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-The pH-induced physico chemical changes in a reconstituted heated (90°C -10 min) skim milk were studied during fermentation by lactic acid bacteria at two temperatures (30°C and 42°C). Different variables were examined during acidification: the water content of ultracentrifugation pellets (150000 g for 70 min at 30 or 42°C), the partition of proteins into pelleted and unpelleted fractions (analysed by RP-HPLC on supernatants) and the dissociation of colloidal salts (determined by atomic absorption spectrophotometry on the ultrafiltrates). The proportion of unpelleted protein was found to be pH-dependent with a gradual linear decrease during acidification. All the caseins pelleted below pH 5.5. As the pH was reduced, calcium was progressively released from the micelles. In contradiction with a number of previous findings, fermentation induced a shift toward acid pH values of the salt dissociation compared with a glucono-delta-lactone acidification. The partition of casein between the pellet and the supernatant due to acidification by fermentation was temperature-dependent, as was the solvation of the pellet. Both were greater at 30°C compared with 42°C. The temperature had no effect on the salt distribution. The results are discussed in terms of transfer phenomena between the colloidal and the solvent fractions and diffusion-limited processes. Milk / yoghurt / pH / mineral / micellar solvation / caseinRésumé -Fermentation du lait reconstitué traité thermiquement par des bactéries lactiques à deux températures. I -Protéines et minéraux. L'effet du pH sur les évolutions physicochimiques de lait écrémé reconstitué et traité thermiquement à 90°C, 10 min a été étudié au cours d'une acidification par fermentation lactique à deux températures (30°C et 42°C). La quantité d'eau a été évaluée par séchage des culots d'ultracentrifugation (150000 g pendant 70 min à 30 ou 42°C). La distribution des protéines entre le culot et le surnageant a été analysée par chromatographie RP-HPLC sur les surnageants avec ou sans dithiothreitol. La dissociation des sels colloïdaux a été déterminée par spectrométrie d'absorption atomique dans les ultrafiltrats de lait. La proportion de protéines dans le surnageant était dépendante du pH avec une diminution graduelle au cours de

The pH-dependent behaviour of soluble protein aggregates produced by the pre-heating of reconstituted skim milk at 90 degrees C for 10 min was studied, in order to understand the role of these aggregates in acid gelation of heated milk. The following milk samples were prepared: (1) control (unheated reconstituted milk, pH 6.5); (2) milk heat-treated at pH 6.5 (mHtd6.5) and (3) milk heat-treated at pH 7.2 (mHtd7.2). They were centrifuged and the supernatants (SPNT 1) pH-adjusted to yield a series of pH values ranging from 6.5 or 7.2 to 4.6 using HCl at 20 degrees C or GDL at 20 and 38 degrees C. pH-Adjusted SPNTs 1 were re-centrifuged. The resulting supernatants (SPNTs 2) were analysed by OD (at 600 and 280 nm) and SDS-PAGE in order to characterise proteins still soluble as a function of pH. Particle size in SPNTs 1 was analysed by Steric Exclusion Chromatography. The OD600 nm revealed that during acidification soluble casein in both control and heat-treated samples exhibits variations in its optical properties or size as previously shown with micellar casein. In heat-treated samples, soluble casein and heat-induced covalent soluble aggregates precipitate at the same pH value. A progressive acidification of the soluble phase did not separate them. Increasing the temperature of acidification from 20 to 38 degrees C resulted in an increase in the precipitation pH of the proteins. However choice of acidifier did not have a significant effect on OD profiles. The soluble covalent aggregates from mHtd7.2 were smaller, more numerous, and had a higher content of kappa-casein than mHtd6.5. Both types of aggregates began to precipitate at the same pH value but precipitation occurred over a narrower pH-range for soluble aggregates prepared from mHtd7.2. This may explain the higher gelation pH of mHtd7.2 compared with mHtd6.5.

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Phenolic compounds contribute to the micronutrient composition of pasture, which in turn may affect animal product composition. To assess the importance and variations in content of these compounds, the polyphenolic and botanical compositions of 24 permanent pastures located in one lowland and two upland regions were studied at equivalent stages of growth. Phenolic fractions were characterized and quantified using HPLC-PDA-ESI-QToF, and the total content was determined by colorimetry over each whole pasture. A rise in altitude was accompanied by a marked increase in total phenolic content, linked to changes in botanical composition, but did not have any influence on the distribution according to molecular class. For all of the pastures, significantly different patterns due to qualitative and quantitative differences among the 92 separate peaks were observed with 31 compounds identified. The involvement of certain plants in the variations of content and composition in phenolic compounds of pastures was statistically evaluated.

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(reçu le 12-11-1987, accepté le 3-6-1988) Résumé -Les constituants de la fraction protéose-peptones du lait de vache sont décrits. Ils sont divisés en 2 classes en fonction de leur origine : d'une part les hydrolysats qui comprennent les composants 5 et 8 ainsi que de nombreux polypeptides résultant de l'hydrolyse des caséines par les protéases natives du lait; d'autre part des protéines, dont le composant 3, ainsi que des protéines majeures résiduelles. La composition et la concentration des protéose-peptones varient au cours de la conservation du lait et pourraient être considérées comme indicateurs de qualité. Les protéose-peptones ont de nombreuses propriétés fonctionnelles. Parmi celles-ci, la capacité du composant 3 d'inhiber la lipolyse dans le lait est probablement liée au caractère hydrophobe de ce composant. Cet article résume également les connaissances des fractions protéose-peptones des laits d'autres mammifères.protéose-peptones -protéine -caséine -protéolyse -plasmine Summary -Review : the proteose-peptone fraction of milk.