa- that there is a dearth of information on modelling of ration techniques in processing industries have drop formation and solidification in spray drying, as been reviewed by Beddow [7]. In food processing, well as on the effects of operating variables of fluid- there are important applications of dry separation ised bed drying in quality of the obtained products. techniques, such as the removal of particles from Finally, fundamental research is needed on the dust-laden air in milling operations, the recovery of general field of improving rehydration and reconsti- the dried product in spray dehydration, and the tution properties of diverse dehydrated foods. cleaning of grains prior to processing. REFERENCES 4 FUTURE RESEARCH TRENDS [1] Merrow, E W: Estimating startup times for sol- Powder technology is of dynamic significance to the ids-processing plants, Chem. Eng. 89 (1988), world economy with a broad range of industries tak- 89-92. ing advantage of the rapidly growing knowledge in [2] Marinelli, J and Carson, J W: Solve solids flow this discipline. Research within universities and problems in bins, hoppers and feeders, Chem. similar institutions, coupled with the vested interest Eng. Prog. 88 (1992), 22-26. from the industrial community, has stimulated rele- [3] Carson, J W, Royal, T A and Goodwill, D J: vant results which have been applied to make more Understanding and eliminating particle segre- efficient a number of processes. Periodical meet- gation problems, Bulk Solids Handl. 6 (1986), ings and specialised publications [8] are spreading 139-142. the most relevant and recent advances in diverse [4] Royal, T A and Carson, J W: How to avoid topics such as materials handling, particle formation, flooding in powder handling systems, Powder mixing, grinding and separation. As mentioned at Handl. Process. 5 (1993), 63-67. the beginning of this contribution, there has been a [5] Jenike, A W: Storage and Flow of Solids. Bulle- rapid growth of adapting knowledge of particle tech- tin No. 123. Utah Engineering Experiment Sta- nology worldwide in recent years. However, for the tion. Salt Lake City, UT, USA (1964). case of application to biological materials, specifi- [6] Niranjan, K: An appraisal of the characteristics cally food powders and particulates, there is much of food mixing. In: Food Process Design and to be done as very few research groups can be Evaluation, Edited by R K Singh. Technomics, identified in this particular field around the globe. Lancaster, PA, USA (1995). Since many strategic food industries, such as those [7] Beddow, J K: Dry separation techniques, based on grain processing, rely heavily on a solid Chem. Eng. 88 (1981), 70-84. understanding of powder technology, there is a lot of [8] Ortega-Rivas, E: Food powder processing. In: potential to establish research programs and UNESCO Encyclopaedia of Life Support Sys- schemes to attend this demand. Many lines of re- tems (EOLSS) Eolss Publishers, Oxford, UK, search could be mentioned as examples, but some http://www.eolss.net (2002).

4.14 FOOD POWDER PROCESSING

E. Ortega-Rivas Graduate Program in Food Science and Technology, University of Chihuahua, Mexico

CONTENTS

1. Introduction: Applied Powder Technology to Food Materials 2. Comminution 2.1. Principles of Size Reduction; Properties of Comminuted Products 2.2. Energy Requirements: Comminution Laws SUMMARY 2.3. Size Reduction Equipment: Features and Operation Process and design aspects of unit operations involving 2.4. Criteria for Selection of Comminution Processes particulate solids applied to foods are discussed. 3. Attrition Theoretical considerations, operating principles, and 3.1. Mechanisms of Attrition applications of different techniques used to process 3.2. Kinetics of the Attrition Process powders in the food industry are reviewed. This industry 3.3. Compaction Characteristics and the Fractal needs to make its processing operations more efficient Approach in order to contribute to a better human environment. 4. Mixing In this sense, future competitiveness may be critically 4.1. Introduction: Statistical Approach to Solids dependent on knowledge originating from research Mixing activities on particle technology, as food processing 4.2. Mixing Mechanisms–Segregation involves a great number of raw materials and products 4.3. Assessment of Mixing Processes: Mixing Index in particulate form. This article attempts to provide 4.4. Powder Mixers criteria and information for students, academics, and 5. Separation and Classification industrialists, who may perceive that future growth in 5.1. Sieving and Screening this strategic industry may be dependent on a deeper 5.2. Dedusting Technology: Cyclones and Filters understanding of particle technology. 5.3. Air Classification 6. Agglomeration and Growth 1. INTRODUCTION: APPLIED POWDER 6.1. Introduction: Size Enlargement Processes TECHNOLOGY TO FOOD MATERIALS 6.2. Aggregation Fundamentals: Strength of Agglomerates The food processing industry is one of the largest 6.3. Agglomeration Methods manufacturing industries in the world. Undoubtedly, 7. Drying and Reconstitution it possesses a global strategic importance and, as 7.1. Powder Dryers: Fluidized Bed Dryers, Spray such, has a critical need for growth based on future Dryers research determined by an integrated interdisciplinary 7.2. Reconstitutability of Dried Powders approach to problems in food process engineering. 8. Conclusion and Further Trends Due to relevant recent developments in instrumen- tation and measuring techniques, a biophysical under- standing of foods is advancing rapidly. Detailed study on the physical properties of foods has revealed their important impact on food processes. Among these properties, those related to bulk particulate systems, such as particle size distribution and particle shape, are directly involved in an important number of unit operations (e.g., size reduction, mixing, agglomeration, dehydration, and filtration). The optimum operation for many food processes relies heavily on a good knowledge of the behavior of particles and particle assemblies, either in dry form or as suspensions.

629 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS

There is a relatively new branch of science and tools to improve knowledge and understanding of this engineering known as Powder Technology or Particle focused discipline. Technology, which deals with the organized study of particulate systems in a broad sense. In the case of 2. COMMINUTION food products and materials, some important appli- cations of powder technology can be mentioned. For 2.1. Principles of Size Reduction; Properties example, particle size in wheat flour is an important of Comminuted Products factor in the functionality of food products: attrition of instant powdered foods reduces their reconstitut- Many food processes frequently require the reduction ability, uneven powder flow in extrusion hoppers may in size of solid materials for different purposes. For affect the rheology of the paste, and appropriate charac- example, size reduction may aid other processes, such terization of fluid-particle interactions could optimize as expression and extraction, or may shorten heat treat- clarification of juices. ments, as in blanching and cooking. Comminution is Research efforts have recently shown tremendous the generic term used for size reduction and includes growth. European and Asian professional associations different operations such as crushing, grinding, milling, and societies have recognized the importance of powder mincing, and dicing. Most of these terms are related to technology for some time. Some of these associa- a particular application, for example, milling of cereals, tions, such as the Institution of Chemical Engineers mincing of beef, dicing of tubers, or grinding of spices. (IChemE) and the Society of Chemical Industry The reduction mechanism deforms the piece of food (SCI) in the United Kingdom, include well established until it breaks or tears. Breaking of hard materials research groups on the topic and organize meetings and along cracks or defects in their structure is achieved by conferences on a regular basis. In the United States, it applying diverse forces. The types of forces commonly was not until 1992 that a division known as the Particle used in food processes are compressive, impact, attrition Technology Forum was formed within the American or shear, and cutting. Institute of Chemical Engineers (AIChE). This forum Compressive forces are used for coarse crushing of has also organized meetings from 1994 to date with a hard materials. Coarse crushing implies reduction in good deal of success. In a more global context, diverse size to about 3 mm. Impact forces can be regarded associations worldwide have organized as many as three as general purpose forces and may be associated with congresses on particle technology. The last of the series, coarse, medium and fine grinding of a variety of food the World Congress on Particle Technology 3, was held in materials. Shear or attrition forces are applied in fine Brighton, UK in 1998, with more than 800 delegates pulverization, when size of the product reaches the from around the globe attending to present the most micrometer range. Sometimes the phrase ultra-fine advanced developments in the area. grinding is associated with processes in which a sub- Regarding powder and particle technology applied micron range of particles is obtained. Finally, cutting specifically to food materials, there has not been as much gives a definite particle size and may even produce a activity as that applied to the inert materials discussed definite shape. above. The main conferences/ meetings of professional As stated earlier, the breakdown of solid material is associations and societies related to food processing do done through the application of mechanical forces that not normally include sessions on food powders. These attack fissures present in the materials original structure. include the Institute of Food Technologists (IFT) in These stresses have been traditionally used to reduce the United States, the Institute of Food Science and the size of hard materials, from either inorganic (e.g., Technology (IFST) in the United Kingdom, and rocks and minerals) or organic origin (e.g., grains and the global International Union of Food Science and oilseeds). In such cases, comminuted particles obtained Technology (IUFoST). Some isolated efforts have been following any size reduction operation will resemble made to promote the exchange of ideas among powder polyhedrons with nearly plane faces and sharp edges technologists with an interest in food and biological and corners. The number of major faces may vary, but is materials. For instance, sessions on food powders were usually between 4 and 7. A compact grain with several included in the conference known as Food Ingredients nearly equal faces can be considered spherical, so the Europe in 1993 and in the 26th Annual Meeting of the term diameter is normally used to describe particle size Fine Particle Society in 1995. With contributions from in these comminuted products. the latter, a special issue of the journal, Food Science and The predictable shapes of products described above Technology International, was published in 1997. have to do with molecular structure, since silicon and This article reviews powder technology as applied carbon, elements of the same group in the periodic to food materials. Similar to other process industries, table, are generally key components of the crystal the food industry deals with a great number of raw units that form the solid matrix. In this sense, a good materials and products in powdered or particulate form. number of food materials will present the hardness Theoretical and application principles of food powder associated with the rigid structure of carbon deriva- processing will be discussed in an effort to provide basic tives and, as such, will fragment following the same

630 FOOD POWDER PROCESSING pattern of their relatives in the inorganic world whose 2.2. Energy Requirements: Comminution Laws structure is due to the presence of silicon compo- nents. An ideal size reduction pattern to achieve a As previously discussed, in the breakdown of hard and high reduction ratio of hard brittle food materials, brittle solid food materials, two stages of breakage are such as sugar crystals or dry grains, could be obtained recognized: (1) initial fracture along existing fissures first by compressing, then by using impact force, and within the structure of the material, and (2) formation finally by shearing or rubbing. Therefore, only these of new fissures or crack tips followed by fracture along hard brittle food materials would produce powders these fissures. It is also accepted that only a small when subjected to different forces in a comminution percentage of energy supplied to the grinding equipment operation, whereas tough ductile food materials, such is actually used in the breakdown operation. Figures of as meat, can only be reduced in size by applying cut less than 2 % efficiency have been quoted and, thus, forces. In fact, cutting is considered a process totally grinding is a very inefficient process, perhaps the most different from comminution because its operating inefficient of traditional unit operations. Much of the principles are quite different from those governing the input energy is lost in deforming the particles within size reduction of hard materials. their elastic limits and through inter-particle friction. A In a comminution operation of food materials, large amount of this wasted energy is released as heat, more than one type of the above-described forces is which in turn may be responsible for heat damage of actually present. Regardless of uniformity of the feed biological materials. material, the product always consists of a mixture of Theoretical considerations suggest that the energy particles covering a range of sizes. Some size reduction required to produce a small change in the size of unit equipment is designed to control the size of the largest mass of material is expressed as a power function of particles in the products, but the finer sizes are not the material size: under control. In spite of the hardness of the commi- dE K nuted materials, the above-mentioned shapes of ___ = – __ (1) produced particles would be subjected to attrition, dx xn due to inter-particle and particle-equipment contacts within the dynamics of the operation. Thus, particle where dE is the change in energy, dx is the change in angles will gradually become smooth with the conse- size, K is the constant, and x is the particle size. quent production of fines. In practice, any feed material Equation (1) is often referred to as the general law will possess an original particle size distribution, while of comminution and has been used by a number of the obtained product will end up with a new particle workers to derive laws more specific, depending on size distribution having a whole range finer than the the application. For example, in the grinding of solids, feed distribution. Rittinger considered that the energy required should Product specification will commonly require that be proportional to the new surface produced, and gave a finished product not contain particles greater than the power n a value of 2, thus by integrating Equation (or smaller than, depending on the application) some (1) obtained the so-called Rittinger’s law: specified size. In comminution practice, particle size is 1 1 often referred to as screen aperture size (Section 5.1). E = K __ – __ (2) The reduction ratio, defined as the relationship between [ x2 x1] average size of feed and average size of product, can be used as an estimate of performance of a commi- where E is the energy per unit mass required for the nution operation. The values for average size of feed production of a new surface by reduction, K is called and product depend on the method of measurement, Rittinger’s constant and is determined for a particular but the true arithmetic mean, obtained form screen equipment and material, x1 is the average initial feed analyses on samples of the feed and product streams, size, and x2 the average final product size. Rittinger’s is commonly used for this purpose. Reduction ratios law has been found to hold better for fine grinding, depend on the specific type of equipment. As a rule, where a large increase in surface results. the coarser the reduction, the smaller the ratio. For Kick considered that the energy required for a given example, coarse crushers have size reduction ratios of size reduction was proportional to the size reduction below 8: 1, while fine grinders may present ratios as ratio, and gave the power n a value of 1. Thus, by high as 100: 1. However, large reduction ratios, such integration of Equation (1), the following relationship, as those obtained when dividing relatively large solid known as Kick’s law is obtained: lumps into ultra-fine powders, are normally attained x1 through several stages using diverse crushing and E = K 1n __ (3) grinding machines. A good example of this is in the [ x2 ] overall milling of wheat grain into fine flour, in which crushing rolls in a series of decreasing diameters are where x1/x2 is the size reduction ratio. Kick’s law has employed. been found to hold more accurately for coarser crushing,

631 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS where most of the energy is used in causing fracture utilized in ultra-fine reduction, and cutting machines along existing cracks. used for exact reduction. The equipment is generally A third version of the comminution law is the one known as a crusher when performing coarse reduction, attributed to Bond, who considered that the work and milling when used for all other applications. necessary for reduction was inversely proportional to the The above-mentioned classification includes several square root of the size produced. In Bond’s consider- categories for each type of machine, so in total a number ation, n has a value of 3/2, yielding the following version of approximately 20 different designs are recognized (Bond’s law), also by integrating Equation (1): in comminution processes. In the food industry, not all equipment has important applications. Figure 1 1 1 E = 2K [ ___ __ – ___ __ ] (4) shows the principal size reduction machines used in √x2 √x1 food processing. In crushing rolls, two or more heavy steel cylinders

When x1 and x2 are measured in micrometers and E in revolve towards each other (Figure 1a) so that particles kWh/ton, K=5Ei, where Ei is the Bond Work Index, in feed are nipped and pulled through. The nipped which is defined as the energy required to reduce a particles are subjected to a compressive force that causes unit mass of material from an infinite particle size to the reduction in size. In some designs, differential speed a size where 80 % passes through a 100 micrometer is maintained to exert shearing forces on the particles. sieve. Bond’s law holds reasonably well for a variety The roller surface can be smooth or can carry corruga- of materials undergoing coarse, medium, and fine size tions, breaker bars, or teeth, so as to increase friction reduction. and facilitate trapping of particles between the rolls. Toothed-roll crushers can be mounted in pairs, like 2.3. Size Reduction Equipment: Features the smooth-roll crushers, or with only one roll working and Operation against a stationary curved breaker plate. Toothed-roll crushers are much more versatile than smooth-roll As previously discussed, size reduction is a unit crushers but have one limitation; they cannot handle operation widely used in a number of processing very hard solids. They operate by compression, impact, industries. Many types of equipment are used in size and shear, and not by compression alone, as do smooth- reduction operations. In a broad sense, size reduction roll crushers. Crushing rolls are widely applied in the machines may be classified as crushers used mainly milling of wheat and in the refining of chocolate. for coarse reduction, grinders employed principally in Figure 1b shows a hammer mill, a piece of equipment intermediate and fine reduction, ultra-fine grinders containing a high-speed rotor turning inside a cylin-

Figure 1. Size reduction equipment used in food processing

632 FOOD POWDER PROCESSING drical case. The rotor carries a collar bearing a number and dropped onto the material being comminuted, of hammers around its periphery. In the rotating action, filling the void spaces between the medium. The the hammers swing through a circular path inside the grinding medium components also tumble over each casing containing a toughened breaker plate. Feed other, exerting a shearing action on the feed material. passes into the action zone with the hammers driving This combination of impact and shearing forces brings the material against the breaker plate, forcing it to pass about a very effective size reduction. As a tumbling mill through a bottom mounted screen by gravity when basically operates in a batch manner, different designs the particles attain a proper size. Reduction is mainly have been developed to make the process continuous. due to impact forces, although under choke feeding In a tube mill, the cylinder has a slope with a horizontal conditions, attrition forces can also play a part in such plane; thus, the material travels a single pass while reduction. The hammers may be replaced with knives, being reduced. Putting slotted transverse partitions in or any other device, making possible the mills handling a tube mill converts it into a compartment mill. One of tough, ductile, or fibrous materials. The hammer compartment may contain large balls, another small mill is a very versatile piece of equipment that gives balls, and a third pebbles, achieving thus a segregation high reduction ratios and can handle a wide variety of of grinding media with a consequent rationalization materials from hard and abrasive to fibrous and sticky. of energy. An efficient way to segregate the grinding In the food industry its applications are quite varied, medium is to use the conical ball mill illustrated in being extensively used for grinding spices, dried milk, Figure 1f. While the feed solid enters from the left sugar agglomerate, cocoa press cake, tapioca, dry fruits, into the primary grinding zone, where the shell is at dry vegetables, and extracted bones. maximum diameter, the comminuted product exits Disc attrition mills, such as those illustrated through the cone at the right end where the shell is in Figures 1c through 1e, use shear forces in size at minimum diameter. As the shell rotates, the large reduction, mainly in the fine size range of particles. balls move toward the point of maximum diameter, There are several basic designs for attrition mills. The and the small balls migrate toward the discharge single disc mill (Figure 1c) has a high speed rotating outlet. Therefore, the initial breakage of feed particles grooved disc, leaving a narrow gap with its stationary is performed via the largest balls dropping the greatest casing. Intense shearing action results in comminution distance, whereas the final reduction of small particles of the feed. The gap is adjustable, depending on feed occurs with the small balls dropping a smaller distance. size and product requirements. In the double disc mill In such an arrangement, the efficiency of the milling (Figure 1d), the casing contains two rotating discs that operation is greatly increased. Among applications of rotate in opposite directions, giving a greater degree tumbling mills in the food industry, the reduction of of shear compared with the single disc mill. The pin- fluid cocoa mass can be named. disc mill carries pins or pegs on the rotating elements. In this case, impact forces also play an important role 2.4. Criteria for Selection of Comminution in particle size reduction. The Buhr mill (Figure 1e), Processes which is an older type of attrition mill, originally used in flour milling, consists of two circular stones mounted In deciding how to crush or grind a food material, on a vertical axis. The upper stone is normally fixed and process engineers should consider factors like size has a feed entry port, while the lower stone rotates. The distribution of the feed and product, hardness and product is discharged over the edge of the lower stone. mechanical structure of the feed, moisture, and temper- The applications of attrition mills in the food industry ature sensitivity of the feed. Regarding size distribu- are quite extensive. They have been employed in dry tions of the materials, each type of crusher or grinder milling of wheat, as well as wet milling of corn for the is intended for a certain size of feed and product. It is separation of starch gluten from the hulls. Other appli- usually possible to exercise some control over the size cations include breaking of cocoa kernels, preparation of feed, but sometimes it must be taken as it comes. As of cocoa powder, degermination of corn, production there is an upper limit on the size accepted by a machine of fishmeal, manufacture of chocolate, and grinding of without jamming, if there is oversized material, a guard sugar, nutmeg, cloves, roasted nuts, peppers, etc. screen is needed to keep pieces too large out of the A tumbling mill is used in many industries for fine crusher or grinder. In the case of too much undersized grinding. It basically consists of a horizontal, slow material, pre-screening of the feed can cut down the speed, rotating cylinder that is partially filled with amount going through the equipment. For small-scale either balls or rods. The cylinder shell is usually made operations such cutting is important, as it decreases the of steel, lined with carbon-steel plate, porcelain, silica capacity required; in large scale equipment, however, rock, or rubber. The balls are normally made out of the undersized particles simply pass through the throat, steel or flint stones, while the rods are usually manufac- where there is always ample room, so their removal tured with high carbon steel. The reduction mechanism does not greatly affect capacity. is carried out as follows: as the cylinder rotates, the One of the major factors governing the choice and grinding medium is lifted up the sides of the cylinder design of size reduction machines is the hardness of the

