Effects of pH and Ethanol on the Kinetics of Destabilisation of Oil-in-Water Emulsions Containing Milk Proteins

Abstract

Soya oil-in-water emulsions (1:4, v/v) were prepared using sodium caseinate or beta-lactoglobulin (5 or 10 g litre-1) as the surfactant. The kinetics of aggregation induced by pH changes or the addition of ethanol were measured by light scattering in diluted systems either under shear or quiescent conditions. Under shear, emulsions containing caseinate were stable between pH 3 and 3.5 and also at pH greater than or equal to 5.3, while those formed with beta-lactoglobulin were stable below pH 4 as well as at pH greater than or equal to 5.6. Under quiescent conditions, the emulsions were also destabilised but at a much slower rate. The destabilisation process generally showed an initial lag period which depended on the pH, followed by an explosive growth stage. In aqueous ethanol (50:50, v/v), under both shear and quiescent conditions, fresh emulsions formed with beta-lactoglobulin were only slightly destabilised. Ageing the emulsions for 24 h, however, increased their susceptibility, and destabilisation was achieved at concentrations of ethanol as low as 30:70, (v/v). Emulsions formed with caseinate on the other hand were destabilised at ethanol concentrations greater than or equal to 40:60 (v/v), with time-courses showing a high initial slope, followed by a final stage at which particle size remained constant. Shear had a relatively small effect on these reactions. The kinetics of pH-induced aggregation in both sets of emulsions could be explained by orthokinetic flocculation while the ethanol-induced association in caseinate emulsions appeared to be a result of Ostwald ripening.