Application of Ultrasound

P676757-Ch13.qxd 6/20/05 5:02 PM Page 323

Application of 13 Ultrasound

Timothy J Mason1, Enrique Riera2, Antonio Vercet3 and Pascual Lopez-Buesa3 1Coventry University, Sonochemistry Centre, School of Science and the Environment, Coventry, UK 2Instituto de Acústica, CSIC, Ultrasonics Department, Madrid, Spain 3Universidad de Zaragoza, Departamento de Producción Animal y Ciencia de los Alimentos, Facultad de Veterinaria, Zaragoza, Spain

Nowadays, power ultrasound is considered to be an emerging and promising technology for food processing in industry. In this chapter a review of the most recent uses of power ultrasound in the food industry will be presented. The potential use of this novel technology to produce permanent changes in the material will be discussed in liquid systems (by the production of intense cavitation) and in gases (by the generation of high-intensity acoustic fields). Mechanical, chemical and biochemical effects produced by the propagation of high- intensity ultrasonic waves through the medium will be discussed. The inactivation of microorganisms and enzymes for food preservation or decontamination by ultrasonic irradiation will demonstrate the benefits of ultrasound (alone or combined with heat and high-pressure techniques) as a food preservation tool. In addition, the increasing number of industrial processes that employ power ultrasound as a processing aid will be described including the mixing of materials; foam formation or destruction; agglomeration and precipitation of airborne powders; the improvement in efficiency of filtration, drying and extraction techniques in solid materials and the enhanced extraction of valuable compounds from vegetables and food products. Finally, the effect of ultrasound on food properties such as flavour, colour and texture will be analysed.

1 Introduction

Probably the first question that might be asked about applications of ultrasound in food technology is, why use ultrasound? For the answer to this we need only think of two properties of sound to appreciate the possibilities. The first is the use of sound as a diagnostic tool, e.g. in non-destructive evaluation and the second is the use of sound as a source of energy, e.g. in sonochemistry. These applications involve different frequency ranges of ultrasound (Figure 13.1) and the uses of both ranges within the food industry continue to be an active subject for research and development (Povey and Mason, 1998).

324 Application of Ultrasound

010102 103 104 105 106 107

Human hearing 16 Hz–18 kHz

Conventional power ultrasound 20 kHz–100 kHz

Extended for special applications 20 kHz–1 MHz

Diagnostic ultrasound 5 MHz–10 MHz

Figure 13.1 Frequency ranges of sound.

Table 13.1 Some uses of power ultrasound in food processing

Mechanical effects crystallization of fats, sugars etc degassing destruction of foams extraction of flavourings filtration and drying freezing mixing and homogenization precipitation of airborne powders tenderization of meat Chemical and biochemical effects bactericidal action effluent treatment modification of growth of living cells alteration of enzyme activity sterilization of equipment

Until recently the majority of applications of ultrasound in food technology involved non-invasive analysis with particular reference to quality assessment. Such applications use techniques that are similar to those developed in diagnostic medicine, or non-destructive testing, using high frequency (1 MHz) low power (1 W/cm2) ultrasound (Mulet et al., 2002). Examples of the use of such technologies are to be found in the location of foreign bodies in food (Cho and Irudayaraj, 2003), the analysis of droplet size in emulsions of edible fats and oils (Coupland and McClements, 2001) and the determination of the extent of crystallization in dispersed emulsion droplets (Hindle et al., 2002). In recent years food technologists have discovered that it is possible to employ a more powerful form of ultrasound (5 W/cm2) at a lower frequency (generally around 40 kHz). This is usually referred to as power ultrasound and its history can be traced P676757-Ch13.qxd 6/20/05 5:02 PM Page 325

Fundamentals of ultrasound 325

back to 1927 when a paper was published entitled ‘The chemical effects of high frequency sound waves: a preliminary survey’, which described the development of power ultrasound for use in a range of processing including emulsification and surface cleaning (Richards and Loomis, 1927). By the 1960s the uses of power ultrasound in the processing industries were well accepted and this interest has continued to develop (Abramov, 1998; Mason, 2000; Mason and Lorimer, 2002). In this chapter we will concentrate on possible applications of power ultrasound in the food industry, an indication of the breadth of which is shown in Table 13.1.

2 Fundamentals of ultrasound 2.1 The physics and chemistry of ultrasound 2.1.1 Power ultrasound in liquid systems The major mechanical effects of ultrasound are provided when the power is sufficiently high to cause cavitation. Like any sound wave, ultrasound is propagated via a series of compression and rarefaction waves induced in the molecules of the medium through which it passes. At sufficiently high power the rarefaction cycle may exceed the attractive forces of the molecules of the liquid and cavitation bubbles will form. Such bubbles grow by a process known as rectified diffusion, i.e. small amounts of vapour (or gas) from the medium enters the bubble during its expansion phase and is not fully expelled during compression. The bubbles, distributed throughout the liquid, grow over the period of a few cycles to an equilibrium size for the particular frequency applied. If the bubbles were only subject to that particular frequency they would remain as oscillating bubbles, however, the acoustic field that influences an individual bubble among the many thousands generated in a cavitating fluid is not uniform. Each bubble will slightly affect the localized field experienced by neigh- bouring bubbles. Under such circumstances the irregular field will cause the cavitation bubble to become unstable and collapse. It is this collapse that generates the energy for chemical and mechanical effects. For example, in aqueous systems at an ultrasonic frequency of 20 kHz, each cavitation bubble collapse acts as a localized ‘hotspot’ generating temperatures of about 4000 K and pressures in excess of 1000 atmospheres. This bubble collapse, distributed through the medium, has a variety of effects within the system depending upon the type of material involved.

2.1.1.1 Homogeneous liquid-phase systems It is not absolutely correct to describe any system within which cavitation occurs as homogeneous since cavitation bubbles must be present. However, it is logical to refer to systems in the state that they were in before ultrasound is introduced. There are two major zones in which cavitation collapse can influence such systems: in the bulk liquid immediately surrounding the bubble where the rapid collapse of the bubble generates shear forces which can produce mechanical effects and in the bubble itself where any species introduced during its formation will be subjected to extreme conditions of temperature and pressure on collapse, leading to chemical effects. P676757-Ch13.qxd 6/20/05 5:02 PM Page 326

326 Application of Ultrasound

In order for cavitation to occur it is necessary to pull molecules of the liquid apart to produce a hole (cavity). A pure liquid would require very high power levels to initiate cavitation, too high to be achieved by normal ultrasonic equipment. However, most normal liquids contain some discontinuities, such as gas bubbles or dust motes, which act as weak spots and allow the bubbles to form. Ultrasonic degassing is normally performed before cleaning equipment is used because it increases the efficiency of cavitation by removing air bubbles which absorb acoustic energy and dampen sonication. After degassing continued sonication will provide continuous and more powerful cavitation that will have other effects. The cavitation bubble does not contain a vacuum; during the process of growth in an acoustic field vapour from the liquid medium or dissolved volatile reagents will have entered the bubble. On collapse, these vapours will be subjected to extremely large increases in temperature and pressure, resulting in molecular fragmentation. In the case of water the extreme conditions are sufficient to cause rupture of the O-H bond, leading to the production of small quantities of oxygen gas and hydrogen peroxide.

2.1.1.2 Solid–liquid systems Unlike cavitation bubble collapse in the bulk liquid, collapse of a cavitation bubble on or near to a surface is asymmetrical because the surface provides resistance to liquid flow from that side. The result is an inrush of liquid predominantly from the side of the bubble remote from the surface, resulting in a powerful liquid jet being formed, targeted at the surface. The effect is equivalent to high pressure jetting and is the rea- son that ultrasound is used for cleaning. This effect can also increase mass and heat transfer to the surface by disruption of the interfacial boundary layers. Acoustic cavitation can also produce dramatic effects on powders suspended in a liquid. Surface imperfections or trapped gas can act as the nuclei for cavitation bubble formation on the surface of a particle and subsequent surface collapse can then lead to shock waves which break the particle apart. Cavitation bubble collapse in the liquid phase near to a particle can force it into rapid motion. Under these circumstances the general dispersive effect is accompanied by interparticle collisions that can lead to erosion, surface cleaning and wetting of the particles and particle size reduction.

2.1.1.3 Liquid–liquid systems The general mechanical effects of cavitation at or close to a liquid–liquid interface lead to very effective emulsification/homogenization.

2.1.2 Power ultrasound in gases There are two difficulties that pertain to the use of ultrasound in gaseous systems. First, there is a greater attenuation (power loss) in the transmission of sound through air compared with that through liquid. Secondly, the transfer of acoustic energy generated in air into a food material is inefficient due to the mismatch between acoustic impedances of gases and solids or liquids. Significant attempts have been made to alleviate these problems by developing very powerful sources of airborne ultrasound that can achieve more efficient energy transmission to the material (Gallego et al., P676757-Ch13.qxd 6/20/05 5:02 PM Page 327

Fundamentals of ultrasound 327

Stepped radiating Piezoelectric plate ceramics

Mechanical Sandwich amplifier transducer

Figure 13.2 Dish emitter for airborne ultrasound.

1994). The system is based on the use of a stepped-plate transducer to generate airborne ultrasound (Gallego et al., 1978) and incorporates an extended titanium stepped-grooved radiating plate (350 mm in diameter) which allows the ultrasonic energy to focus (Figure 13.2). Sound pressure levels (SPL) of about 165 dB (ca 3 W/cm2) have been recorded at a distance of about 330 mm from the centre of the plate, when a maximum power of 150 W is applied to the transducer. Using this type of device, airborne ultrasound has been used in the precipitation of airborne powders, drying and defoaming.