637 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS material being processed. As a general rule, hardness and shear forces, even hard particles will erode and is defined in accordance with the Mohs’ scale, which produce fines until the product presents a particle size is divided into ten grades of hardness. As a rule of distribution instead of a homogeneous distribution. thumb in using the Mohs’ scale, any material would While attrition can be considered quite normal in be considered soft when having a value between 1 comminution, it represents a real problem in agglom- and 3, medium-hard between values 3.5 and 5, and erated systems and some particulate food materials, hard between values 5 and 10. Many food materials, such as instant coffee and ready-to-eat breakfast especially when dry, are brittle and fragile on the Mohs’ cereals. Attrition in agglomerated food powders is scale, in the order of 1 to 2. According to this, ball undesirable but somewhat inevitable. Food powders mills, hammer mills, roller mills, and attrition mills are are agglomerated in order to improve rehydration and very suitable for treating most solid foods commonly to avoid lump formation when reconstitution takes used in the food industry. Knowledge of the mechanical place. In agglomeration of fine powders, of about structure of the feed material is useful in determining 100 micrometers, into particles up to several millim- the most likely force needed in its size reduction. As eters in size, wetting is improved and lump formation mentioned above, many food materials are brittle and avoided, making the agglomerate actually an “instant” fragile, so compressive forces may be employed. Some formulation. other food materials have a fibrous structure and are not Agglomerated systems are brittle and fragile, and easily disintegrated by compressive or impact forces, so can easily break down when exposed to mechanical cutting may be required. motion. The main causes of agglomeration include The presence of moisture can be either beneficial or the kinetic energy absorbed by the particulates when inconvenient in comminution processes. It is well known colliding with each other or against static walls, and that safety problems are caused by dust formation, the mechanical compression of the structure. As previ- arising during the dry milling of many solid materials. ously mentioned, there are three recognized mecha- The presence of small quantities of water has been nisms in the attrition process: shattering, erosion, and found useful in the suppression of dust and, in appli- chipping. Shattering is considered the breaking down cations where the presence of moisture is acceptable, of particles to form smaller ones; erosion is interpreted water sprays are often used to reduce dust formation. as the smoothing of pointed angles to release fines Some other applications allow for large quantities of from individual particles, while chipping is said to be water to be introduced in the size reduction process; a combination of both mechanisms. Figure 2 illustrates wet milling of corn is a good example of this. On the the mechanisms of attrition. other hand, in many cases feed moisture content in As previously mentioned, attrition is a huge excess of 2 to 3 % can lead to clogging of the mill problem in diverse food products. The most negative with consequent effects on throughput and efficiency. aspects of attrition may be summarized as follows: Agglomeration can also be caused by moisture, which creation of fines and consequent loss of product; is undesirable when a free flowing powder is needed to change in product bulk properties; wear in handling control the feed rate. system, a result of particle impact; contamination As stated earlier, comminution is possibly the of processed particles; risk of dust explosion; segre- most inefficient unit operation in the food processing gation of fines from bed structure, adversely affecting industry. The excessive friction present in most size product appearance; and reduction in wetability and reduction machines causes a rising of heat, which can dispersibility of a product. Attrition has an important lead to a considerable rise in temperature of the material being processed. Since food materials are normally heat sensitive, degradation reactions can occur. The release of sticky substances caused by the heat rise may also pose a problem. For these reasons, some crushing and grinding machinery may be equipped with cooling devices, such as jacket, coils, and so on.

3. ATTRITION 3.1. Mechanisms of Attrition

Attrition can be defined as the fractionation of particles, or release of fine particles from a mother solid piece, due to shattering, erosion, or chipping involved in handling or processing of particulate systems (see Physical Properties of Food Powders). When grinding different materials, principally by impact Figure 2. Mechanisms of attrition

638 FOOD POWDER PROCESSING impact on the marketing of food powders. It is one of because the fines would have a size much smaller than the quality factors most often used to remove instant the parent particles and would appear on the graph products from the shelves of supermarkets (a visual as a different population. Finally, chipping would be quantification of fines at the bottom of the container). represented as a bi-modal resultant distribution because Attrition has been detected in several manufacturing fines are also produced. A diagram of such distribution processes involving agglomerated food products, such patterns is presented in Figure 3. as fluidized beds, cyclone separation, bulk material In order to characterize the attrition process, the handling, as well as during bulk product packaging change in particle size distribution shape can be studied and storage. as a function of sample tapping. One of the most convenient ways to present attrition data is through 3.2. Kinetics of the Attrition Process an attrition curve showing the relationship between the percentage of particles retaining original size as The shape of particle size distribution provides infor- a function of characteristic time or number of taps. mation on the dominance of attrition mechanisms Several kinetics models have been proposed to charac- described above. It is known that powders and partic- terize these curves, among which include the single ulate systems may be represented by a particle size term exponential model, the two-term exponential distribution, usually a mono-modal shape. Therefore, model, and the non-exponential model. the effect of shattering would be somewhat equivalent The simple term exponential model is represented to a size reduction process, with the powder subjected by to attrition, and having a particle size distribution that is also mono-modal, but differently positioned on a W(n) = W(0) [e–St] (5) graph of frequency against particle diameter. On the other hand, the considerable amount of fines produced where W(n) is the weight fraction of particles retaining by erosion would result in a bi-modal distribution, original sample size, W(0) is the weight sample of

Figure 3. Particle size distribution patterns in attrition

639 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS mother particles, S is a rate constant, and t is the 3.3. Compaction Characteristics and the time. Fractal Approach The two-term exponential model has been found to be more accurate for particles larger than 16 mesh It is expected that the behavior of agglomerated food and is expressed as: systems under compression loads will differ from other particulate materials, due to a combined effect of attrition _____W(n) – S n – S n = A’ [e 1 ] + (1 – A’)[e 2 ] (6) and compaction (see Physical Properties of Food Powders). W(0) The following expression is considered suitable for low compression stresses in food powders: where S1 and S2 are characteristic rate constants, and ρ σ A’, as well as (1-A’), are fractions of the material disin- s = a’’ + b log (9) tegrating at the rates S1 and S2, respectively. ρ σ The non-exponential model is represented as where s is the density, is the compression stress, follows: a” is a constant, and b is another constant known as compressibility. In the case of compression by tapping, n W(n) = 1 – ______(7) the following model can be applied: k1 + k2n ρ ρ – n/k A – n = A’’ [e ] (10) where k1 and k2 are constants. ρ ρ The above-described analytical functions can be where A and n are the asymptotic and “n” taps bulk represented graphically by an attrition curve, previ- densities, respectively, and A” is a constant. ously mentioned, giving the trends illustrated in Equations (9) and (10) enable calculation of the ρ Figure 4. relationship between asymptotic bulk density, A, and ρ Another way to describe the extent of the attrition initial density, 0, known as the Hausner (HR) ratio phenomena is by the erosion index, Ie: and expressed as

A(n)M(0) ρ 1 I = ______(8) H = __A = _____ (11) e R ρ M(0) – M(n) 0 1 – a’ where A(n) is the weight fraction of fines at “n” taps, where a’ is a constant. and M(0) and M(n) are the initial at time t mode, The Hausner ratio measures how much a powder respectively. Notice that the variable n can be either can be compacted after an infinite number of taps. time or number of taps. This index is a dimensionless For some cases, such as agglomerated coffee, this ratio number ranging from 0, no erosion, to infinity, when also quantifies the extent of attrition after a very large the mode has no change after a certain number of number of taps. taps. A good criteria is to consider that erosion index In addition to size, particle shape has been recog-

Ie < 1 represents a shattering dominant process, while nized as a useful tool to describe the eroding process erosion index Ie > 1 indicates an erosion dominant in particulate materials. Among many characterization process. techniques, Fractal analysis has emerged as a widely used criterion for rugged or re-entrant particles, and has been successfully applied in other disciplines, such as sedimentology and geology. It has been proved that for a self-similar profile, a logarithmic plot of the profiles perimeter against the step length can be fitted into a linear relationship. The Fractal dimension of profile F is defined as

F = 1 – D (12)

where D is the slope of the line. This is a non-integer dimension ranging from 1 to 2 for two-dimensional profiles. Value 1 corresponds to a perfectly straight line and value 2 corresponds to an infinite irregular and self-similar profile, which can only be obtained by computer-generated figures. It has been established that real particles have a Fractal dimension ranging Figure 4. Schematic view of different mathematical models from 1.05 to 1.36, since a lower value indicates a lack describing attrition kinetics of self-similarity of contour and a higher value makes

640 FOOD POWDER PROCESSING the particle mechanically unstable. A true self-similar In practice, however, a perfectly random mixture is profile is one exhibiting the same degree of roughness commonly defined as one in which the probability for any range of scrutiny at which the boundary line is of finding a particle of a constituent in the mixture is examined. Real particles are not true-similar, but do the same for all its points. Over the years, many have exhibit one or several ranges of scrutiny in which this attempted to establish criteria for the completeness and condition holds. degree of mixture. In order to accomplish this, frequent The Fractal dimension concept has been used as sampling of the mix is usually required, and tending to a tool to characterize the attrition process in agglom- be statistical in nature, such an exercise is often of more erated food powders. The ruggedness of individual interest to mathematicians than to process engineers. particles could be quantified in terms of the natural Thus, in practical mixing applications, an ideal mixture Fractal dimensions of silhouettes obtained from their may be regarded as one produced at minimum cost micrographs, or from an optical microscope. Since and one that satisfies the product specifications at the attrition is mainly an erosion process clearly affecting point of use. surface ruggedness, it is feasible to use a Fractal Sampling is a crucial step in the mixing process approach in quantifying the extent of attrition in because any form of control of mixing operations powders after exposure to mechanical solicitations. It involves sampling procedures. The sample must be has been found that a Fractal dimension of the particle representative of the mixture and post-sampling silhouette decreases with the number of taps. This result handling must not alter it. As sampling also has a is valid for particles with original Fractal dimensions statistical aspect, sampling procedures following a pure, significantly higher than in the case of agglomerates. In mathematical approach are not completely practical in other cases, the validity of the method fails because the industrial situations. The confidence placed on any natural Fractal dimension is too low and meaningless, results obtained from the sampling and mixture analysis due to the lack of self-similarity in contours. is greatly influenced by several factors, including the method of sampling, the number of samples, the 4. MIXING size of sample, and the location in the bulk material from which the sample is taken. If sampling is not 4.1. Introduction: Statistical Approach to performed carefully, the mixture determination could Solids Mixing be considered meaningless. It can be demonstrated by statistical means that The unit operation in which two or more materials the larger the number of samples, the more reliable are interspersed in space with each other is one of the results. For example, a statistical theory of the oldest and yet one of the least understood of unit sampling states that the most representative sample operations in process engineering. Mixing is used would approach an infinite number of samples. In in the food industry mainly to reduce differences in other words, the only way to include every member properties such as concentration, color, texture, taste, of the population being sampled is to sample the and so on, among different parts of a system (see entire population. However, for practical purposes in Engineering Properties of Foods, Food Mixing). Since the mixing of food powders, it has been established the components being mixed can exist in any of three that at least 50 samples (but not less than 20) should states of matter, a number of mixing possibilities be taken to obtain representative results. The size of can arise. The mixing cases involving a fluid (e.g., the sample is also important. If a sample particle can liquid-liquid and solid-liquid) are most frequently be drawn from the mixture, no mixing is evident. In encountered so they have been extensively studied. contrast, if the whole mixture is analyzed, provided Despite the importance in many areas of processing the ingredients are present in the correct propor- in the mixing of particulate materials, fundamental tions, complete homogeneity would appear to have work of real value to both designers and users of been achieved. As both extremes are impractical and solids mixing equipment is still relatively sparse. It unreliable, the recommended sample volume, often is through studies conducted in very specific fields, called the scale of scrutiny or characteristic sample size, such as powder technology and multi-phase flow, that falls between the two and is defined as the size of important advances in understanding the mixing of sample taken that corresponds with product usage. solids and pastes have been made. In animal feed manufacture, for instance, the feed In the mixing of particulate solid materials the contains and , balanced with probability of getting an orderly arrangement of particles added nutrients. In a particular feed, an animal must representative of the perfect mixture, is virtually zero. receive the correct balance of components. Provided In practical systems, the best mixture attainable is that the required quantities of necessary ingredients are in which there is a random distribution of ingredients. present in the food consumed at each feed, intimate An ideal random distribution of two solid components mixing is not essential. Thus, the volume of sample in equal proportions would resemble a chess board, i.e., that would give such balance would be the most useful black and white squares in a perfect alternate pattern. one, regardless of its perfection in statistical terms.

641 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS

4.2. Mixing Mechanisms−Segregation act by intermingling two or more separate materials. Once a material is randomly distributed throughout Three mechanisms have been recognized in solids another, mixing may be considered complete. Based mixing: convection, diffusion, and shear. In any on these concepts, a statistical procedure for deter- particular process, one or more of these basic mecha- mining the effectiveness of a mixing operation can be nisms may be responsible for the course of operation. performed.. If spot samples are taken at random from In convective mixing, masses or groups of particles a mixture and analyzed, the standard deviation of the transfer from one location to another; in diffusion analyses, s, on the average value for the specific powder mixing individual particles are distributed over a surface fraction,⎯x, is estimated as follows: developed within the mixture; in shear mixing, groups ______of particles are mixed via the formation of slipping N ∑ (x –⎯x)2 planes developed within the mass of the mixture. Shear s = ______i = 1 i (13) mixing is sometimes considered as part of a convective √ N – 1 mechanism.

A combination of the mechanisms described above where xi is every measured value of fraction of one promotes the mixing of solids in diverse types of mixers. powder and N the number of samples. However, during a mixing operation, particle movement For a completely blended mixture of two powders, can also result in another mechanism that could retard or if N spot samples are taken, each containing n particles even reverse the mixing process, and is known as segre- and an overall fraction of particles of one kind in µ gation. When particles differing in physical properties, the total mix, p, for any given spot sample size the σ particularly size and/or density, are mixed, mixing is theoretical standard deviation e is given by ______accompanied by the tendency to unmix. Thus, in any µ µ mixing operation, mixing and de-mixing may occur σ p (1 – p) e = ______(14) concurrently; the intimacy of the resulting mix depends √ n on the predominance of the former mechanism over the latter. Apart from the properties already mentioned, For granular solids s, the mixing index, Is, is defined as surface properties, flow characteristics, friability, σ moisture content, and tendency to cluster or agglom- __e Is = (15) erate, may also influence the tendency to segregate. The s closer the ingredients are in size, shape, and density, the easier the mixing operation becomes, and the more Substituting from Equations (13) and (14) into intimate the final mix. Once the mixing and de-mixing Equation (15) yields a mixing index for powders as mechanisms reach a state of equilibrium, the condition follows: ______of the final mix is determined, and further mixing will µ µ not produce a better result. p (1 – p)(N – 1) Is = ______(16) The importance of segregation on the degree of √ N n ∑ (x –⎯x)2 homogeneity achieved in solids mixing cannot be i = 1 i over-emphasized. Any tendency that segregation can occur must be recognized when selecting solids mixing By comparing the mixing of powders with the mixing equipment. Segregation in a mixture of dry solids is of pastes, an equation for estimating the time required readily detected by use of a heap test. A well-mixed for any desired degree of mixing is derived, provided sample of solids is poured through a funnel forming the segregation is not severe: a conical heap. Samples taken from the central core __ 1 1 – 1/√ n and from the outside edge of the cone should have t = __ 1n ______(17) essentially the same compositions, if segregation is not k [ 1 – Is ] to be a problem. When the two samples have signifi- cantly different compositions, it can be assumed that where k is a constant. segregation would likely occur, unless a very careful choice of equipment is made. It is generally accepted 4.4. Powder Mixers that the efficiency of a mixing process must be related to both the flow properties of the components, and to In general, mixers used for dry solids have nothing to the selection or design of the mixer. do with mixers involving a liquid phase. According to the previous discussion on mixing mechanisms, 4.3. Assessment of Mixing Processes: solids mixers can be classified into two groups: segre- Mixing Index gating mixers and non-segregating mixers. The former is mainly operated by a diffusive mechanism, while the The degree of uniformity of a mixed product may be latter involves a convective mechanism. Segregating measured by analyzing a number of spot samples. Mixers mixers are normally non-impeller type units, such as

642 FOOD POWDER PROCESSING tumbler mixers, whereas non-segregating mixers may dust hazard, or may be jacketed to allow temperature include screws, blades, and ploughs in their designs, for control. Due to small clearance between the ribbon and example, horizontal trough mixers and vertical screw the trough wall, this kind of mixer can cause particle mixers. damage and consume high amounts of power. Tumbler mixers operate by tumbling the mass of In vertical screw mixers, a rotating vertical screw is solids inside a revolving vessel. These vessels take located in a cylindrical or cone-shaped vessel. The screw various forms, such as those illustrated in Figure 5, may be mounted centrally in the vessel or may rotate which may be fitted with baffles or stays to improve or orbit around the central axis of the vessel near the performance. The shells rotate at variable speeds having wall. Such mixers are schematically shown in Figures values up to 100 rev/min with working capacities 7a and 7b, respectively. The latter arrangement is more around 50 to 60 % of the total. They are manufactured effective and stagnant layers near the wall are elimi- using a wide variety of materials, including stainless nated. Vertical screw mixers are quick and efficient, steel. This type of equipment is best suited for gentle and are particularly useful for mixing small quantities blending of powders with similar physical character- of additives into large masses of material. istics. Segregation can represent a problem if particles Applications for the types of mixers described above vary, particularly in size and shape. include: blending of grains prior to milling, blending Horizontal trough mixers consist of a semi-cylindrical of flours and incorporation of additives, preparation of horizontal vessel in which one or more rotating devices custard powders and cake mixes, blending of soup mixes, are located. For simple operations, single or twin screw and incorporation of additives in dried products. conveyors are appropriate and one passage through such a system may be enough. For more demanding duties, 5. SEPARATION AND CLASSIFICATION a ribbon mixer like the one shown in Figure 6 may be used. The design of the ribbon mixer typically consists of 5.1. Sieving and Screening two counteracting ribbons mounted on the same shaft. One moves the solids slowly in one direction while the Screening is a separation technique that screens other moves it quickly in the opposite direction. There mixtures of various sizes of solids particles into several is a resultant movement of solids in one direction, so fractions, based on size difference. It involves forcing the equipment can be used as a continuous mixer. Some the mixture through a screen of a specific size aperture. other types of ribbon mixers operate on a batch basis. In Small capacity plane screens are often called sieves. By these designs, the troughs may be closed to minimize vibrating or oscillating a screen, particles smaller than

Figure 5. Typical tumbler mixers

643 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS

Figure 6. Plain view diagram of a ribbon mixer

Figure 7. Vertical screw mixers a given aperture can pass through, thus being separated individual wires of a wire mesh screen, is the preferred from the remaining mixture. Screens are made from terminology for screening operations, the former desig- metal bars, perforated or slotted metal plates, woven nation of mesh number, defined as the number of wires wire cloth, or fabric, such as silk bolting cloth. The per lineal inch, is still widely adapted. metals used include steel, stainless steel, bronze, The objective of the screening operation is copper, nickel, and Monel. The screen surface may be to separate a feed stream into two fractions: the plane (horizontal or inclined) or cylindrical. The size underflow, which passes through the screen; and the aperture of a screen ranges from about 0.1 mm to 250 overflow, which is rejected by the screen. An ideal mm, with exceptional cases in which the aperture may screen would sharply separate the feed in such a way be as large as 460 mm, approximately. The material that the smallest particle in the overflow would be just passing through a given screen is termed undersize, larger than the largest particle in the underflow. Such fines, or minus (-) material, while the material retained an ideal separation would define a cut diameter Dpc, in a given size screen is called oversize, tails, or plus (+) which would represent the point of separation between material. Either stream may be the desired (product) the fractions. For an ideal operation, a plot of screen or undesired (reject) stream. Screening has two main opening against the cumulative fraction retained would applications: laboratory technique for particle size have the shape shown in Figure 8b. As can be seen, the analysis, and industrial operation for fractionation and largest particle in the underflow has the same size as the classification of particulate solids. Although the screen smallest particle in the overflow. In practice, however, aperture, which is defined as the space between the the plot would have the shape illustrated in Figure 8c,

