Milk fat influences proteolytic enzyme activity of dairy Pseudomonas species

This study investigated the effect of growth conditions on proteolytic activity of six Pseudomonas strains, (Pseudomonas fragi DZ1, Pseudomonas koreensis DZ138, Pseudomonas rhodesiae DZ351, Pseudomonas fluorescens DZ390, Pseudomonas synxantha DZ832 and Pseudomonas lundensis DZ845), isolated from raw milk. The proteolytic activity of all Pseudomonas strains in dairy media (skim milk and whole milk) was significantly higher (p < 0.05) than in non-dairy media (TSB), with most activity from Pseudomonas grown in whole milk. The proteolytic activity from P. lundensis DZ845 grown in TSB with the addition of 5% (w/v) butter was higher than other dairy ingredients added to TSB and the amount of proteolytic activity increased with increasing concentrations of butter (from 5 to 15%). P. rhodesiae DZ351 showed little proteolytic activity in all TSB supplemented with dairy ingredients. Only four of the six strains produced one protease of 47 kDa when grown in TSB. However, all six strains were able to produce at least one type of proteases in milk medium. For P. lundensis DZ845, a 12% casein zymography gel revealed that the presence of butter could induce proteolytic activity. This is the first study showing the effect of milk fat (butter) on the proteolytic activity of Pseudomonas. This highlights the greater vulnerability of whole milk compared to skim milk to proteolytic activity.