Relation of Milk Serum Proteins and Milk Salts to the Effects of Heat Treatment on Rennet Clotting

Abstract

A study was made of the effects of heat at 85 or 90°C. for 30min. on the rennet coagulability of skimmilk and of artificial systems composed of caseinate centrifuged from skimmilk and dispersed in milk dialysate with and without added milk serum proteins. Such heat treatments of skimmilk cause an immediate prolongation of the rennet clotting time at 35°C. and a further prolongation (hysteresis) when the heated sample is held following heating. These effects occurred with the artificial systems only if they contained β-lactoglobulin. Furthermore, 0.2% β-lactoglobulin added to skimmilk enhanced the magnitude of both the immediate prolongation and the hysteresis. Thus, heat treatment appears to alter β-lactoglobulin or to cause it to complex with casein in such a way that it inhibits rennet clotting. The rate of action of rennet at 16°C. is slower on heated than on raw milk, hence the heat-induced reactions involving β-lactoglobulin appear to interfere with the primary (enzymatic) action of rennet.Heat treatment of eight samples of skimmilk at 85°C. for 30min. caused an average transfer of 6mg. of calcium and 4mg. of phosphorus per 100ml. from the dissolved to the colloidal state. The dissolved calcium and phosphate tended to return to their original concentrations when heated milk was held cold. Dialysis of heated milk against a large volume of raw milk increased the colloidal phosphate and partially restored the rennet coagulability. These changes in state of calcium and phosphate occurred in the absence of β-lactoglobulin and were not enhanced in magnitude by added (0.2%) β-lactoglobulin. Thus, they seem to be independent of that protein. The hysteresis phenomenon may result from loss of colloidal calcium phosphate from the caseinate particles on holding after heating.