2.2 Ultrasonic processing equipment 2.2.1 Laboratory scale Whatever food processing application is to be studied or developed, the essential requirement is a source of ultrasound. The two most common pieces of laboratory equipment used for processing liquids are an ultrasonic cleaning bath, which is inex- pensive and is commonly used to sonicate liquid samples in vessels immersed in the bath (Figure 13.3), or the more powerful probe system, which introduces vibrations directly into the sample (Figure 13.4) (Mason, 1999).

2.2.2 Large scale There are essentially two types of large scale plant: batch and flow types. The results from successful small-scale experiments can be adapted for large scale work provid- ing that information is available on power input and volume treated. Several scale-up designs are available for food processing which can be broadly divided into batch and flow systems. A general review of large scale processors has been published by P676757-Ch13.qxd 6/20/05 5:02 PM Page 328

328 Application of Ultrasound

Reaction mixture in conical flask

Water with detergent

Optional Stainless heater steel tank Transducers bonded to base

Figure 13.3 Ultrasonic bath.

Generator

Casing containing transducer element

Upper fixed horn (booster)

Screw fitting at null point

Detachable horn

Replaceable tip

Figure 13.4 Ultrasonic probe system.

Mason and Peters (2002). Several large scale applications using airborne ultrasound will be discussed later in this chapter.

2.2.2.1 Batch systems Batch systems will generally be based upon the ultrasonic cleaning bath using the whole bath as the reactor. Examples can be found in cleaning and decontamination of P676757-Ch13.qxd 6/20/05 5:02 PM Page 329

Ultrasound as a food preservation tool 329

Region of controlled formation and collapse of cavitation bubbles

Flow from pump

Figure 13.5 Liquid whistle.

equipment, e.g. in the cleaning of chicken shackles to avoid cross-contamination (Quartly-Watson, 1998).

2.2.2.2 Flow systems One of the oldest devices used to achieve emulsification through cavitation is the liquid whistle. Process material is forced under pressure generated by a powerful pump through an orifice from which it emerges and expands into a mixing chamber (Figure 13.5). With no moving parts, other than a pump, the system is rugged and durable (Moser et al., 2001). The systems that are particularly suitable for general usage in the food industry are resonating tube reactors. Essentially the liquid to be processed is passed through a pipe with ultrasonically vibrating walls. In this way the sound energy generated from transducers bonded to the outside of the tube is transferred directly into the flowing liquid. Generally, commercial tube reactors are constructed of stainless steel and the choices for pipe cross-section are rectangular, pentagonal, hexagonal and circular. An alternative arrangement is via the coaxial insertion of a radially emitting bar into the pipe containing the flowing liquid; this would require minimal change to existing pipework. One such system consists of a hollow tube sealed at one end and driven at the other by a standard piezo transducer. Another concept involves a cylin- drical bar of titanium with opposing piezoelectric transducers attached at each end. The design of both inserts is such that the ultrasonic energy is emitted radially at half wavelength distances along their lengths.

3 Ultrasound as a food preservation tool

Food preservation can be defined as the extension of shelf-life of raw materials or prepared foods beyond their ‘natural’ (i.e. without human intervention) decay times. This extension can be considered to be a competition between different physical, chemical P676757-Ch13.qxd 6/20/05 5:02 PM Page 330

330 Application of Ultrasound

or biochemical processes. It is also depends upon the growth and development of different microbial populations, which can be sometimes beneficial or, more commonly, detrimental for maintaining desirable food properties.

3.1 Inactivation of microorganisms The food industry has generally concentrated on inactivating or killing microorganisms and enzymes as a means of preservation by using a number of physical methods, mostly involving heat, with enormous success. However, while heat can aid food preservation it can also cause some deterioration, e.g. the loss of some nutrients and reduction in the organoleptic properties of the material. Thus the scientific com- munity has been searching for alternative methods to preserve foods using different strategies or physical principles. The attempt to use ultrasound for food preservation is not new but it has experienced a relative revival in the last 10 years (Mason et al., 2003). Initial investigations of ultrasound were devoted to its effects on the most important food alteration agent – microbial populations. The destruction of microorganisms by power ultrasound was reported in the 1920s when the work of Harvey and Loomis (1929) was first published. Their work examined the reduction in light emission from a seawater suspension of rod- shaped Bacillus fisheri caused by sonication at 375 kHz under temperature-controlled conditions. Maintaining the temperature during sonication at 19°C prevented re- growth and all the bacteria appeared to be dead when viewed under a microscope. They attributed microbial death to cell disruption caused by cavitation. Other authors (Lepeschkin and Goldman, 1952; Kinsloe et al., 1954) pointed out that microbial inactivation due to ultrasound could be also achieved in the absence of cavitation, which led them to suggest that other inactivation mechanisms could play a significant role. Subsequently, investigations of the effects of ultrasound on different microbial species revealed very different sensitivities, which were related to the shape and size of the microorganisms. With some exceptions, it is generally accepted that bigger cells are more sensitive than smaller ones and that coccal forms more resistant than rod-shaped bacteria. Further, Gram-positive are more resistant than Gram-negative and aerobic are also more resistant than anaerobic bacteria. The physiological state of the cells also plays a role, with younger cells more sensitive than older ones and spores much more resistant than vegetative cells (Paci, 1953; Jacobs and Thornley, 1954; Davies, 1959; Ahmed and Russell, 1975). However, comparisons between the sensitivities of different species or even within the same species is very difficult mostly due to the different types of equipment used for sonication and the conditions used, especially the control of temperature. In the 1970s and 1980s, a research group led by Burgos explored the effects of ultrasound on sporulated and vegetative forms of Bacillus spp. (Burgos et al., 1972). In 1987, the first report of synergy between ultrasound and heat in the inactivation of bacteria was published by the same group and, interestingly, there was a reduction in the effectiveness of ultrasound at elevated temperatures (Ordoñez et al., 1987). In 1989, Burgos suggested that this efficiency loss could be due to the elevation of vapour pressure in the sonicated medium that would impair or at least diminish the P676757-Ch13.qxd 6/20/05 5:02 PM Page 331

Ultrasound as a food preservation tool 331

intensity of cavitational collapse (Garcia et al., 1989). Burgos suggested in that paper, that this problem could be avoided by increasing the applied pressure of the sonicated medium and patented the process in 1993 (Sala et al., 1993) that was termed manothermosonication or MTS (reviewed by Burgos, 1999). The enhanced killing of microorganisms when ultrasound is combined with heat or pressure is generally ascribed to a greater mechanical disruption of cells. Very interesting results have been obtained by the group of Sala (Sala et al., 1995; Mañas et al., 2000) in the inactivation of emerging pathogenic microorganisms such as Listeria monocytogenes, a number of strains of Salmonella spp., Escherichia coli, or Staphylococcus aureus which are increasingly found in outbreaks of food poison- ing from mildly treated and/or refrigerated foods. Ultrasonic treatment at ambient temperature was not very effective on L. monocytogenes giving a decimal reduction time of 4.3 min. By using manosonication, the D-value of the ultrasonic treatment was reduced to 1.5 min. Temperatures up to 50°C did not have any significant effect on inactivation, but at higher temperatures, a considerable enhancing effect was noted (Pagan et al., 1999). Similar results have been obtained with Salmonella (Mañas et al., 2000). One of the most remarkable features of manosonication and manothermosonication is that the factors that make microorganisms more resistant to heat treatment do not seem to affect their resistance to ultrasound. This makes inactivation by ultrasound very interesting for food preservation as its efficacy would be much less dependent on treatment and real food conditions than current heat treatments. The other advantage is that damage to cells caused by heat can be reversible but, in contrast to this, experimental results show that damage caused by manosonication is irreversible (Pagan et al., 1999). Ultrasound has also been used in combination with other non-thermal technologies (San Martin et al., 2001) such as magnetic fields and high pressure together with lisozyme treatments in model systems using pulses of 20 kHz ultrasound to reduce contamination by E. coli ATCC 11775. Inactivation of Saccharomyces with a combination of heat and ultrasound has been found to be almost independent of pH (Guerrero et al., 2001). Continuous ultrasonic treatment systems have been shown to produce similar results to batch treatments (Villamiel and de Jong, 2000a). Another continuous system working in combination with steam injection has been shown to afford between 1.5- and 4-fold higher inactivation rates of E. coli and Lactobacillus acidophilus in several liquid foods such as milk or fruit juices (Zenker et al., 2003). All of the above refers to the preservation of liquid systems, but microbial decontamination of some solid foods can also be achieved by ultrasonic irradiation. Decontamination of minimally processed fruit and vegetables (lettuce, cucumber, carrots, parsley and others) has been attempted (Seymour et al., 2002) using ultrasound to remove microorganisms from the surface of the vegetable pieces which were subsequently killed by the use of chemical sanitizers, generally chlorine. It was suggested that the low increase in cleaning efficiency did not justify the extra cost of the process. A similar approach by Scouten and Beuchat (2002) to decontaminate alfalfa seeds inocu- lated with Salmonella or E. coli O157 showed that the combined treatments of ultrasound and Ca(OH)2 could be an alternative to chlorine treatments to avoid contamination P676757-Ch13.qxd 6/20/05 5:02 PM Page 332

332 Application of Ultrasound

in the sprouts. One to 1.5 log reductions of Salmonella contaminating poultry surfaces have been achieved by using ultrasonic irradiation alone but up to 4 log reductions were achieved by combining the same irradiation intensities with minimal (0.5 ppm) chlorine concentrations (Lillard, 1994). However, Sams and Feria (1991) reported no decrease in total aerobic counts from broiler drumsticks sonicated with and without heat.

