14 Extrusion cooking M. E Camire, University of Maine 14.1 Introduction Extrusion cooking is a relatively recent form of food processing. Forcin material through a hole is the process of extrusion Sausage extruders were devel- oped in the nineteenth century as simple forming machines. Eventually pasta was produced in extruders. Flour and water were added at one end of the machine, and a screw mixed and compressed the dough before extruding it through numer ous holes or dies that gave the pasta its shape. During the 1930s heat was added to the barrel containing the screw puffed corn curl snacks resulted. The pressure developed as the dough moved along the screw; this, together with the heat under pressure, caused the corn to puff upon exiting the dies. As extrusion cookin processed more types of food, extruders became more specialised for food applications. Twin-screw extruders containing two screws were adapted from the polymer industry, and these machines are considerably more versatile than are single screw extruders. Extruded products are often subjected to further process ing, such as frying, baking, and rolling The improved mixing ability of these extruders provided impetus for further product development. Table 14. 1 lists major food categories produced by extru- sion cooking. Extrusion cooking can be performed as either a batch or con- tinuous operation, offering many advantages over conventional food processing methods (Table 14.2)(Harper, 1981). Several manufacturers produce cooking extruders. Laboratory-size extruders have screw diameters of 10-30mm and throughputs of up to a few hundred kilogram per hour. The length of the barrels for these research extruders. which are the most common machines cited in the literature varies from about one to two meters Production-sized extruders can create thousands of kilogram of product per hour
14 Extrusion cooking M. E. Camire, University of Maine 14.1 Introduction Extrusion cooking is a relatively recent form of food processing. Forcing material through a hole is the process of extrusion. Sausage extruders were developed in the nineteenth century as simple forming machines. Eventually pasta was produced in extruders. Flour and water were added at one end of the machine, and a screw mixed and compressed the dough before extruding it through numerous holes or dies that gave the pasta its shape. During the 1930s heat was added to the barrel containing the screw; puffed corn curl snacks resulted. The pressure developed as the dough moved along the screw; this, together with the heat under pressure, caused the corn to puff upon exiting the dies. As extrusion cooking processed more types of food, extruders became more specialised for food applications. Twin-screw extruders containing two screws were adapted from the polymer industry, and these machines are considerably more versatile than are single screw extruders. Extruded products are often subjected to further processing, such as frying, baking, and rolling. The improved mixing ability of these extruders provided impetus for further product development. Table 14.1 lists major food categories produced by extrusion cooking. Extrusion cooking can be performed as either a batch or continuous operation, offering many advantages over conventional food processing methods (Table 14.2) (Harper, 1981). Several manufacturers produce cooking extruders. Laboratory-size extruders have screw diameters of 10–30 mm and throughputs of up to a few hundred kilogram per hour. The length of the barrels for these research extruders, which are the most common machines cited in the literature, varies from about one to two meters. Production-sized extruders can create thousands of kilogram of product per hour
Extrusion cooking 315 Table 14.1 Common food products prepared by extrusion cooking Ready-to-eat breakfast cereals Puffed cereals Flaked cereals Puffed snacks Half-products or pellets(third generation snacks) Confections Chocolate Texturised Soy meat analogues Restructured seafood Processed cheese Infant foods Biscuits Weaning cereals Table 14.2 Unique advantages of extrusion cooking atch or continuous processing High throughput Low labor and energy costs Variety of products produced and types of ingredients that can be processed Control of thermal/mechanical environment Negligible effluent Adapted from Harper(1981). 14.1.1 Unique aspects of extrusion cooking Most extruders act as heat exchangers, and they also shape and form food products. Mixing, dehydration, and pasteurisation and sterilisation are other unit operations that typically occur during extrusion. Aside from thermal destruction of nutrients, the shear that develops within the extruder barrel can damage food chemicals. Temperature can be controlled by many means including limiting direct heating, adding water, and increasing throughput Shear may be reduced by using low-shear screw elements, increasing water or lipid content, modifying screw speed(based on other parameters), and by reducing pressure at the die Extrusion research often focuses upon one to four variables, although screen ing studies should be performed to identify key factors. Extruder operators may select parameters such w speed, feed moisture, and barrel temperature as primary factors, that in turn determine a secondary set of factors: specific mechan ical energy (SME), product or mass temperature(PT) and pressure(Meuser and van Lengerich, 1984). These factors influence the viscosity of the food within the extruder barrel. the residence time of the material in the extruder. and the shear applied to the food(Fig. 14.1). Variations caused by feed
