Dynamic adsorption and interfacial rheology of whey protein isolate at oil-water interfaces: Effects of protein concentration, pH and heat treatment

The effects of bulk protein concentration, C-p, (0.01, 0.1, 1 wt%), pH (3, 4.7 and 7) and heat treatment (unheated or 95 degrees C for 30 min) on whey protein isolate (WPI) stabilized interfaces were examined. The interfacial pressure and shear rheology of WPI-stabilized sunflower oil-water (o/w) interfaces were characterized using a pendant drop tensiometer and a rheometer equipped with a Du Nouy ring. The rate of WPI adsorption was faster at higer C-p and pH 3. Heat-enhanced surface activity was more pronounced at pH 7 compared to pH 3 as a result of greater heat stability of WPI at acidic pH. The elastic modulus of WPI stabilized interfaces increased with Cp (<= 0.1 wt%). A further increase in C-p (to 1 wt%) resulted in monolayer collapse and weaker films. Non-heated (NHT) WPI formed less elastic interfacial films at pH 3 than at pH7. Heat treatment enhanced the elastic behavior of interfacial films with longer relaxation times. This may be associated with the formation of intermolecular beta-sheets. The knowledge gained on the nature of WPI-stabilized interfaces can be used to better understand the stability of dairy emulsions during subsequent processing, digestion or storage.