644 FOOD POWDER PROCESSING

Figure 8. Plots of ideal and actual screening processes in which there is an overlap. The underflow has an the screen functions perfectly, all of material O is in appreciable amount of particles larger than the desired the overflow and all of material U is in the underflow. cut diameter, while the overflow has particles smaller By calculating the ratio of oversize material O that is than the desired cut diameter. It has been observed actually in the overflow and the amount of material that the overlap is small when particles are spherical (or O entering with the feed, screen efficiency is deter- close to a spherical shape) and larger when particles are mined: needle-like, fibrous, or tend to agglomerate. The efficiency of a screening operation may be ____OXO EO = (22) evaluated with simple mass balances. For example, let FXF F be the mass flow rate of feed, O the mass flow rate of tails, and U the mass flow rate of fines. Also, let Similarly, considering the fine material,

XF be the mass fraction of tails in the feed, XO the mass fraction of tails in the overflow, and X the mass ______U(1 – XU) U EU = (23) fraction of tails in the underflow. Furthermore, for F(1 – XF) fractions of fines in the feed, overflow and underflow would be 1-XF, 1-XO and 1-XU, respectively. Since the An overall combined efficiency is defined as the product total material fed to the screen must leave either as of Equations (22) and (23), denoted as E: overflow or underflow, OUX (1 – X ) E = ______O U (24) 2 F = O + U (18) F XF(1 – XF)

The tails in the feed must also leave in two streams, Substituting from Equations (20) and (21) into thus Equation (24) yields

(XF – XU)(XO – XF)XO(1 – XU) FXF = OXO + UXU (19) E = ______(25) 2 XO – XU) (1 – XF)XF Elimination of U from Equations (18) and (19) yields Equation (25) is an alternative expression for evaluating O ( X – X ) __ = ______F U (20) screen efficiency without involving the streams and only F (XO – XU) using the fractions. As an analytical laboratory technique, screening Similarly, elimination of O yields is used to determine the particle size distributions of powdered materials. Standard screens are used to U ( X – X ) __ = ______O F (21) measure the size of particles in a size range between F (XO – XU) about 0.04 and 75 mm. Testing sieves are made of woven wire screens, the mesh and dimensions of The effectiveness of a screen is measured by how well which are carefully standardized. The openings are it performs in the separation of tails and fines. When square and each screen has a formerly designated

645 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS

Figure 9. Different types of industrial screens mesh number, defined as the number of wires per is a series based on an 18 mesh screen with a 1.0 mm lineal inch. The current tendency is to refer to the aperture and screen interval of (2)1/4. An International screen opening or aperture instead, but as mentioned Standard (ISO) scale is also available. A number of before, mesh number is still widely used in laboratory sieves on both the BS and ASTM standards corre- and industry jargon. Screen aperture and the mesh spond to this proposal by the ISO Committee. Many number are not the same because of the thickness food products are characterized in terms of particle of the wires. The screen interval is the relationship size distribution using screening. Examples are baking between successively decreasing openings in a standard flours, sugar, cocoa powder, fruit powders, ground screen series. Several screen series are in use: Tyler vegetables, and spices, etc. Standard is a widely used series based on a 200 mesh Screening as a unit operation may be carried out screen having 0.05 mm diameter wires and a screen in different kinds of equipment. Three types are more aperture of 0.074 mm. The ratio between apertures common: grizzlies (bar screens), screens, and trommels. in consecutive screens is (2)1/2. British Standards (BS) Basic designs for each type of equipment are shown in is a screen series based on a 170 mesh screen with a Figure 9. Grizzlies are used for screening larger particles nominal aperture size of 0.09 mm, and with a screen (pieces greater than 25 mm). They consist of a set of interval of approximately (2)1/4 between consecutive parallel bars, spaced to the desired separation. The screens. American Society of Testing Materials (ASTM) bars are often wedge-shaped to minimize clogging.

646 FOOD POWDER PROCESSING

They may be used horizontally or inclined at angles where N is the number of revolutions of the trommel up to 60°. Vibrating grizzlies are available, the feed per minute and D is the diameter of the trommel in material passing over the screening surface in a series meters. of jerks. Screens come in many types: sifter, vibrating, In the food industry, screening is widely used for shaking, centrifugal, and revolving, to name only a few. cleaning and sorting of diverse commodities. Cleaning Sifter screens can be conveniently divided into circular- may be carried out in trommels or flat-bed screens, the motion, gyratory-motion, and circular-vibrator types. latter being in its simplest form, a pitched stationary They can be mounted in several decks and the rate of frame clad with a screen bed. The operation may be throughput increased by inclining the screen surface, arranged as to retain oversize material such as string, bag- as shown in Figure 9. In centrifugal screens, the surface hairs, etc., from flour, salt, or sugar, while discharging a consists of a vertical cylinder rotating at a constant cleaned product. Alternatively, the screen may be used to speed with a gyratory motion. Gravity moves the retain the cleaned material as oversize while discharging oversize material down the length of the cylinder as undesired material (e.g., in the removal of weed-seeds, fines are forced through the openings. The screens grit, and small stones from cereals). are normally inclined to the horizontal and may be Sorting fruits and vegetables by size is extensively multi-deck units, a series of screens mounted beneath performed also in flat-bed screens, as well as in trommels each other, permitting separation of a given feed stock or drum screens. Simple deck flat-bed screens are used into several size ranges. Reels or trommels are revolving for preliminary sorting of potatoes, carrots, and turnips. cylindrical screens mounted almost horizontally. Multi-deck screens of this type find extensive use in the Again, the screening surface may consist of wire mesh size sorting of raw materials, such as cereals and nuts, as or perforated sheet. Hexagonal cross sections are also well as in part-processes and finished foods like flour, used since these lead to agitation, which aids in the sugar, salt, herbs, and ground spices. Drum screens separation of fine material. The capacity of a trommel are used extensively as size sorters for peas, beans, and increases with increasing speed of rotation until a similar foods that can withstand the tumbling action critical speed is achieved. At speeds greater than this, produced by the drum rotation. Drum sorters are usually the material does not cascade over the surface but is required to separate the feedstock into two or more carried round by centrifugal force and separation is streams and, thus, two or more screening stages are seriously impaired. The critical speed of a trommel is needed. To achieve this, the screens may be arranged to given by operate concentrically or consecutively. The concentric drum screen illustrated in Figure 10a has the advantage 42.3 N = _____ (26) of compactness but, because it is fed at the center, the (D)1/2 highest product loading goes through the smallest

Figure 10. Drum screens for sorting of foods

647 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS screen area. The series-consecutive drum screen shown itself as it finds a reduction in rotation space due to in Figure 10b has the disadvantage of requiring a large the conical shape, creates an upward inner vortex in floor area. Also, since the feed enters at the end with the center of the cyclone, and then exits through the the smallest aperture screen, the whole screen tends to top of the cyclone. In an ideal operation, there is only become overloaded at the inlet end, resulting in ineffi- gas in the upward flow, while in the downward flow cient sorting. The parallel-consecutive drum screen there are all the particles that are fed with the stream. presented in Figure 10c overcomes the disadvantages Cyclone diameters range in size from less than 0.05 m of the previously described designs, by first contacting to 10 m, feed concentrations cover values from 0.1 kg the inlet material with the large-aperture screen, leaving m-3 to about 50 kg m-3, while gas inlet velocities may the following smaller-aperture screens to deal with a be in the order of 15-35 m s-1. reduced quantity of undersized material. Another type of drum screen reported to reduce damage during pea sorting uses spaced, circumferential, wedge-section rods instead of perforated-screen drums. The spacing of these rods increases in steps from inlet to outlet, giving a series-consecutive system. Built-in flights ensure smooth transfer of the peas through the sorter.

5.2. Dedusting Technology: Cyclones and Filters In many food processes and related industries, separating solids from a gas stream is very important. A typical example would be the risk of dust explosion in the dry milling industry. It has been found that not only in this industry but also in many others the atmosphere may become dust-laden with particles from different sources, representing a health risk. In other cases, the suspension of particles in a gas stream has been promoted, as in pneumatic conveying or spray drying. However, at the end of the process there is a need to separate the phases. Separation of solids from a gas is accomplished using many different Figure 11. Schematic diagram of a cyclone devices. Perhaps the devices most commonly used to separate particles from gas streams are cyclones and A cyclone is a settling device in which a strong bags, or gas filters. centrifugal force, acting radially, operates in place of Cyclones are by far the most common type of gas- the relatively weak gravity force that acts vertically. solids separation devices used in diverse industrial Due to the small range of particles involved in cyclone processes. They have no moving parts, are inexpensive separation (smallest particle separated is about 5 µm), compared to other separation devices, can be used at it is considered that the Stokes law primarily governs high temperatures, produce a dry product, have low the settling process. The common form of the Stokes energy consumption, and are extremely reliable. Their law follows: primary disadvantage is that they have a relatively low µ x2(ρ – ρ )g collection efficiency for particles below about 15 m. u = ______s g (27) t µ As illustrated in Figure 11, the cyclone consists of a 18 g vertical cylinder with a conical bottom, a tangential inlet near the top and outlets at the top and bottom, where ut is the terminal settling velocity, x is the particle ρ ρ respectively. The top outlet pipe protrudes into the diameter, s is the solids density, g is the gas density, µ conical portion of the cyclone in order to produce a and g is the gas viscosity. vortex when a dust-laden gas (normally air) is pumped Cyclones can generate centrifugal forces between tangentially into the cyclone body. Such a vortex 5 and 2500 times the force of gravity, depending on develops centrifugal force and, because the particles are the diameter of the unit. When particles enter into much denser than the gas, they are projected outward the cyclone body, they quickly reach their terminal to the wall, flowing downward in a thin layer along the velocities corresponding to their size and radial position wall in a helical path. They are eventually collected at in the cyclone. The radial acceleration in a cyclone the bottom of the cyclone and separated. The inlet gas depends on the radius of the path being followed by stream flows downward in an annular vortex, reverses the gas and is given by

648 FOOD POWDER PROCESSING

g = ω2r (28) using standard cyclone geometries, it is much easier to predict the effects on variable changes, and scale-up where ω is the angular velocity and r is the radius. calculations are greatly reduced. Such calculations may Substituting from Equation (28) into Equation (27) be carried via dimensionless relationships, as follows: yields ∆ ____2 P 2 ρ ρ ω2 Eu = (32) ______x ( s – g) r ρ v2 vt = (29) g 18µ g x2 ρ v Stk = ______50 s (33) 50 µ where vt is the terminal velocity of the particle. 18 gDc Also, the centrifugal acceleration is a function of ω the tangential component of the velocity vtan.= r, and In Equations (32) and (33), Eu and Stk50 are the thus, Equation (29) becomes Euler and Stokes numbers, respectively; ∆P is the pressure drop and x the cut size. D is the cyclone x2(ρ – ρ )v2 50 c v = ______s g tan (30) inside diameter (Figure 12), and v is the characteristic t µ 18 gr velocity, which has various definitions, of which the simplest is based on a cross-section of the cylindrical Multiplying Equation (30) by g/g yields body. Thus,

x2(ρ – ρ )g v2 v2 4Q v = ______s g ___tan = (u ) ___tan (31) v = ____ (34) t µ t π 2 [ 18 g ] gr gr Dc where ut is the terminal settling velocity defined where Q is the gas flow rate. by Equation (27). As can be implied, according to Cyclones operate at relatively high gas velocities at Equation (31), the higher the terminal velocity, the the inlet, so particle attrition can be significant. Erosion easier to “settle” a particle within a cyclone. can also represent a problem and is directly associated For a given particle size, the terminal velocity is at with attrition. It occurs primarily where the particles a maximum in the inner vortex, where r is small, so the first impact the cyclone wall. As mentioned before, finest particles separated from the gas are eliminated cyclones are extensively used in the food industry to in the inner vortex. These migrate through the outer reduce particle loads to safe levels during dry milling. vortex to the wall of the cyclone and drop, passing the They are also employed in recovering fines from spray bottom outlet. Smaller particles that do not have time to drying and fluidized bed drying processes. Another reach the wall are retained by the air and carried to the important application is in pneumatic conveying of top outlet. Although the chance of particle separation diverse food products, such as grains and flours. decreases with the square of particle diameter, the fate of a particle depends also on its position in the cross section of the entering stream and on its trajectory in the cyclone. Thus, the separation according to size is not sharp. A specific diameter, called cut diameter or cut size, can be defined as that diameter for which one half of the inlet particles, by mass, are separated while the other half are retained by the gas. The cut size is a very useful variable used to determine the separation efficiency of a cyclone. Since a given powder to be separated in a cyclone would have an extremely fine half in its distri- bution, this half may not be easily separated using conventional pressure drops. Therefore, it is advisable to make the cut size coincide with the mean size of a powder particle size distribution to guarantee separation of the coarse part of such distribution, as the fine portion may be unattainable due to the small range involved. Experience and theory have shown that certain relationships exist among cyclone dimensions that should be observed for efficient cyclone performance, and which are generally related to cyclone diameter. There are several different standard cyclone “designs” and a very common one is called the Stairmand design; its dimensions are shown in Figure 12. By Figure 12. Dimensions of a Stairmand design cyclone

649 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS

Bag Filters are used for final particulate removal Resistance to the particle layer accumulated during in many processes of the food industry. These filters the filtration cycle is calculated by determining a can capture particles much smaller in diameter than variable known as cake resistance factor K1: a cyclone, so are commonly placed downstream of a ∆P cyclone in diverse applications. A bag filter generally K1 = ___c (36) consists of porous fabric in the shape of a cylinder. Many vfw of the bags are placed in a matrix so that their total ∆ area results in a low gas velocity through the bags and, where Pc is the pressure drop through the powder therefore, a low pressure drop through the filter. The layer and w is the powder mass flow rate approaching gas velocities range from 0.005 to 0.02 m s-1. Particulate the filter. loading for these filters generally ranges from 0.2 to As previously mentioned, bag filters have practically 250 g cm-3. A diagram of a bag filter is shown in Figure the same applications as cyclones, and are normally 13. The collected particles build up on the surface of coupled with these in order to remove the finest tails of the bags and cause a gradual increase in pressure drop particle size distribution in diverse food powders. through the filter. After a certain limited pressure drop is reached, the bags are cleaned by pulsing gas back 5.3. Air Classification through the filter to remove the solids buildup on the bag. Filter media in bag filters are normally woven Air classification is a method of separating powdery, fibers, natural or synthetic, which produce fabrics of the granular, or fibrous materials in accordance with the sort, cotton or wool. There are tables listing properties settling velocity, combined with the influence of particle of filtering media to determine whether they are suitable size, particle density, and particle shape. Ideally, the for applications under diverse conditions, such as high separation effect of an air classifier should be such that temperatures, corrosive and chemical resistance, etc. all particles exceeding the cut point are transported into The operating variables of gas filtration are resistance the coarse fraction and the smaller particles transported to flow, permeability of air to the filtering medium, into the fines fraction. In this sense, air classification and resistance due to particle accumulation. Regarding basically involves dividing the particle size distribution resistance to flow, the pressure drop across the filtering of given powders and, as such, is a technique commonly ∆ medium Pf is represented by used in combination with size reduction equipment, normally to eliminate fines that could affect properties ∆ µ Pf = Kc gvf (35) like wetability and dispersibility. A common design for an air classifier consists of where Kc is a constant depending on the filtering a rotating wheel with zigzag channels on the surface, medium and vf is the gas superficial velocity through each of which comprises six components. As shown the filtering medium. in Figure 14, an air stream is fed from the outside. Two forces radially affect a solid-state element in the classifier wheel: a centrifugal force toward the outside and a frictional force of air toward the inside. Since particles that are quite fine are of interest, assuming Stokes law applicability, the resulting equilibrium between the two mentioned forces for a certain grain size would be the so-called cut size. Particles larger than said cut size are centrifugally extracted as oversize particles toward the outside, while smaller particles are carried inside via the air stream. Assuming a constant air throughput, the fineness of separation in the classifier depends on the peripheral speed of the particle, which in turn is in conformity with the peripheral speed of the rotating wheel. With the speed remaining constant, an increasing air throughput changes the cut point within the coarser range. In principle, any cut point can be achieved by combining two matching values of speed and air throughput. Due to complicated flow conditions in the zigzag wheel, the results of the classifier with given values of speed and air throughput cannot be predetermined. For this reason, the assignment of the classifiers cut point and operating data are determined by experiments Figure 13. Schematic diagram of a bag fi lter with a calibration curve. The various particle sizes of a

650 FOOD POWDER PROCESSING

by many names, including: compaction, granulation, tabletting, briquetting, pelletizing, encapsulation, sintering, and agglomeration. While some operations could be considered somewhat similar (e.g., tabletting and pelletizing), others are relevant to specific types of industry (e.g., sintering in metallurgical processes). In food industry processes, the term agglomeration applies, in which the main objective is to control the porosity and density of materials in order to influence properties like dispersibility and solubility. In this case, the operation is often referred to as instantizing; rehydration and reconstitution are important functional properties in food processes. On the other hand, when size enlargement is used to obtain definite shapes, the food industry takes advantage of a process that can shape and cook at the same time, known as extrusion. In a more general context, however, instantizing and extrusion are the two common categories of agglom- eration: tumble/growth and pressure agglomeration.

6.2. Aggregation Fundamentals: Strength of Agglomerates Agglomeration can be defined as the process by which particles join or bind together in a random way, ending with an aggregate of porous structure much larger in size than the original material. As mentioned above, Figure 14. Mode of operation for centrifugal air classifi er agglomeration is used in food processes mainly to improve properties related to handling and reconsti- given material of known distribution are used to plot tution. Figure 15 represents some binding mechanisms the calibration plot. These particle sizes are separated of agglomeration, showing bridges, or force fields, at by certain conditions of speed and air throughput. The the coordination points between particles. This figure coarse fraction is weighed at the same time. The particle depicts the most basic agglomerate comprising two size corresponding to the coarse grain proportion, and particles. In reality, the structure of agglomerates is legible from the fineness characteristics, is the cut point three-dimensional and contains a large number of according to the operating conditions of the individual particles. Each particle interacts with several others classifier, also known as analytical cut point. For inert surrounding it, and the points of interaction may be materials with densities in the order of 2700 kg m-3, characterized by this contact, or a distance small enough cut points between 1 and 100 µm are obtained with for the development of binder bridges or sufficiently “normal” air throughput, and also by varying the speed high attraction forces caused by one of the short-range between 2500 and 20 000 rev min-1. force fields. The number of total interaction sites for Air classification is used by the food industry in important applications, such as wheat flour fraction- ation to separate the coarse, low protein fraction from the fine, high protein fraction. Other applications include the classification of confectionery products, soy flour, rice flour, lactose, oleaginous fruits, etc.

6. AGGLOMERATION AND GROWTH 6.1. Introduction: Size Enlargement Processes

Size enlargement operations are used in the process industry with different aims, such as improving handling and flowability, reducing dusting or material losses, producing structurally useful forms, enhancing appearance, etc. Size enlargement operations are known Figure 15. Binding mechanisms in agglomerates

651 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS one particle within the agglomerate structure is called erates of approximate spherical shape from buildup the coordination number. during the tumbling of fine particulate solids; the In regular packing of mono-sized spherical particles resulting granules are at first weak and require binders the number of coordination points, as well as the to facilitate formation, and post-treatment is needed void volume between particles, can be calculated. A to reach final and permanent strength. On the other relationship between the coordination number k and hand, pressure agglomeration products are made from porosity, or void volume ε, is expressed by particulate materials of diverse size; they are formed without the need of binders or post-treatment, and kε ≈ τ (37) acquire immediate strength. The mechanism in tumble/growth agglomeration A general relationship describing the tensile strength of is illustrated in Figure 16. As shown, the overall σ agglomerates, t, held together by binding mechanisms growth process is complex and involves both the acting at the coordination points is disintegration of weaker bonds and reagglomeration Σ n by abrasion transfer and coalescence of larger units. ε Ai(x,...) The conditions for tumble/growth agglomeration can σ 1____ – ______i=1 (38) be met using inclined discs, rotating drums, any kind t = k π x2 of powder mixer, and fluidized beds. In general, any piece of equipment or environment creating random where Ai is the adhesion force caused by a particular movement is suitable for carrying out tumble/growth binding mechanism and x is the representative size of agglomeration. In certain applications, very simple particles forming the agglomerate. tumbling motions, such as on the slope of storage Substituting from Equation (37) into Equation (38) piles or other inclined surfaces, are sufficient for the yields formation of crude agglomerates. Σ n The most difficult task in tumble/growth agglom- ε Ai(x,...) eration is to form stable nuclei, due to the presence of σ 1____ – ______i=1 (39) few coordination points in small agglomerates. Also, t = ε x2 since the mass of particles and nuclei are small, their kinetic energy is not high enough to cause microscopic A further simplification results because many binding deformation at the contact points, which enhances mechanisms are a function of the representative particle bonding. Recirculation of undersized fines provides size x. Thus, nuclei to which feed particles adhere more easily to Σ n ε Ai(x,...) σ 1____ – ______i=1 (40) t = ε x

The three dots in parentheses in Equations (2), (3), and

(4) indicate that Ai is also a function of other unknown parameters. Depending on the binding mechanism, Ai is also a function of these parameters. Agglomerates that are completely filled with liquid obtain strength from the negative capillary pressure in the structure. A relationship for this follows:

1 – ε 1 σ = c ____ α __ (41) t ε x0 where c is a correction factor, α is the surface tension of the liquid, and x0 is the surface equivalent diameter of the particle.

