Heat-induced aggregation of milk protein-stabilized emulsions: sensitivity to processing and composition

Heat stability has been studied in model systems of oil-in-water emulsions made at pH 6.8 with commercial whey protein isolate as emulsifying agent (typically 2 or 3 wt%) and n-tetradecane as the oil phase (typically 30 or 45 vol%). Samples heated for various lengths of time (up to 48 h) in the temperature range 70–90°C have been investigated with respect to changes in viscosity and particle-size distribution. Various factors affecting the extent of heat-induced aggregation have been considered: protein/oil ratio, source of whey protein, difference between whey protein and β-lactoglobulin (β-lg), partial replacement of whey protein by sodium caseinate, and aqueous solution conditions (pH and ionic strength). Substitution of just 10% of pure β-lg by caseinate in a heat-sensitive emulsion was found to confer excellent stability towards prolonged heating (many hours at 90°C). A commercial whey protein ingredient that gave unflocculated emulsions at room temperature was found to exhibit time-dependent heat-induced thickening behaviour after being heated for a few minutes at >70°C. Under well-defined conditions, a maximum in apparent viscosity with heating time was observed, whose magnitude was sensitive to the extent and duration of shearing. Replacement of just 5% of whey protein by caseinate in this emulsion led to complete elimination of this heat-induced viscosity maximum. These findings appear to show that a very small proportion of casein can have a surprisingly pronounced influence on the susceptibility of a whey protein-based emulsion to heat-induced destabilization. A plausible physico-chemical mechanism is tentatively proposed to explain this behaviour based on the additional steric stabilization conferred by a low density of dangling casein tails.