**3.4 Lipids**

274 Thermoplastic Elastomers

solution due to the temperature and specific mechanical energy to which the product is

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

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

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

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

submitted (Camire, 2000).

(Ledward & Mitchell, 1988; Mitchell & Areas, 1992).

(Cheftel et al., 1992).

(Guy, 2001).

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 materials, and are rapidly dispersed as fine oil droplets.

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 generation products (directly expanded snacks).

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; Camire, 2000).

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 alignment (Akdogan, 1999).

#### **3.5 Fibres**

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

Thermoplastic Extrusion in Food Processing 277

increase in extrusion temperature; however, the effect of moisture is more significant

Water acts as a plasticizer for the starchy material that displaces itself within the extruder, reducing viscosity and mechanical energy, producing higher density products and inhibiting bubble growth. Studies carried out with corn grits demonstrated that expansion is inversely proportional to the moisture content of the material being extruded (Chinnaswamy, 1993; Colonna et al., 1998). With higher moisture, starch gelatinization is reduced and bubble growth is retarded, resulting in denser and less crunchy final products

Inside the extruder, the product that contains molten starch in its composition, when leaving the extruder, has part of its water rapidly evaporated. This water loss is of 3 to 5% and contributes to cooling the product. Subsequent cooling occurs more slowly due to the low thermal conductivity of the extrudate. Also, when emerging from the die, the extrudate undergoes an abrupt pressure fall that also contributes to its expansion (Colonna et al., 1998). The expanded final product presents air cells that are formed due to superheated water vapour pressure. As the temperature of the extrudate is reduced below its glass transition temperature (Tg), it solidifies and maintains its expanded form (Riaz, 2000).

In high moisture extrudates, expansion occurs when the product exits the die, but the structure collapses before the necessary cooling, resulting in a dense and hard product

Another important parameter for extrudate expansion is process temperature. Products do not expand if the temperature does not reach 100°C. Expansion increases with the increase in temperature when moisture content of the material is close to 20%, due to lower viscosity, permitting a more rapid expansion of the molten mass, or due to an increase in water vapour pressure. At low extrusion temperatures, expansion is reduced because starch is not completely molten. Radial expansion degree is proportional to temperature up to a certain value, decreasing at much higher temperatures. The reduction of expansion at very high temperatures is attributed to an increase in dextrinization, weakening starch structure

At high temperatures, the gel is more elastic, forming a matrix with small uniform cells, while at low moisture, the gel formed is not very elastic and the extruded material has large, not very uniform cells. It is expected that an increase in temperature should reduce viscosity of the molten material, favouring bubble growth and producing low density extrudates,

In high moisture extrusion, the properties of the protein extrudates are strongly influenced by extruder conditions (Thiebaud et al., 1996; Noguchi, 1998). Hayashi et al. (1991) reported that extruder barrel temperature was the most important parameter for the texturization of dehulled soybean. Melt temperature is a critical factor in protein cross-linking reactions. Increasing temperature from 140 to 180°C results in a proportional decrease in disulfide linkages formed in extruded soy protein isolates (Areas, 1992). Temperatures lower than 90°C hinder expansion and layer formation (Cheftel et al., 1992). At a given temperature, higher moisture contents result in softer and less texturized extrudates due to reduced

with finer cells and greater crunchiness (Ding et al., 2005).

(Harper, 1994).

(Ding et al., 2005).

(Harper, 1994).

(Colonna et al., 1998).

fermented by bacteria in the colon. As many physiological effects of fibres seem to be related to their solubility in water, they are frequently classified as "soluble" and "insoluble" (Roberfroid, 1993; Stark & Madar, 1994).

Soluble fibres form a gel network or a viscous network, in determined physicochemical conditions, and thus bond water increasing viscosity, retarding gastric transit, reducing glucose, lipid and sterol absorption rates (Gorinstein et al., 2001). Soluble fibres are also seen as fermentable substrates, as they can modify the pH and the microflora of the colon, leading to a reduction or modification of mutagenic agents (Thebaudin et al., 1997). Insoluble fibres increase faecal volume, thus diluting its contents, which reduces the interaction between the intestinal mucosa and any carcinogenic component present. Apart from this, insoluble fibres reduce intestinal transit time, avoiding that mutagenic agents in the faeces interact with the intestinal epithelium (Thebaudin et al., 1997).