6.3. Agglomeration Methods With few exceptions, agglomeration methods can be classified into two groups: tumble/growth agglom- eration and pressure agglomeration. Also, agglom- erates can be obtained using binders, or in a binderless manner. The tumble/growth method produces agglom- Figure 16. Kinetics of tumble/growth agglomeration

652 FOOD POWDER PROCESSING form agglomerates. In the whole process, tumble/ the solids in air. The heated air acts both as a fluid- growth agglomeration first yields green products. izing and drying medium. As a result, continuity in These wet agglomerates are temporarily bonded via the drying process is possible and drying time may the surface tension and capillary forces of the liquid be shortened. While fluidized bed drying has many binder. It is for this reason, as mentioned before, that advantages, particularly the continuity of operation just in most cases tumble/growth agglomeration requires mentioned, it must be remembered that certain features some form of post-treatment. Some of the following may render it difficult to operate. For example, polydis- processes have been used as post-treatments: drying perse materials cannot be dried uniformly, and the and heating, cooling, screening, adjustment of product stability of hydrodynamic conditions must be ensured characteristics by crushing, re-screening, conditioning, and the dust trapped. The applicability of drying in and recirculation of undersize material. a fluidized bed largely depends on the design of the In tumble/growth agglomeration, no external equipment. Accordingly, it is very important to select forces are applied, whereas in pressure agglomeration, the construction of drying equipment best suited to the pressure forces act on a confined mass of particulate aggregate state of the material and its physicochemical solids that is then shaped and densified. In this properties, as well as to the scale of production. A method, the magnitude of applied pressure varies from number of different designs of fluidized bed dryers a small amount at first, which causes rearrangement have given successful industrial service in recent years. of particles, to a steep rise in a second stage, during Many new designs are undergoing laboratory test and which the brittle particles break and malleable particles field trials. deform plastically. Two important phenomena limit There is great variety in the design of fluidized bed the speed of compaction and, therefore, the capacity dryers, and several criteria are used to classify them. of the equipment: compressed air in both the pores From a technological standpoint, there are two groups: and elastic spring back. Both can cause cracking and dryers for granular materials, and dryers for pastes, weakening or destruction of products. The effect of solutions, suspensions, and molten materials. With both phenomena can be reduced if the maximum regards to operating conditions, they can be classified pressure is maintained for some time, known as dwell into continuous, semi-continuous, and batch dryers. In time, prior to its release. Pressure agglomeration can terms of construction, the dryers comprise the single be carried out with different types of presses, such as and multi-chamber types. As the advantage of fluidized the punch-and-die press, the compacting roller press, bed drying is to give continuity to a traditional batch and the briquetting roller press, as well as with diverse process, a continuous single or multi-chamber operation sorts of extruders, including the screw extruder, the for particulate materials is a regular option of operation intermeshing gears extruder, and the ram extruder. Low in many applications. and medium pressure agglomeration yields relatively One of the simplest and most widely used designs uniform agglomerates of elongated spaghetti-like or is the continuously operated dryer in a cylindrical cylindrical shapes, whereas high pressure agglomeration single-chamber. As shown in Figure 18, wet material produces pillow or almond-like shapes. is continuously delivered from a hopper to the drying As previously mentioned, there are a number chamber that contains a fluidized bed of the material. of equipment possibilities to perform both types of Hot gas from a furnace is mixed with air in the mixing agglomeration. Figure 17 illustrates some machinery chamber and delivered by a fan to the space beneath the commonly used to obtain different agglomerates in the grid. The dry product is discharged through the outlet food industry. located immediately above the grid on the opposite side With regards to instantizing, tumble/growth of the feed hopper. The dust-laden exhaust gas passes agglomeration is used in the food industry to improve to the cyclone and continues to the bag filter where reconstitutability of a number of products, including it is completely cleaned and returned to the atmos- flours, cocoa powder, instant coffee, dried milk, sugar, phere. Fines from the bottom outlets of the cyclone sweeteners, fruit beverages powders, instant soups, and filter are combined with the dry product. Single- and diverse spices. As to shaping, extrusion has been chamber dryers usually operate with shallow beds of extensively used in grain process engineering to obtain materials (300 to 400 mm), high outputs (500 to 1000 an array of products from diverse cereals, principally kg h-1 moisture for 1 m-2 of grid), and low furnace gas ready-to-eat breakfast cereals. consumption (up to 12 kg per kg of moisture). Multi-chamber dryers contain a number of perfo- 7. DRYING AND RECONSTITUTION rated shelves on which mesh or cloth is stretched, such that the flow of hot gas is uniformly distributed 7.1. Powder Dryers: Fluidized Bed Dryers throughout the bed of material. These dryers consist and Spray Dryers of a closed rectangular chamber divided into three sections, one above the other (Figure 19). The two The principle behind fluidized bed dryers is that heated upper sections are drying zones while the lower is a air is forced up through a bed of solids, suspending cooling zone. The grids slope at an angle of 2 to 3° to

653 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS

Figure 17. Equipment used in agglomeration of food products the horizontal. Wet material is delivered to the top may cause undesirable characteristics in the product, chamber; it is gradually dried, passed into the second such as shrinkage or case hardening. Drying in a chamber, and then sent to the cooling zone where it fluidized bed has distinct features. The particles of is finally discharged through a flap valve. A multi- moist material receive heat from a flow of gas that is chamber dryer equipped with a mixer is used to ensure both a heat transfer and fluidizing medium. The gas more uniform fluidization of very wet or finely divided bulk flow rate is limited because the gas velocity in the materials. drying chamber must be restricted to avoid excessive Design equations used to determine drying time entrainment of the material. Difficulties could also arise and dimensions of the drying equipment are required because the motion of individual particles of material in order to develop methods for drying diverse materials in the drying zone is indeterminate and they could in a fluidized bed. These equations should be based rotate, change in size, and agglomerate. Furthermore, on theoretical analysis of a physical model of the a proportion of the gas phase passes through the bed process, as well as on experimental determinations in the form of bubbles. of the influence of the most important factors on its In fluidized bed drying, normally the limiting kinetics. Drying is a complex heat and mass transfer kinetic factor is the process of external transfer of process, including a number of additional effects that surface moisture from the material to the surrounding

654 FOOD POWDER PROCESSING

Figure 18. Single chamber fl uidized bed dryer medium. In this case, temperature gradients and varia- falling rate period. Duration of the constant rate period tions in moisture content within the material are usually is generally calculated from an energy balance using small and, thus, the material temperature is assumed heat transfer equations. The most difficult problem is constant and equal to the wet bulb temperature. The to determine the duration of the falling rate period. In mechanism of heat transfer in the process is therefore practice, experimental drying curves are often used to considered convective. Under these circumstances, it determine the critical moisture content of the material. is assumed that the quantity of heat supplied deter- The tests are carried out on a small model that must mines the quantity of moisture evaporated. In drying reproduce the drying conditions, i.e., temperature, air processes with convective mechanisms, at least two , air velocity, and so on. During this falling drying periods are recognized: constant rate period and rate period, temperatures of the material and drying agent rise at all points in the fluidized bed. Distribution of the heat quantity between the evaporating moisture and the heating of the wet material is a function of the kinetics of internal heat and mass transfer. Equations that include both the constant and falling rate periods have been proposed. For example, in drying grains in a spouted bed, an expression describing isothermal diffusion within the grains is

⎛ n2π2x2 ⎞ ⎝– ___ _⎠ w – w 6 1 9 ______s = __ Σ __ e (42) π2 2 w0 – ws n=1 n [ ]

where ___ S x = ___ p √ Dt (42b) V p

In Equations (42) and (42b), w is the moisture content

of the material at time t, ws is the surface moisture Figure 19. Multi-chamber fl uidized bed dryer content of the material, w0 is the initial moisture content

655 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS of the material, Sp is the surface area of particles, Vp is the for this purpose. Some dryers are fitted with vibrators, particle volume, and D is the diffusion coefficient. or moving rakes or sweeper arms to prevent the product Fluidized bed drying has been applied on a from sticking to the chamber walls and to assist in its commercial and experimental scale to a number of food collection. products, such as peas, beans, carrots, onions, potato In general, substances that can be handled as liquids granules, meat cubes, flour, cocoa, coffee, salt, and in pipes can be dehydrated in spray dryers. Thus, sugar. As described in a previous section, fluidized beds solutions, emulsions, non-settling suspensions, and are also extensively used for agglomeration purposes. slurries (see Food Rheology and Texture) can be spray In spray drying a feed material, in liquid or mobile dried. Some granular materials (e.g., filter cakes) can form, the material is atomized into a large chamber be successfully spray dried, provided the cohesive forces through which hot air (or other gas) is moving. The between the particles are low. The product in spray liquid in the spray is rapidly evaporated, leaving drying is always a powder, but the form of particles particles of solids that must then be separated from depends upon the substance being dried. When the the air. In this context, a spray dryer basically consists feed is in solution or emulsion form, the dried particles of a drying chamber, a device for atomizing the feed, are usually spherical or nearly so, and are frequently an arrangement to introduce hot air into the chamber, hollow. The products of spray drying usually range in and a means for conveying the dried solids out of the size from 1 to 300µm. If the product is in the form of chamber, as well as for removing and cleaning the hot hollow spheres, the bulk density is low. In cases where air. Figure 20 shows the basic diagram of a spray drying the liquid must be dried directly, the choice is normally system. The feed liquor may be sprayed using either a between drum drying and spray drying. However, in nozzle or a spinning disc. Two types of nozzles may be special circumstances, spray drying may be considered employed; in one type, the feed is forced under pressure as an alternative to other de-watering treatments, through small holes, whereas in the second type, spray such as filtration or crystallization, followed by drying is generated by the action of a secondary fluid (e.g., in other types of equipment. Occasionally, the feed compressed air). Several standard discs are provided material may be specially prepared as a liquid if spray with obstructing devices, such as perforated or slotted drying is particularly desired. walls, spokes, reversed impeller blades, and steps. For drying of biological materials, spray drying is Various methods are used to introduce the hot air considered an excellent alternative because of its rapid into the drying chamber. In some designs, the air enters drying time. While the droplets are drying, the temper- through the roof of the chamber and in others through ature remains at the wet bulb temperature of the drying special ports. For the commonly used conical chamber, air. For this reason, drying air at very high temperatures the air is usually given a swirling motion by vanes in can be tolerated in a dryer with minimum damage to the inlet opening. This assists in the intimate mixing of the heat-sensitive components. Furthermore, the rate air with the spray and also promotes a cyclone action, of degradation reactions in foods slows down with centrifuging the product to the chamber walls from low moisture content. Thus, the portion of the drying which it falls to the bottom of the cone where it is process in which product temperature rises above the collected. It is necessary to have a separator within the wet bulb temperature does not result in severe damage exit air stream to collect the product carried over in the to the process. Other advantages to spray drying include: exit air. In some designs, the entire product leaves the continuity and high capacity of process; lack of contact chamber with the exit air to be collected in separators. of material with walls of equipment until dry, reducing Cyclones or bag filters are the means usually employed problems of sticking and corrosion; production of finely divided free-flowing powder; adjustment within limits of particle size by variations in atomizing conditions; and elimination in certain cases of filtration or grinding (necessary in an alternative process). Some disadvan- tages of spray drying include: high heat consumption per unit weight, high capital cost of equipment, large space requirement for equipment, and likely problems involved in recovering dusty product from exit gases. The spray drying process involves the phenomena, heat and mass transfer, particle settling, and particle- fluid separation. The constant and falling rate stages, which are typical of other forms of drying, are also present in spray drying. As the wet droplets leave the atomizer, the surface loses water instantly. Solidified solute and suspended solids rapidly form a solid crust on the particle surface. The diameter of the particle Figure 20. Typical fl ow diagram of a spray dryer usually decreases during this stage. The formation of

656 FOOD POWDER PROCESSING the solid crust constitutes the constant stage of drying. there will be little risk of blockage and the nozzle When the crust is thick enough, it offers considerable atomizer should give satisfactory results, although some resistance to movement of water toward the surface. In erosion may be expected in the long run. However, this moment, the drying rate drops and is controlled this atomizer is relatively inflexible in operation and by the rate of mass transfer. The temperature of the is not advisable where a range of products is being particle increases and the liquid trapped in its interior dried or when the dryer controlled by varying the feed vaporizes and generates pressure. Eventually, a portion rate. In some cases, atomizing nozzles can produce of the crust cracks and the vapor released. For this dry material or larger particle size and, hence, greater reason, spray dried particles are hollow and the resultant bulk density than is possible with a spinning disc, and powder shows low bulk density and good rehydration may be preferred for this reason. Different kinds of characteristics. The drying time for the constant rate spinning atomizers are in use. One type consists of an period tc is calculated by inverted spinning bowl with liquid feed on the inside surface of the bowl and escaping around the periphery. 2ρλ ______4(w0 – wc)r In another type, the material passes to radial (or nearly tc = (43) 3kf (1 + w0)(Ta – Ts) radial) passages in a wheel-shaped atomizer. Rotational speeds are quite high, 15 000 rev min-1 and over being where w0 is the initial moisture content, wc the critical common. The condition of the product can be influ- moisture content, r is the radius of the droplet, ρ is the enced to some extent by the following: atomizing density of the liquid, λ is the latent heat of evaporation, conditions; higher atomizing speeds, giving smaller kf is the thermal conductivity of the film envelope particles; quicker drying time due to larger surface area; around the particle, Ta is the air temperature, and Ts is and (usually) higher bulk density. However, over certain the surface temperature of the droplet. speeds and with some designs of discs, some suspen-

The drying time in the falling rate period, tf, is sions begin to coagulate and large irregular particles evaluated as follows: may appear in the spray. Concerning thermal efficiency, it is desirable to λρ (r )2(w – w) t = ______s c ___c (44) operate with a relatively high air inlet and temperature. f ∆ 3kf ( T) An approximate temperature of 250°C can be used for preliminary estimates. However, in certain cases it may ρ where s is the density of the dry solid, rc is the radius be necessary to limit the inlet temperature because of of the dried particle, w is the final moisture content, explosion hazards or thermal instability, or to avoid and ∆T is the mean temperature between the drying contamination due to burning of stray particles. To air and particle surface during the falling rate period. prevent burning of this sort, certain areas of the The latter may be considered as a log mean between chamber adjacent to the air inlet louvers are sometimes the wet bulb depression and the difference between the cooled, either by means of cold air jets, or by water exit air and product temperatures. jacket cooling. The atomizer may also be cooled with The size of the drying chamber is mainly deter- a cold air jet. Thermally labile materials can usually mined by the requirement that the particles be sensibly be handled by restricting the air-exit temperature as dried before contact with the chamber walls. For the opposed to the air inlet temperature, since it is the same reason, the shape of the chamber should roughly exit temperature the product will attain. Any normal conform to the shape of the spray. Therefore, a spinning method of air heating may be used. Heating by direct disc atomizer giving an umbrella shaped spray requires mixing with products of combustion is quite frequently a chamber of large diameter, but not particularly deep, practiced. For special cases, where the product would whereas the pressure nozzles normally require a high be harmed by contact with hot oxygen, it is possible to chamber of only moderate diameter. These aspects use an inert gas such as nitrogen as the drying agent cause the low reliability of scale-up exercises in the instead of air. In a spray dryer, working at constant spray drying process. For example in a small dryer, the air rate and constant inlet air temperature, the air-exit spray must be very fine so that the particles are dry temperature can be varied by changing the liquid feed before contact with the walls, whereas in a large dryer rate, and this generally results in changing the dryness the droplets could be considerably coarser since they of the product. A too low air-exit temperature will have more time to dry before reaching the wall. Thus, result in the product being too wet, but a high air- although experiments on laboratory spray dryers may exit temperature is not desirable because heat and the be useful for indicating the feasibility of spray drying dryer capacity are wasted, and product degradation may a particular substance, it is always recommended that occur with heat sensitive materials. In order to maintain trials be conducted first on a dryer as near to full size an uniform product, the feed rate may be automati- as possible. cally controlled to maintain the air-exit temperature at The pressure nozzle type of atomizer is generally a constant value. Variation in feed rate is not usually cheaper than the spinning disc type. If the materials possible with atomizing nozzles and a spinning system being treated do not contain dissolved coarse particles, is preferred.