3.2 Inactivation of enzymes An excellent review was published some years ago of ultrasound inactivation or denaturation of enzymes and proteins (El’Piner, 1964), nevertheless, the study of enzyme inactivation by ultrasound has received less attention than microbial inactivation. As is the case of microbial species, enzymes are found to show a huge variety of sensitivities toward ultrasonic irradiation but, for the same reasons as given above, comparisons between different work from different authors are difficult. In 1994 a research group headed by Burgos initiated the study of the application of MTS treatments to model enzymes relevant to the food industry (lipoxygenase, peroxidase and polyphenol oxidase) in model buffer systems (Lopez et al., 1994). MTS treatments proved to be much more efficient than heat treatment for inactivating these enzymes, especially those which are more thermally labile (lipoxygenase and polyphenol oxidase). MTS inactivated peroxidase by splitting its prosthetic heme group, the same inactivation mechanism as heat (Lopez and Burgos, 1995a), whereas lipoxygenase seemed to be inactivated by a free radical mediated mechanism (Lopez and Burgos, 1995b). This group extended their studies to other harmful food enzymes. Proteases and lipases from psychrotrophic Pseudomonas (Vercet et al., 1997), the limiting factor for UHT (ultra heat treated) milk shelf-life, were found to be inactivated up to ten times faster by MTS treatments. Thermostable pectin methylesterase from oranges affects the cloudiness in citrus juices and whose inactivation is therefore mandatory in the citric juice industry, is inactivated almost 500 fold faster by MTS treatments than by heat treatment at an identical temperature. This is probably due to the impairment of substrate (pectin) enzyme stabilization by ultrasound (Vercet et al., 1999). Pectic enzymes of tomatoes, pectin methylesterase and the two endopoly- galacturonase isozymes are also inactivated by MTS treatments with much higher efficiency, both in model systems (Lopez et al., 1998) and in tomato juice (Vercet et al., 2002a). General trends arose from all these enzyme inactivation studies; ther- molabile enzymes are more sensitive to ultrasound than those which are heat resistant. The stabilization mechanisms operating against heat inactivation do not protect against MTS treatments (unlike that which occurs to microbial populations) and small enzymes seem to be more resistant than bigger ones (Vercet et al., 2001a). The use of ultrasound at ambient pressure has also been successfully used to inacti- vate food relevant enzymes. Peroxidase was inactivated by combinations of heat and ultrasound at neutral (Gennaro et al., 1999) or low pH (Yoon-Ku et al., 2000) and lipoxygenase has been shown to be inactivated at low sonication intensities (Thakur and Nelson, 1997). Villamiel and de Jong (2000b) report no effect of ultrasound on endogenous milk enzymes (alkaline phosphatase, g-glutamyltranspeptidase and lac- toperoxidase) at room temperature but synergistic inactivation at 60–75°C. P676757-Ch13.qxd 6/20/05 5:02 PM Page 333

Ultrasound as a processing aid 333

4 Ultrasound as a processing aid 4.1 Mixing and homogenization There are a large number of industrial processes that employ power ultrasound as a means of mixing materials (Canselier et al., 2002). Although the ultrasound can be supplied through bath or horn systems, there is another method of achieving the mixing via mechanical means. The device used for this purpose is often called the liquid whistle and it was developed many years ago for liquid processing, particularly for homogenization. As the name suggests, it operates on the whistle principle in that ultrasonic vibrations are generated via the flow of a liquid. The required operating pressure and throughput is determined by the use of different sized orifices or jets and the velocity can be changed to achieve the necessary particle size or degree of dispersion. A typical emulsion base for soups, sauces or gravies would consist of water, milk powder, edible oil and fat together with flour or starch as thickening agent. These materials are premixed together and, after passing through the homogenizer, produce a fine particle size emulsion with a smooth texture. Ketchup is processed in a similar way to produce a smooth product with increased viscosity and improved taste as a result of the complete dispersion of any clumps of tomato pulp. Often the rate- determining step of a particular process involves an energy interchange through a liquid–solid interface. In the food industry one such process is the extraction of components from vegetable material. A comparison has been made of the oil-in-water emulsions produced by mechanical agitation (Uitra-Turrax, 10 000 rpm, P 170 W) or power ultrasound (ultrasound horn, 20 kHz, 130 W) using the same model system: water/kerosene/polyethoxylated (20 EO) sorbitan monostearate. With ultrasound, the drop size is much smaller than that given by mechanical agitation under the same conditions, which makes insonated emulsions more stable. In addition, for a given drop size, less surfactant is required. These two methods were also used in a process for the encapsulation of liquid cheese aroma (20 per cent) in different carbohydrate matrices by spray-drying (Mongenot et al., 2000). In terms of encapsulation efficiency, the best system of cheese aroma encapsulation was obtained using ultrasound for the emulsification step, which gave a lower microcapsule size and a higher aroma retention than when an Ultra-Turrax mixer was used. The effect of position of the ultrasound source from the interface on emulsion quality has been studied using ultrasonic bath and horn (Mujumdar et al., 1997). Large variation in the emulsion properties with small changes in the position of ultrasound source were observed and this offered a possible explanation for discrepancies in the results of heterogeneous liquid phase systems reported in the literature. A continuous ultrasound emulsification process has been compared with other continuous mechanical emulsifying devices (Behrend et al., 2000). Continuous phase viscosity was varied by means of water-soluble stabilizers (o/w systems) and different oils (w/o systems). At constant energy density, droplet size decreases when adding stabilizers, whereas the viscosity of the oil in w/o emulsions has no effect. Qualitative investigations of the local distribution of cavitation have shown very small penetration P676757-Ch13.qxd 6/20/05 5:02 PM Page 334

334 Application of Ultrasound

depths of cavitation into the liquid which emphasizes the need for improvement of apparatus design to optimize ultrasonic emulsification processes.

4.2 Foam formation and destruction Foam is a dispersion of gas in a liquid in which the distances between the gas bubbles are very small. In fact, foam can be considered as the agglomeration of gas bubbles separated by a very thin liquid film. In a foam system the volume ratio of gas to liquid is very great and the bulk density approaches that of a gas. The generation of a foam may result from the:

• aeration and agitation of liquids (if the gas is already dissolved in the liquid) • vaporization of the liquid phase • action of microbiological or chemical agents that under certain reaction conditions will release a gas.

The various types of foam can be classified into different categories depending on their characteristics (Ghildyal et al., 1988). Among different foam-characterizing factors, stability is the most essential one as this defines its natural or forced rupture (Viesturs et al., 1982). Natural collapse of foams takes place because of liquid escape from foams (or syneresis). This process means liquid drains from the upper layers of the foam to the lower ones, while the capil- lary pressure gradient along the height of the foam column increases, thus preventing running out. Other relevant factors affecting the stability of foams are pH, molecular surface electrical charge, temperature, viscosity and surface tension. Foams are frequently produced in technical situations as unwanted side effects and, in general, they cause difficulties in process control and in equipment handling. Therefore, a great effort has been made either to prevent foam formation or to control it once it has been formed, for example in bioreactors or when gases are released under conditions of sudden pressure relief in a chemical reactor. In the food industry, submerged fermentation processes represent a good example of where foam formation is disadvantageous because it adversely affects productivity, causes fermenter contamination and hinders downstream processing among another effects (Ghildyal et al., 1988; Sandor and Stein, 1993; Freeman et al., 1997). In view of the adverse effects, it is important to control the foam in the reactors. Nowadays, there are several conventional foam control methods employing water sprays, chemical defoamers or mechanical foam breakers, although a combination of chemical and mechanical methods has been found to be more effective in the control of foam. Chemical methods use antifoam agents or defoamers. These products are usually surface-active agents which are unable to produce a stable foam by themselves. They are very effective but sometimes cause adverse effects by contaminating the process. The use of antifoams also tends to be largely based on a ‘hit and miss’ approach both with regard to the type of antifoam and the amount added. Furthermore, the addition of antifoams may also be limited by legislative problems relating to the production of food and pharmaceutical products. P676757-Ch13.qxd 6/20/05 5:02 PM Page 335

Ultrasound as a processing aid 335

Mechanical methods are more widely used mainly to overcome the disadvantages related to the use of antifoam agents. The collapse of bubbles is produced by any mechanical shock (rapid pressure changes, shear force, compressive force, impact force, suction and centrifugal forces). The use of ultrasonic energy can be considered as a mechanical method based on the propagation of high intensity ultrasonic waves and represents a clean method of breaking foams without contact with the liquid. The breaking and destruction of foams by ultrasound based defoamers is assumed to be a combination of: 1 partial vacuum on the foam bubble surface produced by high acoustic pressure 2 impingement of radiation pressure on the bubble surface 3 resonance of the foam bubbles which create interstitial friction causing bubble coalescence 4 cavitation 5 atomization from the liquid film surface 6 acoustic streaming (Boucher and Weiner, 1963; Gallego, 1999). The potential use of ultrasound for foam breaking was first introduced during the 1950s and 1960s by using various types of acoustic defoamers. Most liquids with viscosities up to 500 cp can be acoustically defoamed (Chendke and Fogler, 1975). The majority of them were based on aerodynamic acoustic sources of various types such as the Hartmann whistle, the rotatory siren, etc. The main disadvantages that these systems present are related to noise problems (they usually operate in the hearing frequency range), but they also require high air generation capacity, control and sterilization of the air-flow and involve high energy consumption. A new concept for a high-intensity ultrasonic defoamer has been developed (Rodríguez et al., 1985) based on the use of a stepped-plate transducer to generate airborne ultrasound (see Figure 13.2) (Gallego et al., 1978) and incorporates an extended titanium stepped-grooved radiating plate (350 mm in diameter) which allows the ultrasonic energy to focus. Sound pressure levels (SPL) of about 165 dB (3 W/cm2) were measured at a distance of about 330 mm from the centre of the plate, when a maximum power of 150 W is applied to the transducer. As an example of the potential use of this system, the airborne ultrasound has been successfully applied to the control of excess foam produced during the filling operation of bottles and cans on high-speed canning lines before capping. The focal area covered by the ultrasonic beam is about 3 cm2. Two focused transducers working at 20 kHz were used in parallel to improve this effect in order to cover the can surface. The overflow of foam was also controlled with bottles in a champagne company with powers lower than 100 W due to the great volume ratio of gas to liquid. Other experiments were carried out in milk and beer factories presenting promising results. Details of a more powerful second generation ultrasonic defoamer with a power capacity of 400 W and a radiating plate of 480 mm in diameter was published 2 years later (Rodríguez et al., 1987). The system was applied to defoaming in a beer cylindro- conical fermenter 6 m in diameter located in a brewery company in London. This more powerful system allowed an increase in the SPL in the focal area (12 cm2) at up to 170 dB (ca 10 W/cm2) (Gallego, 1998). The pilot trials demonstrated that the ultrasound was P676757-Ch13.qxd 6/20/05 5:02 PM Page 336

