Physico-chemical characterization of protein stabilized oil-in-water emulsions

Abstract

Emulsifying properties of four different proteins (soy, potato, pea and whey) were analyzed and related to their solubility, surface hydrophobicity, surface tension and their ability to modify aqueous phase viscosity. The influence of protein (emulsifier) type and concentration (1, 4 and 7 %), and pH value of the system (protein native pH and pH 9.0) on droplet size distribution parameters, rheological behavior and stability of fish oil-in-water emulsions (5, 15 and 30 % of dispersed phase) was determined. The examined proteins showed similar surface activity. The highest aqueous solution viscosity was exhibited by soy protein, while the viscosity of pea protein aqueous solution was the lowest. Droplet size measurements of emulsions showed that increase in protein concentration from 1 to 4% resulted in the formation of smaller droplets during the homogenization, while further increase in protein concentration up to 7 % did not maintain the trend of droplets size decrease. Increase in emulsions pH from protein native to pH 9.0 for almost all proteins led to decrease in average droplets diameters of emulsions and formation of smaller droplets (except for emulsions stabilized by whey protein where the decrease was not significant). All investigated emulsions showed shear-thinning flow behavior. The consistency indexes (K) of emulsions, which are indicators of the system viscose behavior, increased with the increase in protein concentration, oil content and pH of the system. Despite low solubility and surface hydrophobicity, the soy protein emulsifier was capable of producing the most stable emulsions with the smallest droplets compared to the other three proteins. The soy protein stabilized emulsions were also characterized with the highest emulsion consistency index and continuous phase viscosity in comparison to emulsions containing other proteins indicating the role of protein as thickening agent in stabilizing emulsion.