916 International Journal of Food Science and Technology 2007, 42, 916–929 Original article Nutritional aspects of food extrusion: a review Shivendra Singh, Shirani Gamlath* & Lara Wakeling School of Science & Engineering, Mount Helen Campus, University of Ballarat, Victoria 3353, Australia (Received 19 February 2006; Accepted in revised form 19 April 2006) Summary Extrusion cooking, as a multi-step, multi-functional and thermal/mechanical process, has permitted a large number of food applications. Effects of extrusion cooking on nutritional quality are ambiguous. Beneficial effects include destruction of antinutritional factors, gelatinisation 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. Heat-labile vitamins may be lost to varying extents. Changes in proteins and amino acid profile, carbohydrates, dietary fibre, vitamins, mineral content and some non-nutrient healthful components of food may be either beneficial or deleterious. The present paper reviews the mechanisms underlying these changes, as well as the influence of process variables and feed characteristics. Mild extrusion conditions (high moisture content, low residence time, low temperature) improve the nutritional quality, while high extrusion temperatures (P200 °C), low moisture contents (<15%) and/or improper formulation (e.g. presence of high-reactive sugars) can impair nutritional quality adversely. To obtain a nutritionally balanced extruded product, careful control of process parameters is essential. Keywords Carbohydrates, dietary fibre, extrusion, Maillard reaction, minerals, protein, vitamins. is able to break the covalent bonds in biopolymers, and Introduction the intense structural disruption and mixing facilitate Health and nutrition is the most demanding and the modification of functional properties of food ingre- challenging field in this era and would continue to be dients and/or texturizing them (Asp & Bjorck, 1989; in the future as well. Maintaining and increasing the Carvalho & Mitchelle, 2000). In addition, the extrusion nutritional quality of food during food processing is process denatures undesirable enzymes; inactivates some always a potentially important area for research. antinutritional factors (trypsin inhibitors, haemaggluti- Deterioration of nutritional quality, owing to high nins, tannins and phytates); sterilises the finished prod- temperature, is a challenging problem in most tradi- uct; and retains natural colours and flavours of foods tional cooking methods. Extrusion cooking is preferable (Fellows, 2000; Bhandari et al., 2001). to other food-processing techniques in terms of con- The process has found numerous applications, inclu- tinuous process with high productivity and significant ding increasing numbers of ready-to-eat cereals; salty nutrient retention, owing to the high temperature and and sweet snacks; co-extruded snacks; indirect expanded short time required (Guy, 2001). Extrusion cooking is a products; croutons for soups and salads; an expanding high-temperature, short-time process in which moist- array of dry pet foods and fish foods; textured meat-like ened, expansive, starchy and/or protenacious food materials from defatted high-protein flours; nutritious materials are plasticised and cooked in a tube by a precooked food mixtures for infant feeding; and con- combination of moisture, pressure, temperature and fectionery products (Harper, 1989; Eastman et al., mechanical shear, resulting in molecular transformation 2001). and chemical reactions (Havck & Huber, 1989; Castells Parallel to the increased applications, interest has et al., 2005). This technology has some unique positive grown in the physico-chemical, functional and nutri- features compared with other heat processes, because tionally relevant effects of extrusion processing. Preven- the material is subjected to intense mechanical shear. It tion or reduction of nutrient destruction, together with improvements in starch or protein digestibility, is clearly *Correspondent: Fax: +61353279240; of importance in most extrusion applications. Nutri- e-mail: [email protected] tional concern about extrusion cooking is reached at its doi:10.1111/j.1365-2621.2006.01309.x Ó 2007 The Authors. Journal compilation Ó 2007 Institute of Food Science and Technology Trust Fund Nutritional aspects of food extrusion S. Singh et al. 917 highest level when extrusion is used specifically to minant of protein quality in adults, according to FAO/ produce nutritionally balanced or enriched foods, like WHO/UNU (1985). Protein digestibility value of extru- weaning foods, dietetic foods, and meat replacers dates is higher than nonextruded products. The possible (Cheftel, 1986; Plahar