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.

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.

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The stability of α‐tocopherol in stripped corn oils with or without added γ‐tocopherol was determined under different levels of relative humidity (RH) ranging from 0 to 93% at 60°C. The degree of oxidation was determined by the headspace oxygen content and conjugated dienoic acid (CDA) assays and the remaining α‐ and γ‐tocopherols were also analyzed. The stability of α‐tocopherol was significantly influenced by the presence of moisture, the concentration of α‐tocopherol, and the presence of γ‐tocopherol. Samples containing up to 100 ppm α‐tocopherol showed the lowest stability under 75% RH whereas those containing 200 ppm α‐tocopherol had the highest stability under 93% RH. Samples under 0% RH had the lowest α‐tocopherol stability in the presence of γ‐tocopherol. Generally, higher concentrations of γ‐tocopherol resulted in the retention of more α‐tocopherol in stripped bulk oils. Practical application: Tocopherol isomers are typical lipophilic antioxidants in oils. Many lipid‐rich foods contain tocopherol isomers to extend the shelf‐life of the products. The stability of α‐tocopherol was significantly influenced by the presence of moisture, the concentration of α‐tocopherol, and the presence of γ‐tocopherol. Generally, higher concentrations of γ‐tocopherol resulted in the retention of more α‐tocopherol in stripped bulk oils. Using the knowledge obtained in this study, shelf‐life of foods containing oils can be enhanced by modifying the moisture content and concentration and types of tocol isomers. Generally, α‐tocopherol and γ‐tocopherol were more stable at RH 32% and RH 75%, respectively in stripped bulk oils.

Effects of beta‐cyclodextrin (β‐CD), chitosan, and collagen on tocopherol and oxidative stability in heated oils were determined under moisture added condition at 180°C in this study. Collagen was added in the form of a mesh structure with different pore sizes (50, 100, 200, and 300 μm) whereas chitosan was added in the form of gel. Presence of 1% w/w β‐CD significantly (p < 0.05) reduced the formation of p‐anisidine compared to controls. Collagens also acted as antioxidants irrespective of pore sizes whereas chitosan gel failed to act as antioxidant or prooxidant. Collagen and β‐CD significantly (p < 0.05) protected the decomposition of total tocopherols while chitosan failed to show such protection. Collagen with pore size of 300 μm significantly (p < 0.05) stabilized γ‐ and δ‐ tocopherols compared to controls and oils added with β‐CD, chitosan, or collagen with pore size of 50 μm. The addition of β‐CD, chitosan, and collagen significantly (p < 0.05) reduced the moisture content in heated oil compared to controls. β‐CD or collagen mesh structure can be used in heated oils like frying condition to control the rates of lipid oxidation. Addition of biopolymers may extend the oxidative stability and shelf‐life of heated oils and help to produce more food products. Practical applications: β‐Cyclodextrin and collagen mesh structure in heated oils can enhance the stability of antioxidants like tocopherols and the oxidative stability were greatly enhanced in heated oils. Therefore, addition of β‐cylcodextrin or collagen mesh structure can be used in heated oils like frying condition to control the rates of lipid oxidation and extend the operation time in frying oils. Addition of β‐cyclodextrin and collagen mesh structure significantly enhanced the oxidative stability in heated oils. The stability of tocopherols especially γ‐ and δ‐tocopherols increases when collagen mesh structure is added. Moisture content is decreased in heated oils when β‐cyclodextrin, chitosan, and collagen are added.