Kinetics of changes in plasmin activity and proteolysis on heating milk

摘要

Heat-induced inactivation of bovine plasmin, denaturation of β-lactoglobulin (β-lg), the interactions between both species and casein micelles and the subsequent net effect on proteolysis of β-casein was studied in a model system consisting of phosphocasein and β-lg in synthetic milk ultrafiltrate. The inactivation of plasmin and denaturation of β-lg were first order reactions, with the rate of inactivation of plasmin being greater than the rate of denaturation of β-lg. The predominant mechanism involved in the denaturation of plasmin in the temperature range 65-80℃ was its interaction with β-lg (k~r at 60℃, 0.0526; E_a, 176 KJ/mol). At the point of complete inactivation of plasmin ～45% of the β-lg remained undenatured. Thermal inactivation of plasmin through other mechanisms was negligible. The association of β-lg with the casein micelles at 60℃ had a rate constant of 3.71 x 10~(-5) min~(-1) and an E_a of 259 KJ/mol; thermal denaturation of β-lg was of much less importance, with a rate constant at 60℃ of the order of 1 x 10~(-10) min~(-1) and an E_a of 250 KJ/mol. On denaturation of all β-lg in the system, a maximum of ～55% was associated with the casein micelles. The effect of heating on the subsequent hydrolysis of β-casein indicated that the level of plasmin activity was the most important factor affecting proteolysis, while the interaction of β-lg with the casein micelles had limited effect. Overall, thermal stability of plasmin in milk is very much dependent upon its interaction with β-lg.