Antioxidant effect of onion peel extracts (Allium cepa L.) on the stability of soybean oil under thermo-oxidative degradation

Onion peels have considerable amounts of bioactive compounds with high antioxidant capacity. This study aimed to assess the antioxidant capacity of yellow, white, and purple onion peel extracts and to analyze their antioxidant effects on the thermo-oxidative stability of soybean oil. The extracts were assessed regarding the total phenolic compounds and antioxidant activities. The oil was supplemented with extracts of onion peel, tocopherol, ascorbyl palmitate, and tert-butylhydroquinone, whether isolated and combined. The treatments were subjected to thermo-oxidation and the samples were analyzed for oxidative stability, total polar compounds, and tocopherols content. The purple onion peel extract showed the highest efficiency in phenolic compounds and antioxidant activity. In the oxidative stability analysis, OAP, OTBHQ and OPE+OTBHQ (synergistic effect) stood out. Total polar compounds were elevated in SO, OTOC, and OTOC+OAP at 8 hours, and all treatments exceeded the 25% limit at 16 hours. δ-tocopherol showed greater retention at the end of 16 hours in the OPE treatment with 51.54%. Therefore, the use of purple onion peel extract can delay oxidation and contribute to the retention of tocopherols, enabling the use of lower concentrations of synthetic antioxidants.