Abstract: The protein content, pH, color, colonies number, average particle size, zeta potential, degree of proteolysis and proteinase activity of ultra-high temperature (UHT) milk with and without protein coagulation and commercial pure milk samples were determined to analyze the reasons for protein coagulation in UHT milk at the end of self-life. The results showed that the total protein content of coagulated milk samples was 2.9 g/100 g, which was not significantly different from that of milk samples without protein coagulation, but the pH decreased slightly. The total color difference (ΔE) and brightness value (L*) decreased to 68.05 and 67.56, respectively, indicating that the color became dark, and the total bacterial count increased to 1.92 (lg(CFU/mL)). The proteinase activity was 2 565.33 U/L, the degree of proteolysis was 17.07%, the average particle size was 685.8 nm, and the zeta potential increased to ?19.97 mV; these data were all significantly higher than those of milk samples without protein coagulation and commercial pure milk. The trend of changes in proteinase activity was basically consistent with that of the degree of proteolysis, particle size and zeta potential for all samples tested. Protein coagulation in UHT milk at the end of shelf life may be attributed to protein hydrolysis due to increased protease activity and physical deposition caused by the enlargement of casein micelles during long-term storage at ambient temperature.

Key words: ultra-high temperature milk; shelf-life; proteinase; protein coagulation

摘要： 对产生蛋白凝块的超高温灭菌（ultra-high temperature，UHT）乳和正常无凝块乳样品以及市售纯牛乳样品贮藏期间的蛋白质含量、pH值、颜色、菌落总数、平均粒径、Zeta电位、蛋白水解度和蛋白酶活性进行测定，分析UHT乳在贮藏末期蛋白凝块产生的原因。结果表明：凝块乳样品中总蛋白含量为2.9 g/100 g，与正常无凝块乳样品相比无显著差异，但凝块乳样品的pH值略有降低，总色差（ΔE）和亮度值（L*）分别降至68.05和67.56，导致色泽变暗，并且菌落总数升高至1.92 （lg（CFU/mL））；产生凝块乳样品中的蛋白酶活性高达2 565.33 U/L，蛋白水解度为17.07%，平均粒径为685.8 nm，Zeta电位升高至－19.97 mV，这些数据都显著高于无凝块乳和市售纯牛乳；不同样品的蛋白酶活性变化与蛋白水解度、粒径和Zeta电位变化趋势基本一致；临近保质期的UHT乳出现蛋白凝块可能是由长期常温贮藏，乳中蛋白酶活性升高导致乳蛋白水解与酪蛋白胶束增大引起的物理沉积共同造成的。

关键词: 超高温灭菌乳；保质期；蛋白酶；蛋白凝块