Research has shown that cooking fibres by extrusion can produce changes in their structural characteristics and physicochemical properties, with the main effect being a redistribution of insoluble fibre to soluble fibre (Camire et al., 1990; Guillon et al., 1992; Larrea et al., 2005). This effect would be the result of the rupture of covalent and non-covalent bonds between carbohydrates and proteins associated to the fibre, resulting in smaller molecular fragments, that would be more soluble (Fornal et al., 1987; Wang et al., 1993).

Various researchers have reported a reduction in expansion index (EI) when dietary fibre is added to the formulation (Hsieh et al., 1989; Ilo et al., 1999; Vernaza et al., 2009). The reduction in the expansion index due to fibre addition can be explained through different mechanisms: (i) fibrous materials found in the formulation of extruded products include materials composed of hemicellulose, cellulose and lignin. In normal extrusion conditions, these materials tend to remain firm and stable during processing, without size reduction. The physical presence of fibres in air cell walls reduces the expansion potential of the starchy film (Guy, 2001); larger particles, such as bran, tend to rupture air cell walls of the extruded product, causing a reduction in expansion index (Riaz, 2000); (ii) according to Colonna et al. (1998), maximum degree of expansion is closely related to starch content, with maximum expansion being obtained for pure starches. As bran contains high fibre content, it reduces the starch content of the formulations; (iii) non-starch polysaccharides, such as fibres, may bind water more strongly than proteins and starch during extrusion. This water binding capacity inhibits water loss at the die, that is, at the exit of the extruder, reducing expansion (Camire & King, 1991); (iv) the starch present cannot be totally gelatinized in the presence of fibre and is thus not capable of supporting expansion (Camire & King, 1991); and (v) the porous structure of the extrudate depends of the plasticity of the mass before the die, for which starch is mainly responsible. Porosity, defined by the existence of fine pores and a tender structure, is influenced by alterations in the plasticity of the mass, affected by the composition of the mix. Formulations can be enriched by plasticizing substances or by non-plasticizing substances that retard expansion by diluting starch, as is the case of fibres (Colonna et al., 1998).

#### **3.6 Moisture and temperature**

In the extrusion process of expanded products with low moisture, the expansion of the final product is inversely related to the moisture of the raw material and directly related to the

fermented by bacteria in the colon. As many physiological effects of fibres seem to be related to their solubility in water, they are frequently classified as "soluble" and "insoluble"

Soluble fibres form a gel network or a viscous network, in determined physicochemical conditions, and thus bond water increasing viscosity, retarding gastric transit, reducing glucose, lipid and sterol absorption rates (Gorinstein et al., 2001). Soluble fibres are also seen as fermentable substrates, as they can modify the pH and the microflora of the colon, leading to a reduction or modification of mutagenic agents (Thebaudin et al., 1997). Insoluble fibres increase faecal volume, thus diluting its contents, which reduces the interaction between the intestinal mucosa and any carcinogenic component present. Apart from this, insoluble fibres reduce intestinal transit time, avoiding that mutagenic agents in

Research has shown that cooking fibres by extrusion can produce changes in their structural characteristics and physicochemical properties, with the main effect being a redistribution of insoluble fibre to soluble fibre (Camire et al., 1990; Guillon et al., 1992; Larrea et al., 2005). This effect would be the result of the rupture of covalent and non-covalent bonds between carbohydrates and proteins associated to the fibre, resulting in smaller molecular fragments,

