Rheology and oxidative stability of whey protein isolate-stabilized menhaden oil-in-water emulsions as a function of heat treatment.

Menhaden oil-in-water emulsions (20%, v/v) were stabilized by 2 wt% whey protein isolate (WPI) with 0.2 wt% xanthan gum (XG) in the presence of 10 mM CaCl(2) and 200 microM EDTA at pH 7. Droplet size, lipid oxidation, and rheological properties of the emulsions were investigated as a function of heating temperature and time. During heating, droplet size reached a maximum at 70 degrees C and then decreased at 90 degrees C, which can be attributed to both heating effect on increased hydrophobic attractions and the influence of CaCl(2) on decreased electrostatic repulsions. Combination of effects of EDTA and heat treatment contributed to oxidative stability of the heated emulsions. The rheological data indicate that the WPI/XG-stabilized emulsions undergo a state transition from being viscous like to an elastic like upon substantial thermal treatment. Heating below 70 degrees C or for less than 10 min at 70 degrees C favors droplet aggregation while heating at 90 degrees C or for 15 min or longer at 70 degrees C facilitates WPI adsorption and rearrangement. WPI adsorption leads to the formation of protein network around the droplet surface, which promotes oxidative stability of menhaden oil. Heating also aggravates thermodynamic incompatibility between XG and WPI, which contributes to droplet aggregation and the accumulation of more WPI around the droplet surfaces as well.