Fermentation by lactic bacteria at two temperatures of pre-heated reconstituted milk.: II -: Dynamic approach of the gel construction

The behaviour of heat-treated skim milk towards acidification by fermentation with lactic bacteria was studied at 30degreesC and 42degreesC. Rheological changes using dynamic oscillations and H-1-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 42degreesC at a shorter time and a higher pH (5.53 +/- 0.01) than at 30degreesC (5.24 +/- 0.13). The maximum in tan delta was higher at 42degreesC than at 30degreesC, 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.