336 Application of Ultrasound

able to break older foam much more effectively than freshly formed foam. There may be several reasons for this:

1 the older foam at the surface would be less stable due to drainage and coalescence 2 the lower liquid content of the less dense foam would cause the foam to reflect less of the ultrasound 3 as the foam was destroyed liquid passed downwards into the lower levels of foam which may have made it more resistant to the ultrasound; any large bubbles formed at the broth surface might make their way to the top of the foam.

Nakamura et al. (1996) developed a device to control the large amount of foam generated at the time of filling bottles in a food machine plant. In this work, carried out at a small scale, the mechanism and energy efficiency of various physical and mechanical defoaming technologies were compared. The tests were carried out in a small cell within which the foam was generated and a rectangular (585 93 mm) flat vibrating plate made in aluminium was used in the experiments to generate ultrasonic waves. The experimental results obtained showed that ultrasonic defoaming is the most effective method when compared with laser and infrared techniques. Recently, a more versatile and powerful ultrasonic defoamer system has been developed and tested to break and control foam growth in big reactors in accordance with a procedure patented by Gallego et al. (2002). The application of the system in the dissipation of foam in reactors has been successfully used in the control of foam in industrial fermenters. More recently, this device has been improved by using several airborne focusing ultrasonic emitters, with an electronically controlled rotation system. Most of the bubbles break almost immediately under the influence of the acoustic intensity beam. From the point of view of manufacture, it is a powerful and compact device, without airflow and therefore it does not interfere with the process being treated and it can be easily sterilized.

4.3 Precipitation of airborne powders Having suspended airborne particles in most gases is generally regarded as undesir- able and a method of removing airborne particles (solid or liquid) is by means of coagulation. One of the most promising methods to agglomerate and precipitate airborne particles is by using high intensity sonic or ultrasonic vibrations (Table 13.2). It is a well-known fact that the application of a high intensity acoustic field on an aerosol originates a coagulation process of the suspended particles. This phenomenon was reported by Patterson and Cawood (1931) when they observed the rapid particle agglomeration in a standing wave sound tube. Shortly afterwards independent acoustic agglomeration experiments were carried out by Brant and Hiedemann (1936) in Germany, Andrade (1936) in the UK, and St Clair in the USA (1949). They found that the microscopically small particles, both liquid and solid, in the aerosol, which were normally light enough to remain suspended for a considerable time, were agglomer- ated by the ultrasonic vibrations, the bigger particles then settling more rapidly under the influence of gravity and causing the aerosol to precipitate out. P676757-Ch13.qxd 6/20/05 5:02 PM Page 337

Ultrasound as a processing aid 337

Although, acoustic agglomeration has been widely studied, the development of this process into an industrial application has been slow. This is probably because of the lack of suitable high-intensity, high efficiency sound sources and appropriate full-scale agglomeration chambers. Methods of obtaining high-intensity levels have involved several types of acoustic sources (Hartmann whistles, St Clair emitter, sirens and stepped plate transducers). The majority of these generators have efficiencies of about 25 per cent and poor directivity in air. Nevertheless, Gallego et al. (1978) and Rodríguez et al. (1987) showed that vibrating stepped plate transducers have very high efficiencies, up to 80 per cent, and can be applied to an industrial process where control and precipitation of fine particles is required. Magill et al. (1992) carried out investigations to remove submicron particles (mean particle diameter of 0.8 m) from a gas stream, using a combined acoustic module and electrostatic precipitator (ESP). Stepped plate transducers with power capacities up to 0.5 kW were used in a pilot-scale plant. The aerosol flow rate ranged from 150 to 1500 m3/h with resulting flow velocities of 0.17–1.7 m/s inside the acoustic chamber and 0.33–3.3 m/s in the ESP. In the experiments, the total particle number concentration was 2 106/cm3 and mass loading of about 1.35 g/m3. The separation efficiency of 0.8 m particles was increased from 87 to 92 per cent when ultrasonic energy was applied at 20 kHz (transducer operating at 400 W). Gallego et al. (1996, 1999a) developed a pilot scale acoustic preconditioning system for fumes of a 0.5 MWt fluidized bed coal combustor. In these tests the objective was to agglomerate not only micron-sized particles but also finer particulates in the submicron range. Fly ash-laden fumes generated by the combustor were let into a 3.6 m long horizontal acoustic agglomeration chamber. The residence time of the aerosol in the acoustic chamber was in the order of 2 s. Four stepped-plate, high-intensity macrosonic transducers were used to achieve average sound pressure levels of about 151 dB at 10 kHz and 152 dB at 20 kHz and peak values of 165 dB, in the entire volume of the chamber. Optical and scanning mobility particle sizers were employed along with cascade impactors to measure a wide range of particles. Using ultrasound (four transducers at 400 W), the reduction in micron-sized particles was up to 70 per cent in terms of the initial number of particles. Reductions of about 30 per cent were obtained in the submicron size distribution. The system can be applied to any industrial process where agglomeration and precipitation of airborne particles is required. The preconditioning procedure is equally useful in combination with other filtering or removal systems such as cyclone filters, bag houses, ceramic filters, etc. Another important benefit is its applicability to high-pressure and high-temperature environments. More recently, Riera et al. (2002a) have investigated the influence of humidity on the ultrasonic agglomeration and precipitation of submicron particles in diesel exhaust by using a linear array of four high-power 20 kHz stepped-plate transducers. At flow rates of 900 Nm3/h, there was a small reduction (25 per cent) in the number concentration of particles at the outlet of the chamber but at a humidity of 0.06 kgwater/kggas the reduction increased to 56 per cent. A general enhancement of the acoustic particle agglomeration was also found with higher levels of initial humidity. P676757-Ch13.qxd 6/20/05 5:02 PM Page 338

338 Application of Ultrasound

4.4 Filtration and drying 4.4.1 Filtration The requirement to remove suspensions of solids from liquids is common to many industries. This separation can be either for the production of solids-free liquid or to isolate the solid from its mother liquors. Conventionally, membranes of various sorts have been employed for these processes ranging from the simple filter pad through semipermeable osmotic type membranes to those which are used on a size-exclusion principle for the purification of polymeric materials. Unfortunately, the conventional methodologies often lead to ‘clogged’ filters and, as a consequence, there will always be the need either to replace filters or stop the operation and clean them on a regular basis. The application of ultrasound enables the filtration system to operate more efficiently and for much longer periods without maintenance through two specific effects. First, sonication will cause agglomeration of fine particles and, secondly, will supply sufficient vibrational energy to the system to keep the particles partly suspended and therefore leave more free ‘channels’ for solvent elution. This so-called acoustic filtration has been studied for many years and in many systems achieving, for example, an 18-fold increase in filtration rate of motor oil through a sandstone filter (Fairbanks and Chen, 1971). There have been a number of developments in acoustic filtration and separation processes (Tarleton and Wakeman, 1997). One such is the application of an electrical potential across the slurry mixture while acoustic filtration is performed (Senapati, 1991). The filter itself is made the cathode while the anode, on the top of the slurry, functions as a source of attraction for the predominantly negatively charged particulate material. An example of its application can be found in the dewatering of coal slurry (50 per cent moisture content). Conventional filtration reduces the moisture to 40 per cent, using ultrasound this was improved to 25 per cent and using electroacoustic filtration further improved to 15 per cent. The potential for this process is clearly enormous when applied to a continuous belt drying process in the deliquoring of such extremes as sewage sludge or fruit pulps.

