Alkyl caffeates as antioxidants in O/W emulsions: Impact of emulsifier type and endogenous tocopherols
Antioxidant addition can be one strategy to limit lipid oxidation in emulsions. Research has proven that an important factor regarding the efficacy of antioxidants is their localization in the emulsion; however, other factors such as interactions with other components can also have an impact. Thus, the aim was to evaluate the impact of emulsifiers (Citrem and Tween80) and presence of endogenous tocopherols on the efficacies of caffeic acid and caffeates (C1–C20) as antioxidants in emulsions. Lipid oxidation was evaluated during storage and partitioning of caffeic acid and caffeates was estimated by measuring their concentrations in the aqueous phase. Partitioning of caffeic acid and caffeates was influenced by emulsifier type and the presence of endogenous tocopherols. Caffeic acid was the most efficient antioxidant in Citrem and Tween stabilized emulsions in the presence of endogenous tocopherol. In contrast, for Tween stabilized emulsions, caffeic acid acted as a prooxidant and the evaluated caffeates acted as strong antioxidants in the absence of endogenous tocopherol. Thus, when endogenous tocopherol was present lipophilization of caffeic acid did not increase its efficacy as an antioxidant. It is suggested that the differences observed in antioxidant efficiency with different emulsifiers and with and without endogenous tocopherols is due to emulsifier–antioxidant interactions and antioxidant–antioxidant interactions in the emulsions. Practical applications: Food emulsions contain endogenous tocopherols, thus, the impact of endogenous tocopherols on the efficacy of applied antioxidants is of interest to the industry. So far the hypotheses about antioxidant in emulsions are based on simple emulsions systems without the presence of tocopherols. The finding in this study revealed that both emulsifier type and the presence of endogenous tocopherol had an impact on the efficacy of caffeic acid and caffeates due to emulsifier–antioxidant and antioxidant–antioxidant interactions. This highlights the importance of evaluating the antioxidant in each emulsion system before selecting antioxidants for optimal protection against lipid oxidation. CAT value determined in oil-in-water micro emulsions with caffeic acid and caffeates (100 μM) and with and without endogenous tocopherol and lipid oxidation is initiated with the water soluble radical AAPH.