**3.2 Starch**

The major difference between extrusion processing and conventional food processing is that in the former starch gelatinization occurs at much lower moisture contents (12-22%).

Starch is contained in a large variety of plant crops, such as cereals (50-80% db starch), legumes (25-50% db), and tubers (60-95% db) (Colonna et al., 1998). The three major cereals in order of world production are wheat, rice and maize; moreover, other important crops are barley, rye, oats and sorghum. All these cereals are available as grains, which are milled to form flours rich in starch once first separated the endosperm from the hull or pericarp (Guy, 2001). Starch is present in endosperm cells, is insoluble in cold water and its main nutritional property is to supply energy (4 kcal/g) (Caldwell et al., 2000; Cheftel, 1986). Each cereal has a different composition of its flour which basically depends on the level of the non-starch components such as protein and fibres. For example, maize and rice flours are generally richer in starch than wheat flour due to the lower protein and fibre contents. Oat flours are high in both oil and fibre presenting the lowest starch content (Guy, 2001). Native starch is found in the form of discrete particles or granules, with defined sizes and shapes depending on the botanical source. The starch granule consists of two different glucose polymers: amylose and amylopectin, responsible for its physicochemical and functional properties (Bornet, 1993; Caldwell et al., 2000). The highly branched structure of amylopectin is more prone to shear, but both amylose and amylopectin molecules may decrease in weight (Collona et al., 1998). Amylose is a basically linear polymer, with linear α-1-4 glucosidic bonds, polymerization degree of 600 to 6000 glucose units and molecular weight of 105-106 Da. Amylopectin is a branched polymer, with linear α-1-4 glucosidic bonds and, at branching points, α-1-6 glucosidic bonds, in a proportion of 5-6%; it consist of approximately 106 glucose units, with a molecular weight of 108 Da. Starch contains different proportions of amylose and amylopectin, depending on its botanical origin. Starch from cereals has an amylose content that varies from 15 to 28% (Bornet, 1993).

Thermoplastic extrusion, depending on process conditions and raw material composition, causes swelling and rupture of the starch granule, completely or partially destroying the organized granule structure, reducing viscosity and releasing amylose and amylopectin (Camire et al., 1990; El-Dash et al., 1983). During thermoplastic extrusion, amylose and amylopectin are partially hydrolysed to maltodextrins, due to the high temperatures and shear inside the extruder (Cheftel, 1986; El-Dash et al., 1983).

An important consequence of starch degradation is the reduction in expansion. Highly expanded products may crumble easily due to thin cell walls, while dense products are often hard (Riaz, 2000).

Larger amylopectin molecules in corn flour had the greatest molecular weight reduction. High molecular weight (>107) starches disappeared during extrusion, and there was a general increase in starch molecules of 105 – 107 (Guy, 2001).

The physical nature of the cereal flour may affect the final extruded product. Some cereals present soft and floury endosperm. In this material, the starch granules and protein layer are only loosely bound together and the endosperm is broken down easily on milling to provide a mixture of separated starch and protein bodies. In the extruder, soft flour will create less mechanical energy between its particles and require less mechanical energy to process through the same screw configuration, a longer time being necessary before melt formation

The major difference between extrusion processing and conventional food processing is that

