Thermal Stability of Whey Proteins – A Calorimetric Study

Abstract

Several factors affecting thermal stability of the proteins present in whey were studied over 2.5 to 6.5 pH range using differential scanning calorimetry and heat-induced precipitation. Heating whey at 95C for 5 min above pH 3.8 to 3.9 produced extensive protein coagulation. When the same heat treatment was applied below pH 3.7, protein precipitation was prevented; however, protein denaturation still occurred in various whey protein fractions. The highest denaturation temperature for an acid whey protein concentrate prepared by ultrafiltration was 88C at pH 3.5, while for an isolated 13-1actoglobulin preparation the highest denaturation temperature, obtained also at pH 3.5, was 81.9C. Presence of milk sugars (lactose, glucose, and galactose) appeared to increase the resistance of 13-1actoglobulin to thermal denaturation. Heat stability of a-lactalbumin was lower at pH 3.5 than in the pH 6.5 to 4.5 range; at all pH denaturation temperatures of a-lactalbumin (61.5 to 58.6C) were lower than those for/3-1actoglobulin or two serum albumin preparations. Sample of serum albumin containing 1.0 to 1.3 mol of fatty acids/mol of albumin showed substantially higher denaturation temperatures than an essentially fatty acid free preparation. Thermal behavior of an ultrafiltration whey protein concentrate appears to be controlled by the dominating /3-1actoglobulin fraction.