Casein compositional studies. III. Changes in Irish milk for manufacturing and role of milk proteinase

Abstract

SUMMARY: The composition of casein in Irish milk for manufacturing was examined throughout the year. Large changes in composition were observed in the final quarter of the year as a result of the influence of late lactation when the proportion of α_s- and β-caseins declined and the proportion of κ- and γ-caseins and of minor casein components increased. Similar changes in casein composition were induced by treatment of milk with porcine plasmin. The activity of plasmin-like milk proteinase in milk was quantified from the rate of conversion of added [¹⁴C]β-casein to [¹⁴C]γ-caseins. This activity was considerably higher in a late lactation milk than in a mid lactation milk and was ∼ 20 times higher at 37 °C than at 4 °C. It was concluded that casein compositional changes in late lactation were largely the result of proteolysis of individual caseins by plasmin-like milk proteinase.