Effect of pH on the thermal denaturation of whey proteins in milk.

The effect of pH on thermal denaturation of four main whey protein fractions in skim milk was examined by gel permeation FPLC. On heating skim milk at 80 degrees C for 0.5-20.0 min over the pH range 5.2-8.8, the extent of denaturation, based on loss of solubility at pH 4.6, increased with heating time and was usually in the order immunoglobulins > serum albumin/lactoferrin > beta-lactoglobulin > alpha-lactalbumin. Rates of denaturation of the immunoglobulins and the serum albumin/lactoferrin fraction were highest at the lower end of this pH range, whereas those of beta-lactoglobulin and alpha-lactalbumin increased over most of the pH range. The effects of pH, addition of Ca, and reduction of disulfide bonds on the rates of the unfolding and aggregation stages of denaturation are discussed.