Effect of high isostatic pressure on the peptidase activity and viability of Pseudomonas fragi isolated from a dairy processing plant

To produce safe and high quality processed milk, high pressure (HP) technology was tested to inactivate undesired microorganisms and their caseinolytic activity. A Pseudomonas fragi strain, isolated from the inner surface of a cheese-making machine from a dairy plant, was shown to harbour the aprX gene and cause casein proteolysis. Single-cycle HP processing of P. fragi-spiked milk at 450 MPa and 25 °C for 20 min decreased bacteria viability to lower levels and reduced peptidase activity by 14%. However when HP processing was performed at 50 °C, a synergistic effect on peptidase was observed, reaching 40% inactivation. Multiple HP treatment cycles at 450 MPa and 25 °C were less effective and reduced peptidase activity by only 23%. HP treatment could aid in the challenge to reduce AprX peptidase activity produced by microbial contaminants, but partial inactivation of peptidase was not effective in preventing UHT milk coagulation during storage at room temperature.