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This study investigated the effects of stripping treatment and the presence of α-tocopherol on oxidative stability of stripped or non-stripped soybean oils containing quercetin or rutin (0.4 mM) stored at 25°C for 45 days. The degree of oxidation of oils was determined by analyzing headspace oxygen content, conjugated dienoic acid, and p-anisidine values. The contents of remaining α-tocopherol and the moisture content in soybean oils were also determined. Added quercetin or rutin acted as an antioxidant in stripped soybean oils and as a prooxidant in non-stripped soybean oils. The presence of 0.05 and 0.1 mM α-tocopherol accelerated the rates of lipid oxidation in non-stripped oils containing quercetin or rutin. Moisture content in non-stripped oils with rutin was significantly higher than that in other samples, and increased with the concentration of α-tocopherol ( p < 0.05). In addition, quercetin and rutin helped increase the moisture content in stripped oils, but not in non-stripped oils. Quercetin and rutin accelerated the decomposition of α-tocopherol in both stripped and non-stripped oils. The oxidative stability of oils containing tocopherols and phenolic compounds depended on the stripping process. Practical application: Phenolic compounds are natural antioxidants and are frequently found in edible oils. The presence of quercetin and rutin, which are aglycone and its glycoside phenolic compound, respectively, altered the oxidative stability of oils depending on the stripping process. The moisture content in oils could play an important role in the oxidative stability of bulk oils. For industrial applications, phenolic compounds should be added with care because they could lower the oxidative stability of non-stripped oils. Added quercetin or rutin acted as an antioxidant in stripped soybean oils and as a prooxidant in non-stripped soybean oils. Quercetin and rutin increased the moisture content in stripped oils, but not in non-stripped oils. Quercetin and rutin accelerated the decomposition of α-tocopherol in both stripped and non-stripped oils. The oxidative stability of oils containing tocopherols and phenolic compounds depended on the stripping process. [ABSTRACT FROM AUTHOR]

The stability of tocopherol homologs and oxidative stability were determined in edible oils including soybean, corn, canola, and olive oils under different relative humidities (RH), ranging from 0 to 93%, at 25°C for 8 months. The degree of oxidation was determined by conjugated dienoic acid (CDA) and p-anisidine values ( p-AV), and any remaining tocopherols were also analyzed. The stability of tocopherols was significantly influenced by the presence of both moisture and other tocopherol homologs. Soybean and corn oils under low moisture conditions had significant remaining tocopherol levels, whereas canola and olive oils had different stabilities of tocopherols. The presence of δ-tocopherol, which is found in soybean and corn oils, seems to play an important role in the stability of α- and γ-tocopherols in vegetable oils stored at 25°C. The moisture content of the tested oils was different even when oils were stored at the same RH. Practical application: Many lipid-rich foods contain tocopherol homologs to extend the shelf - life of the products. The stability of tocopherol homologs was significantly influenced by the presence of moisture, the types of oils, and the concentration of tocopherol homologs. Using the knowledge obtained in this study, shelf-life of real foods containing oils can be enhanced by modifying the moisture content and concentration and types of tocol homologs. (i) Soybean and corn oils under low moisture conditions had significant remaining tocopherol levels, whereas canola and olive oils had different stabilities of tocopherols; (ii) the presence of δ-tocopherol, which is found in soybean and corn oils, played an important role in the stability of α- and γ-tocopherols in vegetable oils stored at 25°C; and (iii) the moisture content of the tested oils was different even when oils were stored at the same RH. [ABSTRACT FROM AUTHOR]