Heat Inactivation of Milk Phosphatase in Dairy Products

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Abstract
The times and temps, required to inactivate the phosphatase normally present in milk and its products were detd. in expts. with a laboratory pasteurizer, comprising a metal coil 30 ft. long and 1/8 inch in internal diam., with a metal holding chamber at the end, immersed in a water bath with the temp. controlled thermostatically. Phosphatase-test data for samples heated at any definite temp. for various periods of time show that the destruction of the enzyme by heat is rapid at first and diminishes to a slower rate with time. The exptl. data for phosphatase destruction show that, in tests on milk and other fluid dairy products, a straight line results when the logarithms of the times of heating are plotted against the corresponding temps. Holding periods required to reduce phosphatase activity to 4 units ([mu]g.) per ml. of whole milk, in this test, were: 37.5 min. at 143[degree]F, 30 min. at 143.7[degree], 24 sec. at 160[degree], and 15 sec. at 161.8[degree], respectively. The temp, required to produce a negative test in any given time was generally about 0.7[degree] lower for skim milk than for whole milk, about 0.7[degree] higher for 20 and 40% cream than for whole milk, about 4.5[degree] higher for ice-cream mix than for whole milk, and about 5.7[degree] higher for sherbet than for whole milk. The time required at 143[degree] F was about- 3 times as long for ice-cream mix as for whole milk. Phosphatase was in-activated at considerably lower temps, and shorter holding times in Cheddar cheese than in milk[long dash]e.g., at 130[degree]F in 13 min. and at 140[degree] in slightly less than 3/4 min. in cheese at pH 5.29. Mixing alkalies with the cheese, increasing the pH, had some effect in stabilizing the enzyme against heat. Adding emulsifiers had a greater effect; e.g., with 1.5% anhydrous disodtum phosphate added and the pH of the mixture 5.56, the temp. required to produce a negative test in approx. 30 sec. was 150[degree] F. Addition of lactose to cheese increased the stability of the enzyme; addition of water decreased its stability slightly. Expts. on milks adjusted to different pH levels and heated showed that the milk phosphatase was most stable towards heat when the reaction was within a range of pH 6.5.7.4. Heating at lower or higher pH levels produced more rapid inactivation.