4.4.2 Drying The reduction of moisture is one of the oldest techniques for food preservation. There are two basic methods to remove the moisture in a solid material: mechanical and thermal. Mechanical drying is based on the application of pressure or centrifugal forces to the material, whereas thermal drying uses heat to evaporate the liquid. The first method acts on the moisture weakly attached to the material and the second one can provide a more complete drying effect on the product. The use of ultrasonic energy in drying is very promising because it can act without affecting the main characteristics and quality of the products. In particular, power ultrasound is an especially attractive means of drying heat-sensitive foods because they can be dried more rapidly and at a lower temperature than in the conventional hot-air driers. Some applications of high-power acoustics in food drying are summarized in Table 13.3 where different authors reported studies of acoustic drying of a number of P676757-Ch13.qxd 6/20/05 5:02 PM Page 339

Ultrasound as a processing aid 339

Table 13.2 Experimental data relating to particle precipitation

Airborne particle Frequency Intensity Reference (kHz) (dB)

Paraffin oil 10 138; 148; 151 Brant and Hiedemann (1936) Carbon black smoke 20.4 159.5 Gallego et al. (1999a) Coal fly-ash 1.42–2.5 144 and 160 Tiwary (1985) Soot; black soot glycerine; 20 133–149 Magill et al. (1991, 1992) glycol fog aerosol 21 140–160 Coal fly-ash 1.42–2.5 144 and 160 Song (1990) Fly-ash calcined limestone 44 160 Hoffmann (1993), Hoffmann et al. (1993) Glycol fog aerosol; soot 10 and 20 Caperán et al. (1995) Glass spheres; quartz particles 0–0.9 Hoffmann and Koopmann (1997) Fly-ash laden fumes 10 and 20 140–165 Gallego et al. (1996) Fly-ash laden; diesel exhausts 10 and 20 140–165 Riera et al. (2002a)

materials using airborne radiation, ultrasonic vibration in contact with food, products immersed in hypertonic solutions, in sugar solutions and in salt brine. In all cases high-intensity acoustic waves accelerate the drying process of food solid materials. Although acoustic drying has been known for more than five decades, its application and development has been very slow due to technical problems in the design of efficient and powerful airborne acoustic generators. Boucher dried gelatin, yeast cake and granulated sugar using a multi-whistle device operating in the frequency range of 10–33 kHz (Boucher, 1961). In these experiments the intensity level is considered the main factor governing the evaporation rate. Boucher suggests a minimum intensity level of about 145 dB for industrial purposes. A clear benefit in the final moisture content of about 50–75 per cent less was obtained with ultrasound compared with conventional drying processing. In order to increase the benefit of ultrasound on food drying, ultrasonic vibrations have been applied in direct contact with the samples. Gallego et al. (1999b) applied direct contact ultrasound together with a static pressure at 22°C in the drying of carrots. Results obtained with carrot slices of 2, 4 and 8 mm in thickness and 14 mm in diameter showed that the drying effect was remarkably improved. A final moisture content of about 1 per cent was obtained. Riera et al. (2002b) also obtained encouraging results applying the same technique to the drying of apples, carrots and mushrooms. Osmotic dehydration is widely used for partial removal of water from food materials by immersion in a hypertonic solution. However, one of the main difficulties when applying this technique is the usually slow kinetics of the process. A classical way to increase the rates of mass transfer is the application of a mechanical agitation system; another possibility is the use of power ultrasound. Simal et al. (1998) reported the application of ultrasound with cubes of apples in a hypertonic solution of sucrose (70°Brix) at four different temperatures (40, 50, 60 and 70°C) and showed that the rates of mass transfer increase with the use of ultrasound at 40 kHz. They found an increase of water loss (14–17 per cent) and a sugar gain rate (23 per cent at 40°C; 11 per cent at 70°C) when ultrasound was applied. Therefore, P676757-Ch13.qxd 6/20/05 5:02 PM Page 340

340 Application of Ultrasound

osmotic dehydration can be carried out at a lower solution temperature to obtain higher water loss and solute gain rates, while preserving the heat-sensitive nutritive components, flavour and colour. Experiments carried out by Sanchez et al. (1999) obtained similar results with cheese immersed in saturated NaCl. Water loss increased by 11 per cent and NaCl was increased by 5 per cent compared with normal brining agitation. Cárcel et al. (2002) studied the mass transfer processes of apple slices immersed in a solution of sucrose (30°Brix) at 30°C when different intensities were applied (from 3.6 to 11.5 W/cm2) at 20 kHz with a horn. The authors (Cárcel et al., 2002) detected an intensity threshold of about 9 W/cm2. Above this intensity the water loss and the solute gain were clearly higher compared with experiments carried out with agitated brine. These experiments were repeated for brining pork loin slices with and without ultrasound at several intensities (from 15 to 76 W/cm2) at 2°C. The results for NaCl gain behaved in a similar way to those that observed for solute gain in apples treated in sugar solution. Above the corresponding intensity threshold, the increase of NaCl gain was proportional to the ultrasonic intensity applied and much higher than in experiments with mechanical agitation of the brine.

4.5 Extraction Power ultrasound has been shown to be a promising and innovative method to assist the extraction of valuable compounds from vegetables and food products (Mason et al., 1996; Mason, 1998; Vinatoru, 2001; Valachovic et al., 2001). It is particularly useful in combination with conventional solvent extraction and a range of examples is given in Table 13.3. The beneficial effects of ultrasound derive from its mechanical effects on the process by increasing the penetration of the solvent into the product and enhancing the mass transfer process to and from interfaces. It is supposed that those benefits are related to the enhancement of diffusion of cellular contents through the disruption of the cell walls produced by acoustical cavitation (Chendke and Fogler, 1975). On collapse, bubbles are capable of producing shock waves. Oscillatory particle motion produced by high-intensity ultrasonic waves can also induce secondary flows, known as acoustic streaming. Moreover, cavitation produces microjets at the surface of the food material that may introduce the liquid into the solid. This effect can increase mass transfer in both directions from the liquid to the solid or in reverse. Therefore, cavitation induced cell disruption and dispersion of suspended solids coupled with enhanced mass transfer rates due to acoustic streaming are believed responsible for the improved extraction (Toma et al., 2001). Herbs provide a source of raw materials for the pharmaceutical, cosmetics and food industries and, more recently, in agriculture for pest control and the effect of ultrasound on various extraction procedures has been reviewed (Vinatoru, 2001). During distillation ultrasound will produce more rapidly boiling centres, but no collapsing bubbles, hence the use of ultrasound during distillation does not produce any significant effect. However, ultrasound can be successfully employed to enhance extraction when a low boiling point solvent is used and the temperature of the extraction mixture is kept below its boiling point. P676757-Ch13.qxd 6/20/05 5:02 PM Page 341

Ultrasound as a processing aid 341

Table 13.3 Application of acoustic energy in the drying of food

Material Assisted drying procedure Reference

Gelatin; yeast cake; granulated Airborne radiation in solid–gas Brun and Boucher (1957) sugar system Boucher (1961) Grated cheese; orange crystals; Airborne radiation in solid–gas Soloff (1964) gelatine beds; rice grains system Potato cylinders Airborne radiation in solid–gas Bartolome et al. (1969) system Rice Airborne radiation in solid–gas Muralidhara et al. (1985) system Apple cubes Hypertonic solution of sucrose in Simal et al. (1998) solid–liquid system Cheese cylinders and Saturated NaCl brine in Sánchez et al. (1999) parallelepipeds; curd solid–liquid system Onions Airborne radiation in solid–gas Da Motta and Palau (1999) system Carrots Airborne radiation and direct Gallego et al. (1999b) contact in solid–gas system Carrots; apples; mushrooms Airborne radiation and direct Riera et al. (2002b) contact in solid–gas system Apple slices 30°Brix solution of sucrose in Cárcel et al. (2002) solid–liquid system Pork loin slices Saturated NaCl brine in Cárcel et al. (2003) solid–liquid system Apples; potatoes Ultrasonic vibration in direct de la Fuente et al. (2003) contact in solid–gas system

A combined ultrasound and microwave-assisted extraction method of essential oil from caraway seeds has been proposed by Chemat et al. (2003). A microwave-ultrasonic (MW-US) reactor was designed for atmospheric pressure extraction of biological and chemical products. Its application has been shown by extraction of carvone and limonene from caraway seeds. The system basically consists of a microwave oven cavity unit operating at 2.45 GHz with a power of 300 W, an open vessel reactor operating at atmospheric pressure with a volume capacity of 20–150 ml and an ultrasonic horn-type transducer working at 20 kHz. The samples were caraway seeds prepared by grinding with liquid nitrogen in a roller mill. Samples were directly introduced into the extraction reactor and conventional solid–liquid extraction was also performed for comparison. The analysis of the extracts was carried out by gas chromatography and the structure of the specimen analysed by scanning electron microscopy (SEM). A significant improvement in extraction was obtained using simultaneous ultrasound and microwave-assisted extraction. Supercritical fluid extraction (SFE) with CO2 is a non-conventional technique that can offer very good yields. This technique is suitable for fragrance extraction, giving better yields and good quality essential oil. Nevertheless, fixed bed SFE of oil from solid matrix is slow even when solute-free solvent is re-circulated and therefore improvements in mass transfer are required. The use of power ultrasound represents a potentially efficient way of enhancing mass transfer processes. This is due to the P676757-Ch13.qxd 6/20/05 5:02 PM Page 342

342 Application of Ultrasound

effects produced by compressions and decompressions, as well as by radiation pressure, streaming, etc. In addition, this is probably the unique practical way to produce agitation in SFE because the use of mechanical stirrers is not possible. Riera et al. (2002c, 2004) examined the effect of ultrasound (20 kHz and 50 W) on the particulate almond oil extraction kinetics using supercritical CO2. As a consequence of the trials (at 280 bar and 55°C) at the end of the extraction time (8 h 30 min) the yield of the oil was significantly increased (20 per cent) when SFE was assisted by ultrasound. Alternatively, mass transfer was speeded up to such an extent that yields comparable to those obtained by SFE alone could be achieved in about 30 per cent shorter time when using ultrasound.

5 Ultrasound effects on food properties

The primary goal of food preservation technologies is to extend food shelf-life, mostly by preventing enzymatic deleterious reactions and microbial spoilage. However, on the other hand, they need also to preserve all those attributes that make food a pleasant and nutritious material to be eaten, i.e. flavour, colour and texture. This is especially true in First World consumer societies where interests lie much more in the pleasure of eating than in the Third World where the primary goal of eating is survival. Ultrasonic irradiation concentrates quite large amounts of energy into very small volumes (hot spots) and this has the potential to change food properties in unexpected ways. There are, however, very few reports of these effects either because they are insignificant or perhaps because the experimental findings do not always show the expected (good) results.

