Casein micelle size in relation with casein composition and αs1, αs2, β and κ casein contents in goat milk

The relationship between micelle size and casein micelle composition was studied an 21 individual goat milks from animals homozygous for alpha(s1)casein variants A, B-2, C, E, F and O. A large variation in milk composition was obtained, as alpha(s1) secretion levels varied from 7.2 g.kg(-1) in A milks to none in O milks. Mean micelle size (MMS), determined by photon correlation spectroscopy, varied between samples from 192 nm to 287 nm. This was explained by the different aspects of the histograms of casein distribution according to the size, determined from transmission electron microscopy data, which showed a maximum either in the low diameter range (20-130 nm) or in the large diameter region (130-260 nm), or even intermediary figures with a bimodal distribution. The caseins, alpha(s1)CN, alpha(s2)CN, beta CN and kappa CN were determined in milks from nitrogen matter determinations (N x 6.38) and RP-HPLC analysis of casein. Polynomial relations were calculated between micelle size and milk compositional parameters. MMS was correlated on one side to the alpha(s1)CN and kappa CN levels in milks (g.kg(-1)) and, on the other side, to the proportions of alpha(s1)CN %, alpha(s2)CN % and beta CN % in total casein. These polynomial relations allowed the prediction of the mean micelle size in milks from the casein levels, with a 15 % accuracy. Monofactorial correlations also showed a significant effect of alpha(s1)CN (r = -0.77), but not any of kappa CN. Mineral composition of milks was determined, calcium by atomic absorption spectrophotometry and phosphorus, by a colorimetric method. Goat milks were characterized by a constant colloidal inorganic P level (12.4 (SD = 1.7) mmol.kg(-1)). In contrast, colloidal Ca (Cac), SerineP and total colloidal P (Pc) were correlated to the total casein content. The ratio Cac/Pc was the most constant parameter in goat milks, amounting 1.22 (SD = 0.05), presumably characterizing an unique mode of association of caseins in milk. No significant correlation was obtained between the colloidal Ca and P levels in milks and the size of micelles. (C) Inra/Elsevier, Paris.