Various researchers have reported a reduction in expansion index (EI) when dietary fibre is added to the formulation (Hsieh et al., 1989; Ilo et al., 1999; Vernaza et al., 2009). The reduction in the expansion index due to fibre addition can be explained through different mechanisms: (i) fibrous materials found in the formulation of extruded products include materials composed of hemicellulose, cellulose and lignin. In normal extrusion conditions, these materials tend to remain firm and stable during processing, without size reduction. The physical presence of fibres in air cell walls reduces the expansion potential of the starchy film (Guy, 2001); larger particles, such as bran, tend to rupture air cell walls of the extruded product, causing a reduction in expansion index (Riaz, 2000); (ii) according to Colonna et al. (1998), maximum degree of expansion is closely related to starch content, with maximum expansion being obtained for pure starches. As bran contains high fibre content, it reduces the starch content of the formulations; (iii) non-starch polysaccharides, such as fibres, may bind water more strongly than proteins and starch during extrusion. This water binding capacity inhibits water loss at the die, that is, at the exit of the extruder, reducing expansion (Camire & King, 1991); (iv) the starch present cannot be totally gelatinized in the presence of fibre and is thus not capable of supporting expansion (Camire & King, 1991); and (v) the porous structure of the extrudate depends of the plasticity of the mass before the die, for which starch is mainly responsible. Porosity, defined by the existence of fine pores and a tender structure, is influenced by alterations in the plasticity of the mass, affected by the composition of the mix. Formulations can be enriched by plasticizing substances or by non-plasticizing substances that retard expansion by diluting starch, as is the case of fibres

In the extrusion process of expanded products with low moisture, the expansion of the final product is inversely related to the moisture of the raw material and directly related to the

the faeces interact with the intestinal epithelium (Thebaudin et al., 1997).

that would be more soluble (Fornal et al., 1987; Wang et al., 1993).

(Roberfroid, 1993; Stark & Madar, 1994).

(Colonna et al., 1998).

**3.6 Moisture and temperature** 

increase in extrusion temperature; however, the effect of moisture is more significant (Harper, 1994).

Water acts as a plasticizer for the starchy material that displaces itself within the extruder, reducing viscosity and mechanical energy, producing higher density products and inhibiting bubble growth. Studies carried out with corn grits demonstrated that expansion is inversely proportional to the moisture content of the material being extruded (Chinnaswamy, 1993; Colonna et al., 1998). With higher moisture, starch gelatinization is reduced and bubble growth is retarded, resulting in denser and less crunchy final products (Ding et al., 2005).

Inside the extruder, the product that contains molten starch in its composition, when leaving the extruder, has part of its water rapidly evaporated. This water loss is of 3 to 5% and contributes to cooling the product. Subsequent cooling occurs more slowly due to the low thermal conductivity of the extrudate. Also, when emerging from the die, the extrudate undergoes an abrupt pressure fall that also contributes to its expansion (Colonna et al., 1998). The expanded final product presents air cells that are formed due to superheated water vapour pressure. As the temperature of the extrudate is reduced below its glass transition temperature (Tg), it solidifies and maintains its expanded form (Riaz, 2000).

In high moisture extrudates, expansion occurs when the product exits the die, but the structure collapses before the necessary cooling, resulting in a dense and hard product (Harper, 1994).

Another important parameter for extrudate expansion is process temperature. Products do not expand if the temperature does not reach 100°C. Expansion increases with the increase in temperature when moisture content of the material is close to 20%, due to lower viscosity, permitting a more rapid expansion of the molten mass, or due to an increase in water vapour pressure. At low extrusion temperatures, expansion is reduced because starch is not completely molten. Radial expansion degree is proportional to temperature up to a certain value, decreasing at much higher temperatures. The reduction of expansion at very high temperatures is attributed to an increase in dextrinization, weakening starch structure (Colonna et al., 1998).

At high temperatures, the gel is more elastic, forming a matrix with small uniform cells, while at low moisture, the gel formed is not very elastic and the extruded material has large, not very uniform cells. It is expected that an increase in temperature should reduce viscosity of the molten material, favouring bubble growth and producing low density extrudates, with finer cells and greater crunchiness (Ding et al., 2005).

