Foods are complex systems composed of different components, among which proteins play important roles in the structure, texture, and stability of many foods (Hemar and others 2001). Protein isolates and their concentrated products are commonly and widely used in the food industry for both their added functionality and their nutritive contributions. The functionality of ingredients is important in preparation, processing, storage, quality and sensory attributes of foods (Culbertson 2007). Aside from these functions, proteins also increase solubility, water-holding capacity, elasticity, gelation, emulsification and foaming (Smith 2003). Protein isolates are generally about 90-95% protein while protein concentrates are about 65 to 70% protein (Thompson and Dinh 2010). The protein concentrates or isolates are added to foodstuffs that lack the inherent properties mentioned above (solubility, gelation, emulsification and foaming). Such foodstuffs include: extruded foods, textured meat products, whipped toppings, and protein-rich drinks (Kinsella, 1994).
The use of a protein concentrate or isolate in a food source is dependent on its functional properties which are influenced by the biochemical nature of the protein and the method of extraction and purification used for its derivation. The structure of the protein is of extreme importance in its ability to function in food system. The amino acid sequence of a protein determines the protein folding and three dimensional (3D) appearances. Additionally, the quaternary structure, defined by the number of subunits and the formation of oligometric unit, along with the 3D structure is very important to the overall functionality of a protein concentrate or isolates (Culbertson 2007; Kinsella 1994).
Soy protein is the most economical source of high nutritional quality protein and therefore is the principal commercially available vegetable protein in the world (Fukushima 2001). Soybean proteins are used in human foods in a variety of forms, including infant formulas, flours, protein concentrates and isolates, and textured fibers. (Liu and Singh 2000). Conventional processing of soy and its products involve heating. It is well known that the thermal treatments induce dissociation, denaturation, and aggregation of soy protein (Kwok and Niranjan 1995). During processing, food products containing soy protein isolates are subjected to various thermal treatments to inactivate anti-nutritional factors, remove allergenic determinants, increase digestibility and to obtain desired functionality, such as solubility or textural properties (Mills and others 2001).
Whey is a byproduct of cheese and casein manufacture, and contains approximately 20% of the original milk protein (McIntosh and others 1998). Like soy proteins, whey proteins are often used to improve food products because of their high nutritional quality and their versatile functional properties. However, the behavior of whey...