DEVELOPMENTS in FOOD EXTRUSION Introduction Extrusion

DEVELOPMENTS IN FOOD EXTRUSION

Introduction

Extrusion is a process which combines several unit operations including mixing, cooking, kneading, shearing, shaping and forming. Extruders are classified according to the method of operation (cold extruders or extruder-cookers) and the method of construction (single- or twin- screw extruders). The principles of operation are similar in all types: raw materials are fed into the extruder barrel and the screw(s) then convey the food along it. Further down the barrel, smaller flights restrict the volume and increase the resistance to movement of the food. As a result, it fills the barrel and the spaces between the screw flights and becomes compressed. As it moves further along the barrel, the screw kneads the material into a semi-solid, plasticised mass. If the food is heated above 100ºC the process is known as extrusion cooking (or hot extrusion). Here, frictional heat and any additional heating that is used cause the temperature to rise rapidly. The food is then passed to the section of the barrel having the smallest flights, where pressure and shearing is further increased. Finally, it is forced through one or more restricted openings (dies) at the discharge end of the barrel as the food emerges under pressure from the die, it expands to the final shape and cools rapidly as moisture is flashed off as steam. A variety of shapes, including rods, spheres, doughnuts, tubes, strips, squirls or shells can be formed. Typical products include a wide variety of low density, expanded snack foods and ready-to-eat (RTE) puffed cereals. Cold extrusion, in which the temperature of the food remains at ambient is used to mix and shape foods such as pasta and meat products. Low pressure extrusion, at temperatures below 100ºC, is used to produce, for example, liquorice, fish pastes, surimi and pet foods. The process involves subjecting the feed material (of intermediate water activity) to elevated temperatures (e.g. 120- 180°C) and pressures, as well as to shear stress. Advantages of extrusion cooking include the combination of a number of unit operations in one process, energy efficiency, the production of no effluent and the ability to process relatively dry viscous materials and unconventional ingredients. It is ideal for the processing of cereals, and is used for the manufacture of ready-to- eat cereals and expanded snack foods. Both single- and twin-screw extrusion cookers are available. Twin-screw extruders began to be used for food processing in the 1970s, and possess greater operating flexibility than their single-screw counterparts. Harper (1979) has described the most important components of an extrusion cooker and their functions. Extrusion cooking is a high-temperature short-time (HTST) process which reduces microbial contamination and inactivates enzymes. However, the main method of preservation of both hot- and cold-extruded foods is by the low water activity of the product (0.1–0.4) The material may reach temperatures of, for example, 120-180°C during processing and the moisture content is typically in the region 12-18% (for cereal materials). Extrusion cooking is

therefore a process involving the application of relatively high temperatures to materials of intermediate water activity, and such conditions favour the Maillard reaction. Extrusion cooking alters the nature of many food constituents, including starches and proteins, by changing their physical, chemical and nutritional properties (Harper, 1979). The resulting changes in conformation, together with the partial degradation of starch and protein, can result in increases in the availability of reactive groups which can go on to take part in reactions, including the Maillard reaction. In addition, reductions in low molecular weight carbohydrates during extrusion cooking have been reported by several authors. For example, Andersson et al. (1981) showed that a decrease in total sugar content (sucrose, glucose and fructose) of 70-80% occurred on extrusion cooking of a mixture of wheat starch, wheat gluten and wheat bran, using a final barrel temperature of 142-155°C. It was suggested that losses of glucose and fructose were at least partly due to these sugars taking part in the Maillard reaction, since higher recoveries were obtained when gluten was excluded from the feed mix. The effects of extrusion cooking on amino acids have usually concentrated on lysine. An increase in processing temperature results in a decrease in available lysine, and most reports conclude that an increase in moisture content (over the range 13-18%) gives an increase in lysine retention (Asp & Bjorck, 1989). Intensity of colour increased on raising the amounts of reducing sugar and amino acid in the extruder feed, increasing the die temperature and decreasing the moisture content. In general, decreasing the moisture content from 18% to 13% had a greater effect on colour intensity than increasing the die temperature from 125°C to 135°C. Food extrusion is a form of extrusion used in food processing. It is a process by which a set of mixed ingredients are forced through an opening in a perforated plate or die with a design specific to the food, and is then cut to a specified size by blades. The machine which forces the mix through the die is an extruder, and the mix is known as the extrudate. The extruder consists of a large, rotating screw tightly fitting within a stationary barrel, at the end of which is the die. Extrusion enables mass production of food via a continuous, efficient system that ensures uniformity of the final product. Food products manufactured using extrusion usually have a high starch content. These include some pasta, breads (croutons, bread sticks, and flat breads), many breakfast cereals and ready-to-eat snacks, confectionery, pre-made cookie dough, some baby foods, full-fat soy, textured vegetable protein, some beverages, and dry and semi- moist pet foods.

