Development of Soy Protein Isolate-Isoflavone Pickering Particles for Oil-in-Water (O/W) Emulsion Stability

Abstract

Many food and beverage systems are colloidal oil-in-water emulsions, which contain dispersed oil droplets throughout an aqueous phase. In particular, Pickering emulsions are stabilized by interfacial solid particles and offer improved stability compared to conventional emulsions via irreversible absorption, which prevents the natural tendency of droplets to coalesce. 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 the use of isoflavones has not been studied. Thus, the objective of this research is to understand the interaction of isoflavones on particle and emulsion stability. Okara (soybean residue) was initially evaluated to determine potential for protein and isoflavone capabilities. Overall extraction from okara was determined to be insufficient for further testing, thus soy protein isolate and isoflavone supplements was used for particle and emulsion formulation. Soy protein fractions were classified and assessed by SDS-PAGE and BCA test. Main proteins found were β-conglycinin (7S) and glycinin (11S) protein fractions. HPLC analysis and the Folin-Ciocalteu method were performed to identify and quantify the primary isoflavones used for particle formulation. Notable isoflavones identified from HPLC analysis were daidzin and genistin. Varying concentrations of isolated isoflavones were complexed to soy proteins through pH differentiation and induced heating. Physical characteristics (size, zeta potential, polydispersity index, and wettability) of soy protein isolate-isoflavone (SPI-I) particles and emulsion droplets (10% oil fraction) were measured using a Zetasizer Nano ZS and goniometer. The optimal concentration of soluble soy proteins for protein (control) particles was 3.3 mg/mL, which provided the smallest size (nm) while maintaining a low polydispersity (PdI). The introduction of a heating step to develop SPI-isoflavone particles greatly assisted in maintaining the size and polydispersity, especially during emulsion stability assessments. Particles formulated with higher concentrations (0.58mg/mL – 1.25mg/mL) of isoflavones had significantly smaller particle sizes. Though, isoflavone concentrations (0.94 mg/mL – 1.25mg/mL) showed a slight reduction of repulsive energy compared to the control. Moreover, the incorporation of isoflavones had also shown to significantly change the contact angle of sessile drops, moving towards a neutral wettability state. Destabilization of emulsions were assessed via creaming index. Results suggest that the use of isoflavones within emulsions decrease creaming within the first hour of being produced. Emulsion droplets formed by SPI-I particles sustained size and zeta potential over a 7-day evaluation. Microscopic observation indicated that emulsion droplets formed with isoflavones exhibited less flocculation and were more stable. In particular, emulsions formulated with 0.94 mg/mL – 1.25mg/mL isoflavones appeared the most stable as they exhibited well dispersed, small droplets. Collectively, these findings indicate that isoflavone addition improved emulsion stability compared to control particles without isoflavones. Overall, these results demonstrate potential for SPI-I particles as functional ingredient for food emulsion applications.