Starch is contained in a large variety of plant crops, such as cereals (50-80% db starch), legumes (25-50% db), and tubers (60-95% db) (Colonna et al., 1998). The three major cereals in order of world production are wheat, rice and maize; moreover, other important crops are barley, rye, oats and sorghum. All these cereals are available as grains, which are milled to form flours rich in starch once first separated the endosperm from the hull or pericarp (Guy, 2001). Starch is present in endosperm cells, is insoluble in cold water and its main nutritional property is to supply energy (4 kcal/g) (Caldwell et al., 2000; Cheftel, 1986). Each cereal has a different composition of its flour which basically depends on the level of the non-starch components such as protein and fibres. For example, maize and rice flours are generally richer in starch than wheat flour due to the lower protein and fibre contents. Oat flours are high in both oil and fibre presenting the lowest starch content (Guy, 2001). Native starch is found in the form of discrete particles or granules, with defined sizes and shapes depending on the botanical source. The starch granule consists of two different glucose polymers: amylose and amylopectin, responsible for its physicochemical and functional properties (Bornet, 1993; Caldwell et al., 2000). The highly branched structure of amylopectin is more prone to shear, but both amylose and amylopectin molecules may decrease in weight (Collona et al., 1998). Amylose is a basically linear polymer, with linear α-1-4 glucosidic bonds, polymerization degree of 600 to 6000 glucose units and molecular weight of 105-106 Da. Amylopectin is a branched polymer, with linear α-1-4 glucosidic bonds and, at branching points, α-1-6 glucosidic bonds, in a proportion of 5-6%; it consist of approximately 106 glucose units, with a molecular weight of 108 Da. Starch contains different proportions of amylose and amylopectin, depending on its botanical origin. Starch

in the former starch gelatinization occurs at much lower moisture contents (12-22%).

from cereals has an amylose content that varies from 15 to 28% (Bornet, 1993).

shear inside the extruder (Cheftel, 1986; El-Dash et al., 1983).

general increase in starch molecules of 105 – 107 (Guy, 2001).

often hard (Riaz, 2000).

Thermoplastic extrusion, depending on process conditions and raw material composition, causes swelling and rupture of the starch granule, completely or partially destroying the organized granule structure, reducing viscosity and releasing amylose and amylopectin (Camire et al., 1990; El-Dash et al., 1983). During thermoplastic extrusion, amylose and amylopectin are partially hydrolysed to maltodextrins, due to the high temperatures and

An important consequence of starch degradation is the reduction in expansion. Highly expanded products may crumble easily due to thin cell walls, while dense products are

Larger amylopectin molecules in corn flour had the greatest molecular weight reduction. High molecular weight (>107) starches disappeared during extrusion, and there was a

The physical nature of the cereal flour may affect the final extruded product. Some cereals present soft and floury endosperm. In this material, the starch granules and protein layer are only loosely bound together and the endosperm is broken down easily on milling to provide a mixture of separated starch and protein bodies. In the extruder, soft flour will create less mechanical energy between its particles and require less mechanical energy to process through the same screw configuration, a longer time being necessary before melt formation

**3.2 Starch** 

and less time for the transformation of the melt in the shearing section (high-pressure zone). On the other hand, in certain cereals, such as hard wheat, durum wheat, vitreous flint maize and some varieties of barley there is a strong bonding between the starch granules and the protein layers forming a hard particle of flour that requires more energy to break down and will generate more heat in the extruder. Therefore, if high expansion is required in a low moisture product, finely milled forms of harder endosperm types will give excellent results. If the product requires low to medium expansion, some of the hard material may be replaced by soft flour; and for low expansion in a dense product such as breading crumb, soft flour may be used (Guy, 2001).

Inside the extruder, starch goes through several stages. First, the initial moisture content is very important to define the desired product type. Once inside the extruder, and at relatively high temperatures, the starch granules melt and become soft, besides changing their structure that is compressed to a flattened form (Guy, 2001). The application of heat, the action of shear on the starch granule and water content destroy the organized molecular structure, also resulting in molecular hydrolysis of the material (Mercier et al., 1998). The starch polymers are then dispersed and degraded to form a continuous fluid melt. The fluid polymer continuum retains water vapour bubbles and stretches during extrudate expansion until the rupture of cell structure. The starch polymer cell walls recoil and stiffen as they cool to stabilize the extrudate structure. Finally, the starch polymer becomes glassy as moisture is removed, forming a hard brittle texture (Guy, 2001). The final expanded product presents air cells that are formed due to superheated water vapour pressure. When the temperature of the extrudate is reduced below its glass transition temperature (Tg), it solidifies and maintains its expanded form (Riaz, 2000).

