The relationship between micelle size and casein micelle composition was studied on 21 individual goat milks from animais homozygous for as1casein variants A, B 2 , C, E, F and O. A large variation in milk composition was obtained, as a sl secretion levels varied from 7.2 g-kg"! in A milks to none in 0 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, as1CN, a s2 CN,~CN and KCN were determined in milks from nitrogen malter determinations (N x 6.38) and RP-HPLC analysis of casein. Polynomial relations were calculated between micelle size and milk composition al parameters. MMS was correlated on one side to the as1CN and KCN levels in milks (g-kg'") and, on the other side, to the proportions of as1CN %, a s2 CN % and~CN % in total casein. These polynomial relations allowed the prediction of the mean micelle size in milks from the casein levels, with aiS % accuracy. Monofactorial correlations also showed a significant effect of as1CN (r = -0.77), but not any of KCN. Mineral composition of milks was determined, calcium by atomic absorption spectrophotometry and phosphorus, by a colorimetrie method. Goat milks were characterized by a constant colloidal inorganic P level (12.4 (SO = 1.7) mmol-kg "). In contrast, colloidal Ca (Cac), SerineP and total colloidal P (Pc) were correlated to the total casein content. The ratio CaclPc was the most constant parameter in goat milks, amounting 1.22 (SO = 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. © InralElsevier, Paris.goat milk 1 casein 1 micelle size 1 a sJ caseinl colloidal calcium and phosphorus Résumé -Taille des micelles du lait de chèvre en relation avec la composition de la caséine et les teneurs en caséines a sJ ' a s2 '~et K dans le lait. La relation entre la taille des micelles de caséine et la composition de la fraction caséine des laits a été étudiée sur 21 laits individuels provenant de * Correspondence and reprints.

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International audienceA HPLC/ESI-MS method was used to separate and analyse casein variants in goat bulk milks. From mass determination, about 25 casein components were characterised in each milk casein sample with up to 7 compounds obtained in the same HPLC fraction. The different caseins were each detected at various phosphorylation levels. Only casein compounds present at levels higher than 1% of total casein were obtained, as confirmed by the analysis of individual goat milks of known composition. The discrimination threshold obtained for the differentiation of molecular masses was quantified by the variation coefficient (CV = 0.02% ). Assignment of masses to known compounds was made by reference to theoretical masses. Some compounds, in $\kappa$CN, $\alpha_{{\rm s1}}$CN and $\beta$CN fractions could not be assigned. It is possible that they were non-described variants. A limitation of the method was revealed when variants present in the same RP-HPLC fraction had the same molecular mass (as was the case for $\alpha_{{\rm s1}}$CNA and $\alpha_{{\rm s1}}$CNB$_1$) or when their masses differed by ~80 g$\cdot$mol$^{-1}$ (as was the case for $\alpha_{{\rm s2}}$CNA and $\alpha_{{\rm s2}}$CNB). In this latter case, the 2 variants could not be differentiated from the different phosphorylation levels of the same, indeed the addition of a phosphoseryl residue increased the molecular mass by 80 g$\cdot$mol$^{-1}$. To overcome these limitations, an improvement of the method was proposed. In the 3 bulk milks analysed the same main variants, $\kappa$CN-Ile 2P, $\alpha_{{\rm s2}}$CN A11P, $\alpha_{{\rm s1}}$CN A and/or E were found, and the same non-identified component in $\beta$CN fraction. However, differences between the 3 bulk milks analysed were observed. A variation in the relative proportions of variants in $\kappa$CN and $\beta$CN fractions was observed in the different milks and it is proposed, if confirmed, that it could be related to the selection stage of the producing goats. Thus, the HPLC/ESI-MS method applied to bulk milks collected throughout a country allowed for an accurate determination of the casein variants with a higher frequency. The analysis of individual milks would allow a fast and accurate characterisation of the casein phenotype of goats.Caractérisation des variants de la caséine contenue dans des laits de chèvre de grand mélange par RP-HPLC/ESI-MS. Une méthode RP-HPLC/ESI-MS a été utilisée pour la mise en évidence et la caractérisation des différents variants de la caséine présents dans des laits de chèvre de très grand mélange. L'identification des composés à partir des masses expérimentales était réalisée par référence aux masses théoriques des composés connus. Environ 25 composants ont été mis en évidence dans chaque lait. Dans une même fraction, jusqu'à 7 composés ont pu être séparés par leur différence de masse. Les différents niveaux de phosphorylation des caséines étaient individualisés. Les composants de la caséine présents à des teneurs supérieures à 1 % de la caséine totale ont été mi...