The effect of extracellular protease secreted by Pseudomonas fluorescens W3 on the quality of UHT milk

The heat-resistant protease produced by psychrotrophic bacteria is harmful to the quality of UHT milk, and the complex structure formed by fat and protein also affects the hydrolysis of protein by protease. The protease secreted by Pseudomonas fluorescens W3 was added to whole milk and skimmed milk before UHT treatment to analyze its effect on the quality of two kinds of UHT milk. During the preservation of UHT milk, increases in proteolysis, viscosity, hardness, sediment content, zeta potential, and particle size were observed. Extracellular protease reduced the whiteness, pH, and cohesiveness of UHT milk during the storage. The protease secreted by P. fluorescens W3 was easier to hydrolyze casein than whey protein, and κ-casein of casein was easily hydrolyzed by the protease. Milk fat accelerated the hydrolysis of casein by protease secreted by P. fluorescens W3, which led to a faster decline in the quality of whole UHT milk. Novelty impact statement: Pseudomonas fluorescens W3 can secrete a thermostable protease AprX with a molecular weight of about 45 kDa. The protease AprX has stronger hydrolysis ability on κ-casein than other caseins. Fat can promote the activity of protease AprX, so whole milk is more susceptible to the damage of protease AprX.