Whey protein aggregates modulate the heat stability of whey protein-stabilized emulsions.
Résumé
Native milk proteins are natural surfactants used to emulsify and to stabilize dairy emulsions. However, at high protein concentrations, they do not sufficiently stabilize emulsions against flocculation, coalescence, aggregation or gelation during processing such as heat treatments and manufacturers used to add non-dairy additives to prevent above mentioned detrimental instabilities. Identification of alternative ways to protect dairy emulsions against instabilities without using non-dairy additives is an active research area in order to match with new market trends (clean label tendency). Alternatives using whey protein aggregates were suggested to be more efficient than native whey proteins to stabilize emulsions at high protein concentration1 but the exact mechanism is not completely clarified especially in an industrial context where whey protein ingredients contain variable amounts of caseins, peptides, etc.
To address this question we investigated the protein interfacial composition and the stability of dairy emulsions reconstituted with 30% milk fat and 3 to 6% model whey protein solutions unheated or heated prior to homogenization. Heating whey protein solutions at pH 7.0 prior to homogenization allowed the production of 70 nm-sized protein aggregates. Emulsion stability was determined by visual observation, granulometry and confocal laser scanning microscopy after heating the samples at 120°C up to 30 min. Emulsions prepared with unheated whey proteins did not show any sign of macroscopic instabilities up to 30 min of heating at low protein concentration (3%), but show a gradual decrease of heat stability when protein concentration increased. Instability at high protein concentration was due to protein aggregation or gelation in the continuous phase with the participation of protein-coated fat globules. In the opposite, emulsions formed with whey protein aggregates were destabilized at 10 min of heating for low protein concentration (3%), but were more stable at higher concentration. The destabilization at low protein concentration was governed by the aggregation of interfacial whey protein aggregates leading to fat globule flocculation and emulsion thickening. The stability of the emulsions at protein concentration higher than 4% was correlated with the stability of the aggregates in the aqueous phase and the decrease in the proportion of whey protein aggregates at the oil/water interface due to an increasing competition with peptides and caseins naturally present in the model whey protein powder.