Extrusion cooking 315 Table 14.1 Common food products prepared by extrusion cooking Category Examples Ready-to-eat breakfast cereals Puffed cereals Flaked cereals High-fiber strands Snacks Puffed snacks Half-products or pellets (third generation snacks) Crispbreads Confections Licorice Chocolate Texturised protein Soy meat analogues Restructured seafood Processed cheese Infant foods Biscuits Weaning cereals Table 14.2 Unique advantages of extrusion cookinga Batch or continuous processing High throughput Low labor and energy costs Variety of products produced and types of ingredients that can be processed Control of thermal/mechanical environment Negligible effluent a Adapted from Harper (1981). 14.1.1 Unique aspects of extrusion cooking Most extruders act as heat exchangers, and they also shape and form food products. Mixing, dehydration, and pasteurisation and sterilisation are other unit operations that typically occur during extrusion. Aside from thermal destruction of nutrients, the shear that develops within the extruder barrel can damage food chemicals. Temperature can be controlled by many means including limiting direct heating, adding water, and increasing throughput. Shear may be reduced by using low-shear screw elements, increasing water or lipid content, modifying screw speed (based on other parameters), and by reducing pressure at the die. Extrusion research often focuses upon one to four variables, although screening studies should be performed to identify key factors. Extruder operators may select parameters such as screw speed, feed moisture, and barrel temperature as primary factors, that in turn determine a secondary set of factors: specific mechanical energy (SME), product or mass temperature (PT) and pressure (Meuser and van Lengerich, 1984). These factors influence the viscosity of the food within the extruder barrel, the residence time of the material in the extruder, and the shear applied to the food (Fig. 14.1). Variations caused by feed composition
316 The nutrition handbook for food processors Primary extrusion factors Extruder model Feed composition, particle size, preconditioning Added water rate arrel temperature Screw configuration and speed Die number and geometry Secondary extrusion factors Mass or product temperature Pressure Specific mechanical energy Nutrient changes Destruction Bioavailability Fig. 14.1 Interrelationships of extruder variables and their potential effects on nutrients. and prior processing of the feed materials are important sources of experimental Extrusion can produce safe, lightweight, shelf-stable foods that can be stored for use during famines and natural disasters. Simple single screw extruders are fairly inexpensive and simple to maintain so these machines can be used in less- developed nations to produce weaning and other foods. Harper and Jansen(1985) have reviewed advantages and limitations of extrusion for weaning foods. fric- tion from the rotation of the screw can cook the food thoroughly, reducing pro- duction costs for fuel sources. Extruders can blend diverse ingredients, permitting government and relief agencies to use donated foods such as dried milk as well as indigenous crops such as beans, millet, and cassava. Extruded pellets can be ground, then mixed with milk or water as needed to form gruel for infants Functional ingredients such as soy and botanicals that are relatively unpal ble alone can be incorporated into new food items by extrusion. Traditional foods such as rye crispbread can be further enhanced by addition of extra dietary fiber or other ingredients during extrusion. A relatively new form of extrusion known as wet extrusion operates at higher moisture contents (>40%)and lower
and prior processing of the feed materials are important sources of experimental variation. Extrusion can produce safe, lightweight, shelf-stable foods that can be stored for use during famines and natural disasters. Simple single screw extruders are fairly inexpensive and simple to maintain so these machines can be used in lessdeveloped nations to produce weaning and other foods. Harper and Jansen (1985) have reviewed advantages and limitations of extrusion for weaning foods. Friction from the rotation of the screw can cook the food thoroughly, reducing production costs for fuel sources. Extruders can blend diverse ingredients, permitting government and relief agencies to use donated foods such as dried milk as well as indigenous crops such as beans, millet, and cassava. Extruded pellets can be ground, then mixed with milk or water as needed to form gruel for infants. Functional ingredients such as soy and botanicals that are relatively unpalatable alone can be incorporated into new food items by extrusion. Traditional foods such as rye crispbread can be further enhanced by addition of extra dietary fiber or other ingredients during extrusion. A relatively new form of extrusion known as wet extrusion operates at higher moisture contents (>40%) and lower 316 The nutrition handbook for food processors Primary extrusion factors Extruder model Feed composition, particle size, preconditioning Feed rate Added water rate Barrel temperature Screw configuration and speed Die number and geometry Secondary extrusion factors Mass or product temperature Viscosity Pressure Specific mechanical energy Nutrient changes Retention Destruction Bioavailability Fig. 14.1 Interrelationships of extruder variables and their potential effects on nutrients
