Interfacial compositions, microstructure and stability of oil-in-water emulsions formed with mixtures of milk proteins and κ-carrageenan: 2. Whey protein isolate (WPI)

The changes in droplet size, surface coverage and composition, and stability of emulsions formed with 30% (w/w) soya oil and aqueous solutions containing 0.5 or 3.0% whey protein isolate (WPI) and various concentrations of κ-carrageenan were examined. At neutral pH, addition of κ-carrageenan up to 0.4% (w/w), before homogenization, slightly increased the average droplet size, but had no significant effect on protein surface coverage and composition. Addition of low levels of κ-carrageenan (approximately 0.025% in the aqueous phase) caused extensive flocculation of oil droplets through depletion interactions, which resulted in very low creaming stability. At higher κ-carrageenan concentrations, the creaming stability increased, apparently due to the formation of a network of flocculated droplets. All emulsions formed without κ-carrageenan showed Newtonian behaviour, but upon addition of κ-carrageenan, the emulsions became pseudoplastic. When the pH of the emulsions was lowered to pH≤6, the apparent viscosity of the emulsions, particularly at low shear rates increased, leading to a marked increase in stability to creaming. This effect was attributed to the bridging of the oil droplets through electrostatic interactions between WPI and κ-carrageenan at the interface of the oil droplets, forming a droplet network.