Conformational changes in casein micelles during demineralization of skim milk by electrodialysis with ion exchange membranes
Kimura, T.; Uchida, Y.; Tomizawa, A.; Hiraoka, Y.; Fukushima, M.; Taneya, S.
Nippon Nogeikagaku Kaishi 65(8): 1213-1222
Casein micelles in skim milk were dissociated into sub-micelles during demineralization, and the relationship between the amount of minerals and distribution casein micelles was studied. Ca and P amounts were determined as ionic, colloidal and binding phases. Casein micelle size-distributions were determined by ultracentrifugation, turbidity intensity and electron microscopy. The skim milk demineralization process in electrodialysis using ion exchange membranes was divided by the Ca demineralization ratio and mole ratio Ca/P of binding casein into three stages: (1) 0-50%, 1.38-1.0; (2) 50-80%, 1.0 and (3) over 80%, under 1.0. Ionic mineral components were demineralized at the first stage, and the mineral components binding casein were demineralized at the second stage. The latter caused casein dissociation from a micelle to sub-micelle state, and decreased micelle size and increased colloidal Ca and P. The further demineralization of Ca binding casein proceeded while organic P remained, and the Ca/P ratio was under 1.0. The size of the casein micelles was reduced during demineralization. The minimum casein micelle size during demineralization was at 80% Ca demineralization ratio measured by turbidity intensity. The amount of Ca binding casein decreased under demineralization ratio. The surface hydrophobicity of the casein particle was maximized at 80% demineralization ratio. The size of casein micelles increased at 90% Ca demineralization ratio because of coagulation of the submicelles.