Direct chemical acidification of milk: correlation between the mobility ofmicellar material and the macro and microstructures of acidified milk.

Direct addition of lactic acid was used to follow the behaviour of reconsti tuted skimmed milk during a chemical acidification. The mobility of the mic ellar materials: minerals and caseins, was correlated with the change in a range of physicochemical parameters such as buffering capacity and conducti vity. It was hypothesized that, for a given pH, the transformations that em erge within milk would be dependent on the biochemical state of: the added lactic acid; the micellar minerals; and of some of the functional groups of caseins. Determination of rheological state equations, calculation of stor age and loss moduli and Visualization by Scanning Electron Microscopy, allo wed the hypothesis to be consolidated and showed the existence of three cha racteristic points around pH 5.8, 5.4 and 5.0. These results permitted some specificities of rapid and direct chemical acidification to be deduced and allowed their comparison with the situation in slow biological acidificati on.