The effects of heating, alone or in the presence of calcium or lactose, on calcium binding to milk proteins

Abstract

The binding of calcium by different milk proteins and the effects of heat were investigated. The influences of calcium and lactose were also investigated. It was shown that the caseins of cow's milk have a high calcium binding capacity, when treated with calcium at 25°C for 30 min, pH 7·00, due to the high number of calcium binding sites. The order of binding capacities is α s1 -casein > β-casein > κ-casein. For whey proteins the order is β-lactoglobulin (β-lg) > α-lactalbumin (α-la). Lactose, in general, when present at 25°C, did not appreciably affect the calcium binding of the major milk proteins. Heating of whole casein at 80 or 95°C for 30 min caused conformational changes of the protein resulting in an increase in its calcium binding ability. The presence of lactose during heating with or without calcium reduced its calcium binding. The calcium binding of α s1 -casein was not affected by heating, throughout the temperature range studied, either in the presence or in the absence of calcium, an indication that α s1 -casein may undergo physical changes at high temperatures, but the accessibility and/or the number of calcium binding centres are not increased. β-Casein, when heated with calcium, was subject to conformational changes which were reflected in changes in the degree of calcium-binding. Heating of β-lg in the presence of calcium did not much affect the amount of calcium bound to it. However, in the absence of calcium an increase of 21·5% in calcium binding was observed after heating at 80°C and a decrease of 40·0% after heating at 95°C, compared to unheated samples.