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Calorimetric Study of Thermal Denaturation of ß-Lactoglobulin

Department of Food Science and Technology, Seoul National University, Korea

Daryl B. Lund

Department of Food Science, University of Wisconsin — Madison, Madison 53706

ABSTRACT

Effects of pH and milk constituents (milk ultrafiltrate and -casein) on denaturation of ß-lactoglobulin were investigated by a dynamic method based on differential scanning calorimetry. The apparent reaction order of ß-lactoglobulin denaturation by the dynamic method was 2.0 over the pH range of 4.0 to 9.0, which is in fair agreement with results by other investigators using more classical methods. ß-Lactoglobulin was more stable in the neutral pH region 5.0 to 7.0. The activation energy for thermal denaturation of ß-lactoglobulin was not dependent on pH from 4.0 to 9.0. Activation energy was estimated to be about 125 kcal/mol. Heat of denaturation is larger for pH 5.0 to 8.0 (4.6 cal/g) than at pH 4.0 and 9.0 (4.1 cal/g). Lactose stabilized ß-lactoglobulin against thermal denaturation, and lactose-free milk ultrafiltrate reduced the thermal stability of ß-lactoglobulin. As compared to buffer at pH 6.6, activation energy of ß-lactoglobulin denaturation in milk ultrafiltrate was constant, but heat of denaturation was larger. The rate of thermal denaturation of ß-lactoglobulin increased as -casein content increased. However, activation energy remained constant, suggesting that the enhanced rate was entropy driven.