Thermostability of peptidases secreted by microorganisms associated with raw milk

Peptidases of psychrotolerant microorganisms are known to be thermostable and withstand the thermal processing of milk products. The thermostability of peptidases from 231 recently isolated raw milk microorganisms was evaluated after cultivation in milk medium at 6 °C. Nearly all peptidases secreted showed residual peptidase activity of at least 5% after heat treatments at 70 and 95 °C for 5 min. Peptidases of three isolates from three novel Pseudomonas species were produced in a bioreactor at conditions simulating the storage of raw milk (6 °C), then purified and characterised biochemically. The protective effect of milk protein and influence of autoproteolysis were shown in extensive studies. The peptidases withstood ultra-high temperature treatment with residual activities of 45.8–58.9% and are able to cause destabilisation of milk products during storage; this was demonstrated for the peptidases from one of the novel Pseudomonas isolates in pilot-scale production of milk treated at ultra-high temperatures.