CHARACTERIZATION OF OIL-IN-WATER EMULSIONS STABILIZED BY TYROSINASE-CROSSLINKED SOY PROTEIN ISOLATE AND ITS FRACTIONS

This study aims to improve the stability of soy proteins-stabilized emulsions in order to advance their functionality in food applications. The influence of crosslinking by tyrosinase from Bacillus megaterium (TyrBm) on emulsions stabilized by soy protein isolate (SPI) and purified β-conglycinin (BCG) was investigated. BCG-rich fractions were isolated according to the method of Nagano et al. (1992) and purified by selective proteolysis according to Nishiwaki et al. (2008). Native and crosslinked BCG were used to produce emulsions by means of high pressure homogenization. Findings show that high-molecular weight fractions of BCG were indeed formed by TyrBm as demonstrated by SDS-PAGE analyses. The emulsions obtained were characterized for their rheological behavior and particle size. The crosslinked-BCG stabilized emulsion showed shear thinning behavior as is the case of polymerization products, as opposed to Newtonian behavior of the native-BCG stabilized emulsion. Viscosity measurements showed that the enzymatic treatment increased emulsion viscosity. Particle size measurements using static light scattering demonstrated elevated particle size distribution by volume in the crosslinked-BCG stabilized emulsion in comparison with the native BCG emulsion. The emulsions were de-flocculated by addition of 1% SDS and agitation that abolished all bonds except for covalent bonds created by TyrBm. To characterize the emulsion structure, the oil phase was labelled with Nile red and the emulsions were observed under an inverted microscope. While the native-BCG stabilized emulsions were properly de-flocculated after SDS treatment, aggregates were still visible in the crosslinked-stabilized emulsion. We assume that the aggregates are the result of covalent bonds created by TyrBm. The properties of BCG-stabilized emulsions were compared to TyrBm crosslinked-glycinin emulsions and SPI-stabilized emulsions in order to establish possible future applications.