657 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS

In a given spray dryer, a high air volume rate results vegetables, their reconstitutability largely depends on in a high evaporative capacity. However, high air rates the structure of the dried pieces and the extent to which also result in shorter residence times in the dryer. In the water holding components have been affected by order to obtain satisfactory results the air rate must be the drying operation. Freeze-dried foods often recon- limited to some extent. Normal retention times in spray stitute rapidly because of their porous structure, which dryers, based on the chamber volume and the air flow, is a feature of this method of drying. On the other are up to about one minute. To increase the thermal hand, severe heat damage during drying can cause a efficiency of drying, it may be desirable in certain cases reduction in the water holding capacity of the material, to thicken the original liquor by evaporation, which is with a resulting poor appearance and texture of the more thermally efficient and cheaper process than spray reconstituted product. In the case of powdered dried drying. Also, in some instances, it may be advanta- biological materials, a number of properties may geous to preheat the feed liquor before spraying it into influence the overall reconstitution characteristics. For the dryer (e.g., to give better atomization; preheating instance, wetability describes the capacity of the powder will also increase slightly the capacity of a given size particles to absorb water on their surface, thus initi- of dryer). ating reconstitution. Such a property largely depends on Normally, spray dryers must be cleaned at frequent particle size; small particles representing a large surface intervals as established by experience; at least 10 % area mass ratio may not be wetted individually. In fact, downtime should be allowed in production estimates. they may clump together sharing a wetted surface layer. The parts requiring more regular attention are the This layer reduces the rate at which water penetrates atomizer and air inlet ports of the louvers. The walls of into the particle clump. Increasing particle size and/or the spray dryer do not usually need as much cleaning, for agglomerating particles can reduce the incidence of blowing with compressed air may generally be adequate. clumping. The nature of the particle surface can also In spray dryer operation, some conditions may be varied affect wetability. For example, the presence of free fat to ensure production efficiency and appropriate product on the surface reduces wetability. The selective use of properties. In many cases, a primary variable is found surface-active agents, such as lecithin, can sometimes to affect one factor, which in turns alters other factors. improve the wetability of dried powders containing For example, an increase in the content of the feed fat. Another important property is sinkability, which solids will increase the feed rate, which then increases describes the ability of the powder particles to sink the particle size of the dried product. quickly into the water. This mainly depends on the Spray dryers are extensively used in the food size and density of the particles; larger, denser particles industry. Many food products are spray dried, such as sink more rapidly than finer, lighter ones. Particles milk, whey, ice cream mix, butter, cheese, milk-based with a high content of occluded air may be relatively baby foods, coffee, tea, eggs, fruit juices, edible proteins, large but exhibit poor sinkability because of their low meat extracts, and grain products. density. Finally, dispersibility describes the ease with which a powder may be distributed as single particles 7.2. Reconstitutability of Dried Powders over the surface and throughout the bulk of the recon- stituting water, while solubility refers to the rate and Powdered and particulate food ingredients are used in extent to which the components of the powder particles a number of applications. Diverse food commodities dissolve in the water. Dispersibility is reduced by clump are dehydrated in different forms, for example, cubes formation and is improved when the sinkability is high, and powders, which may then be utilized in diverse whereas solubility depends mainly on the chemical formulations for preparing different foodstuffs. In composition of the powder and its physical state. many applications, these food ingredients need to be Dried food powders and particulates are normally reconstituted or rehydrated. Rehydration properties are reconstituted for consumption. For a dried product to considered functional properties within the context of exhibit good reconstitution characteristics a correct food technology, but have not been as widely studied as balance between the individual properties discussed other functional properties related to nutritional aspects above is needed. In many cases, alteration of one (e.g., fat replacement, low calorie sweetening, etc). In or two of these properties can markedly change the terms of rehydration, handling of dried food products rehydrating behavior. Several measures can be taken may lead to attrition with a consequent production of in order to improve the reconstitutability of dried food fines, which may affect reconstitution of the original products. As mentioned earlier, the selected drying ingredients. method and adjustment of drying conditions can result In the context of , reconstitutability is in a product with good rehydration properties. For the term used to describe the rate at which dried foods example, it is well known that freeze-drying involves pick up and absorb water, reverting to a condition the production of ice crystals and their sublimation that resembles the undried material, when placed in at very low pressures. This procedure results in food contact with an excessive amount of said liquid. In the particles with an open pore structure that can absorb case of dried foods in piece form, as in sliced or diced water easily when reconstituted. Another alternative

658 FOOD POWDER PROCESSING is the so-called combined method, such as osmotic as granulation, drying, and coating, and understanding dehydration followed by conventional drying. In the effects of size, shape, and particle interactions on the osmotic dehydration, food particles are immersed in a rheology of extrudates. Concerning solids mixing, more concentrated solution. By osmotic pressure, the water reliable prediction methods are needed for blending inside the particles tends to migrate to the solution in time and mixing indices. Cyclones generally operate order to equate water activities on both sides of the at low pressures with relatively low solid loading, but cellular wall. This partial dehydration will aid in the may be required to function at higher solid loading final drying stage, and textural damage to the biological and system pressures. Systematic data on how these materials will be minimized. In this sense, biological variables affect cyclone operation are not available. It materials dehydrated by combined methods will also can be also stated that a dearth of information exists on have an open pore structure and, similar to freeze-dried the modeling of drop formation and solidification in materials, will present good reconstitution properties. spray drying, as well as on the effects of the operating Compared to freeze-drying, combined methods variables in fluidized bed drying on the quality of represent an economic alternative for the production of obtained products. Finally, fundamental research is high quality food products. The most efficient method needed to improve the rehydration and reconstitution used to improve the rehydration characteristics of dried properties of diverse dehydrated foods. food powders is probably agglomeration, which has Although the above description is far from been previously discussed. exhaustive, it stresses the importance of food powder processing within the context of established disciplines, 8. CONCLUSION AND FUTURE TRENDS such as particle technology, and food and chemical engineering. There will be a lot of activity in funda- Powder technology is of dynamic significance to the mental and applied research in the years to come, world economy with a broad range of industries taking providing the food industry with the theoretical tools advantage of the rapidly growing knowledge in this needed to increase competitiveness among the great discipline. Research within universities and similar number of processes involving food powders. institutions, coupled with the vested interest of the industrial community, has stimulated relevant results ACKNOWLEDGMENT that have been applied to make a number of processes more efficient. Periodical meetings and specialized The author wishes to express his gratitude to Ramon publications are spreading the most relevant and recent Olivas-Gastelum for help provided in preparing the advances in diverse topics, such as materials handling, figures. particle formation, mixing, grinding, and separation. As mentioned at the beginning of this contribution, GLOSSARY there has been rapid growth in adapting the knowledge of particle technology worldwide in recent years. Agglomeration: aggregation of small particles into However, in the case of application to biological larger entities by different binding mechanisms. materials, specifically food powders and particulates, Attrition: fractionation of pieces into smaller entities there is a lot to be done, as very few research groups from effects of handling, vibration, etc. can be identified in this field around the globe. Since Bag filter: equipment including a filtering medium many strategic food industries, such as those based in supported by a rigid mesh structure, used for grain processing, rely heavily on a solid understanding retention of particles suspended in a gas stream. of powder technology, there is much potential in estab- Bi-modal distribution: graphic representation of lishing research programs and schemes to attend this particle population in the shape of a single hump. demand. Many research activities could be mentioned Chipping: mechanism of fractionation of pieces into as examples, but some urgent needs are summarized smaller ones, which are also divided to give a third in the following. range of smaller particles. With regards to comminution, improving the Comminution: generic term used to describe size energy efficiency of the operation is necessary, as well reduction processes. as predicting the particle size and shape. Attrition is Crusher: equipment used to perform a coarse size causing important losses in different food industries, reduction of normally hard materials. mainly in the cereal industry. Therefore, a means of Crushing: operation of size reduction on the coarse translating particle properties to single and multi- range scale. particle breakage would be quite useful in manipu- Cut diameter: size of particle having a 50 % probability lating the process variables in order to prevent such of being separated in a centrifugal settling device. undesirable phenomenon. Size enlargement is widely Cut size: same as cut diameter. applied in diverse food processes, and some necessary Cyclone: conical-cylindrical separating device in which research includes: better understanding of melt granu- particles are settled in the centrifugal field created lation; development of more combined operations such by a vortex swirling inside.

659 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS

Dehydration: removal of nearly all water present in a Particulate: related to particles. wet solid. Particle technology: branch of engineering dealing Dicing: cutting of pieces into a cubical shape. with the study of particulate systems. Dispersibility: ease with which a powder is distributed Pelletizing: shaping of powder materials into pills or as single particles throughout a volume of liquid. pellets. Drying: process of removal of liquid from a solid Powder technology: same as particle technology. material, to reduce content to a low level. Reconstitutability: rate at which dried materials revert Encapsulation: gives particulate materials a capsule to a condition resembling the undried material. shape, or confines fine powders into capsules. Rehydration: absorption of water from a dehydrated Entrainment: carrying of particles by a stream of material to a level close to the original. fluid. Rheology: study of fluids and flow behavior in a broad Erosion: wearing of pieces by rubbing, handling, sense. contact, etc. Screen: surface containing a number of equally sized Expression: separation of liquids from solids by apertures used to separate particulate solids into applying compressive forces. different sizes. Extraction: removal of constituent from a solid or Segregation: isolate or separate components of a liquid by means of a liquid solvent. mixture. Extrusion: forcing of a material through a restriction Shattering: fractionation into small pieces of brittle by a screw rotating inside a barrel in order to obtain particulates or aggregates by handling, rubbing, different shapes, such as ribbons or ropes. etc. Filtration: retention of particles in suspension or Sieve: utensil, or device with wire network or gauze, solution in a fluid by forcing such system to pass to separate powdered or particulate materials into through a porous medium. fractions. Fluidized bed: layer of solids that may be suspended Sifter: equipment used for sieving or fractionation of by a flowing stream of gas. solid particles into different sizes. Food engineering: scientific discipline studying indus- Sinkability: ability of powder particles to settle quickly trial processes dealing with food materials. into a liquid. Food processing industry: industrial sector that trans- Size reduction: breakdown of solid materials through forms raw food materials into products. the application of mechanical force. Grinder: equipment used to perform medium to fine Spray drying: removal of most of the liquid in a slurry size reduction of solid materials. or solution by atomizing and heating it with a Grinding: operation of size reduction on a medium to stream of hot gas. fine range scale. Stairdman design cyclone: a cyclone with fixed dimen- Grizzlies: set of parallel bars, often wedge-shaped, sions related to the diameter of the cylindrical part, used for retaining large particles or pieces. said to give efficient performance Hopper: conical, or inverted-pyramid shaped device, Tabletting: shaping of powdered solids into tablets by used to feed particulate materials into containers means of compression. or processing equipment.Instantizing: conditioning Trommel: a rotating or tumbling inclined cylinder of fine powders to attain good reconstitution or with surface perforations to separate fractions of rehydration characteristics. particulate solids Mesh number: number of wires per lineal inch of Vortex: whirling fluid inside conical devices woven wire used as sieve or screen. Wetability: ability of a powder particle to absorb water Mill: equipment used for intermediate to fine size on its surface. reduction of solid materials. Milling: industrial operation of several stages of size ABBREVIATIONS reduction in order to obtain pulverized products. Mincing: chopping of elastic materials into small pieces AIChE American Institute of Chemical with a knife or machine with revolving blades. Engineers Mixing: interspersing of two or more materials with ASTM American Society of Testing Materials one another to achieve uniform distribution of BS British Standards components. IChemE Institution of Chemical Engineers Mohs’ scale: classification of solid materials according IFST Institute of Food Science and to hardness from 1 (softer) to 10 (hardest). Technology Mono-modal distribution: graphic representation IFT Institute of Food Technologists of particle population in the shape of a double IUFoST International Union of Food Science and hump. Technology Particle: smallest possible quantity of a solid ISO International Standards Organization material. SCI Society of Chemical Industry

660 FOOD POWDER PROCESSING

SYMBOLS vt Terminal velocity of particle vtan Tangential component of velocity A Weight fraction of fines W(n) Weight fraction of particles retaining original

Ai Adhesion force sample size A(n) Weight fraction of fines at “n” taps W(0) Weight sample of mother particles A’ Fraction of material disintegrating at a given w Moisture content at time t

rate wc Critical moisture content A” Constant ws Surface moisture content a’ Constant w0 Initial moisture content a” Constant XF Mass fraction of coarse particles in the feed b Compressibility XO Mass fraction of coarse particles in the c Correction factor overflow

D Slope of line, trommel diameter, diffusion coeffi- XU Mass fraction of coarse particles in the cient underflow

Dc Cyclone diameter x Particle size, parameter defined by Equation E Energy, overall screen efficiency (42b)

Ei Bond work index xi Every measured value of particle size EO Oversize screen efficiency x0 Surface equivalent diameter of particle Eu Euler number x50 Cut size EU Undersize screen efficiency F Fractal dimension of profile, mass flow rate of Greek letters feed g Acceleration due to gravity α Surface tension of liquid ∆ HR Hausner ratio difference ε Ie Erosion index porosity λ Is Mixing index Latent heat of evaporation µ K Constant g Gas viscosity µ Kc Constant p Fraction of a given particle ρ K1 Cake resistance factor Liquid density ρ k Constant A Asymptotic density ρ kf Thermal conductivity of film envelope around g Gas density ρ particle n Bulk density at “n” taps ρ M(n) Mode at time t s Solid density ρ M(0) Initial mode 0 Initial density N Number of samples, revolutions per minute σ Compression stress σ n Power, number of taps, number of particles e Theoretical standard deviation σ O Mass flow rate of overflow t Tensile strength of agglomerate P Pressure ω Angular velocity

Pc Pressure over powder layer Pf Pressure over filtering medium BIBLIOGRAPHY Q Volumetric flow rate r Radius, droplet radius Barbosa-Cánovas G.V., Malave J., and Peleg M. 1987. rc Radius of dry particle Density and compressibility of selected food powders S Rate constant mixtures. Journal of Food Process Engineering 10, 1- Sp Surface area of particles 19. [This paper reviews in detail the property of Stk50 Stokes number compressibility of common food powders]. s Standard deviation Barletta B.J., Knight K.M., and Barbosa-Cánovas T Temperature G.V. 1993. Review: attrition in agglomerated

Ta Air temperature coffee. Revista Española de Ciencia y Tecnología Ts Surface temperature of droplet de Alimentos 33, 43-58. [Although it focuses on t Time coffee, the paper is an excellent review of attrition tc Constant rate period time of food powders in general]. tf Falling rate period time Bemrose C.R. and Bridgewater J. 1987. A review of U Mass flow rate of underflow attrition and attrition methods. Powder Technology ut Terminal settling velocity 49, 97-126. [This presents a comprehensive review Vp Particle volume and discussion on attrition]. v Cyclone characteristic velocity Coulson J.M. and Richardson J.F. 1978. Chemical rd vf Gas superficial velocity Engineering Vol. 1. 3 ed. Oxford: Pergamon

661 ENCYCLOPEDIA OF LIFE SUPPORT SYSTEMS

Press. [This text provides one of the most complete Svarovsky L. 1981. Solid-Gas Separation. Amsterdam: volumes on powder technology appearing in a Elsevier. [A comprehensive and authoritative book chemical engineering book series]. about solid-gas separations]. Masters K. 1985. Spray Drying Handbook. 4th ed. London: George Godwin. [Possibly the most BIOGRAPHICAL SKETCH complete guide to spray drying]. Niranjan K. 1995. Mixing in food industry. The Enrique Ortega-Rivas is a Professor at the University Chemical Engineer 591, 20-22. [This paper presents of Chihuahua, Mexico. He holds the status of “National a comprehensive review of different methods of Researcher”, which is the maximum recognition mixing, including powder blending]. conferred by the Mexican government to academics Ortega-Rivas E. and Svarovsky L. 1994. Centrifugal based on professional merits and achievements. Dr. air classification as a tool for narrowing the spread Ortega-Rivas also has been recently listed in “Who’s of particle size distributions of powders. Bulk Solids Who in Science and Engineering”. His research Handling 14, 811-813. [This presents an interesting interests include food engineering, powder and particle application of air classification]. technology, solid-liquid separation techniques, and Ortega-Rivas E. 1997. Guest Editor: Handling and fruit processing. Dr. Ortega-Rivas has participated in Processing of Food Powders and Suspensions. Special national and international academic assignments, such issue of Food Science and Technology International as organizing professional meetings, serving on panels, 3, 317-390. [This issue consists of selected contri- and representing his institution in diverse committees. butions by international researchers]. Dr. Ortega-Rivas has published numerous technical Pietsch W. 1991. Size Enlargement by Agglomeration. papers in refereed journals and chapters in technical Chichester, UK: John Wiley. [The classic text on books. He also is a reviewer of manuscripts for different size enlargement methods]. international journals.

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Food Bioprocess Technol (2009) 2:28–44 DOI 10.1007/s11947-008-0107-5

REVIEW PAPER

Bulk Properties of Food Particulate Materials: An Appraisal of their Characterisation and Relevance in Processing

Enrique Ortega-Rivas

Received: 20 September 2007 /Accepted: 4 June 2008 /Published online: 3 July 2008 # Springer Science + Business Media, LLC 2008

Abstract The food industry is one of the largest commer- systems. For the case of food products and materials, cial enterprises in the world today representing important some important applications of particle technology can contributions of the gross national product of many be mentioned. For example, particle size in wheat flour is countries. Numerous raw materials and products in this an important factor in functionality of food products industry are in powdered or particulate form, and their (McDonald 1994), attrition of instant powdered foods optimum characterisation for processing purposes, rely reduces their reconstitutability (Hogekamp and Schubert heavily in a deep knowledge of particle technology. 2003), and uneven powder flow in extrusion hoppers may Characterisation of the main bulk properties affecting be considered one of the factors affecting the rheology of a processing, such as failure properties, bulk density and paste (Pordesimo et al. 2007). compressibility, are discussed in this article. Testing of Particle characterisation, i.e., description of primary these properties is far from standardised so the different properties of food powders in a particulate system, under- manners of measurement are reviewed along with theoret- lies all work in particle technology. Primary particle ical considerations, operating principles, and applications. properties such as particle shape and particle density, The food industry should make more efficient use of its together with the primary properties of a fluid (viscosity many processes involving powders and particulates in order and density), and also with the concentration and state of to provide high quality products. In this sense, future dispersion, govern the secondary properties such as settling competitiveness may be critically dependent on knowledge velocity of particles or rehydration rate of powders. As originated by research activities in particle technology many relationships in powder technology are rather applied to food materials. complex and often not yet available in many areas, particle properties are mainly used for qualitative assessment of the Keywords Food powders . Bulk density . Compaction . behaviour of suspensions and powders, for example, as an Failure properties . Reconstitution equipment selection guide. Since a powder is considered to be a dispersed two-phase system consisting of a dispersed phase of solid particles of different sizes and a gas as the Introduction continuous phase, complete characterisation of powdered materials is dependent on the properties of a particle as an There is a relatively new branch of science and engineering individual entity, the properties of the assembly of particles, known as Particle Technology. Such discipline deals in the and the interactions between those assemblies and a fluid. It broader sense with the systematic study of particulate is mainly for this reason that bulk properties have important effects on many processes and unit operations dealing with E. Ortega-Rivas (*) powders and particulates. Postgraduate Programme in Food Science and Technology, The objective of this review article is to discuss the main Autonomous University of Chihuahua, techniques used in characterisation of food powders and University Campus I, CP 31170 Chihuahua, Chih., México particulates, as well as the critical issues of such charac- e-mail: [email protected] terisations in their further handling and processing. Food Bioprocess Technol (2009) 2:28–44 29

