Variation in the protein composition of bovine casein micelles and serum casein in relation to micellar size and milk temperature

Abstract

SummaryThe caseinate complex in bovine milk was partitioned by differential centrifugation at both 20 and 4 °C into 4 micellar fractions and a fraction representing serum casein, and the protein composition of the fractions determined. At both temperatures the relative amount of κ-casein in the micellar caseins increased markedly and that of β-casein decreased appreciably with decreasing micelle size. The relative amount of αs2-casein also tended to decrease with decreasing micelle size, but the relative amounts of αs1- and γ-caseins, and an unidentified casein fraction, showed little systematic variation. The serum casein differed appreciably in composition from the micellar caseins, being very rich in β-casein and comparatively poor in αsl- and αs2-caseins, and the amount present at 4 °C was considerably greater than at 20 °C, with the increase being due almost entirely to β-casein, but with γ-casein also making a significant contribution. The changes in the composition and distribution of micellar and serum caseins induced by cooling milk at 4 °C were completely reversible when the milk was re-equilibrated at 20 °C for 18 h.