Properties of emulsions stabilized with milk proteins: overview of some recent developments

The physico-chemical properties of oil-in-water emulsions that are stabilized by milk proteins are determined largely by the nature of the adsorbed layer at the surface of the dispersed droplets. Detailed information on the structure of the adsorbed layer is now available for some of the pure milk proteins, especially beta-casein and beta-lactoglobulin. Recent analysis of the segment density profiles that are normal to the surface has shown clear, differences in layer structure between the disordered casein and the globular whey protein. The phosphoserine residues of beta-casein are essential in providing the thickness and steric-stabilizing properties of the layer. Surface coverage and segment density profile are sensitive to bulk protein concentration, pH, ionic strength, and calcium ion content. Functional properties of commercial milk protein ingredients are influenced also by compositional and structural heterogeneity. Unadsorbed protein may make additional contributions to the properties of casein-stabilized emulsions, especially creaming and flocculation behavior. Experimental studies of small oil-in-water droplets emulsified with pure milk proteins or commercial milk protein ingredients (i.e., sodium caseinate and whey protein concentrate) have provided useful insight into the major factors (temperature, pH, protein content, and calcium ions) affecting the rheological behavior of dairy emulsions and the key instability processes of creaming, flocculation, and coalescence. Insight into the effects of interactions of proteins and lipids and competitive adsorption on emulsion properties has been derived from rheological and stability experiments on systems containing mixtures of milk proteins and various small molecule emulsifiers (sorbitan esters, monoglycerides, and lecithins). Such experiments have demonstrated the sensitivity to emulsifier type and concentration of the orthokinetic coalescence stability of whey protein emulsions and the viscoelasticity of heat-set whey protein emulsion gels.