A non-vacuum short goods pasta extruder

Process: In the extrusion process, raw materials are first ground to the correct particle size, usually the consistency of coarse flour. The dry mix is passed through a pre-conditioner, in which other ingredients are added depending on the target product; these may be liquid sugar, fats, dyes, meats or water. Steam is injected to start the cooking process, and the preconditioned mix (extrudate) is then passed through an extruder. The extruder consists of a large, rotating screw tightly fitting within a stationary barrel, at the end of which is the die. The extruder's rotating screw forces the extrudate toward the die, through which it then passes. The amount of time the extrudate is in the extruder is the residence time. The extruded product usually puffs and changes texture as it is extruded because of the reduction of forces and release of moisture and heat. The extent to which it does so is known as the expansion ratio. The extrudate is cut to the desired length by blades at the output of the extruder, which rotate about the die openings at a specific speed. The product is then cooled and dried, becoming rigid while maintaining porosity. The cooking process takes place within the extruder where the product produces its own friction and heat due to the pressure generated (10–20 bar). The process can induce both protein denaturation and starch gelatinization, depending on inputs and parameters. Many food extrusion processes involve a high temperature over a short time. Important factors of the extrusion process are the composition of the extrudate, screw length and rotating speed, barrel temperature and moisture, die shape, and rotating speed of the blades. These are controlled based on the desired product to ensure uniformity of the output. Moisture is the most important of these factors, and affects the mix viscosity, acting to plasticize the extrudate. Increasing moisture will decrease viscosity, torque, and product temperature, and increase bulk density. This will also reduce the pressure at the die. Most extrusion processes for food processing maintain a moisture level below 40% that is low to intermediate moisture. High-moisture extrusion is known as wet extrusion, but it was not used much before the introduction of twin screw extruders (TSE), which have a more efficient conveying capability. The most important rheological factor in the wet extrusion of high-starch extrudate is temperature. The amount of salt in the extrudate may determine the colour and texture of some extruded products.The expansion ratio and airiness of the product depend on the salt concentration in the 3

extrudate, possibly as a result of a chemical reaction between the salt and the starches in the extrudate. Colour changes as a result of salt concentration may be caused by "the ability of salt to change the water activity of the extrudate and thus change the rate of browning reactions". Salt is also used to distribute minor ingredients, such as food colours and flavours, after extrusion; these are more evenly distributed over the product's surface after being mixed with salt. The first extruder was designed to manufacture sausages in the 1870s.Packaged dry pasta and breakfast cereals have been produced via extrusion since the 1930s, and the method was applied to pet food production in the 1950s. It has also been incorporated into kitchen appliances, such as meat grinders, herb grinders, coffee grinders, and some types of pasta makers. A similar functional process occurs when using pastry bags. Examples of extruded foods: Types of product Examples Cereal-based products Expanded snackfoods RTE and puffed breakfast cereals Soup and beverage bases, instant drinks Weaning foods Pre-gelatinised and modified starches, dextrins Crispbread and croutons Pasta products Pre-cooked composite flours Chewing gum Sugar-based products Liquorice Toffee, caramel, peanut brittle Fruit gums Protein-based products Texturised vegetable protein (TVP) Semi-moist and expanded petfoods and animal feeds and protein supplements Sausage products, frankfurters, hot dogs Surimi Caseinates Processed cheese

Effects:

Extrusion enables mass production of food via a continuous, efficient system that ensures uniformity of the final product. This is achieved by controlling various aspects of the extrusion process. It has also enabled the production of new processed food products and "revolutionized many conventional snack manufacturing processes".The extrusion process results in "chemical reactions that occur within the extruder barrel and at the die". Extrusion has the following effects: 1. Inactivation of raw food enzymes. 2. Destruction of certain naturally occurring toxins 3. Reduction of microorganisms in the final product 4

4.Slight increase of iron-bioavailability. 5. Creation of insulin-desensitizing starches (a potential risk-factor for developing diabetes). 6. Loss of lysine, an essential amino acid necessary for developmental growth and nitrogen management. 7. Simplification of complex starches, increasing rates of tooth decay. 8. Increase of glycemic index of the processed food, as the "extrusion process significantly increased the availability of carbohydrates for digestion". 9. Destruction of Vitamin A (beta-carotene) 10.Denaturation of proteins.

