Analysis of Reasons for Protein Coagulation in Ultra-High Temperature Milk at the End of Shelf-life

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 2565.33 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.