According to Colonna et al. (1998), maximum expansion degree is closely related to starch content. Maximum expansion is obtained with pure starches (an increase of 500% in product diameter), followed by whole grains (400%) and with lower expansions for seeds or germ (150-200%); the starch content of these products is 100, 65-78, 40-50 and 0-10, respectively. The minimum starch content for expansion is 60-70% (Riaz, 2000).

#### **3.3 Proteins**

Proteins are biopolymers with a great number of chemical groups when compared to polysaccharides and are therefore more reactive (Mitchel & Areas, 1992) and undergo many changes during the extrusion process, with the most important being denaturation (Camire, 2000). Proteins are formed from chains of amino acids and have a wide range of physical sizes and forms in native raw materials. Proteins in general are classified, with respect to their solubility, in albumins, globulins, prolamines and glutelins with solubility in water, saline solution, alcohol solution and acid or alkaline solutions, respectively (Pereda et al., 2005).

During extrusion, disulfide bonds are broken and may re-form. Electrostatic and hydrophobic interactions favour the formation of insoluble aggregates. The creation of new peptide bonds during extrusion is controversial. High molecular weight proteins can dissociate into smaller subunits (Guy, 2001).

Enzymes, also proteins, lose their activity after being submitted to the extrusion process due to high temperatures and shear. Also, proteins lose their solubility in water and saline

Thermoplastic Extrusion in Food Processing 275

reactions occur and possibly, some covalent bonds form at high temperatures (Areas, 1992). Thermal plasticization of the protein mix at high moisture contents (60%) is possible at relatively high extrusion temperatures (>150°C). At moisture levels lower than 60%, plasticization requires higher temperatures. Apart from this, hydrophobic and electrostatic interactions favour the formation of insoluble aggregates, like the fibrous structure of meat analogues, for example (Li & Lee, 1996; Tilley et al., 2000; Li-Chan, 2004; Sluimer, 2005).

Protein reactions, including both non-covalent and disulfide bonds, form upon cooling. Protein–protein interactions may be enhanced by decreased temperature and by macromolecular alignment. Crystalline aggregation leads to parallel fibre formation of varying length and thickness. A wide range of interaction energy is possible for protein cross-linking with protein and other molecules due to the diversity of amino acids. Therefore, hydrophobic, cation–mediated electrostatic interactions, and covalent bonds also contribute to the stabilization of the three dimensional network formed during extrusion

Also, during the extrusion process high temperatures are normally used, and these favour the Maillard reaction. Reducing sugars can be produced during the process and they can

Fats and oils can be described as lipids. Lipids have a powerful influence in extrusion cooking processes by acting as lubricants, because they reduce the friction between particles in the mix and between the screw and barrel surfaces and the fluid melt (Guy, 2001). In the extruder, fats and oils become liquid at temperatures > 40°C, being mixed with the other

The presence of lipids in quantities lower than 3% does not affect expansion properties, however, in amounts above 5%, reduction in expansion rate is considerable (Harper, 1994). Collona et al. (1998) suggest that the increase in lipid content can be corrected through the reduction in conditioning moisture content, so as not to affect the expansion index of second

The type of starch and lipid present in the raw material influences the formation of the amylose-lipid complex, with free fatty acids and monoglycerides being more favourable to the formation of this complex than triglycerides (Mitchel & Areas, 1992; Harper, 1994;

Moreover, in wet protein extrusion, the presence of lipids does not support protein fibre formation since the lubricating effect of lipids decreases the shear effects and particle

The term "fibres" covers a great variety of substances with different physical, chemical and physiological properties. Dietary fibre consists of fractions of vegetable cells, polysaccharides, lignin, and associated substances, which are resistant to hydrolysis by enzymes present in the digestive system of humans; however, some types of fibres may be

react with the free amine groups of lysine or other amino acids (Camire, 2000).

materials, and are rapidly dispersed as fine oil droplets.

generation products (directly expanded snacks).

(Areas, 1992).

**3.4 Lipids** 

Camire, 2000).

**3.5 Fibres** 

alignment (Akdogan, 1999).

solution due to the temperature and specific mechanical energy to which the product is submitted (Camire, 2000).

