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

Abstract

Summary: Five 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 α_s1-Cn^AB, showed higher concentrations of both total and α_s1-casein. The remainder of the group were either heterozygous α_s1-Cn^BE or, more probably, homozygous α_s1-Cn^E and produced milk of a generally lower protein concentration. Both FPLC and PAGE results showed that the relative amounts and concentrations of α_s2-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.