Micellar transition state in casein between pH 5.5 and 5.0

pH-induced changes in casein micelles during direct acidification and bacterial fermentation of reconstituted skim milk at 20 degrees C were monitored by scanning electron microscopy (SEM) in combination with biochemical and rheological methods. For SEM casein micelle observations, an original method of milk sample preparation with porous inorganic membranes was developed. Micrographs suggested that different stages of micellar association were related to pH and that between pH 5.5 and 5.0 casein micelles coalesced. Correlations between microstructural and biochemical changes in casein micelles, and theological behavior of milk or gel, help to explain the different steps leading to the final protein network of the acid milk gel.