Particle Size particle size distribution, but most of them would fall into four general methods: sieving, microscope counting tech- There are several single particle characteristics that are very niques, sedimentation, and stream scanning. In particle size important to product properties. They include particle size, measurement, two most important decisions have to be particle shape, surface, density, hardness, adsorption prop- made before a technique is to be selected for the analysis; erties, etc. Amongst these mentioned features, particle size these are concerned with the two variables measured, the is the most essential and important one. The term size of a type of particle size and the occurrence of such size. powder or particulate material is very relative, and it is Particle size was previously discussed, and emphasising often used to classify, categorise or characterise a powder. what was already presented, is important to bear in mind The selection of a relevant characteristic particle size to that great care must be taken when making a selection of start any analysis or measurement often poses a problem particle size, as an equivalent diameter, in order to choose (Kaye 1997). In practice, the particles forming a powder the most relevant to the property or process which is to be will rarely have a spherical shape. Many industrial powders controlled. The occurrence of amount of particle matter are of mineral (metallic or non metallic) origin and have belonging to specified sizes may be classified or arranged been derived from hard materials by size-reduction pro- by diverse criteria as to obtain tables or graphs. In powder cesses. They are generally known as inert powders and their technology, the use of graphs is convenient and customary comminuted particles resemble polyhedrons with nearly for a number of reasons. For example, a particular size plane faces, in a number of 4 to 7, and sharp edges and which is to be used as the main reference of a given corners (McCabe et al. 2005). The particles may be material is easily read from a specific type of plot. compact, with length, breadth and thickness nearly equal but, sometimes, they may be plate-like or needle-like. As Particle Shape particles get smaller, and by influence of attrition due to handling, their edges may become smoother, and thus, they All geometrical features of individual powders’ particles are may be considered close to a spherical shape. The term related to the intimate structures of their forming elements, diameter is, therefore, often used to refer to the charac- whose arrangements are normally symmetrical with definite teristic linear dimension, and a variable defining how shapes like cubes or octahedrons for inert powders. On the particles approximate to a spherical shape is known as other hand, powdered food materials would be mostly sphericity. This variable, described thoroughly in the organic in origin, and their individual grain shapes could literature (Kaye 1994, 1997; Allen 1997; McCabe et al. have a great diversity of structures, since their chemical 2005), takes values of unity for perfect spheres and 0.7–0.8 compositions would be more complex than those of for most crushed materials. inorganic industrial powders. Shape of food powders vary Particle size, as an independent property is useless from extreme degrees of irregularity (ground materials like because there is no particulate material having a single spices and sugar), to an approximate spherical shape (starch particle size. Any powder would consist of a population of and dry yeast) or well-defined crystalline shapes (granulated particles of the same chemical composition, but with a wide sugar and salt). General definitions of particle shapes are range of individual sizes. Particle size distribution mea- listed in Table 1. It is obvious that such simple definitions surement is a common method in any physical, mechanical are not enough to do the comparison of particle size or chemical process because it is directly related to material measured by different methods or to incorporate it as behaviour and/or physical properties of products. Foods are frequently in the form of fine particles during processing and marketing (Schubert 1987). The bulk density, com- Table 1 General definitions of particle shape (Adapted from Barbosa- Cánovas et al. 2005) pressibility and flowability of a food powder are highly dependent on particle size and its distribution (Barbosa- Shape name Shape description Cánovas et al. 1987). Segregation will happen in a free- Acicular Needle shape flowing powder mixture because of the differences in Angular Roughly polyhedral shape particle sizes (Barbosa-Cánovas et al. 1985). Size distribu- Crystalline Freely developed geometric shape in a fluid medium tion is also one of the factors affecting the flowability of Dentritic Branched crystalline shape food powders (Peleg 1977). For quality control or system Fibrous Regularly or irregular thread-like property description, the need to represent the particle size Flaky Plate-like distribution of food powders becomes paramount and also Granular Approximately equidimensional irregular shape proper descriptors in the analysis of the handling, process- Irregular Lacking any symmetry Modular Rounded irregular shape ing and functionality of each food powder. There are many Spherical Global shape different types of instruments available for measuring 30 Food Bioprocess Technol (2009) 2:28–44 parameters into equations where particle shapes are not the of powder flowability investigations is to provide both same (Herdan 1960; Allen 1997). Shape, in its broadest qualitative and quantitative knowledge of powder behav- meaning, is very important in particle behaviour, and just iour, which can be used in equipment design and in looking at the particle shapes, with no attempts at equipment performance prediction (Sutton 1976). The flow quantification, can be beneficial. Shape can be used as a characteristics of powders are of great importance in many filter before size classification is performed. For example, problems encountered in bulk material handling processes as shown in Fig. 1, all rough outlines could be eliminated in the agricultural, ceramic, food, mineral, mining and by using the ratio: perimeter/convex perimeter or all pharmaceutical industries because the ease of powder particles with an extreme elongation ratio. The earliest conveying, blending and packaging depends on flow methods of describing the shape of particle outlines used characteristics (Chen 1994). In designing plants involving length L, breadth B and thickness T, in expressions such as handling and processing of food powders, diverse difficul- the elongation ratio (L/B) and the flakiness ratio (B/T). The ties may arise that can cause severe operating problems. drawback with simple, one number shape measurements is Flow problems related to food powders, such as arching, the possibility of ambiguity; the same single number may ratholing and erratic flow are thoroughly described and be obtained from more than one shape. Nevertheless, a reviewed by Marinelli (2005). measurement of this type which has been successfully The common flow problems in hoppers and silos can be employed for many years is the so-called sphericity, summarised as follows: (a) no flow, (b) segregation, (c) previously mentioned. flooding and (d) structural failure. Lack of discharge in the Due to the great variability of shape of food powders’ no flow situation can be attributed to the formation of a particles, interactions between assemblies of particles and stable arch over the outlet or a stable cavity called a fluid may become more complex in food powders than in “rathole” (Marinelli and Carson 1992). With regard to inert powders. Bulk property determinations may be segregation, many materials experience separation of fine considered, therefore, it is a very critical issue for food and coarse particles (Carson et al. 1986), and such powder characterisation, as well as for evaluating their separation can seriously compromise the quality of the effects on processing. final product as well as the efficiency of the process. Flooding can be caused by the collapse of a rathole in a bin containing fine powder, resulting in uncontrollable flow of Failure Properties material, loss of product and clouds of dust (Royal and Carson 1993), among other problems. Pertaining to Basic Concepts of Powders Flow structural failure, each year over 1,000 silos, bins and hoppers fail in North America alone. Most of these failures Powder flow is defined as the relative movement of a bulk could have been prevented with proper and careful design, of particles among neighbouring particles or along the in which the loads imposed by the bulk solid being stored, container wall surface (Peleg 1977). The practical objective had been well considered. The design of bins, hoppers and silos has never been given the attention it deserves. Approaches using properties such as angle of repose or angle of spatula in design considerations are ineffective because the resulting values bear no relation to the design parameters needed to ensure reliable flow, since particulate solids tend to compact or consolidate when stored. The attempt of trying to model bulk solids as fluids also leads to a bottleneck, due to the fact that flowing bulk solids generate shear stresses and are able to maintain these stresses even when their flow rate is changed dramatically. It is also improper to consider bulk solids as having viscosity since almost all bulk solids exhibit flow properties that are flow-rate independent. The systematic approach for designing powder handling and processing plants started in the mid-1950s by the pioneering work of Andrew W. Jenike. His concept was to model bulk solids using the (a) (b) principles of continuum mechanics. The resulting compre- Fig. 1 Relation between: a perimeter, and b convex perimeter of a hensive theory (Jenike 1964) describing the flow of bulk particle solids has been applied and perfected over the years, but is Food Bioprocess Technol (2009) 2:28–44 31 generally recognised worldwide as the only scientific guide the surrounding relative humidity, so liquid bridges will to bulk solids flow. disappear, and flowability may turn back to normal. Food Practically, for all fine powders, the attractive forces powders are predominantly soluble, so solid bridges may between particles are large when compared with the weight remain even after humidity decrease, causing the powder to of individual particles, so they are said to be cohesive and cake and aggravating flow problems. Furthermore, due to normally present flow problems (Adhikari et al. 2001). their chemical composition, food powders are normally Cohesion occurs when interparticle forces play a significant more sensitive to all the ambient factors previously role in the mechanics of the powder bed. Flow problems described and physicochemical changes largely depend on occur in any kind of cohesive powder, but may be more their temperature–moisture histories. For instance, increas- serious with food powders because they are more sensitive ing the temperature of a food powder increases dissolution to physical and physicochemical phenomena which may of particles, facilitating changes in crystalline form that may affect their composition and properties (Fitzpatrick 2005). also result in caking and flow problems (Teunou and The most common effects of such phenomena in food Vasseur 1996). powders lead to situations which aggravate flow properties, as they normally relate to releasing of sticky substances or Determination of Failure Properties to the presence of hygroscopic behaviour. Cohesive powders are, normally, also adhesive. In cohesion, the To assure steady and reliable flow, it is crucial to accurately contact surface is similar (particle–particle), while in characterise the flow behaviour of powders. Obtaining adhesion, the contact surface is different (particle—any useful reproducible data has always been a difficult task surface material). For example, adhesion takes place since, as previously discussed, it is not just a powder to be between drying droplets and the drying wall of a spray tested, it is also a powder/air mixture, and the method of dryer, whereas cohesion is responsible for caking of the packing, consolidating and shearing will greatly affect the powder during processing and storage. In food powder, measurements (De Silva 2000). The forces involved in processing cohesion is also termed caking, and adhesion is powder flow are gravity, friction, cohesion (interparticle also referred as stickiness. attraction) and adhesion (particle–wall attraction). Further- Stickiness is a prevalent situation that can cause more, particle surface properties, particle shape and size problems in operation, equipment wear and product yield distribution, and the geometry of the system, are factors (Adhikari et al. 2001). Interaction of water with solids is the affecting the flowability of a given powder. It is, therefore, prime cause of stickiness and caking in low-moisture food quite difficult to have a general theory applicable to all food powders. Water provides the necessary plasticity to food powders in all possible conditions that may develop in polymeric systems, to reduce viscosity and promote practice (Peleg 1977). The first requirement is to identify molecular mobility. Chemical caking is caused by chemical the properties which characterise the flowability of a reactions in which a compound has been generated or particular material and to specify procedures for measuring modified, as in hydration, recrystallisation or sublimation. them. The common belief that the flowability of a powder Plastic-flow caking occurs when the particles’ yield values is a direct function of the angle of the repose is misleading are exceeded, and they stick together or merge into a single and should be avoided because, as stated earlier, most particulate form, as in amorphous materials like gels, lipids industrial powders show different grades of cohesiveness. or waxes. Melting and solidification of fats may also cause The angle of repose of a cohesive material is indeterminate, solid bridges and enhance caking in food powders. In this being dependent on the previous story of a given sample, case, an increase in temperature causes solid fats to melt and is irrelevant to the flow behaviour of the material in any producing a liquid that can redistribute itself among the particular situation (Jenike 1964). The important feature of particles. If the temperature is subsequently reduced, the a particulate material is the way its shear strength varies liquid fat will solidify and form solid bridges between with the consolidating stress, and the properties used to the particles. identify and quantify such interactions are commonly In general terms, the condition of the surrounding known as the failure properties of a powder. atmosphere affects secondary properties of powders, such Failure properties can be determined using a shear cell. as flowability, causing diverse type of problems. Small Two basic types of shear cells are available for powder differences in factors like moisture content, particle size, testing: the Jenike shear cell, also known as the transla- storage time and temperature, can make a significant tional shear box, and the annular or ring shear cell, also difference in flowability. If relative humidity increases, called the rotational shear box. The Jenike shear cell is any kind of powder tends to absorb water that may form circular in cross-section, with an internal diameter of liquid bridges between particles reducing flowability. Inert 95 mm. As shown in Fig. 2 (Thomson 1997), it consists powders, however, would desorb water with a decrease in of a base and a ring, which can slide horizontally over the 32 Food Bioprocess Technol (2009) 2:28–44

Fig. 2 Diagram of Jenike’s shear cell: a cell components, b testing steps

base. The ring and base are filled with the powder, and a lid and it is a line which gives the stress conditions needed to is placed in position. By means of a weight carrier, a produce flow for the powder when compacted to a fixed vertical compacting load can be applied to the powder bulk density. A yield locus represents the result of a series sample (Fig. 2). The lid carries a bracket with a projecting of tests on samples which have the same initial bulk pin, and a measured horizontal force is applied to such density. If the material being tested is cohesive, the yield bracket, causing the ring and its content along with the lid, locus is not a straight line and does not pass through the to move forward at a constant speed. The shear force origin. It can be shown that the graph when extrapolated needed to cause the powder to flow can thus be obtained. downwards cuts the horizontal axis normally. As shown in The actual steps to carry out a test in a translational shear Fig. 3, the intercept T is known as the tensile strength of the box are: (a) a standardised procedure is used to fill the shear powder, and the intercept C is called the cohesion of the cell with a powder specimen consolidated in some powder; the yield locus ends at the point A. More yield loci reproducible manner, (b) a vertical load is applied in the can be obtained by changing the sample preparation lid, (c) the horizontal force is applied to the bracket pushing procedure, and in this way a family of yield loci can be forward the upper part of the cell, and the maximum shear obtained. This family of yield loci contains all the force needed to initiate movement is measured, (d) the cell information needed to characterise the flowability of a is emptied, and a new sample is formed by the same particular material. For many powders, yield locus curves procedure as in the first step, (e) a different vertical load is can be described by the empirical Warren–Spring equation applied to the lid, (f) the procedure described in the third (Chasseray 1994):  step is repeated. Five or six different incremental vertical t n s loads are applied to a set of identical samples, and the shear ¼ 1 ð1Þ C T force needed to initiate flow is found in each case. The forces are divided by the cross-sectional area of the cell to where t is the shear stress, C the material’s cohesion, σ the give stresses, and the shear stress is plotted against the normal stress, T the tensile strength, and n the shear index normal stress. The resulting graph of plotting shear stress (1

operation and possibility of time consolidation measure- ments using a consolidation bench (Schulze 1996; Schulze and Wittmaier 2007). Another important failure property, the failure function, A can be measured using a split cylindrical die as shown in Fig. 5. The bore of the cylinder may be about 50 mm, and its height should be just more than twice the bore. The Shear stress cylinder is clamped so that the two halves cannot separate, and it is filled with the powder to be tested, which is then C scraped off to be at al level with the top face. By means of a plunger, the specimen is subjected to a known consolidat- ing stress. The plunger is then removed, and the two halves of the split die are separated, leaving a free standing T cylinder of the compacted powder. A plate is then placed on Normal stress top of the specimen, and an increasing vertical load is Fig. 3 The Jenike yield locus applied to it until the column collapses. The stress at which this occurs is the unconfined yield stress, i.e., the stress that has to be applied to the free vertical surface on the column solids to a practical technique for measuring their flow to cause failure. If this is repeated for a number of different properties (Wright 1999), resulting in the prediction of the compacting loads and the unconfined yield stress is plotted minimum acceptable values of the important hopper and against the compacting stress, the failure function of the silo design parameters. Correct choice of these parameters powder will be obtained. Although the results of this ensures collapsing of any cohesive arch at any container’s method can be used for monitoring or for comparison, the hopper outlet, under its own weight. There are, however, failure function obtained will not be the same as that given some disadvantages of the method, such as laborious by shear cell tests, due to the effect of die wall friction sample preparation prior to testing, unequal stress distribu- when forming the compact. A method of correcting for tion causing progressive failure and inaccuracy of the friction has been described (Williams et al. 1971). method at low loads due to tilting of the lid (Wright 1999). The failure function, also referred as the flow function, is Some of the difficulties of the translational shear tester a measure of how the unconfined yield strength developed described above can be overcome by the use of a ring shear within the powder, varies with maximum consolidation tester. Ring, or annular, shear testers have been used in bulk stress. This variation is illustrated in Fig. 6 and forms the solids technology for an extended period of time (Carr and basis of the Jenike classification of powders. Jenike used Walker 1967; Münz 1976; Gebhard 1982; Höhne 1985) the inverse slope of the failure function, known as flow with an increasing range of applications (Schulze 1994a, b). index or flow factor ff, to classify powder flowabilty. In annular shear cells, the shear stress is applied by rotating According to the value of the flow index, powders are the top portion of an annular shear, as represented in Fig. 4. categorised as very cohesive (ff<2), cohesive (2

Fig. 5 Device for direct mea- surement of failure function: a mounted measuring device, b securing halves and filling with sample, c compacting with plunger, d separating halves and stable column of powder, e application of normal force, f collapsing of material

(a) (b) (c)

(d) (e) (f)

Stickiness can be evaluated by several methods includ- The optical probe method is based on the changes in ing the Jenike shear method previously described, but since optical properties of a free-flowing powder (Lockemann shear cell methods are normally used for studying the flow 1999). The motion of a food powder in a constantly rotating behaviour of powders through chutes and hoppers, they tube is observed with a fibre-optic sensor. Both tube and have limited applications to characterise the stickiness of sensor are immersed in an oil bath to maintain the food powders (Bhandari and Hartel 2005). Other available temperature. A sharp increase in reflectance of a free- methods for stickiness measurement are the propeller- flowing powder is observed at its sticky point. The method driven method, the optical probe method, the blow test seems appropriate for coloured foods, but has not been and the fluidisation method. The propeller driven method, applied to food materials tending to be transparent on originally developed by Lazar et al. (1956), basically softening or melting. It seems to be a promising technique comprises a test tube containing powder with known for evaluation of already dried powders (Bhandari and moisture content. The test tube is immersed in a water bath, Hartel 2005). and a machine-driven impeller stirs the powder. The tem- A blow test method proposed by Paterson et al. (2001) perature of the water bath is slowly risen to record a maximum measures the velocity of air needed to blow a channel into a force of stirring at a point known as the sticky point. pack bed of powder and the stickiness is based on the air velocity range. The blow test apparatus consists of a multi- segmented circular distributor where the preconditioned sample is packed. Air, at a given temperature and humidity, No flow is blown from a 45° angle onto each segment with More increasing flow rate to a maximum of 22 l/min, until a difficult to flow channel is formed. The result from this method would represent further the caking behaviour of food powders, but is physically demanding and several steps are required before each measurement. The fluidised method, described by Bloore (2000), comprises a small fluidised bed set-up to study the stickiness property of a powder at different temperature Unconfined yield strength

Easy flow and humidity conditions. The positive feature of this method, compared to other tests, is that the powder is in a dynamic condition, closer to spray drying and fluidised bed drying situations. The stickiness observed by this method is governed by the cohesive properties of the particles, and is Maximum consolidating stress useful to represent conditions in spray and fluidised bed- Fig. 6 The failure function of a food powder drying processes. Food Bioprocess Technol (2009) 2:28–44 35

Factors Influencing Food Powder Flowability 14 12 Food powder flowability has been characterised as a function of some of the factors affecting it previously 10 described. Particle size has a major influence on powder flowability. From a critical size of about 200 μm and 8 downwards, flowability is seriously affected (Fitzpatrick 6 2005). A noticeable change in capacity to flow is observed 1 day if the size is reduced by an order of magnitude, for example 4 from 100 to 10 μm. This reduction in flowability may be 3 days 7 days attributed to the increase surface area per unit mass of 2 Unconfined yield strength (kPa) solids, as the particle becomes smaller. There is more 0 surface area or surface contacts available for cohesive 51015202530 forces, in particular, and frictional forces to resist flow. The Maximum consolidated stress (kPa) particle size distribution also has an impact of flow possibility. A significant amount of fines in the distribution Fig. 7 Effect of consolidation time on flowability of self-rising wheat will aggravate flow problems and powder flowability may flour (adapted from Teunou and Fitzpatrick 2000) be worse that that expected from the mean size of the powder. Pertaining to particle shape, there is a dearth of information in the literature on this aspect. Bumiller et al. powder flowability, provided no melting of components (2002) described studies of particle shape influence on occurs, or no component exceeds its glass transition point hopper angle and outer size requirements for mass flow. temperature. Some food powders undergo a glass transition Storage conditions including storage temperature, expo- at elevated temperatures, where they change from crystal- sure to air humidity, storage time and consolidation, all play line material to an amorphous solid (Aguilera et al. 1995; an important role in food powder flowability. Powders in Bhandari and Hartel 2005). Amorphous particles become confinement tend to consolidate, which often leads to soft, and solid contact pressures cause particles to bind increased strength within the powder and increase adhesion together, affecting thus flowability. Glass transition tem- between the powder and the hopper wall. Teunou and perature can characterise the stickiness during powder Fitzpatrick (2000) reported effects of storage time and storage (Roos and Karel 1991). Many food powders will consolidation on flowability of flour, tea and whey normally show an effect of temperature in flow capacity as permeate. In agreement with other reports (Walker 1967; presented in Fig. 9. The presence of glassy low-molecular- Goelema et al. 1993; Fitzpatrick et al. 2004; Barbosa- weight materials (glucose, fructose and sucrose) in spray- Cánovas and Juliano 2005a), they found that the ability to dried fruit powders makes them particularly prone to flow was reduced with consolidation time. For typical food powders, the unconfined yield strength increases with consolidation time for a given maximum consolidating 3 stress, as shown in Fig. 7. Effects of relative humidity have also been investigated 2.5 (Teunou and Fitzpatrick 1999; Heng and Stainforth 1988; Tomas and Schubert 1982). Generally, in bulk materials, 2 moisture absorption causes an increase in cohesion, with a consequent decrease in flowability. Normally, local mois- ture content is the direct cause of cohesion. For instance, 1.5 crystalline products can cake significantly when exposed to 1 humid conditions due to formation of liquid bridges RH = 20% between particles, followed by drying. By a mechanism of RH = 44% 0.5 dissolution of soluble material and the transport of water Unconfined yield strength (kPa) RH = 76% vapour away from solid surfaces, new crystal structures are formed between particles (Johanson 2005). The effect of 0 1.5 2 2.5 3 3.5 4 4.5 different moisture levels on flowability of food powders will generally follow the trend illustrated in Fig. 8. Maximum consolidated stress (kPa) In general, varying the storage temperature form above Fig. 8 Effect of moisture content on flowability of self-rising wheat freezing to 30–40°C does not have a major impact on flour (adapted from Teunou and Fitzpatrick 1999) 36 Food Bioprocess Technol (2009) 2:28–44

