RHEOLOGY AND FLOCCULATION OF OIL-IN-WATER EMULSIONS MADE WITH MIXTURES OF ALPHA(S1)-CASEIN PLUS BETA-CASEIN

The influence of the composition of a mixed binary protein emulsifier composed of alpha(s1)-casein + beta-casein on the rheology of concentr ated oil-in-water emulsions (45 vol% oil, 5 wt% protein, pH 7) has bee n investigated over the temperature range 0-40 degrees C. Controlled s tress viscometric data are reported over the shear stress range 0.1-30 Pa for systems with alpha(s1)-casein/beta-casein ratios of 100:0, 98: 2, 95:5, 90:10, 75:25, 50:50, and 0:100. The pure casein emulsions sho wed substantially different temperature-dependent rheology, and there was observed to be a pronounced maximum in the small-deformation compl ex modulus of the pure alpha(s1)-casein emulsion in the range 30-40 de grees C. In the emulsions containing greater than or equal to 90% alph a(s1)-casein in the emulsifier mixture, all of the beta-casein present was found to be associated with the surface of the droplets. Average droplet sizes and protein surface coverages were higher in the mixed c asein systems than in the equivalent pure casein systems. The strongly pseudoplastic character of the emulsions is consistent with extensive reversible flocculation caused probably by a depletion mechanism invo lving unadsorbed protein. The degree of flocculation is sensitive to t emperature and to the alpha(s1)-casein/beta-casein ratio. The results can be interpreted in terms of changes in protein self-assembly and ad sorbed layer structure which influence the strength of the interdrople t interactions and hence the rheological behavior of the emulsions. Th ere is some evidence of a specific role for alpha(s1)-casein-beta-case in complexes in these systems. (C) 1998 Academic Press.