Modification of colloidal stability of casein micelles by enzymatic hydrolysis

Destabilization of casein micelles in reconstituted skim milk due to the enzymatic hydrolysis was studied at 32°C with solutions containing from 0.090 to 0.110 g cm^-3 of skim milk powder at pH 6.4. Destabilized micelles were precipitated by the acetate buffer and dissolved in NaOH solutions, where their concentration was determined spectrophotometrically. Within the narrow range of skim milk concentrations around 0.10 g cm^-3 the kinetics of the micelle destabilization process changed rapidly and qualitatively, indicating changes in the destabilization mechanism. While in the solution with the 0.090 g cm^-3 skim milk concentration the destabilized micelles were produced shortly after beginning of the hydrolysis, in the more concentrated mixture a substantial lag phase exists, leading to a considerably different rime evolution of the destabilized micelle concentration. According to the analysis based on the mathematical model of the destabilization process, these changes can be attributed to the shift of the hydrolysis kinetics from the first order to a two-step mechanism and to the change of character of the spatial enzyme activity from the locally correlated action to the spatially random hydrolysis.