Thermal Stability of Whey Proteins – A Calorimetric Study

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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 95.degree. C 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 88.degree. C at pH 3.5, while for an isolated .beta.-lactoglobulin preparation the highest denaturation temperature, obtained also at pH 3.5, was 81.9.degree. C. Presence of milk sugars (lactose, glucose, and galactose) appeared to increase the resistance of .beta.-lactoglobulin to thermal denaturation. Heat stability of .alpha.-lactalbumin was lower at pH 3.5 than in the pH 6.5 to 4.5 range; at all pH denaturation temperatures of .alpha.-lactalbumin (61.5 to 58.6.degree. C) were lower than those for .beta.-lactoglobulin 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 essentially fatty acid free preparation. Thermal behavior of an ultrafiltration whey protein concentrate appears to be controlled by the dominating .beta.-lactoglobulin fraction.