Mixed plant-based emulsifiers inhibit the oxidation of proteins and lipids in walnut oil-in-water emulsions: Almond protein isolate-camellia saponin

The oxidation of lipids and proteins often occurs at the oil-water interface in emulsions, and so emulsifiers may influence the oxidative stability of these systems. The objective of this work was to assess the influence of mixed plant-based emulsifiers, almond protein isolate (API) and camellia saponin (CS), on the physical and oxidative stability of walnut oil-in-water emulsions. Initially, a 5 wt% walnut oil emulsion was formulated using 0.5 wt% API as an emulsifier, then different levels of CS (1.0–2.0 wt%) were added. Experiments were carried out under conditions where the API and CS had different charges: pH 3 (protein positive, saponin negative) and pH 7 (both negative). Analysis of the surface-potential and interfacial protein concentration indicated that mixed API-CS interfacial layers were present at the oil droplet surfaces, with the main driving forces for adsorption being electrostatic and/or hydrophobic interactions. Emulsions prepared with the mixed emulsifier system had greater resistance to droplet flocculation than those stabilized by API alone when incubated at 45 °C for 0, 3, and 6 days. Markers of lipid oxidation (hydroperoxides and TBARS) and protein oxidation (carbonyl formation, sulfhydryl loss, intrinsic fluorescence loss, and electrophoresis) were recorded during storage. These measurements showed that API-CS-coated droplets were more resistant to oxidation than API-coated ones. Oxidation was faster at pH 3 than pH 7, which was mainly linked to the higher water-solubility and chemical reactivity of the transition metals under acidic conditions. Our results indicate that a combination of almond protein and camellia saponin is suitable for forming plant-based emulsions with high physical and chemical stability.