One of the main applications of extrusion in high protein content foods is protein texturization. Texturization processes by extrusion can be used to obtain products that imitate the texture, taste, and appearance of meat or seafood with high nutritional value (Cheftel et al., 1992).

The use of raw materials with high protein contents in extrusion began around the 1970s, with the use of soy for the production of texturized soy products and meat analogues (Ledward & Mitchell, 1988; Mitchell & Areas, 1992).

In extrusion, the proteins that have been found to form a continuous structure are globular proteins from oilseeds such as soybeans, sunflower seeds, common beans, peas and cottonseed and from cereals, especially wheat gluten proteins (Riaz, 2000; Strahm, 2006).

The extrusion process, physically, converts protein bodies into a homogeneous matrix, while chemically, the process recombines storage proteins in some way into structured fibres (Stanley, 1998). Low moisture (up to 35%) extrusion of vegetable protein can be used to elaborate products to partially or totally substitute meat. Usually, these products are expanded and need to be re-hydrated before consumption. On the other hand, high moisture (>50%) extrusion results in products that do not need to be re-hydrated and can be consumed directly. In general, dry extrusion is applied when the aim is to produce meat extenders and wet extrusion is used for meat analogues (Noguchi, 1998). In dry extrusion, when the conditioned material passes through the die at a high temperature, the water in the material is changed into superheated steam, which expands the extrudate immediately. Water also makes the extrudate very soft, by reducing its viscosity drastically, so the material just after the die is not self-supporting. Therefore, cooling at the die is essential to increase the viscosity of the hot melt and reduce its fluidity so that the necessary pressure and temperature before the die can be maintained. When cooling is done appropriately, the correct amount of extrudate elasticity and fluidity can be obtained to allow a continuous "rope" structure without explosive puffing and the destruction of product integrity (Noguchi, 1998). The mechanism for structure creation with proteins is similar to that with starch in the sense that proteins must be dispersed from their native bodies into a free flowing continuous mass. Texturization occurs between the molecules as they flow in the streamlines to form laminar cross-linked products. Evaporation of water in the mass creates gas bubbles that form alveolar structures held in place by cross-linking in the protein layers (Guy, 2001).

Denaturation during the extrusion process of proteins results in reduction of protein solubility, favours digestibility and inactivates antinutritional factors (such as antitrypsin factor, lectins, etc.). Also, the extrusion of soy protein reduces the bitter taste and the undesirable volatile compounds related to this protein (Areas, 1992; Kitabatake & Doi, 1992).

During extrusion, protein structures are disrupted and altered under high shear, pressure, and temperature (Harper, 1984). In the extrusion of proteins, disulfide bonds are cleaved and undergo reorganization and polymerization. Disulfide bonds, non-specific hydrophobic and electrostatic interactions are the main bonds and interactions responsible for protein texturization by extrusion (Areas, 1992). Protein solubility decreases and cross-linking reactions occur and possibly, some covalent bonds form at high temperatures (Areas, 1992). Thermal plasticization of the protein mix at high moisture contents (60%) is possible at relatively high extrusion temperatures (>150°C). At moisture levels lower than 60%, plasticization requires higher temperatures. Apart from this, hydrophobic and electrostatic interactions favour the formation of insoluble aggregates, like the fibrous structure of meat analogues, for example (Li & Lee, 1996; Tilley et al., 2000; Li-Chan, 2004; Sluimer, 2005).

Protein reactions, including both non-covalent and disulfide bonds, form upon cooling. Protein–protein interactions may be enhanced by decreased temperature and by macromolecular alignment. Crystalline aggregation leads to parallel fibre formation of varying length and thickness. A wide range of interaction energy is possible for protein cross-linking with protein and other molecules due to the diversity of amino acids. Therefore, hydrophobic, cation–mediated electrostatic interactions, and covalent bonds also contribute to the stabilization of the three dimensional network formed during extrusion (Areas, 1992).

Also, during the extrusion process high temperatures are normally used, and these favour the Maillard reaction. Reducing sugars can be produced during the process and they can react with the free amine groups of lysine or other amino acids (Camire, 2000).