The material of which an extrusion die is made can affect the final product. Compared to stainless steel dies, a pasta machine with bronze dies produces a rougher surface. This is considered to give an improved taste, as it better retains pasta sauces. "Bronze die" pasta is labelled as such on retail packages, to indicate a premium product. The effects of "extrusion cooking on nutritional quality are ambiguous", as extrusion may change carbohydrates, dietary fibre, the protein and amino acid profile, vitamins, and mineral content of the extrudate in a manner that is beneficial or harmful. High-temperature extrusion for a short duration "minimizes losses in vitamins and amino acids". Extrusion enables mass production of some food, and will "denature antinutritional factors",such as destroying toxins or killing microorganisms. It may also improve "protein quality and digestibility", and affects the product's shape, texture, colour, and flavour. It may also cause the fragmentation of proteins, starches, and non-starch polysaccharides to create "reactive molecules that may form new linkages not found in nature". This includes Maillard reactions which reduce the nutritional value of the proteins. Vitamins with heat lability may be destroyed. As of 1998, little is known about the stability or bioavailability of phytochemicals involved in extrusion. Nutritional quality has been found to improve with moderate conditions (short duration, high moisture, low temperature), whereas a negative effect on nutritional quality of the extrudate occurs with a high temperature (at least 200°C), low moisture (less than 15%), or improper components in the mix. A 2012 research paper indicates that use of non-traditional cereal flours, such as amaranth, buckwheat or millet, may be used to reduce the glycemic index of breakfast cereals produced by extrusion. The extrudate using these cereal flours exhibits a higher bulk and product density, had a similar expansion ratio, and had "a significant reduction in readily digestible carbohydrates and slowly digestible carbohydrates". A 2008 paper states that replacing 5% to 15% of the wheat flour and white flour with dietary fibre in the extrudate breakfast cereal mix significantly reduces "the rate and extent of carbohydrate hydrolysis of the extruded products", which increased the level of slowly digested carbohydrates and reduced the level of quickly digested carbohydrates.

Products: Extrusion has enabled the production of new processed food products and "revolutionized many conventional snack manufacturing processes". The various types of food products manufactured by extrusion typically have a high starch content. Directly expanded types include breakfast cereals and corn curls, and are made in high temperature, low moisture conditions under high shear. Unexpanded products include pasta, which is produced at intermediate moisture (about 40%) and low temperature. Texturized products include meat analogues, which are made using

plant proteins ("textured vegetable protein") and a long die to"impart a fibrous, meat-like structure to the extrudate", and fish paste. Confectionery made via extrusion includes chewing gum, liquorice, and toffee. Some processed cheeses and cheese analogues are also made by extrusion. Processed cheeses extruded with low moisture and temperature "might be better suited for manufacturing using extrusion technology" than those at high moisture or temperature. Lower moisture cheeses are firmer and chewier, and cheddar cheese with low moisture and an extrusion temperature of 80°C was preferred by subjects in a study to other extruded cheddar cheese produced under different conditions. An extrudate mean residence time of about 100 seconds can produce "processed cheeses or cheese analogues of varying texture (spreadable to sliceable)". Other food products often produced by extrusion include some breads (croutons, bread sticks, and flat breads), various ready-to-eat snacks, pre- made cookie dough, some baby foods, some beverages, and dry and semi-moist pet foods. Specific examples include macaroni, jelly beans, sevai, and some french fries. Extrusion is also used to modify starch and to pellet animal feed. Extrusion processing has become an important food process in the manufacture of pasta, ready- to-eat cereals, snacks, pet foods, and textured vegetable protein (TVP). Extrusion is a process which combines several unit operations including mixing, cooking, kneading, shearing, shaping and forming. An extruder consists of tightly fitting screw rotating within a stationary barrel. Preground and conditioned ingredients enter the screw where they are conveyed, mixed, and heated by a variety of processes. The product exits the extruder through a die where it usually puffs and changes texture from the release of steam and normal forces. Mathematical models for extruder flow and torque have been found useful in describing exclusion operations. Scale-up can be facilitated by the application of these models. A variety of food extruder designs have developed. The differences and similarity of design are discussed. Pertinent literature on the extrusion of cereal/snack products, full-fat soy, TVP, pet foods (dry and semi-moist), pasta, and beverage or other food bases are discussed. In many of these applications, the extruder is a high temperature, short time process which minimizes losses in vitamins and amino acids. Color, flavor, and product shape and texture are also affected by the extrusion process. Extrusion has been widely applied in the production of nutritious foods. Emphasis is placed on the use of extrusion to denature antinutritional factors and the improvement of protein quality and digestibility. The principles of operation are similar in all types: raw materials are fed into the extruder barrel and the screw(s) then convey the food along it. Further down the barrel, smaller flights restrict the volume and increase the resistance to movement of the food. As a result, it fills the barrel and the spaces between the screw flights and becomes compressed. As it moves further along the barrel, the screw kneads the material into a semi-solid, plasticized mass. If the food is heated above 100ºC the process is known as extrusion cooking (or hot extrusion). Here, frictional heat and any additional heating that is used cause the temperature to rise rapidly. The food is then passed to the section of the barrel having the smallest flights, where pressure and shearing is further increased. Finally, it is forced through one or more restricted openings (dies) at the discharge end of the barrel as the food emerges under pressure from the die, it expands to the final shape and cools rapidly as moisture is flashed off as steam. A variety of shapes, including rods, spheres, doughnuts, tubes, strips, squirls or shells can be formed. Typical products include a wide variety of low density, expanded snack foods and ready-to-eat (RTE) puffed cereals. Cold extrusion, in which the temperature of the food remains at ambient is used to mix and shape foods such as pasta and meat products. Low pressure extrusion, at temperatures

