Effect of temperature and pH on the interactions of whey proteins with casein micelles in skim milk

Skim milk was heated at temperatures in the range 75-90 degrees C, at pH values of 6.8, 6.2 and 5.8. The amounts of alpha-lactalbumin and beta-lactoglobulin which interacted with the casein micelles during heat treatment were quantified by SDS-polyacrylamide gel electrophoresis of the micellar fractions isolated by ultracentrifugation. Both alpha-lactalbumin and beta-lactoglobulin appeared to interact similarly with casein micelles at temperatures up to 85 degrees C. The amount of whey protein complexed with micelles increased with time, reaching plateau values that, at the highest temperatures, were comparable with the quantity present in the original skim milk. In general, faster reaction of the whey proteins with the micelles was found at lower pH and higher temperatures. The rates and extent of the reaction changed also when additional alpha-lactalbumin and beta-lactoglobulin isolates were added to milk before heating. The reaction between alpha-lactalbumin and casein micelles depended to a relatively small extent upon environmental variations (pH and temperature), while beta-lactoglobulin interactions were more affected, so that a more complex behaviour may be attributed to the latter protein. Copyright (C) 1996 Canadian Institute of Food Science and Technology