Changes in casein composition of goats' milk during the course of lactation: physiological inferences and technological implications

Abstract

SummaryFive British Saanen goats were milk sampled during the first 39 weeks of lactation to determine changes in casein composition. Caseins were separated by anion- and cation-exchange FPLC to determine the relative amounts of the individual caseins. Acid, alkaline and SDS-PAGE were used to determine possible genetic polymorphisms and observe any lactational changes. Total casein nitrogen was determined using a micro-Kjeldahl method and this allowed the concentrations of individual caseins to be calculated. The milk of one animal, which had the deduced genotype αs1-CnAB, showed higher concentrations of both total and αs1-casein. The remainder of the group were either heterozygous αs1-CnBE or, more probably, homozygous αs1-CnE and produced milk of a generally lower protein concentration. Both FPLC and PAGE results showed that the relative amounts and concentrations of αs2-casein decreased with stage of lactation, consistent with its susceptibility to proteolysis. The relative amounts of the breakdown products of plasmin attack on β-casein, γ-caseins, were highly negatively correlated with milk yield (r = –0·942, P < 0·001) in the declining phase of lactation, reflecting the gradual involution of the gland at this time. The relative amount of κ-casein increased by ∼ 50% after peak lactation and its concentration almost doubled near the end of lactation. These compositional changes may alter the processing qualities of goats' milk in relation to cheese production.