Physicochemical and Functional Properties

Among cereal and other legume species, soybean has the highest protein content (around 40%) and second highest fat content (about 20%). The protein is of very high quality because it contains all of the essential amino acids, thus making it very important for vegans. Other valuable components found in soybean include phospholipids, vitamins and minerals (Gopalan et al., 1989). Soybean is a good source of antioxidants such as lecithin and vitamin E. It is also rich in magnesium, which has important functions in the bones, heart and arteries. The approximate composition of full-fat soybean flour is 41.0% protein, 20.0% fat, 5.3% ash, 2.7-3.9% crude fibre and 25.0% carbohydrate (Mustakes, 1971; Kellor, 1971; Krishna et al., 2003), whereas that of de-fatted soybean flour is approximately 50.5-52.0% protein, 1.0-1.5% fat, 3.0-3.2% crude fibre and 5.7% ash. Michael and Alison

© CAB International 2010. The Soybean: Botany, Production and Uses

(2003) reported the approximate composition of low-fat soybean flour is 52.05% protein, 6.31% ash and 7.39% fat.

To utilize soybean ingredients effectively, food processors should have detailed information on the methods of preparation and processing of soybean products, because these affect the composition and functional properties of the component proteins. Protein solubility is closely related to the functional properties needed for bakery food application. Heat treatment, especially moist heat, rapidly insolubilizes soybean protein. The more dispersible types of soybean flours are used in bakery and cereal products, by adding them directly to dough. Enzyme-active soybean flour has a minimum water solubility of 70% (Pingle, 1974). The ability of protein to aid in the formation and stabilization of emulsions is critical in the preparation of meat sausages and cake batters. In general, the emulsifying capacity of soybean protein products is enhanced by rising solubility. Accordingly, soybean proteins progressively reduce interfacial tension as the concentration is increased (Kinsella, 1979).

Foaming - the capacity of proteins to build stable foams with gas by forming impervious protein films - is an important property in some food applications, including beverages, angel cakes and sponge cakes. The foaming properties of various soybean protein products have been studied and soybean isolates have been found to be superior to soybean flour and concentrates (Kinsella, 1979). Processing conditions can vary the amount of water that can be absorbed. Soybean proteins differ considerably from wheat proteins in their chemical composition, as well as in physical properties (such as their total lack of elasticity). Adding soybean proteins to wheat flour thus dilutes the gluten proteins and the starch. On the other hand, soybean proteins exhibit a strong binding power that provides some resistance to dough expansion. This can be partially overcome by increasing the amount of water used in dough making and by a longer proofing time. The binding power of soybean flour is closely related to its high water-absorption capacity, which in the case of the de-fatted product is equivalent to 110% by weight. With full-fat flour, however, no measurable increase in dough absorption results from normal usage levels of the soybean product (Pyler, 1988).

Gels are characterized by relatively high viscosity, plasticity and elasticity. The ability of a gel structure to provide a matrix to hold water, fat, flavour, sugar and other food additives is useful in variety of products, such as chicken or ham analogues made from textured soy protein and fibrous soybean protein. Soybean flour and concentrates form soft, fragile gels, whereas soy isolates form firm, hard and resilient gels. The general procedure for producing a soy protein gel involves heating the protein solution at 80-90°C for 30 min followed by cooling at 4°C (Kinsella, 1979). Heating reduces the gel-forming capacity of isolated soybean protein and at >100°C there is complete loss of gel structure (Shemer, 1974).

Water-holding capacity is a measure of trapped water that includes both bound and hydrodynamic water. It affects the texture, juiciness and taste of the product. Water-holding capacities of soybean flour, concentrate and isolate have been reported as 2.6, 2.75 and 6.25 g g-1 of solids, respectively (Kinsella, 1979). All soy protein concentrates, regardless of the process used, have certain fat-and water-holding characteristics. This ability of soy protein enhances the shelf life of bakery products.

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