pH-induced changes in casein micelles during direct acidification and bacterial fermentation of reconstituted skim milk at 20ЊC 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 rheological behavior of milk or gel, help to explain the different steps leading to the final protein network of the acid milk gel.

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Reconstituted skim milk with varying concentrations of total solids was coagulated using glucono‐δ‐lactone (GDL). Microscopic, turbidimetric and rheological procedures were used to examine mineral solubilization, buffering capacity, casein dissociation and micellar solvation during gelation. Total solids of the milk affected pH of the onset of gelation attributable to differences in colloïdal calcium phosphate in the casein particles during acidification. Firmness and elasticity of the resulting gel increased with total solids from a more direct contribution of dry matter during the last stage of acid milk gel formation.

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Soluble proteins and Ca and P salts in the fouling deposit on a mineral microfiltration membrane obtained in static conditions were analysed and compared with those found using dynamic filter conditions in a laboratory pilot installation for milk processing. The serum milk proteins were retained by the porous micellar deposit under both static and dynamic conditions, resulting in progressive fouling of the dynamic micellar membrane, enabling milk to be processed with a mineral microfiltration membrane. Ca and P salts increased fouling probably by allowing better adsorption of casein micelles on the alumina and by acting as intermicellar bonds in the deposit. Fat globules modified porosity, permeability and resistance to matter transfer of the deposit. The dynamic conditions gave a deposit of the same type and structure as with the static conditions although made thinner by the velocity effect.The main factor limiting the permeation flow in tangential membrane filtration is the formation of a polarization and fouling layer on the surface of the filter (Porter