In high moisture extrusion, the properties of the protein extrudates are strongly influenced by extruder conditions (Thiebaud et al., 1996; Noguchi, 1998). Hayashi et al. (1991) reported that extruder barrel temperature was the most important parameter for the texturization of dehulled soybean. Melt temperature is a critical factor in protein cross-linking reactions. Increasing temperature from 140 to 180°C results in a proportional decrease in disulfide linkages formed in extruded soy protein isolates (Areas, 1992). Temperatures lower than 90°C hinder expansion and layer formation (Cheftel et al., 1992). At a given temperature, higher moisture contents result in softer and less texturized extrudates due to reduced

Thermoplastic Extrusion in Food Processing 279

The effects of extrusion cooking on nutritional quality are ambiguous. Benefits include destruction of antinutritional factors, gelatinization of starch, increased soluble dietary fibre and reduction of lipid oxidation. On the other hand, Maillard reactions between protein and sugars reduce the nutritional value of the protein, depending on the raw material types, their composition and process conditions. Besides, heat-labile vitamins may be lost to

Starch digestibility is largely dependent on complete gelatinization. High starch digestibility is essential for specialized nutritional foods such as infant and weaning foods. Creation of resistant starch by extrusion may have value in reduced calorie products (Guy, 2001;

The nutritional value of vegetable proteins is generally enhanced by mild extrusion cooking conditions due to the increase in digestibility (Asp and Björck 1989; Arêas, 1992), probably a result of protein denaturation and the inactivation of enzyme inhibitors present in raw materials, by the exposure of new active sites for enzyme attack (Colonna et al., 1989).

Processing nutritional food products at moisture levels below 20% has been proven to be uneconomical and nutritionally undesirable. Low-moisture extrusion results in production of certain undesirable dextrins as a result of increased shear energy inputs. Losses of vitamins and reduced amino acid availability are greatly accelerated as extrusion moistures are decreased. For this reason, vitamins and heat-sensitive nutrients are usually added post

Mild extrusion conditions (high moisture content, low residence time, low temperature) improve nutritional quality, while high extrusion temperatures (higher than 200°C), low moisture contents (lower than 15%) and/or improper formulation (e.g. presence of highreactive sugars) can affect nutritional quality adversely. Also, to obtain a nutritionally balanced extruded product, careful control of process parameters is essential (Singh et al.,

A benefit derived from extrusion-cooking is the partial or total destruction of potentially antinutritional factors, especially protease inhibitors, haemagglutinins, tannins and phytates, which limit utilization of nutrients in legume seeds. However, chemical alteration produced by thermal treatment could also result in decreased nutrient assimilation,

Vitamin losses in extruded foods vary according to the type of food, moisture content, processing temperature and retention time. Generally, losses are minimal in cold extrusion. The HTST conditions in extrusion cooking, the short residence time of the extrudate and the rapid cooling as the product emerges from the die, cause relatively small losses of vitamins

Extrusion cooking was reported by Saalia & Phillips (2011) as an efficient process to destroy or inactivate aflatoxins, if special conditions (high shear, high temperature, and adequate

Zhu et al. (1996) emphasized that by extrusion cooking, soybeans can be converted into high quality food ingredients. The short residence time and high temperature in an extruder

including lower apparent absorption of certain minerals (Alonso et al., 2001).

and essential amino acids (Fellows, 2000).

extrusion when processing at low moisture conditions (Huber, 2001).

**5. Influence on nutritional quality** 

varying extents (Singh et al., 2007).

Riaz, 2000).

2007).

pH) are used.

protein–protein interactions and lower viscosity. At relatively lower moisture contents, higher barrel temperatures (140 to 180°C) result in better textures. At higher moisture levels, temperature needs to be decreased as moisture flash-off may cause considerable water loss if a cooling die is not used (Thiebaud et al., 1996).

High moisture levels combined with elevated temperatures yield extrudates that are very soft and not self-supporting after the die. However, a specially designed die which provides cooling at this section will increase the viscosity of the hot extrudate before exiting, contributing to the correct elasticity and fluidity required for texturization (Noguchi, 1998). The temperature at which solidification occurs is related to the plasticization temperature.

Low moisture (15 to 30%) extrusion tends to result in processes with greater generation of mechanical energy and products with lower density, while high moisture (50 to 70%) extrusion results in products with higher density, and is normally used in pellet production (Guy, 2001).
