Extrusion cooking 319 reported when glucose-citric acid mixtures were extruded at different barrel tem- peratures(Hwang et al, 1998) Longer cellulose fibers added to cornstarch decreased starch solubility Chinnaswamy and Hanna, 1991). Removal of insoluble dietary fiber from wheat flour in combination with 20% protein addition resulted in pasta with significantly delayed dextrin release under in vitro digestion conditions(Fardet et al, 1999), possibly due to enhanced protein-starch interactions. Amylose forms complexes with lipids during extrusion, thereby reducing both starch and lipid availability. This phenomenon will be addressed in section 14.4 The term dietary fiber is used to describe nondigestible carbohydrates and associated compounds such as lignin. Although a global definition of dietary fiber does not yet exist, there is a consensus that adequate fiber consumption is essen tial for good health. Analytical methods for quantitating dietary fiber vary con- siderably. The AOAC total dietary fiber method used for US nutritional labeling does not measure compounds that are soluble in 80%o aqueous ethanol such as ertain fructans and polydextrose, and this procedure does not detect changes in extruded fiber solubility. If different dietary fiber fractions are not analysed sepa rately, it is possible to overlook important changes in dietary fiber composition and functionality caused by extrusion. Like starch, branched dietary fiber molecules are susceptible to shear. The smaller fragments may be soluble in water. Fragments may also combine to form large insoluble complexes that may be analysed as lignin. Although extrusion did not affect pectin, both soluble and insoluble nonstarch polysaccharides(Nsp)were increased in extruded oatmeal and potato peels( Camire and Flint, 1991). C orn meal fiber was unaffected by extrusion under the same conditions as the other foods. Extruded beans(Phaseolus vulgaris L) had total fiber values comparable to those before extrusion but a redistribution of insoluble to soluble fiber occurred (Martin-Cabrejas et al, 1999). Sugar beet pectin and hemicellulose molecular weight decreased with extrusion, and water solubility of those compounds in- creased by 16.6 to 47.5%(Ralet et al, 1991). Extrusion increased the solubility of beta-glucans in regular and waxy barley cultivars( Gaosong and Vasanthan, 2000) Does the 'soluble fiber created during extrusion have the same health ben- efits as natural forms such as pectin and B-glucan? Viscous gels formed in the small intestine trap bile acids and thus may contribute to lower serum cholesterol levels; the soluble fiber matrix is also thought to slow glucose absorption from e small intestine. Extrusion increased the viscosity of aqueous suspensions of wheat, oats and barley (Wang and Klopfenstein, 1993). Although increased in vitro viscosity was correlated with higher levels of soluble citrus peel fiber after extrusion( Gourgue et al, 1994), in vitro starch digestion and glucose diffusion were unaffected. Extrusion of wheat flakes containing guar gum did not reduce e guar gums ability to lower post-prandial blood glucose and insulin in healthy adults(Fairchild et al, 1996). In an intervention study involving middle-aged men with hyperlipidemia, baked goods fortified with 92 g/day extruded dry white beans did not lower serum lipoproteins(Oosthuizen et al, 2000)
reported when glucose-citric acid mixtures were extruded at different barrel temperatures (Hwang et al, 1998). Longer cellulose fibers added to cornstarch decreased starch solubility (Chinnaswamy and Hanna, 1991). Removal of insoluble dietary fiber from wheat flour in combination with 20% protein addition resulted in pasta with significantly delayed dextrin release under in vitro digestion conditions (Fardet et al, 1999), possibly due to enhanced protein–starch interactions. Amylose forms complexes with lipids during extrusion, thereby reducing both starch and lipid availability. This phenomenon will be addressed in section 14.4. The term dietary fiber is used to describe nondigestible carbohydrates and associated compounds such as lignin. Although a global definition of dietary fiber does not yet exist, there is a consensus that adequate fiber consumption is essential for good health. Analytical methods for quantitating dietary fiber vary considerably. The AOAC total dietary fiber method used for US nutritional labeling does not measure