Nonvolatile triacylglyceride (TAG) oxidation products play an important role in the oxidative degradation of lipids. They serve as a reservoir of oxygen-containing species and hence can act as off-flavor precursors or as initiators for further oxidation reactions. Possible nonvolatile lipid oxidation products are TAG with a hydroperoxy, hydroxy, epoxy, or oxo (ketone or aldehyde) group or combination of these groups. The breakdown of TAG hydroperoxides yields nonvolatile glyceride species with two intact fatty acid chains and one short chain mostly ending in an aldehyde or hydroxy group (2 1 / 2 glycerides). By means of normal-phase high-performance liquid chromatography (HPLC) with mass spectrometric (MS) detection, nonvolatile lipid oxidation products can be separated according to polarity. This results in separation into classes of TAG oxidation products, such as epoxy-TAG, oxo-TAG, hydroperoxy-TAG, hydroxy-TAG and 2 1 / 2 glycerides, which can be identified using selected ion chromatograms. The retention times of TAG oxidation products on the normal-phase HPLC system and the signal intensity of the MS detector are stable enough to enable quantitative analysis based on external calibration. The normal-phase HPLC-MS method is very suitable for the characterization and quantitation of nonvolatile TAG oxidation products in oxidized TAG reference compounds as well as in real oils or oil phases isolated from emulsions, spreads, or other fat-based food products. This method can give detailed information for the study of lipid oxidation mechanisms.
Triacylglyceride hydroperoxides (HPO-TAG), the primary autoxidation products of triacylglycerides (TAG), have been analyzed in polyunsaturated vegetable oils by means of nonaqueous reversed-phase high-performance liquid chromatography (HPLC) with ultraviolet detection. Using a retention time model based on equivalent carbon numbers, mono-and bishydroperoxy TAG and hydroxy TAG could be identified. The correlation between the peroxide value (POV) determined by iodometric titration and quantitative HPLC results for HPO-TAG was established for sunflower oil samples with POV between 0.5 and 50 meq/kg. The recovery of HPO-TAG in the HPLC procedure was found to be close to 100% in the POV range of 4 to 71 meq/kg. Absolute quantitative results for HPO-TAG in sunflower oil samples could not be obtained accurately, as molar extinction coefficients of HPO-TAG occurring in natural oils deviate from those of available HPO-TAG reference compounds.The oxidation of polyunsaturated lipids has received great attention due to increased consumption of vegetable oils with high unsaturation levels. Polyunsaturated lipids are oxidatively more labile than saturated and monounsaturated lipids and undergo autoxidation and photooxidation easily in the presence of initiators. Following a free radical chain mechanism, triacylglyceride hydroperoxides (HPO-TAG) are formed as primary products of triacylglyceride (TAG) oxidation. It is well established that HPO-TAG readily decompose into a wide range of aldehydes, ketones, hydrocarbons, and other volatile compounds which contribute to the flavor deterioration of lipid-containing food.Extensive oxidation experiments with subsequent analysis of the oxidation products using pure fatty acids and TAG as well as real oils have been carried out in order to elucidate the mechanism of lipid oxidation. For oxidation studies of vegetable oil TAG, triolein, trilinolein (LLL), and trilinolenin were frequently used as model systems. The main autoxidation products of triolein and LLL were identified as mono-, bis-, and trishydroperoxides, which are formed by sequential oxygen addition (1,2). The monohydroperoxides of triolein are composed of 8-, 9-, 10-, and 11-isomers (3). Autoxidation of LLL yields a mixture of 9-and 13-hydroperoxides as primary products (2,3; Scheme 1). LLL produces, in addition to mono-, bis-, and trishydroperoxides, substantial amounts of hydroperoxy epidioxides. The monohydroperoxides are composed of 9-, 12-, 13-, and 16-hydroperoxides (3,4). The cyclic peroxides are composed of 9-and 16-hydroperoxy epidioxides (4).Numerous attempts have been made to analyze hydroperoxides with respect to the position of the hydroperoxy group on the fatty acid chain, the sn-position in the TAG molecule, and the number of hydroperoxy groups per TAG molecule or per fatty acid chain (1-5). Park et al. (5) compared normalphase high-performance liquid chromatography (NP-HPLC) and reversed-phase (RP) HPLC for the analysis of autoxidized LLL and some vegetable oils. NP-HPLC of oxidized LLL revealed the geometri...
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