Effect of heat-resistant protease from psychrotrophic bacteria in raw milk on shelf-life quality of UHT milk

The deterioration of ultra-high-temperature (UHT) milk quality during shelf life such as protein hydrolysis, is closely related to the residual activity of heat-resistant protease from psychrotrophic bacteria. This study aimed to evaluate the effect of heat-resistant protease activity derived from psychrotrophic bacteria on the quality of whole milk. Different concentrations of heat-resistant protease from Aeromonas sp .were added to UHT milk, and the effect on milk quality was investigated during storage time at different temperatures. The results indicate that as the shelf life extended, the pH, stability, and fluorescence intensity of all samples gradually decreased, while the degree of hydrolysis, average particle size, precipitation, and zeta potential significantly increased. The results of SDS-PAGE indicated that κ -casein was preferentially hydrolyzed by proteases. Moreover, the degree of deterioration in UHT milk was correlated with the concentration of proteases. As the protease concentration in milk reached 0.08 U/mL, the samples reached the end of the shelf life when stored at 37°C within 28 d, while no significant deterioration was observed after 98 d at 4°C. The results of this study provide valuable evidence for controlling quality of UHT milk during the storage of UHT milk and extending its shelf life.