This review paper is focused on the relative antioxidant activities of tocopherols and tocotrienols in oils and fats and certain food systems. a-Tocopherol generally showed better antioxidant activity than c-tocopherol in fats and oils, but at higher concentrations c-tocopherol was found to be a more active antioxidant. The results of studies on the optimum antioxidant concentrations of tocopherols in oils and fats indicated that the optimal level for a-tocopherol is usually lower than other tocopherols, meaning less a-tocopherol is needed for maximum antioxidant protection. There are comparatively very few studies related to the antioxidant activities of tocotrienols in oils and fats. It has been stated that generally c-tocotrienol has higher antioxidant effect than a-tocotrienol, and tocotrienols may be better antioxidants than their corresponding tocopherols in certain oils and fats systems. Studies on the antioxidant activity of various tocopherols in food systems are varied and cannot be uniformly evaluated because experiments have generally focused on different foods and used various methods for the detection of antioxidant activities. Depending on the food system, in certain cases tocopherols were better antioxidants than synthetic antioxidants such as butylhydroxy toluene (BHT) or butylhydroxy anisole (BHA). However, in certain other food systems the synthetic antioxidants were more effective to increase the shelf life and the stability of foods than those containing tocopherols.

The formation of 4-hydroxy-2-trans-nonenal (HNE), a mutagenic and cytotoxic product of the peroxidation of linoleic acid, was monitored in soybean oil that was heated at 185°C for 2, 4, 6, 8, and 10 h. Unheated soybean oil contained no HNE and a relatively low concentration of polar lipophilic secondary oxidation products (aldehydes and related carbonyl compounds), measured as 2,4-dinitrophenylhydrazine derivatives by HPLC. An increase in the concentration of both HNE and total lipophilic polar oxidation products was observed with increased exposure to frying temperature. A considerable concentration of HNE had already formed at 2 h and the concentration continued to increase at 4 and 6 h of heating. After 6 h the concentration of HNE decreased, possibly due to degradation of the aldehyde with further exposure to high temperature. The loss of endogenous tocopherols was also monitored in the heated oil, and the tocopherol concentration decreased as the secondary lipid oxidation products increased.

A very sensitive high-performance liquid chromatography (HPLC) method was developed for the simultaneous separation and measurement of nonpolar and polar lipophilic secondary lipid peroxidation products in vegetable oil. Seventeen nonpolar and 13 polar lipophilic aldehydes and related carbonyl compounds, derived from thermally oxidized soybean oil as 2,4-dinitrophenyl hydrazones, were separated simultaneously by reversed-phase HPLC. Detection limit for the individual compounds is 1 ng. Thirteen of the nonpolar carbonyl compounds were identified as: butanal, 2-butanone, pentanal, 2-pentanone, hexenal, hexanal, 2,4-heptadienal, 2-heptenal, octanal, 2-nonenal, 2,4-decadienal, decanal, and undecanal. Three of the polar carbonyl compounds were identified as: 4-hydroxy-2-hexenal, 4-hydroxy-2-octenal, and 4-hydroxy-2-nonenal. The detection of the toxic 4-hydroxy-2-nonenal, a major compound, and 4-hydroxy-2-hexenal, a minor compound, in heated soybean oil is of particular importance because these toxic compounds have been shown to be absorbed from the diet. a The solvent systems used for co-chromatography of the individual polar carbonyl compounds are indicated by an asterisk.

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The toxic aldehyde 4‐hydroxy‐2‐trans‐nonenal (HNE) is an oxidation product of linoleic acid and is formed during the thermal oxidation of soybean oil at frying temperature. This investigation was conducted to determine whether HNE would be incorporated into food fried in thermally oxidized soybean oil. Commercially available liquid soybean oil was heated at 185°C for 5 h prior to frying uniform pieces of potato (1×0.5×7 cm). The oil was sampled prior to and after frying and was analyzed for the presence of HNE and other polar lipophilic aldehydes and related carbonyl compounds by HPLC. The oil was also extracted from the fried potato pieces and was analyzed identically to the frying oil. HNE was found to be a major polar lipophilic compound in the thermally oxidized frying oil, as previously published by this laboratory, and in the oil extracted from the fried potato. Similar concentrations of HNE were found in the oil prior to and after frying and in the oil extracted from the fried potato (57.53±16.31, 52.40±6.10, and 59.64±11.91 mg HNE per 100 g oil, respectively). These results indicate that toxic HNE was readily incorporated into food fried in thermally oxidized oil; extensive consumption of such fried foods could be a health concern.