Proteins have an important role in the nutritional, structural, chemical and functional properties of food. The food proteins provide the essential amino acids, which are needed for growth and various metabolic pathways (Murad, 2007). Proteins are made up of carbon, hydrogen, nitrogen, oxygen and/ or sulfur (Barret, 1985).
Proteins present in dairy products have a significant impact on the flavor profile of the products. Milk proteins are used to improve the smoothness, whippability and texture of frozen dairy desserts (Damodaran, 1997). Milk contains six major gene products of the mammary gland, which are αs1-, αs2-, β-, and κ-caseins, β-lactoglobulin, and α-lactalbumin. Protease-peptones and γ-caseins are formed due to the presence of blood plasmin and plasminogen in milk ( Swaisgood, 1995). Rowland (1938) developed one of the first chemical methods to determine the distribution of nitrogen among five milk fractions. The method involved precipitation by saturation with magnesium sulfate and it was concluded that mature bovine milk had nitrogen distributed as follows: 78.3 % casein, 9.1 % albumin, 3.5 % globulin, 4.1 % protease peptone and 5 % non protein.
The αs1-, αs2-, β-, and κ-casein are the major casein proteins. Apart from dairy products like cheese and yogurt, isolated casein is used in a variety of non-dairy foods like beverages, dessert-types, bakery, pasta, confectionery and meat products, pharmaceutical and health care products (Mulvihill and Ennis, 2003). It was observed by Warner (1944), that casein could be precipitated from alkaline solution by acidification. Caseins exist in milk as polydisperse, almost spherical particles, called casein micelles. The caseins, as a whole, can be separated from the rest of the milk proteins by altering the pH of the solution to 4.6. At this pH, the solubility of casein is very low because it is the isoelectric point of the casein. Also, casein can be separated from other milk proteins by taking the advantage of the interaction of casein with calcium. The αs1-, αs2- and β-casein are the group of Ca-sensitive caseins and precipitate easily by mM concentrations of Ca2+. However, the κ-casein is not sensitive to Ca2+ and if it is present in sufficient amount, it will protect the other Ca-sensitive caseins against precipitation (Swaisgood, 1992).
When the caseins are separated from the milk by alteration of pH, the straw colored serum remaining is called whey. β-lactoglobulin and α-lactalbumin are the major whey proteins. The β-lactoglobulin accounts for 7-12% of total milk proteins and the ratio of α-lactalbumin to β-lactoglobulin in bovine milk is 1:3 (Kilara and Hilwalkar, 1996). Individual whey proteins can be isolated on a laboratory scale by salting out, ion exchange chromatography and/or crystallization. In 1983, Pearse developed a method which exploited the low heat stability of α-lactalbumin, to separate it from the other whey proteins. α-lactalbumin is a Ca-containing metalloprotein...