below 100ºC, is used to produce, for example, liquorice, fish pastes, surimi and pet foods. Extrusion cooking is a high-temperature short-time (HTST) process which reduces microbial contamination and inactivates enzymes. The main method of preservation of both hot- and cold- extruded foods is by the low water activity of the product (0.1–0.4), and for semi-moist products in particular, by the packaging materials that are used. The principles of operation are similar in all types: raw materials are fed into the extruder barrel and the screw(s) then convey the food along it. Further down the barrel, smaller flights restrict the volume and increase the resistance to movement of the food. As a result, it fills the barrel and the spaces between the screw flights and becomes compressed. As it moves further along the barrel, the screw kneads the material into a semi-solid, plasticized mass. If the food is heated above 100ºC the process is known as extrusion cooking (or hot extrusion). Here, frictional heat and any additional heating that is used cause the temperature to rise rapidly. The food is then passed to the section of the barrel having the smallest flights, where pressure and shearing is further increased. Finally, it is forced through one or more restricted openings (dies) at the discharge end of the barrel as the food emerges under pressure from the die, it expands to the final shape and cools rapidly as moisture is flashed off as steam. A variety of shapes, including rods, spheres, doughnuts, tubes, strips, squirls or shells can be formed. Typical products include a wide variety of low density, expanded snack foods and ready-to-eat (RTE) puffed cereals. Cold extrusion, in which the temperature of the food remains at ambient is used to mix and shape foods such as pasta and meat products. Low pressure extrusion, at temperatures below 100ºC, is used to produce, for example, liquorice, fish pastes, surimi and pet foods. Extrusion cooking is a high-temperature short-time (HTST) process which reduces microbial contamination and inactivates enzymes. The main method of preservation of both hot- and cold-extruded foods is by the low water activity of the product (0.1–0.4), and for semi-moist products in particular, by the packaging materials that are used. Texturized soy protein (TSP) has been widely used in food and feed industry because of its high imitation of texture, taste, appearance of meat and its high nutritional value (Akdogan, 1999; Cheftel et al., 1992), and it is one of the most important applications of extrusion cooking to produce TSP (Akdogan, 1999; Areas, 1992). However, despite the increased use of extrusion technology, the extrusion process is still an unmastered and complicated system (Ding et al., 2006). Small variations in processing conditions affect process variables and product quality (Desrumaux et al., 1999). Specific mechanical energy (SME) is the amount of work input from driver motor into the extruded raw material (Harper, 1989). SME is an important factor for design engineering and directly affects the final product quality (Bagley and Christianson, 1989; Godavarti and Karwe, 1997). Kaletunç and Breslauer (1993) reported that the extent of corn flour fragmentation increases with increased SME input, which further affects the crispness and Denseness of extrudates. De Mesa et al. (2009) showed that the bulk density and piece densities of corn starch–soy protein concentrate (CS–SPC) extrudates have a high negative correlation with SME. A higher SME also typically leads to increased melt temperature in the barrel, which induces a greater driving force for the expansion of CS–SPC extrudates. Moreover, Chen et al. (2010) studied the response between system parameters and product properties of soy protein during extrusion. They suggested that SME has significantly positive correlations with the tensile strength, hardness, and chewiness of soy protein extrudates. According to Fang et al. (2013), the weight-average molecular weight (Mw) of soy protein isolates (SPI) increases by 74–104% after extrusion treatments. However, the value decreases from 2.688 _ 106 to 2.340 _ 106 g/mol with increased SME from 839.81 to 1277.01 kJ/kg. The increase in SME also causes a proportional