et al., 2003). Many researchers cause might be the denaturation of proteins and inacti- have reported the positive and negative effects of the vation of antinutritional factors that impair digestion. extrusion process on the nutritional quality of food and The nutritional value in vegetable protein is usually feed mixtures using different extruder conditions (tem- enhanced by mild extrusion cooking conditions, owing perature, feed moisture, screw speed and screw confi- to an increase in digestibility (Srihara & Alexander, guration) and raw-material characteristics (composition, 1984; Hakansson et al., 1987; Colonna et al., 1989; particle size). Reviews of various chemical changes Areas, 1992). It is probably a result of protein denatur- affecting the nutritional quality of food during extrusion ation and inactivation of enzyme inhibitors present in cooking have been published by Cheftel (1986), Asp & raw plant foods, which might expose new sites for Bjorck (1989), Camire et al. (1990) and Areas (1992). enzyme attack (Colonna et al., 1989). All processing However, none of the publication offers a comprehen- variables have different effects in protein digestibility. sive review of all nutritional aspects. The findings are summarised in Table 1. The present paper reviews the updated and more Among the process variables, the feed ratio has the advanced mechanisms and new concepts about the maximum effect on protein digestibility, followed by nutritional changes during the extrusion process. The process temperature in the extrusion of fish–wheat flour effect on proteins and amino acid profile, carbohydrates, blend. Tripling the ratio of fish to wheat increases the dietary fibre, vitamins, mineral content and some non- digestibility of the extrudates by 2–4% (Bhattacharya nutrient healthful components of food are discussed. et al., 1988; Camire et al., 1990). Increase in extrusion This paper also indicates the gaps in the available temperature (100–140 °C) enhances the degree of inac- extrusion literature and some of the future opportunities tivation of protease inhibitors in wheat flour, and for research to make extrusion processes more efficient consequently, the protein digestibility values are in terms of retention of nutritional quality of food. increased. Extrusion, even at 140 °C, does not have any adverse effect on protein digestibility, which might be attributed to the lesser residence time of food dough Nutritional changes within the extruder. The effect of other process varia- bles, such as length to diameter ratio and screw speed on Protein protein digestibility values appears to be insignificant Every animal, including humans, must have an adequate (P ¼ 0.05) (Bhattacharya et al., 1988). Increased screw source of protein in order to grow or maintain itself. speed may have increased the protein digestibility of Proteins are a group of highly complex organic com- extruded corn-gluten, because the increase in shear pounds that are made up of a sequence of amino acids. forces in the extruder denatures the proteins more easily, Among the twenty-two amino acids that make up most thus facilitating enzyme hydrolysis (Bhattacharya & proteins, isoleucine, leucine, lysine, methionine, phenyl- Hanna, 1985). alanine, threonine, tryptophan, and valine are consid- An advantage of extrusion cooking is the destruction ered as essential amino acids. of antinutritional factors, especially trypsin inhibitors, Protein nutritional value is dependent on the quantity, haemagglutinins, tannins and phytates, all of which digestibility and availability of essential amino acids. inhibit protein digestibility (Bookwalter et al., 1971; Digestibility is considered as the most important deter- Lorenz & Jansen, 1980; Armour et al., 1998; Alonso Table 1 Effect of processing parameter on protein digestibility Processing parameter Protein digestibility Food source References Process temperature › with increasing Corn gluten–whey Fapojuwo et al. (1987), extrusion temperature blends, sorghum and Bhattacharya & Hanna (1985), fish–wheat blends Bhattacharya et al. (1988) Feed ratio › with increasing Fish and wheat flour Bhattacharya et al. (1988) & animal protein Camire et al. (1990) Screw speed Insignificant effect Fish and wheat flour Bhattacharya et al. (1988) › with increasing Corn gluten–whey blend Camire et al. (1990) screw speed Length to diameter ratio Insignificant effect Fish and wheat flour Bhattacharya et al. (1988) ›, increase. Ó 2007 The Authors. Journal compilation Ó 2007 Institute of Food Science and Technology Trust Fund International Journal of Food Science and Technology 2007 918 Nutritional aspects of food extrusion S. Singh et al. et al.,