Effect of egg albumin on physical, rheological and microstructure of oil-in-water emulsions

Kinetically stable emulsions can be formed by using emulsifiers and/or thickening agents. Emulsifiers are mainly surface-active proteins which can adsorb on the droplet surfaces lowering surface tension and preventing the droplets from coming close enough together to aggregate. Biopolymers like proteins with emulsifier property and the ability to increase the viscosity of the emulsion systems are the essential ingredients in the formation of a stable emulsion. Emulsifiers have surface-active properties with ability of formation a compact and thick interfacial layer after the adsorption on the droplet surfaces, which results in a strong steric and electrostatic stabilization against coalescence and flocculation/aggregation. Egg white albumin (EWA) is one the best choices among proteins for fabrication of food emulsions with desirable properties, owing to its great functional possessions. In food based on emulsion systems, generally, steady state flow measurements and oscillatory (dynamic) measurements are used for the characterization of food emulsions. Steady state flow measurements are used for characterizing the shear thinning nature of food stabilizer. Oscillatory experiments are used to obtain information on the viscoelastic and gel characteristics of foods. Concerning emulsion rheological behaviour, it ranges between Newtonian and pseudoplastic. In this study, the effect of different concentrations (0.1, 0.3, 0.5, 1, 3 and 5%) of albumin on physical, rheological properties, as well as microstructure of oil-in-water (O/W) emulsions (20% wt/wt) were investigated. The emulsions were prepared by dispersing 20% (w/v) sunflower oil in the albumin solution (0.1-5%). Primary emulsions were prepared using an Ultra-Turrax blender at 11,000 rpm for 2 min. Fine emulsions were then prepared by homogenization of the primary emulsions using an ultrasonic apparatus for 2 min. Creaming was measured during 12 days. The rheological properties of samples were measured using rheometer. Frequency dependency of storage modulus and loss modulus can be calculated by Power law model. The amount of b and d is closer to zero when the frequency dependency is low, the higher amount of b represents the elastic characteristic of the samples, it is more than zero for physical gels, and zero for the covalent gels. Microstructure of the emulsions was studied at room temperature using light microscope. The experimental data were analyzed using SPSS statistical software. The rheological data fitting was done using Rheoplus software (Version 3.4, Ostfildern, Germany). Significant differences between the means of different treatments were determined by Duncan’s multiple range tests (P<0.05). The results showed that the stability of the emulsions containing 0.1% albumin was 67% while it increased to 95% with increasing the concentration of albumin to 5%. After 12 days of the storage at ambient temperature, there was less creaming observed for the emulsions containing higher concentration of albumin. Results of the flow behaviour test demonstrated a decrease in the viscosity of the samples resulted from the shear rate increasing, leading to the emulsions exhibiting a shear thinning behaviour. These behaviors can be explained with ordering of emulsion droplets along the flow direction due to overcoming Brownian motion with increasing shear rate, which causes a decrease in resistance to flow and thus, decreases the
viscosity. Due to a hysteresis area, the emulsions presented a thixotropic behaviour. Carreau was recognized as the best model for describing the rheological data in the current experiment. Emulsions containing 0.3% albumin had the highest zero shear viscosity, hysteresis area, and apparent viscosity (measured at constant shear rate of 50 s-1). Data of the strain sweep test showed that LVE was about 0.1%. In the emulsions containing different amounts of albumin, storage modulus was more than loss modulus and both increased with the frequency. In the case of the emulsion containing 0.3% albumin, the storage modulus was the maximum. Tan (δ) and complex viscosity increased and decreased, respectively, at frequency sweep test. Power low model described the frequency sweep data properly. Microstructure of emulsion showed that emulsions containing high albumin content (5%) demonstrate a considerably uniform size of droplets and amount of large droplet was decreased. Taken together, the results of the current study showed that the addition of albumin, as the natural emulsifier, up to 5% could improve the stability of the egg albumin emulsions, due to its emulsifying properties. This sample can be used in foods based on emulsion system.