increase in low Mw fractions from 35.02% to 52.63%. Changes in the SME input also affect the Maillard reaction during the extrusion of soy protein, thereby affecting the color of extruded product (Guerrero et al., 2012).

Fig.1 SDS–PAGE of proteins extracted from native and extruded SPIs (soy protein isolate) with different specific mechanical energy (SME) levels. Lanes: (1) Marker; (2) Native SPI; (3–7) Extruded SPIs 1–5 corresponding to SME 819.70, 878.15, 957.78, 1035.10 and 1258.70 kJ/ kg, respectively. (Y. Fang et al.2014)

COEXTRUSION: Coextrusion is the process of extruding two or more materials simultaneously or in tandem. It allows a combination of an ingredient such as wheat flour, which is inexpensive and easily enriched with vitamins and minerals, with dairy protein, which provides functionality and texture. For example, an early coextrusion of wheat flour and rennet casein was performed by van de Voort et al. (1984), who obtained products with varying characteristics depending on process parameters. Coextrusion of whey protein and corn flour was shown to reduce the specific mechanical energy input into the extrusion process and increase the expansion and breaking strength of a protein-enriched extrudate (Onwulata et al., 2001b). Altering the extrusion moisture and shear led to improvement in expansion and breaking strength. When corn, rice, and potato flours were extruded in combination with whey proteins, the whey protein could be substituted for up to 25% of the flour without affecting the quality of the resulting snack product (Onwulata et al., 1998, 2001a,b). Again, the inclusion of whey protein reduced the specific mechanical energy input. Further research using moistures ranging from 20% to 70% by weight and extrusion temperatures of 50, 75, and 100 ºC showed that temperature affected the degree of protein texturization (Onwulata et al., 2010). WPC and WPI liquefied when extruded at 50 ºC, became soft at 75 ºC and high moisture, and were solid at 100 ºC. These results reflected the level of denaturation induced by heat and shear (Onwulata, 2009). Another product, defatted corn germ flour was coextruded at 150 or 170 ºC with 5% milk protein to produce a puffed nutrient snack (Peri et al., 1983). The addition of the milk protein at the lower temperature 8

improved the organoleptic characteristics of the extrudates but adversely affected the product expansion and consistency of the samples at the higher temperature. Supercritical fluid extrusion Low process impact extrusion may be accomplished through the introduction of supercritical CO2. Supercritical fluid extrusion (SCFX) has been used over a wide range of texturization temperatures for different whey protein fractions with starch. In this process, supercritical CO2 is injected into dough in the extruder barrel, the temperature and pressure are adjusted to control bubble nucleation, and the degree of cell growth is manipulated by selecting the appropriate die and controlling cooling and drying after extrusion (Rizvi et al., 1995). SCFX mitigates some of the harsh environmental conditions in the extruder, such as destruction of heat- and shear- sensitive compounds. SCFX below 90ºC transforms whey proteins into cold-setting gels (Manoi and Rizvi, 2008, 2009). The process could be used to deposit vitamins directly on a cooked and cooled melt that can be puffed with CO2 upon exiting the die and then dried to obtain breakfast cereal. The authors suggest that fast-cooking pasta can be obtained by SCFX, as the product is not precooked in the extruder (Rizvi et al., 1995).

