Phase separation in milk protein and amylopectin mixtures

The phase separation behaviour of milk protein (colloidal casein) and amylopectin mixtures was studied. The phase boundary between stable and unstable mixtures was determined by visual observation and confocal microscopy (CSLM). In the latter case, an image analysis procedure (the variance measure) was used. The phase boundary was also calculated using depletion theory as developed by Vrij, A. (1976). Polymers at interfaces and the interactions in colloidal dispersions. Pure and Applied Chemistry, 48, 4, 471–483. Good agreement was obtained between calculated and measured phase boundaries. The CSLM study revealed the appearance of protein aggregate structures when phase separation occurred. Qualitatively, the appearance of the structures was similar to that of rennet- or acid-induced milk protein aggregates. At high concentrations, the aggregates form a network with gel-like properties. The occurrence of aggregate structures has not been reported before for phase separating milk protein–polysaccharide mixtures. So far, emulsion-like microstructures have been observed with milk proteins concentrated in spherical droplets. The observed microstructural difference could be explained using earlier experimental observations reported by Sperry, P. R. (1984). Morphology and mechanism in latex flocculated by volume restriction. Journal of Colloid and Interface Science, 99, 1, 97–108 and a theoretical description of phase separation in colloid–polymer mixtures developed by Lekkerkerker, H. N. W., Poon, W. C. K., Pusey, P. N., Stroobants, A., and Warren, P. B. (1992). Phase behaviour of colloid+polymer mixtures. Europhysics Letters, 20, 6, 559–564 and is ascribed to a low value of the polymer–colloid size ratio.