Kinetics of changes in plasmin activity and proteolysis on heating milk.
Heat-induced inactivation of bovine plasmin, denaturation of beta-lactoglobulin (beta-lg), the interactions between both species and casein micelles and the subsequent net effect on proteolysis of beta-casein was studied in a model system consisting of phosphocasein and beta-lg in synthetic milk ultrafiltrate. The inactivation of plasmin and denaturation of beta-lg were first order reactions, with the rate of inactivation of plasmin being greater than the rate of denaturation of beta-lg. The predominant mechanism involved in the denaturation of plasmin in the temperature range 65-80 degrees C was its interaction with beta-lg (kr at 60 degrees C, 0.0526; Ea, 176 KJ/mol). At the point of complete inactivation of plasmin approximately 45% of the beta-lg remained undenatured. Thermal inactivation of plasmin through other mechanisms was negligible. The association of beta-lg with the casein micelles at 60 degrees C had a rate constant of 3.71 x 10-5 min-1 and an Ea of 259 KJ/mol; thermal denaturation of beta-lg was of much less importance, with a rate constant at 60 degrees C of the order of 1 x 10-10 min-1 and an Ea of 250 KJ/mol. On denaturation of all beta-lg in the system, a maximum of approximately 55% was associated with the casein micelles. The effect of heating on the subsequent hydrolysis of beta-casein indicated that the level of plasmin activity was the most important factor affecting proteolysis, while the interaction of beta-lg with the casein micelles had limited effect. Overall, thermal stability of plasmin in milk is very much dependent upon its interaction with beta-lg.
Crudden
Anthony
A
Department of Food and Nutritional Sciences, University College Cork, Ireland. a.kelly@ucc.ie
Oliveira
Jorge C
JC
Kelly
Alan L
AL
D000818:Animals; D002364:Caseins / Q000032:analysis / Q000378:metabolism; D002417:Cattle; D005260:Female; D005341:Fibrinolysin / Q000378:metabolism*; D006358:Hot Temperature; D007700:Kinetics; D007782:Lactoglobulins / Q000032:analysis*; D008433:Mathematics; D008823:Micelles; D008892:Milk / Q000737:chemistry*; D008894:Milk Proteins / Q000032:analysis* / Q000378:metabolism; D011489:Protein Denaturation
D002364:Caseins; D007782:Lactoglobulins; D008823:Micelles; D008894:Milk Proteins; D005341:Fibrinolysin