Changes in the solubility and SDS-PAGE profile of whey proteins during storage at different temperatures: A kinetic study

Whey is the soluble fraction of milk separated from the caseins during manufacture of rennet-coagulated cheese, (sweet whey) or use of acids (acid whey). Whey proteins are widely used in a variety of foods, primarily for their superior functional properties such as gelling and emulsification. Solubility is the major protein attribute affecting all functional properties. In this study, the effect of storage at different temperatures on sweet and acid whey proteins solubility and gel electrophoresis pattern was investigated. Acid whey was produced by adjusting the pH of pasteurized milk to 4.6 and sweet whey was produced by adding microbial rennet to the pasteurized milk held at 37°C for 1 h. Whey samples were stored at -18, 4, 25, 40 and 60°C. At different time intervals, samples were removed and used for protein solubility determination and SDS-PAGE. Results showed that the solubility of proteins of  both  whey  types   stored   at -18 and 60°C significantly decreased, such that at -18oC, 50 and 61% decrease for sweet and acid whey protein solubility, respectively, occurred after 6 weeks. These decreases were 44 and 55% for samples stored at 60°C for 5 days. The decrease in solubility at 4, 25 and 40°C was less drastic. At all temperatures, decrease in protein solubility followed a first order kinetic. The activation energies for loss of protein solubility were 6.3 to 6.4 kcal/mol for sweet and acid whey proteins. The SDS-PAGE patterns indicated significant denaturation and aggregation of major whey proteins during storage.   Key words: Whey protein, solubility, kinetics, SDS-PAGE, activation energy.