Calorimetric Study of Thermal Denaturation of β-Lactoglobulin

Abstract

Effects of pH and milk constituents (milk ultrafiltrate and .kappa.-casein) on denaturation on .beta.-lactoglobulin were investigated by a dynamic method based on differential scanning calorimetry. The apparent reaction order of .beta.-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. .beta.-Lactoglobulin was more stable in the neutral pH region 5.0 to 7.0. The activation energy for thermal denaturation of .beta.-lactoglobulin was not dependent on pH from 4.0 to 9.0. Activation energy was estimated to be .apprx. 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 .beta.-lactoglobulin against thermal denaturation and lactose-free milk ultrafiltrate reduced the thermal stability of .beta.-lactoglobulin. As compared to buffer in pH 6.6, activation energy of .beta.-lactoglobulin denaturation in milk ultrafiltrate was constant, but heat of denaturation was larger. The rate of thermal denaturation of .beta.-lactoglobulin increased as .kappa.-casein content increased. However, activation energy remained constant, suggesting that the enhanced rate was entropy driven.