Effect of protein, nonprotein-soluble components, and lactose concentrations on the irreversible thermal denaturation of β-lactoglobulin and α-lactalbumin in skim milk

The effect of protein, nonprotein-soluble components, and lactose concentrations on the irreversible denaturation of beta-lactoglobulin (beta-LG) and alpha-lactalbumin (alpha-LA) in reconstituted skim milk samples was studied over a wide temperature range (75- 100 degrees C). The irreversible thermal denaturation of beta-LG had a reaction order of 1.5 and that of alpha-LA had a reaction order of 1.0 in all systems and under all conditions. The rates of irreversible denaturation of beta-LG and alpha-LA were markedly dependent upon the composition of the milk. At all temperatures, the irreversible denaturations of beta-LG and alpha-LA were enhanced at a higher protein concentration and were retarded when the nonprotein-soluble components and lactose concentrations were increased. The effects of increasing the concentrations of lactose and nonprotein-soluble components were interpreted using the preferential hydration theory and allowed for the interpretation of the changes in the denaturations of beta-LG and alpha-LA when the milk total solids concentration was increased.