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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-The aim of this study was to compare the rheological properties of stirred acid milk gels, acidified using two different acidification procedures. Pre-heated milks were acidified either by means of glucono-δ-lactone (GDL gel) at 20• C or by incubation with lactic acid bacteria (lactic gel) at 38• C. The latter procedure gave a harder acid set gel with a more heterogeneous structure. Once formed, the gels were filtered and stirred, then immediately characterised using the low-amplitude dynamic oscillation method for 20 h at 4• C or creep measurement at 4• C. The stirred gels were stored at 4• C for 28 d, during which the pH and the viscosity at 64 s −1 were measured. After stirring, the lactic gel made at 38• C had a higher instantaneous elastic modulus, a higher viscosity and showed a greater increase in the elastic modulus over 20 h compared with the stirred GDL gel. The viscosity of both stirred gels progressively increased during storage at 4• C, but that of GDL acidified gels increased 3.5 times more than that of lactic gels. The procedure used to prepare the set acid gels had notable effects on the interactions formed within 20 h and 28 d of storage of stirred gels. Résumé -Effet de la procédure d'acidification sur les propriétés rhéologiques des gels acides brassés. Le but de ce travail est de comparer les propriétés rhéologiques des gels acides brassés acidifiés par 2 procédures différentes. Le lait traité thermiquement était acidifié soit par addition de glucono delta lactone à 20• C (gel GDL), soit par incubation avec des bactéries lactiques à 38• C (gel lactique). Cette dernière procédure donnait des gels plus fermes avec une structure davantage hétérogène. Une fois formés, les gels étaient filtrés et brassés, puis immédiatement caractérisés en utilisant la méthode dynamique d'oscillations en faible amplitude pendant 20 h à 4• C et par fluage à 4• C. Les gels brassés étaient stockés à 4• C pendant 28 j pendant lesquels le pH et la viscosité à 64 s −1 étaient mesurés. Après brassage, le gel lactique préparé à 38• C avait un plus grand module élastique instantané, une plus grande viscosité et montrait une plus grande augmentation du module élastique pendant 20 h, comparé au gel GDL brassé. La viscosité des 2 types de gel brassé augmentait progressivement pendant le stockage à 4• C, mais elle augmentait 3,5 fois plus dans le gel GDL que dans le gel lactique. La procédure appliquée pour préparer le gel acide avait des effets notables sur les interactions formées à 20 h et à 28 j de stockage des gels brassés. yaourt brassé / yaourt ferme / gel acide / rhéologie

The aim of the present study was to identify the nature of bonds established between protein particles after stirring that are responsible for the texture improvement of stirred yoghurts, called rebodying. Using a constant model yoghurt at pH 4.4, the effects of changes in the physicochemical conditions at stirring were studied on the subsequent rebodying. Short term rebodying was measured as the changes in viscoelastic properties at 4 degrees C during 20 h after stirring, while long-term rebodying was measured as the viscosity changes during 28 d storage at 4 degrees C. Moreover, stirred gels obtained from either set gels that were allowed time or not for ionic equilibration were compared. Increasing or decreasing ionic strength did not change the properties of stirred gels. Calcium chloride addition significantly decreased G'0 h, G'20 h and tan20 h but did not induce changes in the gel microstructure as observed by confocal scanning microscopy. Yoghurt rebodying could not be explained by fulfilling ionic equilibrium. Moreover, N-ethyl maleimide addition had no effect on the stirred yoghurt. Attractive electrostatic and disulphide interactions were not involved in the gel rebodying and increasing calcium concentration in the set gel limited rebodying.