Extrusion cooking 317 barrel temperatures(Akdogan, 1999). These conditions permit extrusion and texturisation of high-protein materials since protein denaturation is limited. Very little has yet been published on the effects of wet extrusion on nutrient retention but nutrient destruction should be considerably less than in conventional extru on cooking 14.2 Impact on key nutrients: carbohydrates Reducing sugars such as glucose and lactose participate in Maillard reactions which will be discussed further in section 143. The shear forces during extru- sion can also create reducing sugars from complex carbohydrates as well as from sucrose and other sugars. Sucrose losses of up to 20% were found in protein- enriched biscuits(Noguchi and Cheftel, 1983). While sucrose loss may affect product color and flavor, there is an opportunity to reduce the content of indigestible oligosaccharides that can cause flatulence. Sucrose, raffinose and stachyose decreased significantly in extruded pinto bean high-starch fractions (Borejszo and Khan, 1992). Corn-soy snacks had lower levels of both stachyose and raffinose compared to unextruded soy grits and flour, but values were not corrected for the 50-60% corn present(Omueti and Morton, 1996). Starch and stachyose were lower in extruded peas compared to raw peas(Alonso et al, 2000), but an increase in total free sugars did not fully account for these losses (Fig.14.2) □Raw 闔 Extruded Stachyose Total free sugars Fig. 14.2 Carbohydrate changes (g/kg dry matter) due to extrusion of peas ( sativum L)at an exit temperature of 145C and 25% feed moisture(Adapted from etal,2000)
barrel temperatures (Akdogan, 1999). These conditions permit extrusion and texturisation of high-protein materials since protein denaturation is limited. Very little has yet been published on the effects of wet extrusion on nutrient retention, but nutrient destruction should be considerably less than in conventional extrusion cooking. 14.2 Impact on key nutrients: carbohydrates Reducing sugars such as glucose and lactose participate in Maillard reactions, which will be discussed further in section 14.3. The shear forces during extrusion can also create reducing sugars from complex carbohydrates as well as from sucrose and other sugars. Sucrose losses of up to 20% were found in proteinenriched biscuits (Noguchi and Cheftel, 1983). While sucrose loss may affect product color and flavor, there is an opportunity to reduce the content of indigestible oligosaccharides that can cause flatulence. Sucrose, raffinose and stachyose decreased significantly in extruded pinto bean high-starch fractions (Borejszo and Khan, 1992). Corn-soy snacks had lower levels of both stachyose and raffinose compared to unextruded soy grits and flour, but values were not corrected for the 50–60% corn present (Omueti and Morton, 1996). Starch and stachyose were lower in extruded peas compared to raw peas (Alonso et al, 2000), but an increase in total free sugars did not fully account for these losses (Fig. 14.2). Extrusion cooking 317 500 450 400 350 300 250 200 150 100 50 0 Starch Stachyose Total free sugars * * * Raw Extruded g/kg Fig. 14.2 Carbohydrate changes (g/kg dry matter) due to extrusion of peas (Pisum sativum L) at an exit temperature of 145 °C and 25% feed moisture. (Adapted from Alonso et al, 2000)
318 The nutrition handbook for food processors Starch is usually the major food constituent in extruded foods such as break- fast cereals, snacks and weaning foods. Humans and other monogastric species do not readily digest native or ungelatinised starch. Unlike many thermal processes, extrusion cooking gelatinise starch at fairly low(12-22%)moisture levels. Removal of cooking water is not a problem, and leaching of water-soluble nutrients is avoided. Increased temperature, shear, and pressure during extrusion increase the rate of gelatinisation, but lipids, sucrose, dietary fiber and salts can retard gelatinisation (Jin et al, 1994). While full gelatinisation may not occu during extrusion, digestibility is often improved (Wang, S et al, 1993) During extrusion, starch molecules can be physically broken into smaller, more digestible fragments. For example, amylopectin branches can be sheared off the main molecule, with larger molecules experiencing the greatest effect Politz et al, 1994b). Both amylose and amylopectin molecules may be affected, however. Molecular weight in extruded wheat starch was retained better under processing conditions of higher die temperature(185C)and feed moisture(20%) (Politz et al, 1994a). Screw configurations using more reverse and high-shearele ments favor starch breakdown(Gautam and Choudhoury, 1999) Lower molecular weight starch fragments may be sticky, thereby increasing the risk for