Antioxidant potential of non-modified and glycated soy proteins in the continuous phase of oil-in-water emulsions

Food emulsions with a high omega-3 polyunsaturated fatty acid content are desirable from a nutritional point of view. However, such products are particularly prone to lipid oxidation and have thus a limited shelf-life. The use of natural antioxidants is a promising and consumer-oriented strategy to counteract lipid oxidation. The addition of an excess of proteins to the continuous phase may be considered in that respect. Starting emulsions were prepared with either Tween 20 (a nonionic surfactant) or whey protein isolate (WPI). They were then supplemented with non-modified or dextran-glycated soy protein isolate (SPI) added to the continuous phase. As controls, emulsions with excess WPI or unreacted SPI/dextran mixture were also prepared. The addition of these compounds did not significantly affect the physical stability of emulsions, while the lipid oxidation inhibition capacity was, starting from the highest, in the order glycated SPI mixture ≈ SPI/dextran mixture > SPI > WPI. This suggests that SPI ingredients and dextran hold potential for mitigating lipid oxidation in emulsions. The antioxidant mechanisms involved include iron-binding and free radical-scavenging activities; the former effect is predominant by preventing transition metals from approaching the oil-water interface. Furthermore, compared to WPI-stabilized emulsions, the antioxidant potential of excess proteins is boosted in Tween 20-stabilized emulsions. Interaction of surfactants with proteins could lead to a conformational change of proteins, which could increase their ability to bind molecules involved in the reaction cascade. This study shows that it is possible to tune emulsions towards greater oxidative stability by adjusting protein localization and continuous phase composition, which reduces the need for synthetic antioxidants.