Formation and physical characterization of soy protein-isoflavone dispersions and emulsions

Abstract

Proteins stabilize emulsions by adsorbing at the interface to prevent the natural tendency of droplet coalescence. Polyphenols (plant-derived secondary metabolites) are capable of complexing with proteins through structural interactions. Certain polyphenols have shown influence on interfacial properties of proteins, yet isoflavones specifically have not been widely investigated. Thus, the objective of this research was to study the effect of isoflavones on protein-stabilized oil-in-water emulsions. Soy protein fractions, assessed via SDS-PAGE and bicinchoninic acid assay, contained primarily β-conglycinin (7S) and glycinin (11S). Notable isoflavones identified from HPLC analysis were daidzin, genistin, and glycitin. Physical characteristics (size, zeta potential, polydispersity index, and wettability) of soy protein isolate-isoflavone (SPI–I) dispersions and emulsion droplets were measured with a Zetasizer Nano ZS and goniometer. Isoflavone addition significantly increased the contact angle of sessile drops towards neutral wettability. Heating appeared to significantly improve colloidal stability and emulsion droplets formed with heated SPI-I dispersions sustained size (∼200–245 nm) and zeta potential (∼-30-35 mV) over 7 days, with higher concentrations of isoflavones being generally correlated with smaller emulsion droplets. Collectively, these findings indicate that isoflavone addition improved emulsion stability compared to the control without isoflavones and that they can contribute to colloidal stability of food products.