Effects of spray drying on physicochemical properties of milk protein-stabilised emulsions

The effect of spray drying and reconstitution has been studied for oil-in-water emulsions (20.6% maltodextrin, 20% soybean oil, 2.4% protein, 0.13 M NaCl, pH 6.7) with varying ratios of sodium caseinate and whey protein, but with equal size distribution (d(32) = 0.77 mum). When the concentration of sodium caseinate in the emulsion was high enough to entirely cover the oil-water interface, the particle size distribution was hardly affected by spray drying and reconstitution. However, for emulsions of which the total protein consisted of more than 70% whey protein, spray drying resulted in a strong increase of the droplet size distribution. The adsorbed amount of protein ranged from 3 mg m(-2) for casein-stabilised emulsions to 4 mg m(-2) for whey protein-stabilised emulsions with a maximum of 4.2 mg m(-2) for emulsions containing 80% whey protein on total protein, which means that for all these emulsions about one quarter of the available protein was adsorbed at the oil-water interface. The adsorbed amount of protein was hardly affected by spray drying. After emulsion preparation casein proteins adsorbed preferentially at the oil-water interface. As a result of spray drying, the relative amount of beta-lactoglobulin in the adsorbed layer increased strongly at the expense of alpha(s1)-casein and beta-casein. Percentages of alpha(s2)-casein and kappa-casein in the adsorbed layer remained largely unchanged. The changes in the protein composition of the adsorbed layer as a result of spray drying and reconstitution were the largest when beforehand hardly any whey protein was present in the adsorbed layer and hardly any sodium caseinate in the aqueous phase. Apparently, during spray drying conditions have been such that beta-lactoglobulin could unfold, aggregate, and react with other cystein-containing proteins changing the particle size distribution of the emulsions and the composition of the adsorbed layer. It seemed, however, that non-adsorbed sodium caseinate in some way was able to protect the adsorbed casein proteins from being displaced by aggregating whey protein. (C) 2003 Elsevier B.V. All rights reserved.