5.1 Effects of ultrasound on dairy products Milk is one of the most studied raw materials in relation to the ultrasonic irradiation of foods. Villamiel and de Jong (2000b) report effects on sonication at ambient pressure on several of its constituents. Milk fat globules are finely homogenized, individual caseins seem not to be affected, although the authors do not give any indication about their multimeric structures, the micelles. Serum proteins -lactalbumin and -lactoglobulin, on the other hand, are denatured more extensively when ultrasound is combined with heat than with these two treatments performed separately. Similar results were obtained in fat globule homogenization when applying continuous manothermosonication. This also results in a slight change in milk colour according to instrumental measurements, but this is probably just a consequence of different light reflection (Vercet et al., 2002b). Milk is also a good source of thiamine and riboflavin. The former is stable to light and oxidation but it is the least heat stable vitamin. Meanwhile, riboflavin is thermostable but it is rather sensitive to oxidation and degradation by light. Another parameter to take into account is the Maillard reaction because, in milk, lactose usually reacts with amino groups of lysine to form an Amadori product. This reaction is the first step of a complicated group of reactions P676757-Ch13.qxd 6/20/05 5:02 PM Page 343

Ultrasound effects on food properties 343

that eventually lead to browning of milk and milk products. However, the ultimate result is not only the changes in colour but also the loss of the bioavailability of lysine (an essential aminoacid). There is no significant loss of riboflavin and thiamine in MTS treated milk compared with samples treated only with heat (under otherwise identical conditions). However, there was a dramatic change in the kinetics of browning when ultrasound was applied with and without pressure, even at the relatively low temperature of 92°C. Although the kinetics was very different, it proved impossible to identify the precise changes induced by ultrasound in the browning pathways (Vercet et al., 2001b). Another problem arose with the modification of milk flavour induced by MTS treatment which had an extreme cooked flavour (Vercet and Lopez Buesa, unpublished observation) very similar to that of UHT milk just after thermal treatment. The cooked flavour of UHT milk is due to exposure of sulphydril volatile groups derived from proteins and disappears after a short storage (a few days) period. Strong flavour changes have also been found in sonicated sunflower oil: they were due to the apparition of volatile derivatives of fatty acids such as hexanal and hept-2-enal (Chemat et al., 2004). Using MTS-treated milk for yoghurt production, better textural properties have been found than in control milks homogenized with standard methods (Vercet et al., 2002c). No flavour defect could be observed in these yoghurts (Vercet and Lopez Buesa, unpublished observation). A similar finding has been reported by Wu et al. (2001) using milk sonicated at ambient pressure. These authors (Wu et al., 2001) attribute the effect to more effective milk homogenization, but work by Vercet et al. (2002c) showed that control yoghurts were even more finely homogenized than MTS yoghurts. The change was attributed to protein modification by ultrasound which agrees also with the cooked flavour derived from proteins in MTS treated milk and with the fact that MTS-treated milk does not coagulate in cheese making (milk coagulation for cheese production is mostly a protein dependent phenomenon) (Vercet and Lopez Buesa, unpublished observation).

5.2 Effects of ultrasound on juices The textural properties of tomato juices after manothermosonication treatments compared with controls have been thoroughly studied (Vercet et al., 2002a). MTS resulted in higher consistency and initial apparent viscosities. Tomato juices are, from a rheological point of view, pseudogels, whose flow properties depend on the interaction or entanglement of cell particles (mostly cell walls), soluble pectin concentration and the chemical properties of the latter. MTS treatments of pure pectin solutions yielded molecules with lower apparent viscosities due to a size reduction (Lopez Buesa, unpublished observation). A similar result was observed by Seshadri et al. (2003). It is difficult to predict what might be expected from a modification of pectin properties in gels, or pseudogel derived from pectin. Longer molecules show higher resistance to flow but shorter ones can interact in a different way with suspended particles leading also to increased resistance to flow. An untrained panel detected some flavour differences in MTS-treated tomato juice but these were not considered detrimental by the panellists, just different. Instrumental P676757-Ch13.qxd 6/20/05 5:02 PM Page 344

344 Application of Ultrasound

colour measurements of tomato treated juice also revealed some differences from control samples. This is most probably due to the disappearance of almost 70 per cent of the initial lycopene, the main carotene responsible for tomato red colour (Lopez Buesa, unpublished results). However, no appreciable change was observed in the tomato juice content of another readily oxidable and important molecule, ascorbic acid. Zenker et al. (2003) also found that sonication had no effect on the ascorbic acid content in milk and orange juice. This is somewhat surprising taking into account that MTS treatments are able to produce substantial amounts of hydroxyl radicals (Vercet et al., 1998). The explanation could lie in a competition (different reaction rates) between hydroxyl groups and all the other oxidizable substrates available (i.e. proteins or sugars). A comparison study has been made of the effect of the heating and sonication of orange juice in terms of: 1 ascorbic acid, the most important vitamin in orange juice which is quite sensitive to oxidation 2 carotenoid content which is important for both colour and as a nutrient (provita- min A and functional molecules) and 3 non-enzymatic browning that, in this case, is related to sugars and ascorbic acid degradation. Indeed, browning intermediates derived probably from sugar degradation were found only in MTS treated orange juice. Also a 10 per cent decrease was found in carotenoid content. Only a slight decrease in ascorbic acid was found in MTS treated juice (Vercet et al., 2001b).

5.3 Effects of ultrasound on egg products Liquid eggs have been also submitted to MTS treatments without any noticeable change in functional properties (Mañas, 1999). However, there was a detectable diminution of the gelling properties of isolated ovalbumin submitted to MTS treatments (a 50 per cent decrease in storage modulus of ovalbumin gels). The gelling properties of manosonicated ovalbumin samples recovered partially after 24 hours cold storage which points to protein unfolding and refolding processes occurring during and after MTS treatments, respectively (Sánchez et al., 2002).

6 Conclusions

The effectiveness of ultrasound as a food processing tool has been proven in the laboratory and there are a number of examples of scale-up. In most cases the frequency used has been that which is available commercially, i.e. 20 or 40kHz and this has proved quite satisfactory. In such cases the variable parameters are temperature, treatment time and acoustic power. Little attention has been paid to the use of different frequencies except in a few cases. One such is the use of ultrasound in food preservation P676757-Ch13.qxd 6/20/05 5:02 PM Page 345

References 345

using the bactericidal action of sonication combined with other techniques such as heat, ultraviolet light and the use of a biocide. The results presented in this chapter should provide a starting point for more com- prehensive research and development leading to the introduction of power ultrasound into a wider range of industrially significant food processing operations.

References

Abramov OV (1998) High-Intensity Ultrasound: Theory and Industrial Applications. London: Gordon and Breach. Ahmed FIK, Russell C (1975) Synergism between ultrasonics waves and hydrogen peroxide in killing microorganisms. Journal of Applied Microbiology 39, 31–40. Andrade EN da C (1936) The coagulation of smoke by supersonic vibration. Transactions of Faraday Society, 134, 1111–1115. Bartolome LG, Hoff JE, Purdy KR (1969) Effect of resonant acoustic vibrations on drying rates of potato cylinders. Food Technology, 23, 47–50. Behrend O, Ax K, Schubert H (2000) Influence of continuous phase viscosity on emulsification by ultrasound. Ultrasonics Sonochemistry, 7, 77–85. Boucher RMG (1961) Drying by airborne ultrasonics. Ultrasonic News, 3, 8–9, 14–16. Boucher RMG, Weiner AL (1963) Foam control by acoustic and aerodynamic means. British Chemical Engineering, 8, 808–812. Brant O, Hiedemann E (1936) The aggregation of suspended particles in gases by sonic and supersonic waves. Transactions of Faraday Society, 42, 1101–1110. Brun E, Boucher RMG (1957) Research on the acoustic air-jet generator: A new development. Journal of Acoustical Society America, 29, 573–583. Burgos J, Ordoñez JA, Sala FJ (1972) Effect of ultrasonic waves on the heat resistance of Bacillus cereus and Bacillus licheniformis spore. Applied Microbiology, 24, 497–498. Burgos J (1999) Manothermosonication. In Encyclopedia of Food Microbiology (Robinson RK, Batt CA, Patel PD, eds). New York: Academic Press, pp. 1462–1469. Canselier JR, Delmas H, Wilhelm AM, Abismail B (2002) Ultrasound emulsification – An overview. Journal of Dispersion Science and Technology, 23, 333–349. Caperan Ph, Somers J, Richter K, Fourcaudot S (1995) Acoustic agglomeration of a glycol fog aerosol: influence of particle concentration and intensity of the sound field at two frequencies. Journal of Aerosol Science, 26, 575–594. Cárcel JA, Golás Y, Benedito J, Mulet A (2002) Influence of power ultrasound in osmotic dehydration of apple slices. Proceedings of the International Drying Symposium (IDS 2002). Beijing, China, pp. 1281–1286. Cárcel JA, Benedito J, Mulet A, Riera E (2003) Mass transfer effects during meat ultrasonic brining. Proceedings of World Congress on Ultrasonics WCU, Paris, France, pp. 431–434. Chemat S, Lagha A, AitAmar H, Chemat F (2003) Combined ultrasound and microwave- assisted extraction of essential oil from caraway seeds. Proceedings of Fourth P676757-Ch13.qxd 6/20/05 5:02 PM Page 346