dental caries, since bacteria in the mouth rapidly ferment these dextrins. Toothpack, the amount of material retained on teeth, has been used as an indication of the severity of extrusion processing. Bjorck and co-workers (1984) found that white wheat flour extruded under 'mild'and'severe'condi tions caused drops in dental plaque pH comparable to those obtained with gluco e while easily-digested starch is desirable for infants and invalids, the resulting rapid post-prandial rise in blood sugar and insulin levels is thought to be a risk factor for development of insulin insensitivity and Type Il, or adult-onset diabetes. Extrusion offers the ability to reduce the high glycemic index(GD) of some foods by converting starch to digestion-resistant starch(RS). Theander and Westerlund (1987) reported transglycosidation in extruded wheat flour, presum- ably from attachment of sheared amylopectin branches to other reactive sites. The resulting novel bonds would be resistant to digestion by enzymes. Addition of high amylose starch also reduces digestibility. As much as 30%o resistant starch was reported when high amylose starch was reacted with pullulanase prior to extrusion( Chiu et al, 1994). Extruded high amylose rice noodles had lower starch digestibility and reduced GI(Panlasigui et al, 1992 An evolving area of research involves the use of additives to promote RS for nation. Adding 30% corn, potato or wheat starch did not increase RS values in cornmeal, but RS and fiber values more than doubled when 7. 5% citric acid was used, and 30% high-amylose cornstarch with 5 or 7.5% citric acid resulted in values of 14%0, compared with slightly more than 2% in 100%o cornmeal (Unlu and Faller, 1998). Polydextrose may have been formed during extrusion. Limi tations to this approach would be the expenses of the additives and sour taste of the extrudates. Yields of up to 93. 7% oligosaccharides and polydextrose were
Starch is usually the major food constituent in extruded foods such as breakfast cereals, snacks and weaning foods. Humans and other monogastric species do not readily digest native or ungelatinised starch. Unlike many thermal processes, extrusion cooking gelatinises starch at fairly low (12–22%) moisture levels. Removal of cooking water is not a problem, and leaching of water-soluble nutrients is avoided. Increased temperature, shear, and pressure during extrusion increase the rate of gelatinisation, but lipids, sucrose, dietary fiber and salts can retard gelatinisation (Jin et al, 1994). While full gelatinisation may not occur during extrusion, digestibility is often improved (Wang, S et al, 1993). During extrusion, starch molecules can be physically broken into smaller, more digestible fragments. For example, amylopectin branches can be sheared off the main molecule, with larger molecules experiencing the greatest effect (Politz et al, 1994b). Both amylose and amylopectin molecules may be affected, however. Molecular weight in extruded wheat starch was retained better under processing conditions of higher die temperature (185 °C) and feed moisture (20%) (Politz et al, 1994a). Screw configurations using more reverse and high-shear elements favor starch breakdown (Gautam and Choudhoury, 1999). Lower molecular weight starch fragments may be sticky, thereby increasing the risk for dental caries, since bacteria in the mouth rapidly ferment these dextrins. Toothpack, the amount of material retained on teeth, has been used as an indication of the severity of extrusion processing. Björck and co-workers (1984) found that white wheat flour extruded under ‘mild’ and ‘severe’ conditions caused drops in dental plaque pH comparable to those obtained with glucose. While easily-digested starch is desirable for infants and invalids, the resulting rapid post-prandial rise in blood sugar and insulin levels is thought to be a risk factor for development of insulin insensitivity and Type II, or adult-onset, diabetes. Extrusion offers the ability to reduce the high glycemic index (GI) of some foods by converting starch to digestion-resistant starch (RS). Theander and Westerlund (1987) reported transglycosidation in extruded wheat flour, presumably from attachment of sheared amylopectin branches to other reactive sites. The resulting novel bonds would be resistant to digestion by enzymes. Addition of high amylose starch also reduces digestibility. As much as 30% resistant starch was reported when high amylose starch was reacted with pullulanase prior to extrusion (Chiu et al, 1994). Extruded high amylose rice noodles had lower starch digestibility and reduced GI (Panlasigui et al, 1992). An evolving area of research involves the use of additives to promote RS formation. Adding 30% corn, potato or wheat starch did not increase RS values in cornmeal, but RS and fiber values more than doubled when 7.5% citric acid was used, and 30% high-amylose cornstarch with 5 or 7.5% citric acid resulted in values of 14%, compared with slightly more than 2% in 100% cornmeal (Unlu and Faller, 1998). Polydextrose may have been formed during extrusion. Limitations to this approach would be the expenses of the additives and sour taste of the extrudates. Yields of up to 93.7% oligosaccharides and polydextrose were 318 The nutrition handbook for food processors