A Quantitative Evaluation of Changes Occurring during Heat Treatment of Skimmilk at Temperatures Ranging from 170° to 300° F

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Abstract
Skim milk was heated in a high-temperature-short-time tubular heater (which is described). Sixty samples were secured representing holding times of 1, 6, 16, 30, 64, and 96 seconds at 180[degree], 190[degree], 200[degree], 210[degree], 220[degree], 230[degree], 240[degree], 260[degree], 280[degree], and 290[degree]F. Additional samples were heated at 300[degree]F for some holding times and further samples were heated in stainless steel test tubes at 170[degree] and 185[degree]F for 5, 10, 15, 20 and 30 minutes. Samples were analyzed for serum protein denaturation by the Harland-Ashworth method, total and free sulfhydryls, lactose-protein interaction products and baking quality. Plots of log time vs. temperature required for a given extent of denaturation of serum proteins were curvilinear with "z" values of 13.5 in the range 145-175[degree]F and 19 in the range 175-195[degree]F. Heat treatments for production of near maximum free sulfhydryl groups with a minimum of lactose-protein interaction were defined. The loaf vol. depressant factor in skim milk can be inactivated by high-temperature-short-time heat treatment. Although inactivation of this factor is not strictly proportional to serum protein denaturation, a specification of 80-85% serum protein denaturation should insure adequate baking quality.