COLD EXTRUSION: The elevated cooking temperatures used in normal extrusion lead to discoloration of whey proteins from the Maillard reaction, racemization of protein during cross-linking, destruction of the sulfur-containing amino acids, cysteine and methionine, and other problems (Pordesimo and Onwulata, 2008). As shear without heating has been found to be adequate to induce texturization of particulate whey (Walkenstrom et al., 1998), some investigations have been made in the area of cold or nonthermal extrusion. Cold extrusion is defined as extrusion in which the process temperature is below 50 ºC. Molten gel temperatures are not reached in cold extrusion, which produces shear-induced gels similar to cold-set gels (Cho et al., 1997). Cold-denatured proteins are in a state similar to the molten globular state exhibited by heat-denatured proteins (Kunugi and Tanaka, 2002). WPI is denatured in 45–90 s when cold extruded at 50 ºC, and the degree of denaturation may be adjusted through manipulation of moisture, shear rate, and temperature. Digestibility, functionality, and protein value are retained when WPI is extruded at ≤50 ºC (Pordesimo and Onwulata, 2008). Investigating coextrusion of corn meal and WPI, Onwulata et al. (2003b) found that the melt temperature of the extrudate was more of an indicator of physical properties than specific mechanical energy. Quality attributes such as breaking strength, color, and expansion index were related to melt temperature measured at the die. In a different study, it was determined that the concentration range of whey protein required for a fibrous texture suitable for meat extenders. Consumer evaluation showed that 48% whey protein was the optimal level, with no benefits obtained by raising the protein concentration. The effects of the drying conditions on extruded WPC and WPI were examined by Nalesnik et al. (2007), who found no changes in color when extruded material was dried at 40 or 70 ºC but did observe differences in force-time curves when performing texture analyse. Extrusion is an effective means of denaturing whey proteins to create texturized products. TWP may be used as an ingredient to improve the characteristics of many foods. The production of snack foods with enhanced protein levels is possible by direct extrusion of WPC or WPI. Thermal extrusion at elevated temperatures is usually employed, and co-extrusion with flour and other ingredients reduces mechanical energy input. Extrusion using supercritical CO2 or cold extrusion (at or below 35 ºC) is another option. Manipulating the extrusion process may create new food products with enhanced functional properties and nutritional profiles. Extrusion processing

texturizes WPCs, WLAC, and WPI, but the greatest amount of texturing occurred with WPI. Texturized or denatured WPI retained its native protein value, functionality, and digestibility when extruded below 50 ºC; changes in functionality occur at 65 ºC and above. Through careful selection of extrusion conditions of temperature and moisture, TWPs with unique functionality can be produced. The degree of texturization increased with increasing temperature, but temperatures higher than 100 ºC may be needed to form fibrous structures with WPI. It is demonstrated here that extrusion is an effective tool for texturing whey proteins to create new functions for dairy proteins and that thermally denatured WPI is a unique ingredient that can be used in large amounts in nontraditional applications for non-TWPI.

Effects on flavor and other components:

Flavor retention is a concern with extrusion due to thermal degradation in the barrel and volatilization at the die (Riha and Ho, 1996). Moreover, flavor generation from Maillard and other reactions may occur. Maga and Kim (1989) extruded sodium caseinate, WPC, and other proteins with cornstarch and found that low-temperature and high-moisture extrusion resulted in the generation of more flavor compounds than high temperatureand high moisture. Flavorings may be added to the material before or after extrusion to enhance desirable flavors and mask unwanted ones generated during extrusion cooking (Maga and Kim, 1989). Carbohydrates are gelatinized during extrusion, and starch may be degraded to dextrins, which are carbohydrates with lower molecular weight (Bjorck and Asp, 1983). Starch granules gelatinize and melt durin extrusion because hydrogen bonding in the polysaccharide chains is disrupted by heat and moisture (Camire et al., 1990). Gelatinization plays an important role in the characteristics of the final product (van de Voort et al., 1984). Lipids are hydrolyzed by moisture and heat into free fatty acids, though hydrolytic enzymes may be deactivated by extrusion. Also, unsaturated fatty acids may undergo oxidative rancidity (Camire et al., 1990). Vitamins, microorganisms, and enzymes are susceptible to inactivation or destruction in an extruder. Removal of microorganisms and enzymes is desirable in most cases, but vitamin retention is important for nutritional considerations (Bjorck and Asp, 1983). Survival of vitamins increases if moisture is increased and if temperature, screw speed, and specific energy input decrease (Killeit, 1994). Vitamin loss may be compensated by adding more than the necessary amount of preextrusion or by applying a vitamin coating, filling, or spray postextrusion. Vitamins differ greatly in structure, and degradation of vitamins depends on processing conditions, but minimizing temperature and shear protects most vitamins during processing (Singh et al., 2007). Riaz et al. (2009) and Bjorck and Asp (1983) reported losses of vitamins during high-temperature, high-shear extrusion processing at 80–180 ºC, but in a more recent review of food extrusion and nutrition, Singh et al. (2007) showed that the effect of extrusion on nutritional quality was ambiguous, both beneficial or deleterious depending on processing conditions. Controlled changes can be induced on proteins by mild heat treatments, pH changes, and shear during food manufacturing to favorably alter them biologically and functionally by modifying specific amino acids (Onwulata et al., 2006). For example, acidic conditions affect glutamine and asparagines, while alkaline conditions affect cysteine, serine, and threonine forming lysinoalanine and D-amino acids.Heating proteins in the presence of reducing sugars results in nonenzymatic browning. Although most thermal denaturation is irreversible, a-LA denaturation is primarily reversible