2 mechanics. The void ratio VR, defined as the ratio between 1.8 volume of voids and volume of solids, gives another form 1.6 of mass balance for a given unit volume of solids as 1.4 follow: ðÞþ ¼ þ ð Þ 1.2 VR 1 rb rb raVR 4 1 and if ρa is again neglected, VR can be calculated from: 0.8 ðÞ rs rb 0.6 T = 40°C VR ¼ ð5Þ rb 0.4 T = 30°C ° 0.2 T = 5 C As can be seen by comparison with Eq. 3, this equation Unconfined yield strength (kPa) has the bulk density in the denominator instead of the 0 1.5 2 2.5 3 3.5 particle density as in the case of porosity. The relationship between void ratio and porosity is from Eqs. 3 and 5 and is Maximum consolidated stress (kPa) as follows: Fig. 9 Effect of temperature on flowability of self-rising wheat flour (adapted from Teunou and Fitzpatrick 1999) VR ¼ rs ð Þ " 6 rb stickiness, due to their elevated hygroscopic behaviour and One fundamental difference between void ratio and the chemical reactions occurring at elevated temperatures. porosity is that the latter can never be greater than unity, while the former can be as high as 1.3 in some cases (oats for example). It is preferable to use porosity whenever Bulk Density and Porosity possible because its definition is more logical and less prone to confusion (as a volume fraction of the whole). Definitions of Bulk Density Definitions and relationships between different types of densities are still confusing, and differences among mea- When a powder just fills a vessel of known volume V, and suring techniques can lead to considerable errors when the mass of the powder is m, then the bulk density of the determining them (Fasina 2007). Over the years, in order of powder is m/V. However, if the vessel is tapped, it will be increasing values, three classes of bulk density have found in most cases that the powder will settle, and more become conventional: poured, aerated and tap (Barbosa- powder needs to be added to have once more a complete Cánovas and Juliano 2005b). Each of these depends on the fill. If the mass now filling the vessel is m′ then the bulk treatment to which the sample was subjected, and although density is m′/V>m/V. Clearly, this change in density just there is a move towards standard procedures, these are far described has been caused by the influence of the fraction from universally adopted. There is still some confusion in of volume not occupied by a particle, known as porosity the open literature as to how these terms are interpreted. (Barbosa-Cánovas et al. 2005). The bulk density is, Some consider the poured bulk density as loose bulk therefore, the mass of particles that occupies a unit volume density, while others refer to it as apparent density. Aerated of a bed, while porosity or voidage is defined as the volume density can also be considered to be a quite confusing term. of the voids within the bed divided by the total volume of Strictly speaking, aerated should mean that the particles are the bed. These two properties are, in fact, related via the separated from each other by a film of air and not being in particle density in that, for a unit volume of the bulk direct contact with each other. Some authors interpret the powder, there must be the following mass balance: term as meaning the bulk density after the powder has been r ¼ r ðÞ1 " r " ð2Þ aerated. Tap density, the bulk density after a volume of b s a powder has been tapped or vibrated under specific ρ ρ where b is the powder bulk density, s is the particle density, conditions, can also be regarded as compact density. ε is the porosity and ρa is the air density. As the air density is small relative to the powder density, it can be neglected, and Measurement of Bulk Density the porosity can thus be calculated simply as: ðÞr r Poured density is widely used and would simply mean to " ¼ s b ð3Þ determine the mass–volume ratio of a powder sample by r s weighing a container of known volume without the sample Porosity, as defined above can also be termed voidage, and then with the freely poured powder. The measurement but it may be confused with the void ratio often used in soil is often performed in a manner found suitable for the Food Bioprocess Technol (2009) 2:28–44 37 requirements of the individual company or industry. In some cases, the volume occupied by a particular mass of powder is measured, but the elimination of operation judgment, and thus possible error, in any measurement is Scoop advisable. To achieve this, the use of a standard volume and the measurement of the mass of powder to fill it are needed. Certain precautions which should be taken are clear, e.g., it Screen cover is better to use a density cylinder with a 2:1 length to diameter ratio, the powder should always be poured from the same height, and the possibility of bias in the filling Screen should be made as small as possible. Although measuring Spacer ring of poured bulk density is far form standardised, many industries use a sawn-off funnel with a trap door or stop, to pour the powder through into the measuring container. Vibrating The aerated bulk density is, in practical terms, the chute density when the powder is in its most loosely packed form, while the tap bulk density, as is implied by its name, is the bulk density of a powder, which has been settled into a closer packing than existed in the poured state by tapping, jolting or vibrating the measuring vessel. Aerated and tap Stationary bulk densities can both be determined by use of a powder chute tester. A commonly used tester, which has been designed in compliance by norms established by the American Asso- Air-borne ciation of Testing and Materials (ASTM) is the Hosakawa fines powder tester. For determination of the aerated bulk density, a loosely packed form can be achieved by dropping a well-dispersed cloud of individual particles down into a measuring vessel. Standard 100 cc cup The structure within the vessel is held by the cohesive Cup forces between the particles and can be extremely fragile. location Levelling off the surface of the powder at the top of the vessel is difficult to achieve without causing particle movement leading to error, as some structure collapses.

As shown in Fig. 10 (Abdullah and Geldart 1999), the Hosakawa powder tester comprises an assembly of screen cover, a screen, a spacer ring and a chute attached to a Fig. 10 Determination of aerated bulk density mains-operated vibrator of variable amplitude. A stationary chute is aligned with the centre of a pre-weighed 100 ml cup. The powder is poured through a vibrating sieve and to determine tap bulk density includes a standard cup allowed to fall a fixed height of 25 cm approximately (100 ml) and a cam-operated tapping device which moves through the stationary chute into the cylindrical cup. The the cup upward and drops it periodically (once in every amplitude of the vibration is set so that the powder will fill 1.2 s). A cup extension piece has to be fitted and powder the cup in 20 to 30 s. The excess powder is skimmed from added during the sample preparation so that, at no time, the the top of the cup using the sharp edge of a knife or ruler, powder packs below the rim of the cup. After the tapping, without disturbing, or compacting, the loosely settled excess powder is scraped from the rim of the cup and the powder. bulk density determined by weighing the cup. For measuring the tap bulk density, as with poured bulk Approximate values of poured bulk density of different density, the volume of a particular mass of powder may be food powders are given in Table 2. As can be seen, with observed, but it is generally better to measure the mass of very few exceptions, food powders have poured bulk powder in a fixed volume. Although tapping can be done densities in the range of 300 to 800 kg/m3. The solid manually, it is better to use a mechanical tapping device so density of most food powders is about 1,400 kg/m3,so that the conditions of sample preparation are more these values are an indication that food powders have high reproducible. The version of the Hosokawa powder tester porosity, which can be internal, external or both. There are 38 Food Bioprocess Technol (2009) 2:28–44

Table 2 Approximate bulk density and moisture of different food cannot always be anticipated. There is an intricate relation- powders (adapted from Barbosa-Cánovas et al. 2005) ship between the factors affecting food powder bulk Powder Poured bulk Moisture density, as well as surface activity and cohesion. density (kg/m3) content (%) With regard to the moisture factor, also included in Table 2, moisture sorption is generally associated with Baby formula 400 2.5 increased cohesiveness mainly due to interparticle bridges. Cocoa 480 3–5 Coffee (ground and roasted) 330 7 Many food powders are highly hygroscopic, and therefore, Coffee (instant) 470 2.5 high moisture contents would result in lower loose bulk Coffee creamer 660 3 densities. However, this decrease would only be detected in Corn meal 560 12 freshly sieved or in flowing powders, where the same Corn starch 340 12 interparticle forces are not allowed to cause caking of the Egg (whole) 680 2–4 mass. Whereas sugar and salt are examples of powders Gelatin (ground) 680 12 which lower their densities as a result of increasing Microcrystalline cellulose 610 6 humidity, fine powders which are very cohesive even in Milk 430 2–4 Oatmeal 510 8 their dry form (baby formula and coffee creamer) do not Onion (powdered) 960 1–4 present such a trend. For these powders, it appears that the Salt (granulated) 950 0.2 bed array has reached maximum voidage at low moisture Salt (powdered) 280 0.2 contents, and further lowering of the density becomes Soy protein (precipitated) 800 2–3 impossible. It is also worthwhile to remember that Sugar (granulated) 480 0.5 excessive moisture levels, especially in powders containing Sugar (powdered) 480 0.5 soluble crystalline compounds, may result in liquefaction of Wheat flour 800 12 Wheat (whole) 560 12 the powder with the consequent increase in its density. Whey 520 4.5 Anticaking agents, also known as flow conditioners, are Yeast (active dry baker’s) 820 8 supposed to reduce interparticle forces, and as such, they Yeast (active dry wine) 8 are expected to increase the bulk density of powders (Peleg and Mannheim 1973). It has been observed that there may be an optimal concentration beyond which the effect will diminish or will be practically unaffected by the anticaking many published theoretical and experimental studies of concentration (Hollenbach et al. 1982). It can also be porosity as a function of the particle size, distribution and observed that, for a noticeable effect on the bulk density shape. Most of them pertain to free-flowing powders or (i.e., an increase in the order of 10% or more), the agent models (e.g., steel shots and metal powders), where and host particles must have surface affinity. If this is not porosity can be treated as primarily due to geometrical the case, the conditioner particles may segregate and, and statistical factors only (Gray 1968; McGeary 1967). instead of reducing interparticle forces, will only fill Even though in these cases porosity can vary considerably, interparticle space. Examples of effects of moisture and depending on factors such as the concentration of fines, it is anticaking agents on the bulk properties of selected food still evident that the exceedingly low density of food powders are given in Tables 4 and 5. powders cannot be explained by geometrical considerations only. Most food powders are known to be cohesive, and therefore, an open bed structure supported by interparticle Compressibility forces is very likely to exist (Moreyra and Peleg 1981; Scoville and Peleg 1980; Dobbs et al. 1982). Since the bulk In most cases, a material’s bulk density varies continuously density of food powders depends on the combined effect of as a function of the consolidating pressure acting on it. As a interrelated factors, such as the intensity of attractive result, it is not sufficient to describe a material simply as interparticle forces, the particle size and the number of contact points (Rumpf 1961), it is clear that a change in any Table 3 Comparison of two types of bulk density for different food of the powder characteristics may result in a significant powders (adapted from Teunou and Fitzpatrick 2000) change in the powder bulk density. For example, in Table 3, Powder Poured bulk Tap bulked variations in poured and tap density of three food powders density (kg/m3) density (kg/m3) are presented (Teunou and Fitzpatrick 1999). As can be seen, significant difference exists in the values of the two Self-rising wheat flour 601 695 Fine tea powder 617 913 types of density measured for the three powders tested. The Wheat-permeate powder 516 622 magnitude of change in bulk density of food powders Food Bioprocess Technol (2009) 2:28–44 39

Table 4 Effect of moisture content on mechanical charac- Powder Moisture (%) Poured bulk Compressibility Cohesion 3 2 teristics of some food powders density (kg/m ) (b in Eq. 9) (g/cm ) (adapted from Barbosa- Cánovas et al. 2005) Glass beads Dry 1,720 ∼0 ∼0 (175 μm) 1.0 1,200 0.23 15 Powdered salt Dry 1,260 0.02 ∼0 (100/200 mesh) 0.6 780 0.12 50 Powdered sucrose Dry 620 0.152 ∼10 (60/80 mesh) 0.1 500 0.185 ∼14 Starch Dry 810 0.12 ∼6 18.5 690 0.15 ∼13 Powdered onion Dry 510 0.03 5 (80/120 mesh) 5.2 510 0.05 15 Baby formula Dry 520 0.08 37 (commercial) 2.7 410 0.08 Too cohesive Coffee creamer Dry 460 0.08 49 (commercial) 7.0 450 0.19 32 Active dry 5.2 520 0.05 ∼0 Baker’s yeast 8.4 520 0.08 14 13.0 490 0.26 Too cohesive loose or compacted. Instead, the bulk density-to-pressure tions aimed at obtaining defined shapes are usually required relationship can be often expressed as a straight line on a in some processes. Theoretical and empirical considerations log–log plot as illustrated in Fig. 11. Food powders can be of vibratory compaction have been mainly focused to compacted by tapping or by mechanical compression. nonfood powders (Hausner et al. 1976). Sone (1972) These processes can occur either unintentionally as a result reported the following relationship for food powders: of handling or transporting or intentionally as when tabletting or agglomerating. In the food industry, uninten- ¼ V0 Vn ¼ abn ð Þ gn 7 tional compression is normally undesirable, while opera- V0 1 þ bn

Table 5 Effect of anticaking agents on bulk density and Powder Agent Concentration Poured bulk Compressibility compressibility of selected density (kg/m3) (b in Equation 9) food powders (adapted from Barbosa-Cánovas et al. 2005) Sucrose (powdered) None – 700 0.066 Calcium stearate 0.5 870 0.039 Silicon oxide 0.5 750 0.052 Tricalcium phosphate 0.5 760 0.044 Salt (powdered) None – 1,010 0.080 Calcium stearate 0.1 1,140 0.032 Silicon oxide 0.1 1,100 0.045 Tricalcium phosphate 0.1 1,160 0.025 Soup mix None – 700 0.27 Aluminium silicate 2.0 750 0.15 Calcium stearate 2.0 630 0.27 Gelatin (powdered) None – 680 ∼0 Aluminium silicate 1.0 700 0.016 Microcristalline cellulose None – 350 0.017 Aluminium silicate 1.0 360 0.030 Corn starch None – 620 0.109 Calcium stearate 1.0 590 0.099 Silicon oxide 1.0 670 0.077 Tricalcium phosphate 1.0 610 0.062 Soy protein None – 270 0.040 Calcium stearate 1.0 270 0.041 Silicon oxide 1.0 270 0.036 Tricalcium phosphate 1.0 310 0.024 40 Food Bioprocess Technol (2009) 2:28–44

1000 the particles themselves (Nyström and Karehill 1996). The second stage involves filling of smaller voids by particles that are deformed elastically and/or plastically, and eventu- ally broken down (Kurup and Pilpel 1978; Carstensen and Hou 1985; Duberg and Nyström 1986). Most of the organic 500 compounds show consolidation behaviour consisting on )

3 particle fragmentation during the initial loading, followed by elastic and/or plastic deformation at higher loads. In food agglomerates, on the other hand, compression takes place in three distinct stages: agglomerate particle re- arrangement to fill the voids similar or larger in size than the agglomerates, agglomerate deformation or brittle break- down, and primary particle rearrangement, elastic/plastic

Bulk density (kg/m density Bulk deformation and fracture (Mort et al. 1994; Nuebel and Peleg 1994). Both compression mechanisms in fine and agglomerated food powders are influenced by particle size and size distribution, particle shape and surface properties. If the material is packed in a loose state, considerable compressibility is shown, and when compressive forces are 100 applied, they are transmitted at the contact points. 0.1 1 10 100 Compression tests have been used widely in food powders, as a simple and convenient technique to measure Consolidation pressure (kPa) physical properties such as compressibility and flowability. Fig. 11 Bulk density of a food powder as a function of consolidation In order to get the pressure–density relationship for a given pressure powder, a set of compression cells (usually a piston in a cylinder) is used. The tested powder is poured into the cylinder and compressed with a piston attached to the cross- where γn is the volume reduction fraction, V0 is the initial head of a TA-XT2 Texture Analyzer or Instron Universal volume, Vn is the volume after n taps, and a and b are Testing Machine. Normally, a force–distance relationship constants. during a compression test will be recorded by the The applicability of Eq. 7 was tested through its fit to the instrument. It is relatively easy to change this relationship following linear form: into a pressure–density relationship to get the com- n 1 n pressibility after data treatment, when the cross section ¼ þ ð8Þ area of the cell and the initial powder weight are known. g ab a n The pressure–density for powders in a compression test at The constant a in Eqs. 7 and 8 represents the asymptotic low pressure range can be described by the following level of the volume change (the level obtained after a large equation (Barbosa-Cánovas et al. 1987): number of tapings or a long time in vibration). The constant b is representative of the rate at which this compaction is rsðÞr 0 ¼ a þ b log s ð9Þ achieved, i.e., 1/b is the number of vibrations necessary to r0 reach half of the asymptotic change. In general, this form of data presentation is very convenient for systems com- where ρ(σ) is the bulk density under the applied normal parisons, since it only involves two constants. stress σ, ρ0 the initial bulk density, and a and b constants. A very common undesirable aspect of compressibility is The constant b represents, specifically, the compressibility its negative influence on flowing capacity. In powder of a given powder. Compression tests are useful in technology, great attention has been paid to the general characterising the flowability of powders because the behaviour of powders under compressive stress (Peleg interparticle forces that enable open structures in powder 1977; Barbosa-Cánovas and Juliano 2005a, b). It has been beds succumb under relatively low pressures. As shown in observed that the compressive mechanisms in fine food Eq. 9, the constant b representing the change in bulk powders are different from those in food agglomerates density by the applied stress is referred to as the powder (Barbosa-Cánovas and Juliano 2005b). The first stage of compressibility. It has been found that b can be correlated compression in fine powders involves the movement of with cohesion of a variety of powders and therefore could particles toward filling voids similar to or larger in size than be a simple parameter to indicate flowability changes Food Bioprocess Technol (2009) 2:28–44 41

(Peleg 1977). Generally, the higher the compressibility, the exhibit good reconstitution characteristics, there needs to poorer the flowability is, but if quantitative information be a correct balance between the individual properties about capacity to flow is required, shear tests are necessary discussed above. In many cases, alteration of one or two of (Schubert 1987). these properties can markedly change the rehydrating One of the standard methods to evaluate the flowability behaviour. Several measures can be taken in order to of a particulate system is to calculate the Hausner ratio after improve reconstitutability of dried food products. The tapping. This ratio is defined as the ratio of a powder selected drying method and adjustment of drying conditions system’s initial bulk density to its tapped bulk density. It is can result in a product with good rehydration properties. easy to calculate the Hausner ratio and evaluate the For example, it is well known that freeze drying consists of flowability when the loose and tapped volumes of the test production of ice crystals and their sublimation at very low material are known. For a Hausner ratio of 1.0∼1.1, the pressures (Heldman and Singh 1981). This procedure powder is classified as free flowing; 1.1∼1.25, medium results in food particles with an open pore structure which flowing; 1.25∼1.4, difficult flowing and >1.4, very difficult absorb water easily when they are reconstituted. Another flowing (Hayes 1987). Flowability of food powders can be alternative is the use of the so-called combined methods, improved by agglomeration followed by drying in an air such as osmotic dehydration followed by conventional stream. Apart from improving flowability, agglomerated drying. In osmotic dehydration, food particles are immersed powders may show better wettability and dispersibility in in a concentrated solution. By osmotic pressure, the water liquids, and tend to be dust-free (Masters 1976). inside the particles tends to migrate to the solution in order to equate water activities on both sides of the cellular wall (Monsalve-González et al. 1993). This partial dehydration Reconstitution Properties will aid in the final stage drying, and textural damage will be minimised. In this sense, food materials dehydrated by Reconstitutability is the term used to describe the rate at combined methods will also have an open pore structure, which dried foods pick up and absorb water reverting to a and similar to freeze dried materials, will present good condition which resembles the un-dried material, when put reconstitution properties. Beltran-Reyes et al. (1996) devel- in contact with an excessive amount of this liquid (Masters oped an apple powdered ingredient by grinding dried 1976). In the case of dried-powdered foods, a number of apples obtained by osmotic dehydration followed by properties may influence the overall reconstitution charac- conventional heated air drying. They determined that the teristics (Hogekamp and Schubert 2003). For instance, firmness of the rehydrated mash was a direct function of the wettability describes the capacity of the powder particles to particle size. For the same ingredient, it has been reported absorb water on their surface, thus initiating reconstitution. (Ortega-Rivas and Beltran-Reyes 1997) that rehydration Such a property depends largely on particle size. Small improved as particle size decreased, as shown in Fig. 12. particles, representing a large surface area: mass ratio, may The most efficient method to improve the rehydration not be wetted individually but may clump together sharing characteristics of dried food powders is probably the use of a wetted surface layer. This layer reduces the rate at which agglomeration (Barletta and Barbosa-Cánovas 1993). In water penetrates into the particle clump. Increasing particle order to improve wettability of food powders, their particles size and/or agglomerating particles can reduce the inci- can be increased in size by agglomeration. A critical size of dence of clumping. The nature of the particle surface can also affect wettability. For example, the presence of free fat 7 in the surface reduces wettability. Another important property is the sinkability, which describes the ability of 6 the powder particles to sink quickly into the water. This 5 depends mainly on the size and density of the particles. 4 Larger denser particles sink more rapidly than finer, lighter ones. Particles with a high content of occluded air may be 3 relatively large but exhibit poor sinkability because of their 2 Rehydration ratio low density. Finally, dispersability describes the ease for a 1 powder to be distributed as single particles over the surface 0 and throughout the bulk of the reconstituting water, while 0.6 0.7 0.8 0.9 1 1.1 solubility refers to the rate and extent to which the components of the powder particles dissolve in water. Particle size (mm) Food dried powders and particulates are normally Fig. 12 Dehydration ratio as a function of particle size of a food reconstituted for consumption. For a dried product to powder (adapted from Ortega-Rivas and Beltran-Reyes 1997) 42 Food Bioprocess Technol (2009) 2:28–44 particles to get wet is about 100 μm. Generally, powders Stickiness and caking are common problems prevailing in with particle sizes below 100 μm are difficult to wet food powder handling and processing, so their flowability because of their small inter-particulate space (Bhandari and results are quite indeterminate. Reliable characterisation of Hartel 2005). Thus, increasing median sizes to a larger secondary properties would aid to understand better flow range between 500 and 3,000 μm will have a positive behaviour of food powders under different conditions. impact in wettability. This agglomeration process is called instantisation, and it is generally accepted that “instant” Notation powders will get wet quickly and will disperse in water faster than “non-instant” powders. Instantisation can be (Dimensions given in terms of mass, M, length, L, and carried out by removing particles from a spray drier at high time, T) moisture content (in thermoplastic state) and allow them to a constant, asymptotic level of the volume change stick in a fluidised bed at elevated temperature. Another b constant, rate at which compaction is achieved, method of instantisation involves rewetting the surface of compressibility of given powder individual particles allowing them to come in contact and − C cohesion (ml 2) stick together, and then drying to remove water and cause n shear index, number of taps stuck particles to become stable agglomerates. − − T tensile strength (ml 1 t 2) Wetting time is the most important variable in evaluating V volume after n taps (l3) instant properties. Standard procedures for measuring n VR void ratio instant properties need to consider a series of aspects such V initial volume (l3) as specific solvent temperature, liquid surface area, amount 0 of material to dissolve, method for depositing a certain Greek symbols amount of material on the liquid surface, unassisted or γn Volume reduction fraction predetermined mixing steps, and timing procedure (Pietsch ε Porosity 1999). Different techniques have been reported to evaluate −3 ρa Air density (ml ) food powder instant properties (Barbosa-Cánovas and −3 ρb Powder bulk density (ml ) Juliano 2005b) and include the penetration speed test and −3 ρs Particle density (ml ) the standard dynamic wetting test. In the penetration test, a −3 ρ0 Initial bulk density (ml ) cell with screen carrying a layer of agglomerates, retained ρ(σ) Bulk density under applied normal stress (ml−3) with a plexiglass cylinder, is placed into water. 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Modelling the strength and the flow behavior and crystallization of amorphous food model. Journal properties of most soluble materials. Aufbereitungstechnik, 9, of Food Science, 56,38–43. 507–515. Royal, T. A., & Carson, J. W. (1993). How to avoid flooding in Walker, D. M. (1967). A basis for bunker design. Powder Technology, powder handling systems. Powder Handling & Processing, 5, 1, 228–236. 63–67. Williams, J. C., Birks, A. H., & Bhattacharya, D. (1971). The direct Rumpf, H. (1961). The strength of granules and agglomerates. In W. measurement of the failure function of a cohesive powder. A. Knepper (Ed.), Agglomeration (pp. 39–418). New York: Powder Technology, 4, 328–337. Industrial Publishers. Wilms, H. (1999). Equipment design based on solids flowability data. Schubert, H. (1987). Food particle technology part I: Properties of Powder Handling & Processing, 11,37–42. particles and particulate food systems. JournalofFood Wright, H. (1999). Proper design for reliable flow from hoppers and Engineering, 6,1–32. silos. Bulk Solids Handling, 19, 181–187. Eng. Life Hydrocyclone Separation Sci. Review