compounds that are soluble in 80% aqueous ethanol such as certain fructans and polydextrose, and this procedure does not detect changes in extruded fiber solubility. If different dietary fiber fractions are not analysed separately, it is possible to overlook important changes in dietary fiber composition and functionality caused by extrusion. Like starch, branched dietary fiber molecules are susceptible to shear. The smaller fragments may be soluble in water. Fragments may also combine to form large insoluble complexes that may be analysed as lignin. Although extrusion did not affect pectin, both soluble and insoluble nonstarch polysaccharides (NSP) were increased in extruded oatmeal and potato peels (Camire and Flint, 1991). Corn meal fiber was unaffected by extrusion under the same conditions as the other foods. Extruded beans (Phaseolus vulgaris L) had total fiber values comparable to those before extrusion, but a redistribution of insoluble to soluble fiber occurred (Martín-Cabrejas et al, 1999). Sugar beet pectin and hemicellulose molecular weight decreased with extrusion, and water solubility of those compounds increased by 16.6 to 47.5% (Ralet et al, 1991). Extrusion increased the solubility of beta-glucans in regular and waxy barley cultivars (Gaosong and Vasanthan, 2000). Does the ‘soluble’ fiber created during extrusion have the same health benefits as natural forms such as pectin and b-glucan? Viscous gels formed in the small intestine trap bile acids and thus may contribute to lower serum cholesterol levels; the soluble fiber matrix is also thought to slow glucose absorption from the small intestine. Extrusion increased the viscosity of aqueous suspensions of wheat, oats and barley (Wang and Klopfenstein, 1993). Although increased in vitro viscosity was correlated with higher levels of soluble citrus peel fiber after extrusion (Gourgue et al, 1994), in vitro starch digestion and glucose diffusion were unaffected. Extrusion of wheat flakes containing guar gum did not reduce the guar gum’s ability to lower post-prandial blood glucose and insulin in healthy adults (Fairchild et al, 1996). In an intervention study involving middle-aged men with hyperlipidemia, baked goods fortified with 92 g/day extruded dry white beans did not lower serum lipoproteins (Oosthuizen et al, 2000). Extrusion cooking 319
320 The nutrition handbook for food processors 14.3 Proteins Two reviews of protein extrusion have been published(Camire, 1991; Areas 1992). The effects of extrusion on protein nutrition have been studied extensively for animal feeds and for human weaning foods. Total protein changes very little during most extrusion operations. Changes in nutritional quality may be over- looked if only total nitrogen is assayed; animal feeding studies or in vitro protein digestibility testing should be performed on products that are designed to provide significant amounts of high-quality protein. Disulfide and other covalent cross- linking, aggregation and fragmentation are among reactions reported in the litera ture. Free radical reactions are significant during wheat flour extrusion(Schaich and Rebello, 1999) Excessive Maillard browning can result in losses of lysine up to approximately 0%(de la Gueriviere et al, 1985). High barrel temperature, low moisture, and high shear promote Maillard reactions. Browning may occur even when reduc ing sugars are excluded from formulations because new reducing sugars may be formed from hydrolysis of sucrose, starch, and other polysaccharides In a model system of wheat starch, glucose and lysine, low pH increased Maillard reactions ( Bates et al, 1994). Lysine can be preserved, however, if extruder operating onditions and formulations are carefully balanced. Corn-soy blends extruded for reconstitution as porridge or gruel had good lysine retention(Konstance et al Extrusion may improve protein digestibility by denaturating proteins, ex posing enzyme-accessible sites. Enzymes and enzyme inhibitors generally lose activity due to denaturation. Reductions in protease inhibitors can contribute to better plant protein utilisation. Although a single test for protein denaturation is not used internationally, protein solubility in water or aqueous solutions is com- monly used to assess the extent of denaturation. High shear extrusion conditions in particular promote denaturation(Della Valle et al, 1994), although mass tem- perature and moisture are also