Stability of whey protein emulsions to heat treatments is mainly governed by the stability of the proteins in the aqueous phase

Milk proteins are natural surfactants used to emulsify and to stabilize dairy emulsions. However, in numerous cases, they do not sufficiently stabilize emulsions against flocculation, coalescence, aggregation or gelation during processing such as heat treatments. By analogy to Pickering stabilization mechanism, it is suggested that whey protein aggregates could be more efficient than native whey proteins to stabilize emulsions. To confirm this hypothesis we investigated the stability of dairy emulsions reconstituted with 30% milk fat and 3 to 6% whey protein (commercial WPI) solutions unheated or heated prior to homogenization. Heating WPI solutions under specific physicochemical conditions prior to homogenization allowed the production of 2 types of heat-induced protein aggregates exhibiting differences in size and morphology. Emulsions stability was determined by visual observation, granulometry and confocal laser scanning microscopy (CLSM) after heating the samples at 120°C up to 30 min. Emulsions prepared with unheated WPI did not show any sign of macroscopic instabilities up to 30 min of heating at 120°C at low protein concentration (3%) but a gradual decrease of heat stability when protein concentration increased. Instability at high protein concentration is mainly due to protein aggregation or gelation in the aqueous phase entrapping the fat globules. In the opposite, emulsions formed with whey protein aggregates are destabilized as soon as 5 min of heating at 120°C for low protein concentration (3%) but are more stable at higher concentration. The stability of the emulsions at protein concentration higher than 4% is correlated to the stability of the aggregates in the aqueous phase. In conclusion, it is possible to form stable whey protein emulsions in a large protein concentration scale by a proper structuration of the proteins before emulsification even if a Pickering stabilization mechanism is excluded. It undoubtedly opens interesting perspectives in the formulation of dairy emulsions without additives.