346 Application of Ultrasound

Conference on the Application of Power Ultrasound in Physical and Chemical Processing, Becanson France, pp. 349–353. Chemat F, Grondin I, Shum Cheong Sing A, Smadja J (2004) Deterioration of edible oils during food processing by ultrasound. Ultrasonics Sonochemistry, 11, 13–15. Chendke PK, Fogler HS (1975) Macrosonics in Industry: 4. Chemical processing. Ultrasonics, 13, 31–37. Cho BK, Irudayaraj JMK (2003) Foreign object and internal disorder detection in food materials using noncontact ultrasound imaging. Journal of Food Science, 68, 967–974. Coupland JN, McClements DJ (2001) Droplet size determination in food emulsions: comparison of ultrasonic and light scattering methods. Journal of Food Engineering, 50, 117–120. Da Motta VM, Palau E (1999) Acoustic drying of onion. Drying Technology, 17, 855–867. Davies R (1959) Observations on the use of ultrasonic waves for the disruption of microorganisms. Bioochimica et Biophysica Acta, 33, 491–493. de la Fuente S, Riera E, Gallego JA, Gómez TE, Acosta VM, Vázquez F (2003) Parametric study of ultrasonic dehydration process. Proceedings of World Congress on Ultrasonics WCU, Paris, France, pp. 61–64. El’Piner (1964) Ultrasound: Physical, Chemical and Biological Effects. New York: Consultants Bureau. Fairbanks HV, Chen WI (1971) Ultrasonic acceleration of liquid flow through porous media. Chemical Engineering Symosium Series, 67, 108. Freeman GJ, Reid AI, Valdecantos C, Lynch FJ, Gallego JA (1997) The use of ultrasonic radiation to suppresses foaming in fermenters. Proceedings of twenty sixth European Brewery Convention, EBC Congress, pp. 405–412. Gallego JA (1998) Some applications of air-borne power ultrasound to food processing. In Ultrasonics in Food Processing (Povey MJW, Mason TJ, eds). London: Thomson Science, pp. 127–143. Gallego JA (1999) High-power ultrasound. In Wiley Encyclopedia of Electrical and Electronics Engineering (Webster JG, ed.). New York: John Wiley & Sons, Inc., 9, 49–59. Gallego JA, Rodríguez G, Gaete L (1978) An ultrasonic transducer for high-power applications in gases. Ultrasonics, 16, 267–271. Gallego JA, Rodríguez G, San Emeterio JL, Montoya F (1994) Electroacoustic unit for generating high sonic and ultrasonic intensities in gases and at interfaces. US Patent 5299175. Gallego JA, Riera E, Rodriguez G et al. (1996) Pilot scale acoustic preconditioning of coal combustion fumes to enhance electrostatic precipitator performance. In High Temperature Gas Cleaning (Schmidt E, Gäng P, Pilz T, Dittler A, eds). Karlsruhe: Inst. Mech. Verfahrenstechnik und Mechanik, pp. 60–68. Gallego JA, Riera E, Rodríguez G et al. (1999a) Application of acoustic agglomeration to reduce fine particle emissions from coal combustion plants. Environmental Science and Technology, 33, 3843–3849. Gallego JA, Rodriguez-Corral G, Moraleda JCG, Yang TS (1999b) A new high-intensity ultrasonic technology for food dehydration. Drying Technology, 17, 597–608. Gallego JA, Rodríguez G, Acosta VM, Andrés E, Blanco A, Montoya F (2002) Procedimiento y sistema ultrasónico de desespumación mediante emisores con placa vibrante escalonada, Spanish Patent 200202113. P676757-Ch13.qxd 6/20/05 5:02 PM Page 347

References 347

Garcia ML, Burgos J, Sanz B, Ordoñez JA (1989) Effect of heat and ultrasonic waves on the survival of two strains of Bacillus subtilis. Journal of Applied Bacteriology, 67, 619–628. Gennaro LD, Cavella S, Romano R, Masi P (1999) The use of ultrasound in food technology I: inactivation of peroxidase by thermosonication. Journal of Food Engineering, 39, 401. Ghildyal NP, Lonsane BK, Karanth NG (1988) Foam control in submerged fermentation: state of the art. Applied Microbiology, 33,173–223. Guerrero S, López-Malo A, Alzamora SM (2001) Effect of ultrasound on the survival of Saccharomyces cerevisiae: influence of temperature, pH and amplitude. Innovative Food Science and Emerging Technologies, 2, 31–39. Harvey E, Loomis A (1929) The destruction of luminous bacteria by high frequency sound waves. Journal of Bacteriology, 17, 373–379. Hindle SA, Povey MJW, Smith KW (2002) Characterizing cocoa-butter seed crystals by the oil-in-water emulsion crystallization method. Journal of the American Oil Chemists Society, 79, 993–1002. Hoffmann TL (1993) Visualization of particle interaction and agglomeration in an acoustic field. PhD Dissertation, The Pennsylvania State University. Hoffmann TL, Koopmann GH (1996) Visualization of acoustic particle interaction and agglomeration: Theory and experiments. Journal of the Acoustical Society of America, 99, 2130–2141. Hoffmann TL, Chen W, Koopmann G, Song L, Scaroni AW (1993) Experimental and numerical analysis of bimodal acoustic agglomeration. Transactions of ASME Journal of Vibrations and Acoustics, 115, 232–240. Jacobs SE, Thornley MJ (1954) The lethal actions of ultrasonic waves on bacteria suspended in milk and another liquids. Journal of Applied Bacteriology, 17, 38–55. Kinsloe H, Ackerman E, Reid JJ (1954) Exposure of microorganisms to measured sound fields. Journal of Bacteriology, 68, 337–380. Lepeschkin WW, Goldman DE (1952) Effects of ultrasound on cell structure. Journal of Cellular Composition and Physiology, 40, 393–397. Lillard HS (1994) Decontamination of poultry skin by sonication. Food Technology, 44, 73–74. Lopez P, Sala FJ, Fuente JL, Condon S, Raso J, Burgos J (1994) Inactivation of peroxidase, lipoxygenase, and polyphenol oxidase by manothermosonication. Journal of Agriculture and Food Chemistry, 42, 252–256. Lopez P, Burgos J (1995a) Lipoxygenase inactivation by manothermosonication: effect of sonication physical parameters, pH, KCl, sugars, glycerol, and enzyme concentration. Journal of Agriculture and Food Chemistry, 43, 620–625. Lopez P, Burgos J (1995b) Peroxidase stability and reactivation after heat treatment and manothermosonication. Journal of Food Science, 60, 451–455. Lopez P, Vercet A, Sanchez AC, Burgos J (1998) Inactivation of tomato pectic enzymes by manothermosonication. Zeitschrift für Lebensmittel-Untersuchung und Forschung A, 207, 249–252 Magill J, Caperan Ph, Somers J et al. (1991) Frequency dependence of acoustic agglomeration rate of a glycol fog. Journal of Aerosol Science, 22, S27–S30. P676757-Ch13.qxd 6/20/05 5:02 PM Page 348

348 Application of Ultrasound

Magill J, Caperan Ph, Somers J et al. (1992) Characteristics of an electro-acoustic precipitator (EAP). Journal of Aerosol Science, 23, S803–S806. Mañas P (1999) Liquid whole egg hygienization by heat and ultrasound. Doctoral Thesis. Universidad de Zaragoza (Spain). Mañas P, Pagan R, Raso J, Sala FJ, Condon S (2000) Inactivation of Salmonella typhimurium and Salmonella seftenberg by ultrasonic waves under pressure. Journal of Food Protection, 63, 451–456. Mason TJ (1998) Power ultrasound in food processing. In Ultrasound in Food Processing (Povey MJW, Mason TJ, eds). London: Blackie Academic & Professional, pp. 105–150. Mason TJ (1999) Sonochemistry. Oxford University Primer Series No. 70. Oxford: Oxford Science Publications. Mason TJ (2000) Large scale sonochemical processing: aspiration and actuality. Ultrasonics Sonochemistry, 7, 145–149. Mason TJ, Lorimer JP (2002) Applied Sonochemistry. Weinheim: Wiley VCH. Mason TJ, Peters D (2002) Practical Sonochemistry, Power ultrasound uses and applications, 2nd edn. Chichester: Ellis Horwood Publishers. Mason TJ, Paniwnyk L, Lorimer JP (1996) The uses of ultrasound in food technology. Ultrasonics Sonochemistry, 3, S253–S260. Mason TJ, Paniwnyk L, Chemat F (2003) Ultrasound as a preservation technology. In Food Preservation Techniques (Zeuthen P, Bøgh-Sørensen L, eds). Cambridge: Woodhead Publishers, pp. 303–337. Mongenot N, Charrier S, Chalier P (2000) Effect of ultrasound emulsification on cheese aroma encapsulation by carbohydrates. Journal of Agricultural and Food Chemistry, 48, 861–867. Moser WR, Find J, Emerson SC, Krausz IM (2001) Engineered synthesis of nanostruc- tured materials and catalysts. Advances in Chemical Engineering, 27, 1–48. Mujumdar S, Kumar PS, Pandit AB (1997) Emulsification by ultrasound: Relation between intensity and emulsion quality. Indian Journal of Chemical Technology, 4, 277–284. Mulet A, Benedito J, Golas Y, Carcel JA (2002) Non-invasive ultrasonic measurements in the food industry. Food Reviews International, 18, 123–133. Muralidhara MS, Ensminguer D, Putnam A (1985) Acoustic dewatering and drying (low and high frequency). State of the art review. Drying Technology, 3, 529–566. Nakamura S, Abe S, Iwata H, Itoh Y (1996) Development of foam breaking systems in ultrasonics. Mitsubishi Heavy Industries, INNOPAC, 33, 374–377. Ordoñez JA, Aguilera MA, Garcia ML, Sanz B (1987) Effect of combined ultrasonic and heat treatment (thermosonication) on the survival of a strain of Staphylococcus aureus. Journal of Dairy Research, 54, 61–67. Paci C (1953) L´emploi des ultra-sons pour l´assainissement du lait. Le Lait, 33, 610–615. Pagan R, Mañas P, Alvarez I, Condon S (1999) Resistance of Listeria monocytogenes to ultrasonic waves under pressure at sublethal (manosonication) and lethal (manothermosonication) temperatures. Food Microbiology, 16, 139–148. Patterson HS, Cawood W (1931) Phenomena in sounding tube. Nature, 124, 667–680. Povey MJW, Mason TJ (1998) Ultrasound in Food Processing. London: Blackie Academic and Professional. P676757-Ch13.qxd 6/20/05 5:02 PM Page 349

