Gel-Forming Characteristics of Milk Proteins. 1. Effect of Heat Treatment

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Abstract
Skim milk with or without preheating (60 to 80.degree. C for 30 min) were acid coagulated at 60 to 80.degree. C for 1 h with glucono-delta-lactone. Preheating below 70.degree. C has no effect on gel firmness and water-holding capacity. When coagulated below 70.degree. C, the gels were weak and had low water-holding capacity. When coagulated at 80.degree. C, the gels were solid and had high water-holding capacity. Gels prepared from skim milks preheated to above 80.degree. C had a different quality: when coagulated at less than 70.degree. C, gel firmness increased slightly, and when coagulated at 80.degree. C, gel firmness decreased sharply. Change in the accessibility of sulfhydryl groups in milk protein caused by heating, was also measured using Ellman''s reagent. Changes in the gel-forming property of milk protein, caused by the heat treatment, were closely related to increase in available sulfhydryl groups in milk proteins, and also were related to heat denaturation of whey protein or the formation of .beta.-lactoglobulin/.kappa.-casein complex.