Effect of heat denaturation of whey proteins on the rheological properties of cornstarch-milk systems
Heating cornstarch with milk that has received little or no previous heat treatment produces a two-step increase in Amylograph viscosity resulting in higher final viscosity after further heating. The second step disappears as increasing temperatures are used for preheating the milk. Amylograms of 6% slurries of cornstarch in the preheated milks were compared with gel electrophoresis patterns of the whey proteins remaining undenatured in the same preheated milk samples. As temperature of preheating was increased, smaller portions of each whey protein remained undenatured and the second increase in viscosity shown by the Amylograph became less. After all the whey proteins except (alpha)-lactalbumin and (beta)-lactoglobulin had been completely denatured by preheat treatment at 82(DEGREES)C, the second step in the viscosity curve no longer appeared but was replaced by a gradual viscosity increase. This increase was eliminated when the temperature of preheat treatment was raised to 95(DEGREES)C. Addition of more (alpha)-lactalbumin to milk preheated at 82(DEGREES)C failed to raise the viscosity, but addition of (beta)-lactoglobulin did increase viscosity. The Amylogram made with the addition of (beta)-lactoglobulin was similar to that obtained with raw milk or milk that had been preheated at only 74(DEGREES)C. Microscopy and measurement of the swelling power of the starch in the preheated milk samples indicated differences in viscosity were not caused by effects on swelling of the starch granules. Entanglement of protein molecules, especially (beta)-lactoglobulin, with starch molecules inside and outside the swollen granules as denaturation progresses is proposed as an explanation for the viscosity differences.