important factors. Protein solubility is reduced in pasta despite the low process temperatures used in pasta making(Ummadi et al, 1995) The mechanism for cholesterol lowering by a diet with soy protein is not well understood, but the lysine/arginine ratio may play an important role. Health effects of food proteins could be significantly affected by extrusion cooking if lysine is selectively lost via Maillard reactions. Extrusion-texturised soy isolate fed to rats had similar effects as nonextruded soy on serum cholesterol, choles- terol and steroid fecal excretion, or protein nutrition(Fukui et al, 1993). In another rat study, amino acid-supplemented extruded pea( Pisum sativum L, cv. Ballet) seed meal lowered total and LDL cholesterol as well as did supplemented raw seeds compared with a control diet(Alonso et al, 2001). The peas were extruded under fairly mild conditions(145C exit temperature and feed moisture of 25%0), but antinutritional factors were adequately inactivated, as evidenced by lower pancreatic weights in rats fed the extruded peas. Amaranth protein has a lysine/arginine ratio similar to soy LDL cholesterol in rabbits fed an extruded
14.3 Proteins Two reviews of protein extrusion have been published (Camire, 1991; Arêas, 1992). The effects of extrusion on protein nutrition have been studied extensively for animal feeds and for human weaning foods. Total protein changes very little during most extrusion operations. Changes in nutritional quality may be overlooked if only total nitrogen is assayed; animal feeding studies or in vitro protein digestibility testing should be performed on products that are designed to provide significant amounts of high-quality protein. Disulfide and other covalent crosslinking, aggregation and fragmentation are among reactions reported in the literature. Free radical reactions are significant during wheat flour extrusion (Schaich and Rebello, 1999). Excessive Maillard browning can result in losses of lysine up to approximately 50% (de la Gueriviere et al, 1985). High barrel temperature, low moisture, and high shear promote Maillard reactions. Browning may occur even when reducing sugars are excluded from formulations because new reducing sugars may be formed from hydrolysis of sucrose, starch, and other polysaccharides. In a model system of wheat starch, glucose and lysine, low pH increased Maillard reactions (Bates et al, 1994). Lysine can be preserved, however, if extruder operating conditions and formulations are carefully balanced. Corn-soy blends extruded for reconstitution as porridge or gruel had good lysine retention (Konstance et al, 1998). Extrusion may improve protein digestibility by denaturating proteins, exposing enzyme-accessible sites. Enzymes and enzyme inhibitors generally lose activity due to denaturation. Reductions in protease inhibitors can contribute to better plant protein utilisation. Although a single test for protein denaturation is not used internationally, protein solubility in water or aqueous solutions is commonly used to assess the extent of denaturation. High shear extrusion conditions in particular promote denaturation (Della Valle et al, 1994), although mass temperature and moisture are also important factors. Protein solubility is reduced in pasta despite the low process temperatures used in pasta making (Ummadi et al, 1995). The mechanism for cholesterol lowering by a diet with soy protein is not well understood, but the lysine/arginine ratio may play an important role. Health effects of food proteins could be significantly affected by extrusion cooking if lysine is selectively lost via Maillard reactions. Extrusion-texturised soy isolate fed to rats had similar effects as nonextruded soy on serum cholesterol, cholesterol and steroid fecal excretion, or protein nutrition (Fukui et al, 1993). In another rat study, amino acid-supplemented extruded pea (Pisum sativum L., cv. Ballet) seed meal lowered total and LDL cholesterol as well as did supplemented raw seeds compared with a control diet (Alonso et al, 2001). The peas were extruded under fairly mild conditions (145 °C exit temperature and feed moisture of 25%), but antinutritional factors were adequately inactivated, as evidenced by lower pancreatic weights in rats fed the extruded peas. Amaranth protein has a lysine/arginine ratio similar to soy. LDL cholesterol in rabbits fed an extruded 320 The nutrition handbook for food processors