Article

Abstract

-The behaviour of heat-treated skim milk towards acidification by fermentation with lactic bacteria was studied at 30°C and 42°C. Rheological changes using dynamic oscillations and 1 H-NMR relaxation time, T2, were studied with time and pH. Front-face intrinsic fluorescence measurements studied with principal component analysis were performed during fermentation. The gel was formed at 42°C at a shorter time and a higher pH (5.53 ± 0.01) than at 30°C (5.24 ± 0.13). The maximum in tan d was higher at 42°C than at 30°C, but the G' values at pH 4.6 were the same at the two temperatures. The changes in T2 with pH were not greatly different at the two temperatures, but the dT2/dpH curves showed some differences with the temperature. The changes in T2 were due to the changes in the particle structure caused by the fermentation and were correlated with calcium solubilisation. Front-face fluorescence can detect a first shift of the maximal intensity toward lower wavelengths and a second shift toward longer wavelengths. These shifts were separated by the gel point and were due to changes in the environment of tryptophan residues in the protein chains. The discussion of these results and the comparison with GDL-induced gels take into account the limitation of the transfer of the acid and the protons from the aqueous to the colloidal phase and the subsequent heterogeneity of the gel.Milk / yoghurt / pH / 1 H-NMR / rheology / intrinsic fluorescence Résumé -Fermentation de lait reconstitué et chauffé par des bactéries lactiques à deux températures. II. Approche dynamique de la construction du gel. Le comportement du lait écrémé traité thermiquement au cours de l'acidification par des bactéries lactiques est étudié à 30 et 42°C. Les modifications rhéologiques par oscillations dynamiques et le temps de relaxation T2 en RMN du proton sont étudiés en fonction du temps et du pH. La fluorescence frontale intrinsèque étudiée conjointement avec l'analyse en composantes principales a été suivie au cours de la fermentation. La gélification du lait apparaît à un temps plus court et un pH plus élevé à 42°C (5,53 ± 0,01) qu'à 30°C (5,24 ± 0,13). Le pic de maximum de tan d était plus marqué à 42 qu'à 30°C, mais le G' à pH 4,6 avait la même valeur aux deux températures. Les changements de T2 au cours * Correspondence and reprints E-mail: famelart@rennes.inra.fr 308 A. Laligant et al.de l'acidification étaient peu différents aux deux températures étudiées, mais les courbes dT2/dpH montrent quelques différences entre 42 et 30°C. Les changements de T2 sont dus à des changements de structure de la particule provoqués par la fermentation et sont corrélés à la solubilisation du calcium. La fluorescence frontale détecte un premier décalage des spectres vers des longueurs d'onde plus petites puis un décalage vers des longueurs d'onde plus grandes. Le point associé à l'inversion de l'environnement des tryptophanes des chaînes protéiques correspond au point de gel. Ces résultats et les différences avec des gels acides obtenus par l'addition ...

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