Quantification of the proteolytic and lipolytic activity of microorganisms isolated from raw milk

Heat-resistant enzymes of psychrotolerant microorganisms are associated with the spoilage of UHT-milk and milk products. In this study, we investigated the extracellular peptidase, esterase and lipase activity after submerged cultivation at 6 °C of 231 recently isolated microorganisms in milk medium. In contrast to the widely used agar diffusion tests for secretion of hydrolases, here a more realistic liquid screening approach was used. The advantages of the latter are the possibility of quantifying the enzyme activity in volumetric units and the opportunity to simulate the growth conditions of the microorganisms to the storage conditions of raw milk. The majority of enzymatically active isolates belonged to Gram-negative bacteria, especially the genus Pseudomonas. Surprisingly, among them, twelve novel Pseudomonas species were discovered. In this study, we demonstrated that numerous raw milk isolates, including bacteria and yeasts, produce extracellular enzymes that may cause spoilage problems for the dairy industry.