Proteolysis in milk: the significance of proteinases originating from milk leucocytes and a comparison of the action of leucocyte, bacterial and natural milk proteinases on casein

Abstract

The caseinolytic activities at pH 6·8 of polymorphonuclear (PMN) and mononuclear leucocyte homogenates (equivalent to a level of 106 cells/ml milk) were less than the levels of natural milk proteinase activity found in milk from healthy cows. Bulk milks contained ∼ 4 times more milk proteinase activity than the composite milks from individual healthy cows. Isolated blood leucocytes, when added to raw milk of good bacteriological quality and stored at 5 °C, did not readily degenerate and had no detectable effect on the milk proteins even when these cells were completely disrupted by homogenization of the milk. Pasteurization of milk which contained leucocytes caused loss of cell vitality. Extracellular proteinases of psychrotrophic bacteria growing in milk were not detected until the early stationary phase of growth. The total viable count at which this occurred varied greatly. Proteinase production by a pure culture of Pseudomonas fluorescens was not detected in milk stored at 5 °C until a viable count of ∼ 109 colony forming units (c.f.u.)/ml was obtained, whilst normal bulk milks stored at 5 °C produced detectable levels of extracellular proteinase(s) when the psychrotrophic flora reached 107–108 c.f.u./ml. Casein proteolysis by PMN and mononuclear leucocyte homogenates resulted in similar polypeptide maps, but plasmin and bacterial proteinase isolated from a strain of Serratia marcescens resulted in polypeptide maps different from each other and from that produced by the leucocyte proteinase(s). The rate of proteolysis of caseins by the different proteinase sources appeared to be in the order αsl- > β- > > κ-casein for the leucocyte extracts, β- > αsl- > > > κ-casein for bovine plasmin and β- ≈ κ- > αsl-casein for S. marcescens proteinase.