(80–90%) above pH 3.3 depending on the presence of calcium. Below pH 3.3 or in the presence of calcium chelators, its reversibility is reduced (Korhonen et al., 1998).

Extrusion has gained in popularity for the following reasons:

•Versatility. A very wide variety of products are possible by changing the ingredients, the operating conditions of the extruder and the shape of the dies. Many extruded foods cannot be easily produced by other methods. •Reduced costs. Extrusion has lower processing costs and higher productivity than other cooking or forming processes. Some traditional processes, including manufacture of cornflakes and frankfurters, are more efficient and cheaper when replaced by extrusion. • High production rates and automated production. Extruders operate continuously and have high throughputs. For example, production rates of up to 315 kg h-1 for snack foods, 1200 kg h-1 for low-density cereals and 9000 kg h-1 for dry expanded pet foods are possible. • Product quality. Extrusion cooking involves high temperatures applied for a short time and the limited heat treatment therefore retains many heat sensitive components. •No process effluents. Extrusion is a low-moisture process that does not produce process effluents. This eliminates water treatment costs and does not create problems of environmental pollution.

Major Developments in Extrusion:Improving extrudate expansion: To improve the interaction of whey proteins with other food components such as starches, flours, and nondairy proteins, different methods have been explored primarily to increase expansion of the extrudate. For example, extreme extrusion process conditions of high shear and low moisture were used to directly expand high-protein corn meal containing 30 wt. % WPC (Onwulata et al., 2001a,b). In a similar process, an expanded extrudate was made using low temperature (<100 ºC) and low shear with supercritical CO2 extrusion (Rizvi andMulvaney, 1992). The range of use for unmodified whey proteins in puffed extrudates is extended with difficulty in amounts greater than 10 wt. % (Onwulata et al., 1998). Manufacturing of expanded snacks in large amounts using non-TWPs had been only marginally successful until recently (Singh et al., 1991) but if proteins are texturized prior to adding them to the starch matrix, or if both are texturized together, an improved product with better functionality and preferred texture can be created (Mohammed et al., 2000).

denaturation may be adjusted through manipulation of moisture, shear rate, and temperature. Digestibility, functionality, and protein value are retained when WPI is extruded at 50ºC (Pordesimo and Onwulata, 2008).

SUPERCRITICAL FLUID EXTRUSION: Low process impact extrusion may be accomplished through the introduction of supercritical CO2. Supercritical fluid extrusion (SCFX) has been used over a wide range of texturization temperatures for different whey protein fractions with starch. In this process, supercritical CO2 is injected into dough in the extruder barrel, the temperature and pressure are adjusted to control bubble nucleation, and the degree of cell growth is manipulated by selecting the appropriate die and controlling cooling and drying after extrusion (Rizvi et al., 1995). SCFX mitigates some of the harsh environmental conditions in the extruder, such as destruction of heat- and shear- sensitive compounds. SCFX below 90 ºC transforms whey proteins into cold-setting gels (Manoi and Rizvi, 2008, 2009). The process could be used to deposit vitamins directly on a cooked and cooled melt that can be puffed with CO2 upon exiting the die and then dried to obtain breakfast cereal. The authors suggest that a fast-cooking pasta can be obtained by SCFX, as the product is not precooked in the extruder (Rizvi et al., 1995). Other investigations investigating coextrusion of corn meal and WPI, Onwulata et al. (2003b) found that the melt temperature of the extrudate was more of an indicator of physical properties than specific mechanical energy. Quality attributes such as breaking strength, color, and expansion index were related to melt temperature measured at the die. In a different study, it was determined that the concentration range of whey protein required for a fibrous texture suitable for meat extenders. Consumer evaluation showed that 48% whey protein was the optimal level, with no benefits obtained by raising the protein concentration. The effects of the drying conditions on extruded WPC and WPI were examined by Nalesnik et al. (2007), who found no changes in color when extruded material was dried at 40 or 70 ºC but did observe differences in force-time curves when performing texture analyses.

