Effect of Plasmin, Milk Somatic Cells and Psychrotrophic Bacteria on Casein Fractions of Ultra High Temperature Treated Milk

The present study examined the enzymes responsible for proteolysis of casein in milk. Reversed-phase HPLC was used to differentiate the peptide products, of each proteinase, soluble in 12% trichloroacetic acid and in pH 4.6 milk filtrates. Peptides produced by bacterial proteinase were less hydrophobic and eluted early in the RP-HPLC chromatogram, while peptides produced by plasmin and somatic cell proteinase were more hydrophobic and eluted later. β-caseins were preferential substrates for plasmin, whereas α s-and κ-caseins were hydrolysed to a lesser extent. Proteases from bacterial origin predominantly affected κ-casein, while s-casein and α s-casein were less susceptible. Somatic cell proteinase degraded mainly β-casein, followed by α s-casein and κ-casein. When milk was contaminated by bacterial proteinases, chromatograms of the TCA 12% filtrate showed early peaks, while the pH 4.6 filtrate showed early and late peaks when proteolysis was caused by either plasmin, somatic cell proteinase or bacterial proteinase.