Caseins in emulsions: interfacial properties and interactions

Milk proteins have excellent emulsifying and emulsion stabilizing properties, and one of the most convenient of milk protein ingredients for use in food processing is sodium caseinate. Since alpha(s1)-casein and beta-casein are the main individual protein components coexisting at the oil-water interface in many dairy-based colloids, it is instructive to compare the interfacial structure of these two pure proteins. Also, since many emulsion formulations contain both adsorbed and unadsorbed milk proteins, it is instructive to consider the effect of protein/oil ratio on the stability of caseinate-based emulsion systems. Recent theoretical analysis of casein adsorption at flat hydrophobic surfaces has suggested substantially different adsorbed monolayer structures for the two major caseins over the pH range 5.5-7.0. Expressed in the simplest terms, the predominant molecular conformation for adsorbed beta-casein on a single surface is a long tail at the N-terminus extending far into the aqueous solution, whereas that for alpha(s1)-casein is a loop which extends less far from the surface. The interaction potential between beta-casein layers is predicted to be strongly repulsive at all ionic strengths, which is consistent with the good combined steric and electrostatic stabilisation of beta-casein-coated emulsion droplets. In contrast, the interaction potential between alpha(s1)-casein layers is predicted to be attractive at intermediate separations above a certain ionic strength (ca. 0.05 M) which is consistent with the poorer stability of alpha(s1)-casein-coated droplets towards flocculation by simple electrolytes (NaCl or CaCl2). Recent experimental investigations of caseinate emulsions at pH 7 and low ionic strength have shown that stability is very sensitive to the protein/oil ratio. In emulsions containing protein at concentrations well below that required for monolayer saturation coverage, the system exhibits time-dependent bridging flocculation and coalescence. At protein contents around that required for saturation monolayer coverage, the system is Newtonian and unflocculated, and is very stable towards creaming and coalescence. At higher protein contents, the creaming stability of the pseudoplastic system is greatly reduced due to depletion flocculation of protein-coated droplets by unadsorbed sub-micellar caseinate. And, at even higher protein contents, there is partial restabilization of the flocculated emulsion in the form of a strong particle gel network. (C) 1999 Elsevier Science Ltd. All rights reserved.