Proteolysis of Milk Protein by Psychrotrophic Bacteria Isolated from Refrigerated Raw Milk

Four strains of psychrotrophic bacteria (Pseudomonas A, Pseudomonas B, Flavobacterium and Klebsiella) were isolated from bulk tank milk. Their proteolytic actions on milk protein were investigated. Lyophilized sodium caseinate and ultracentrifugally separated casein micelle were suspended in ultrafiltrated milk. Whey protein solution was prepared from acid whey. They were inoculated with these strains and incubated at 7°C. The maximum periods of incubation were 10 days for sodium caseinate and casein micelle, and 20 days for whey protein. The changes of milk protein were mainly detected by polyacrylamide gel electrophoresis (PGE). The results were as follows: Proteolytic actions of psychrotrophic bacteria on sodium caseinate solution were evident, that is Pseudomonas A almost completely broke down αs- and β-casein after 5 days-treatment, while Pseudomonas B and Flavobacterium did αs-and β-casein after 10 days-treatment. Proteolytic actions on casein micelle solution were also recognized. Three strains except Klebsiella broke down αs- and β-casein after 5 days-treatment. But these psychrotrophic bacteria hardly affected whey protein. Only Pseudomonas B and Flavobacterium specifically broke down β-lactoglobulin after 10 days-treatment.