APPLICATIONS: PUFFED SNACKS: Whey may be substituted for starch by as much as 25% in extruded corn snacks, but the product does not puff as much as corn alone, as the water-holding whey protein does not react with the starch matrix (Onwulata et al., 1998). WPCs or isolates can be added along with starch to create expanded snack foods with boosted nutritional content; however, without texturization, whey proteins in amounts larger than 15% may interfere with expansion, making the products less crunchy. To counter this effect, whey proteins can be texturized with starch to improve their interaction with other food components in a formulation, principally to increase extrudate expansion. In one successful application, between 25% and 35% of the flour was replaced with whey protein (Onwulata et al., 2001a,b). Texturization enables the creation of more expanded products with boosted protein levels, which are texturally firmer and crispier products, easier to break than the typical cornmeal or cornmeal without TWPI.

CHEESE ANALOGS: Calcium caseinate and butter oil have been extruded directly at 50–60% moisture levels to obtain a cheese analog with no surface water or fat (Cheftel et al., 1992). The fat emulsification and melting ability increased with screw speed or barrel temperature. The texture of the extruded

analogs was similar to those obtained by batch cooking and was affected by pH (Cheftel et al., 1992) and emulsifying salts (Cavalier-Salou and Cheftel, 1991). The product can be used as adjuncts for hamburger, pizza, and sauces. Extruded WPI has been used as a fat mimic. The formation of microparticles is required for a creamy sensation in the mouth (Jost, 1993), and this was achieved by extruding at acidic pH (Queguiner et al., 1992). A cranberry syrup was combined with sucrose, pectin, citric acid, and TWP to obtain a protein-fortified confection (Faryabi et al., 2008). Extruded whey crisps containing between 30% and 70% protein were developed (Taylor et al., 2005). The whey crisps had a lighter color, lower aroma, and different flavor profile than soy crisps, which allow for easier customization of color and flavor (Taylor et al., 2005). Nutritional bars containing cold- extruded whey have been developed (Joseph et al., 1995). Extrusion was conducted at 37 _C to produce a lowcalorie product with high nutrient value. A weaning food was obtained by extruding WPC, WPI, or a-LA with taro flour, which is derived from a tropical root tuber (Onwulata et al., 2002). The extrudates were pulverized, made into powders, and rehydrated into pastes. WPI coblended extrudates produced the best consistency. Dairy proteins can be used to boost the protein content of starch-based puffed snacks made from cornmeal; they bind water and form doughy pastes with the starch, but not the non-TWPs. A wide possibility exists for creating new foods with texturized dairy proteins due to the availability of an extensive range of achievable states (Onwulata et al., 2010).

CONCLUSION: Extrusion is an effective means of denaturing whey proteins to create texturized products. TWP may be used as an ingredient to improve the characteristics of many foods. The production of snack foods with enhanced protein levels is possible by direct extrusion of WPC or WPI. Thermal extrusion at elevated temperatures is usually employed, and coextrusion with flour and other ingredients reduces mechanical energy input. Extrusion using supercritical CO2 or cold extrusion (at or below 35 ºC) is another option. Manipulating the extrusion process may create new food products with enhanced functional properties and nutritional profiles. Extrusion processing texturizes WPCs, WLAC, and WPI, but the greatest amount of texturing occurred with WPI. Texturized or denatured WPI retained its native protein value, functionality, and digestibility when extruded below 50 ºC; changes in functionality occur at 65 ºC and above. Through careful selection of extrusion conditions of temperature and moisture, TWPs with unique functionality can be produced. The degree of texturization increased with increasing temperature, but temperatures higher than 100 ºC may be needed to form fibrous structures with WPI. It is demonstrated here that extrusion is an effective tool for texturing whey proteins to create new functions for dairy proteins and that thermally denatured WPI is a unique ingredient that can be used in large amounts in nontraditional applications for non-TWPI. This review covers the use of extrusion texturized dairy ingredients in foods; however, there are other examples of the successful use of this technique along with the product, TWPI in different types of non-food applications, such as in biodegradable films, and bioplastics.