Applications of the Liquid Cyclone in Biological Separations

By Enrique Ortega-Rivas*

Hydrocyclone technology has been suggested as a practical alternative in solid/liquid separations involving biological materi- als. This paper reviews applications of hydrocyclones in food processing, considering the non-Newtonian nature of most sus- pensions treated in the food industry. The hydrocyclone is easy to install and operate, and requires very limited space. It re- presents an unsophisticated piece of equipment, which runs in a continuous manner and it can be operated at lower costs than most solid/liquid separation techniques. Hydrocyclones have been used in the food industry for the refining of starch, to separate gossypol from cottonseed protein in cottonseed oil processing, and for some other applications, such as multi-stage mixer/separator extraction systems for soluble coffee. More recently, some other applications in biological systems, which will be discussed in this article, have also been tested.

1 Introduction Overflow

Centrifugation is a well-known unit operation, which has numerous applications in the separation of biological mate- rials. The clarification of fruit juices, separation of crystals from mother liquors, treatment of effluents, and downstream processing are examples of processes where phase separa- tion can be performed using centrifuges of various types. In Feed many of these applications, due to the fineness of some suspended solids, as well as the approximate equality of the solid and liquid densities in the suspensions, total removal of such solids by centrifugation cannot be practically accom- plished. Another way of separating suspended solids from a liquid is the hydrocyclone, which takes advantage of centrifugal forces. As shown in Fig. 1, a hydrocyclone consists of a cono-cylindrical body that promotes the formation of a vor- tex when a suspension is pumped through it. The vortex creates a centrifugal force, which causes coarse particles to migrate against the cyclone wall and to be dis- Underflow charged by the underflow orifice. Fine particles remain Figure 1. Schematic of a hydrocyclone. around the central axis of the cyclone and are carried out by the overflow stream. Hydrocyclones are easily manufactured and modified, and have been widely tested in thickening, efficiency G(X) curve shows a value of 50 %. A grade effi- clarification, classification, and other operations in many in- ciency curve for a hydrocyclone is derived from screen ana- dustries [1]. lysis data on the feed, overflow, and underflow streams, and Similarly to centrifuges, hydrocyclones may be evaluated is a continuous representation of the overall mass recovery in terms of separation efficiency by means of the cut size or as a fraction of the mass flow rate. Since most suspended cut point (X or X ). The cut size, which is the only single c 50 powders and fine particulate systems can be represented by number that in some way represents the separating capabili- a continuous size distribution, the grade efficiency curve is ty of a hydrocyclone, is the particle size at which the grade really derived from a step-wise calculation, drawing a line through the mid points of the size intervals [2]. For hydrocy- clones and dynamic separators, consequently, the grade effi- ± ciency curve is a cumulative plot in which the 50 % point [*] E. Ortega-Rivas (author to whom correspondence should be addressed, e-mail: [email protected]), Food and Chemical Engineering Program, represents a limit value. Thus, particles with this limit size University of Chihuahua, Apdo Postal 1542-C, Chihuahua, Chih., Mexico. have a 50 % probability of being separated. In other words,

Eng. Life Sci. 2004, 4,No.2 DOI: 10.1002/elsc.200402004  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 119 Eng. Life Sci. Review most particles above the cut size are generally discharged in 2 The Scaleup Separation Model the underflow, whereas most of those below the cut size are normally carried away in the overflow. Ortega-Rivas has Since hydrocyclones do not have any rotating parts and reviewed some of the numerous expressions utilized for the vortex action to produce centrifugal force is obtained by determining cut size, which have been reported in the litera- pumping the feed suspension tangentially into the cono-cy- ture [3]. lindrical body, the literature includes numerous studies of The grade efficiency curve, as described above, gives a the effects of the relative geometric proportions on pressure plot that does not pass through the origin. This can be ex- drop or capacity, and separation efficiency. Using this infor- plained bearing in mind that a hydrocyclone is a flow divi- mation, particular hydrocyclone geometries could be se- der, so the underflow always contains a certain quantity of lected to obtain an optimum performance in terms of cut very fine particles, which simply follow the flow, and are split size. In this sense, possibly the best opportunity to predict in the same ratio as the liquid. The apparent finite efficiency hydrocyclone performance is the use of a dimensionless sca- for fine particles is therefore equal to the underflow-to- leup model, which has been described elsewhere [1, 3±5]. As throughput ratio Rf. A ªcorrectedº or ªreducedº cut size, many as three dimensionless groups can be used to describe X'50, will practically assess the performance of hydrocy- hydrocyclone operation and performance: The Euler num- clones when derived from the reduced grade efficiency ber Eu, the Reynolds number Re, and the Stokes number G'(X). All these definitions are given in Fig. 2. Stk50. For the best application of the relationships among di- mensionless groups, certain proportions must remain un- changed. Such proportions are generally reported as a func- 100 tion of the diameter of the hydrocyclone. There are several different standard hydrocyclone designs in which the propor- Actual grade tions remain the same regardless of size. One of the most efficiency, G(X) efficient designs for separation is called the Rietema [6] cy- clone, whose proportions are given in Fig. 3. Reduced grade efficiency, G’(X) 50

0.34Dc Grade efficiency, % Grade efficiency,

0.28Dc R f X50 X’50 0.4Dc 0

Particle size, X Figure 2. Grade efficiency and reduced grade efficiency curve for a hydrocy- Dc clone.

5Dc The sharpness of the separation can be related to the shape of the grade efficiency curve in a number of manners. Knowing this curve, comparisons of the classification sharp- 20º ness of different units can be made by plotting G(X) against the dimensionless size relation X/X50, and comparing the re- sultant normalized curves. To define the curve steepness, a ratio of two sizes corresponding to two different percentages 0.2Dc on either side of the grade efficiency curve, and 50 % equidi- Figure 3. Dimensions of a Rietema's standard hydrocyclone. stant, can be used. This parameter is called the sharpness index H25/75. The maximum obtainable efficiency, related to particle The Euler number, which is a pressure loss factor, is de- size, would be the minimum particle size with 100 % prob- fined as the limit of the maximum characteristic velocity, v, ability of being accounted for in the underflow. Graphically, obtained by a certain pressure drop, DP, across the cyclone. by extrapolating the end part of the grade efficiency curve to It can be expressed as the horizontal axis, this size will be obtained. It has been 2ÁP proven [2] that in practice, the maximum efficiency is around Eu ˆ (1) rv2 98 %, and the minimum size, corresponding to this efficien- cy, is represented by X98 and is known as the approximate where r is the liquid density and v is the superficial velocity limit of separation. in the cyclone body.

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The Reynolds number defines the flow features of the the following expression for the effect of high solids concen- system and, in the case of hydrocyclones, the characteristic trations in the feed dimension may be taken as the cyclone body diameter, Dc, Stk r†ˆk 1 À R † exp k C† (7) D vr 50 1 f 2 Re ˆ c (2) l where Stk50(r) includes the previously described reduced cut where r is the liquid density and l is the average viscosity. size, which takes the ªdead fluxº effect into account whereby The Stokes number may be derived from basic fluid me- very fine particles simply following the flow and split in the chanics theory [3] and is defined as follows same ratio as the liquid. As stated earlier, Rf is the under- flow-to-throughput ratio. The correlation has been proven X2 r Àr†v Stk ˆ 50 s (3) to hold well for concentrations above 8 vol.-%, and the val- 50 18lD ±5 c ues of the constants k1 and k2 were found to be 9.05´10 and 6.461, respectively, for limestone and an AKW (Amber where X is the cut size, r is the solids density, and D is 50 s c Kaolinwerke GmbH, Hirschau, Germany) hydrocyclone of the hydrocyclone diameter. 125 mm in diameter. All the above equations use the superficial velocity in the An exhaustive study for concentrations up to 10 vol.-% cyclone body as the characteristic velocity, i.e., was carried out by Medronho [11] in order to verify the ap- 4Q plicability of Eqs. (5±7). He employed three geometrically v ˆ (4) pD2 similar hydrocyclones using Rietema's optimum geometry c [6] and obtained the following relations where Q is the feed volumetric flow rate. The dimensional analysis gives two basic relationships be- ÂÃ0:74 tween the dimensionless groups mentioned above [4] Stk50 r†Eu ˆ 0:047 ln 1=Rf † exp 8:96C† (8)

Eu ˆ 71 Re†À0:116 D =D †À1:3 exp 2:12C† (9) Stk50 Eu ˆ constant (5) i c

np R ˆ 1218 D =D †À4:75 Eu†À0:30 (10) Eu ˆ kp Re† (6) f u c where kp and np are constants derived for a family of geo- where Di,Dc, and Du are the inlet, body, and underflow di- metrically similar hydrocyclones. Eqs. (5) and (6) have been ameters of the hydrocyclone, respectively. tested over a range of conditions by various workers [7±9], For concentrations higher than 10 vol.-% many of the slur- and their characteristic plots are as shown in Fig. 4 for ries used show non-Newtonian behavior and it can be shown Eq. (6). that the Reynolds and Stokes numbers can be rewritten to At higher concentrations the feed concentration as a frac- consider such behavior [4]. The correlations derived are the tion of the volume, C, has to be included as an additional di- following mensionless group. Svarovsky and Marasinghe [10] reported ÂÃ2:37 Stkà r†Eu ˆ 0:006 ln 1=R † exp 6:84C† (11) 50 f

4 10 Eu ˆ 1686 Reà †À0:035 exp À3:39C† (12)

1:53 à À0:34 Rf ˆ 32:8 Du =Dc † Re † exp 3:70C† (13)

103 * * where Stk 50(r) and Re are the generalized Stokes and Rey- nolds numbers, meaning that they include the parameters of characterization of non-Newtonian suspensions, i.e., the

Euler number fluid consistency index K' and the flow behavior index, n, in- 2 10 stead of the medium viscosity [4]. The term generalized is used to imply that, for Newtonian suspensions, the Stokes and Reynolds numbers above would reduce to the common forms normally found in the literature. While for Eq. (12), as 101 compared with Eq. (9), the diameters term did not show sig- 2 3 4 5 6 10 10 10 10 10 nificant correlation, for Eq. (13), in contrast with Eq. (10), Reynolds number the best fit was obtained by correlating the Reynolds num- Figure 4. A typical plot of Eu versus Re for hydrocyclones. ber instead of the Euler number.

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3 Applications in Biological Systems Apart from the fact that in Eq. (16) there was no signifi- cant concentration effect, both expressions showed adequate In terms of biological fluids and suspensions, the standard similarity to those suggested elsewhere [4, 10]. This may be application for small hydrocyclones is in starch refining. considered an important verification of the applicability of They have been extensively used in corn and potato starch the dimensionless scaleup model of hydrocyclone operation, refining, giving good results [12]. Hydrocyclones have been due to the use of real systems in the work reported here. used to separate gossypol from the protein in cottonseed oil processing. In addition they are utilized as separators in mul- ti-stage mixer-separator extraction systems for soluble cof- 4 Conclusions fee, and with promising results, in the thickening of waste- water sludge [13, 14]. Hydrocyclones have been used in Hydrocyclones have tremendous potential for applications Sephadex, , and blood cells separation [15], and effi- in biological processes where, typically, centrifugation has ciently in yeast recovery in different fermentation processes been the only alternative considered. Some real examples of [16±18]. Another reported application is that of kieselguhr the use of hydrocyclone technology in biological processing recycling for filters in the brewing industry [19]. have been discussed in this paper. Equations were derived The previously described dimensionless scaleup model has using real, non-Newtonian slurries considering the dimen- been tested in applications of apple juice clarification [20] sionless scaleup approach of hydrocyclone separation. and wastewater treatment [14]. In the case of the juice clari- Many solid/liquid separations in biological applications fication, low Reynolds numbers values were obtained due to rely on the use of centrifugation. If hydrocyclones prove to the high viscosity of the feed, so the flow was not as turbu- be sufficiently efficient at completely separating fine parti- lent as is commonly found in small diameter hydrocyclone cles (several such situations have been reported in this operation. The Euler number values were practically con- paper), which is not possible by centrifugation, they will rep- stant, which may also be attributed to the highly consistent resent an inexpensive and unsophisticated choice for indus- feed suspensions. For these reasons, the relationship be- tries dealing with biological materials. tween Eu and Re was not properly derived. It is believed Despite the complex nature of biological suspensions, that the behavior of the feed suspension was not satisfactory, some reported results show that a significant reduction in due to the use of an inappropriate pump. A higher capacity suspended solids in biological systems can be obtained by slurry pump for viscous fluids could have provided better means of hydrocyclones. Since only the basic geometry of pressure drops, with the consequent effect of having more the solid/liquid cyclone was employed, and gave promising well defined cut sizes according to the established pressures, results, the use of the so-called liquid/liquid hydrocyclone and achieved more variability in the Euler and Reynolds may be of interest. Since hydrocyclones are easy to manufac- numbers. Despite all the difficulties mentioned, a relation- ture, install, maintain, and operate, a number of body modi- ship similar to Eq. (7) was obtained from experimental data fications can be attempted. [20] as follows The verification of the applicability of the dimensionless scaleup model of hydrocyclone operation, using real systems, À5 is also important. Since hydrocyclones are solid/liquid se- Stk50 r†ˆ7 Á 10 1 À Rf † exp À6:47C† (14) parators, which are more efficient at lower diameters, the As can be seen, the constant values in Eq. (7) and Eq. (14) use of a scaleup model is of great relevance because they show remarkable similarity. Apart from the opposite trend may only be adapted to an industrial scale by operating them in the concentration correction factor, such values are practi- in series including hundreds, or even thousands, of units. cally the same. Taking into account that the derivation con- Received: December 11, 2003 [CET 2004] ditions for both equations were totally different, i.e., inert against biological material and different hydrocyclone diam- eter and operating variables, the good parallel observed References gives reason to believe that the dimensionless approach may be extended to the separation of biological materials. [1] L. Svarovsky, Hydrocyclones, Hold Rinehart and Wilson, London 1984. [2] H. F. Trawinski, in Solid/Liquid Separation Equipment Scale-up Considering the case of wastewater [14], practically all the (Ed: D. B. Purchas), Uplands Press Ltd, Croydon, UK, 1977. feed suspensions were pseudo-plastic. Therefore, a series of [3] E. Ortega-Rivas, PhD Thesis, University of Bradford 1989. dimensionless relationships like those described by Ortega- [4] E. Ortega-Rivas, L. Svarovsky, Fluid Part. Sept. J. 1993, 6, 104. [5] L. Svarovsky, in 3rd International Conference on Hydrocyclones (Ed: Rivas and Svarovsky [4] were attempted. The correlations P. Wood), Elsevier, London 1988. developed are presented below [6] K. Rietema, Chem. Eng. Sci. 1961, 15, 198. [7] M. Antunes, R. A. Medronho, in Hydrocyclones-Analysis and Applica- tions (Eds: L. Svarovsky, M. T. Thew), Kluwer, Dordretch, The Eu ˆ 3:388 Á 109 Reà †À1:542 exp À1:325C† (15) Netherlands, 1992. [8] K. Nezhati, M. T. Thew, in 3rd International Conference on Hydro- cyclones (Ed: P. Wood), Elsevier, London 1988. 4:182 à À0:269 Rf ˆ 59:326 Du =Dc † Re † (16) [9] W. Chen, N. Zydek, F. Parma, Chem. Eng. J. 2000, 80, 295.

122  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.els-journal.de Eng. Life Sci. 2004, 4,No.2 Eng. Life Hydrocyclone Separation Sci.

[10] L. Svarovsky, B. S. Marasinghe, BHRA Conference on Hydrocyclones, [16] J. J. Cilliers, S. T. L. Harrison, in Hydrocyclones 96 (Eds: D. Claxton, Cambridge, UK, October 1980. L. Svarovsky, M. T. Thew), Mechanical Engineering Publications, [11] R. A. Medronho, L. Svarovsky, 2nd International Conference on London 1996. Hydrocyclones, Bath, UK, September 1984. [17] H. Yuan, M. T. Thew, D. Rickwood, in Hydrocyclones 96 (Eds: [12] S. Bednarski, in Hydrocyclones-Analysis and Applications (Eds: D. Claxton, L. Svarovsky, M. T. Thew), Mechanical Engineering L. Svarovsky, M. T. Thew), Kluwer, Dordretch, The Netherlands, 1992. Publications, London 1996. [13] S. Bednarski, in Hydrocyclones 96 (Eds: D. Claxton, L. Svarovsky, [18] V. Martins da Mata, R. A. Medronho, Bioseparation 2000, 9, 43. M. T. Thew), Mechanical Engineering Publications, London 1996. [19] D. Rickwood, G. J. Freeman, M. McKechnie, in Hydrocyclones 96 (Eds: [14] E. Ortega-Rivas, S. P. Medina-Caballero, Powder Hand. Process. 1996, 8, D. Claxton, L. Svarovsky, M. T. Thew), Mechanical Engineering 355. Publications, London 1996. [15] D. Rickwood, J. Onions, B. Bendixen, I. Smyth, in Hydrocyclones- [20] E. Ortega-Rivas, F. Meza-Velµsquez, R. Olivas-Vargas, Food Sci. Analysis and Applications (Eds: L. Svarovsky, M. T. Thew), Kluwer, Technol. Int. 1997, 3, 325. Dordretch, The Netherlands, 1992.

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