References 349

Quartly-Watson T (1998) The importance of power ultrasound in cleaning and disinfection in the poultry industry – a case study. In Ultrasound in Food Processing (Povey M, Mason TJ, eds). London: Blackie Academic and Professional, pp. 144–150. Richards WT, Loomis AL (1927) Chemical effects of high frequency sound waves. Journal of the American Chemical Society, 49, 3086–3100. Riera E, Elvira L, González I, Rodríguez JJ, Muñoz R, Dorronsoro JL (2002a) Investigation of the influence of humidity on the ultrasonic agglomeration of submicron particles in diesel exhaust, Ultrasonics, 38, 642–646. Riera E, Gallego JA, Rodríguez G, Acosta VM, Andrés E (2002b) Application of high- power ultrasound for drying vegetables, Revista de Acústica Vol. XXXIII (3-4), CD- ROM, ULT-05-004, pp.1–6. Riera E, Gallego JA, Montoya F et al. (2002c) Procedimiento para procesos de separación o extracción con fluidos supercríticos asistidos por ultrasonidos de alta intensidad. Spanish Patent 200201822. Riera E, Golás Y, Blanco A, Gallego JA, Blasco M, Mulet A (2004) Mass transfer enhancement in supercritical fluids extraction by means of power ultrasound. Ultrasonics Sonochemistry, 11, 241–244. Rodríguez G, Gallego JA, Ramos A, Andrés E, San Emeterio JL, Montoya F (1985) High- power equipment for industrial defoaming. Proceedings of Ultrasonics International 85, Guildford: Butterworth Scientific Ltd, pp. 506–511. Rodríguez G, San Emeterio JL, Gallego JA (1987) Focused high-power ultrasonic transducer with stepped-plate radiator for industrial application in gases. Proceedings of Ultrasonics International 87, Guildford: Butterworth Scientific Ltd, pp. 794–799. Sala FJ, Burgos J, Condón S, López P, Ordoñez JA, Raso J (1993) Procedimiento para la destrucción de microorganismos y enzimas: proceso MTS. Spanish patent 93/00021. Sala FJ, Burgos J, Condón S, López P, Raso J (1995) Effect of heat and ultrasound on microorganisms and enzymes. In New methods of food preservation (Gould W, ed.). London: Blackie Academic & Professional, pp. 176–204. Sams AR, Feria R (1991) Microbial effects of ultrasonication of broiler drumstick skin. Journal of Food Science, 56, 247–248. San Martín MF, Harte FM, Lelieveld H, Barbosa-Cánovas G, Swanson BG (2001) Inactivation effect of an 18-T pulsed magnetic field combined with other technologies on Escherichia coli. Innovative Food Science & Emerging Technologies, 2, 273–277. Sánchez ES, Simal S, Femenia A, Benedito J, Rosselló C (1999) Influence of ultrasound on mass transport during cheese brining. European Food Research and Technology, 209, 215–219. Sánchez AC, Burgos J, Lopez-Buesa P (2002) Modification of gelation properties of ovalbumin by conventional methods and by ultrasound. Presented at First Innovative Food Centre Conference ‘Emerging Technologies for the Food Industry’. Madrid (Spain). Sandor N, Stein N (1993) Foam destruction by ultrasonic vibrations. Journal of Colloid and Interface Science, 161, 265–267. Scouten AJ, Beuchat LR (2002) Combined effects of chemical, heat and ultrasound treatments to kill Salmonella and Escherichia coli O157:H7 on alfalfa seeds. Journal of Applied Microbiology, 92, 668–674. P676757-Ch13.qxd 6/20/05 5:02 PM Page 350

350 Application of Ultrasound

Senapati N (1991) Ultrasound in chemical processing. In Advances in Sonochemistry, 2 (Mason TJ, ed.). London: JAI Press, pp. 187–210. Seshadri R, Weiss J, Hulbert GJ, Mount J (2003) Ultrasonic processing influences rheological and optical properties of high-methoxyl pectin dispersions. Food Hydrocolloids, 17, 191–197. Seymour IJ, Burfoot D, Smith RL, Cox LA, Lockwood A (2002) Ultrasound decontamination of minimally processed fruits and vegetables. International Journal of Food Science and Technology, 37, 547–557. Simal S, Benedito J, Sánchez ES, Rosselló C (1998) Use of ultrasound to increase mass transport rates during osmotic dehydration. Journal of Food Engineering, 36, 323–336. Soloff RS (1964) Sonic drying. Journal of the Acoustic Society America, 36, 961–965. Song L (1990) Modelling of acoustic agglomeration of aerosol particles. PhD Dissertation, The Pennsylvania State University. St Clair HW (1949) Agglomeration of smoke, fog, or dust particles by sonic waves. Industrial and Engineering Chemistry, 41, 2434. Tarleton ES, Wakeman RJ (1997) Ultrasonically assisted separation processes. In Ultrasound in Food Processing (Povey MJW, Mason TJ, eds). London: Thomson Science, pp. 193–218. Thakur BR, Nelson PE (1997) Inactivation of lipoxygenase in whole soy flour suspension by ultrasonic cavitation. Die Nahrung, 41, 299–301. Tiwary R (1985) Acoustic agglomeration of micron and submicron fly-ash aerosols. PhD Dissertation, The Pennsylvania State University. Toma M, Vinatoru M, Panywnyk L, Mason TJ (2001) Investigation of the effects of ultrasound on vegetal tissues during solvent extraction. Ultrasonic Sonochemistry, 8, 137–142. Valachovic P, Pechova A, Mason TJ (2001) Towards the industrial production of medical tincture by ultrasound assisted extraction. Ultrasonic Sonochemistry, 8, 111–117. Vercet A, Lopez P, Burgos J (1997) Inactivation of heat-resistant lipase and protease from Pseudomonas fluorescens by manothermosonication. Journal of Dairy Science, 80, 29–36. Vercet A, Lopez P, Burgos J (1998) Free radical production by manothermosonication. Ultrasonics, 36, 615–618. Vercet A, Lopez P, Burgos J (1999) Inactivation of heat-resistant pectinmethylesterase from orange by manothermosonication. Journal of Agriculture and Food Chemistry, 47, 432–437. Vercet A, Burgos J, Crelier S, Lopez-Buesa P (2001a) Inactivation of proteases and lipases by ultrasound. Innovative Food Science and Emerging Technologies, 2, 139–150. Vercet A, Burgos J, Lopez-Buesa P (2001b) Manothermosonication of food and food resembling systems: effect of nutrient content and non-enzymantic browning. Journal of Agriculture and Food Chemistry, 49, 483–489. Vercet A, Sanchez C, Burgos J, Montañes L, Lopez-Buesa P (2002a) The effects of manothermosonication on tomato pectic enzymes and tomato paste rheological properties. Journal of Food Engineering, 53, 273–278. Vercet A, Oria R, Negueruela I, Crelier S, Lopez-Buesa P (2002b) Manothermosonication effects on physical and optical properties of milk. Presented at First Innovative Food Centre Conference ‘Emerging Technologies for the Food Industry’, Madrid (Spain). P676757-Ch13.qxd 6/20/05 5:02 PM Page 351

References 351

Vercet A, Oria R, Marquina P, Crelier S, Lopez-Buesa P (2002c) Rheological properties of yoghurt made with milk submitted to manothermosonication. Journal of Agriculture and Food Chemistry, 50, 6165–6171. Viesturs UE, Kritapsons MZ, Levitans ES (1982) Foam in microbiological processes. In Advances in Biochemical Engineering (Fiechter A, ed.). Berlin: Springer-Verlag, pp. 169–224. Villamiel M, de Jong P (2000a) Inactivation of Pseudomonas fluorescens and Streptococcus thermophilus in tripticase soy broth and total bacteria in milk by continuous-flow ultrasonic treatment and conventional heating. Journal of Food Engineering, 45, 171–179. Villamiel M, de Jong P (2000b) Influence of high-intensity ultrasound and heat treatment in continuous flow on fat, proteins and native enzymes of milk. Journal of Agriculture and Food Chemistry, 48, 472–478. Vinatoru M (2001) An overview of the ultrasonically assisted extraction of bioactive principles from herbs. Ultrasonics Sonochemistry, 8, 303–313. Wu H, Hulbert GJ, Mount JR (2001) Effects of ultrasound on milk homogenisation and fermentation with yoghurt starter. Innovative Food Science & Emerging Technologies, 1, 211–218. Yoon-Ku J, Park SO, Bong-Soo N (2000) Inactivation of peroxidase by hurdle technology. Food Science and Biotechnology, 9, 124–129. Zenker M, Heinz V, Knorr D (2003) Application of ultrasound assisted thermal processing for preservation and quality retention of liquid foods. Journal of Food Protection, 66, 1642–1649.