Study On Mechanism Of Color Reversion In Vegetable Oils And Fats During Storage

Vegetable oils are often light yellow when freshly refined. However, the color developsand darkens during storage and consuming. This phenomenon is called color reversion. Themechanism and inhibition method of color reversion have been problems for a long time onacademia and industry oils and fats. The current status, factors, main components andmechanism of color reversion were investigated in this study. The safety of color reversion wasstudied by cytotoxicity test.Firstly, the color change of360samples of seven oil categories was analyzed in6months.These oils included soybean oil, rapeseed oil, corn oil, sunflower oil, peanut oil, palm oil andblended oil. All seven categories were involved in color reversion, but the level of them varied.Color reversion was happened to about60%samples. Most of them darkened during storage.Corn oil was found to be the worst one with95.5%samples darken. Color reversion was weaklycorrelated to refining, TBHQ and peroxides and was not correlated to fatty acids profile andfatty acid value.Secondly, the factors on color reversion were studied in the model of MCT and stripedcorn oil. It was found that triacylglycerols oxidation, carotenes, phospholipids and phytosterolswere not the key and direct factors resulting color reversion of oils. Although oxidation of puretriacylglycerols did not lead to color reversion, the color reversion was found alwaysaccompanied by it. In this case, the production of radicals during oxidation of triacylglycerolswas preconditions of color reversion. γ-tocopherols was found to be the key factor resultingcolor reversion and α-tocopherols could inhibit color reversion by scavenging radicals firstlyand protecting the oxidation of γ-tocopherols. Furthermore, the ratio of α-and γ-tocopherolsaffected the level of color reversion. The more α-tocopherol, the less color reversion. When α-tocopherol: γ-tocopherols is higher than1:3, increasing of α-tocopherol could help to inhibitcolor reversion, but may promote oxidation of oils.Thirdly, the purification of compounds of color reversion was also conducted by TLC andFlash chromatogram and characterized by UV, IR,1HNMR, UPLC-MS-MS and UPLC-QTOF/MS. The result indicated that the major color reversion compounds were γ-tocopherolquinone and oxidized trialcylglycerols (mostly LLL, PPL, LLO, PPO, OOO) with onehydroxide. When γ-tocopherol quinone and oxidized trialcyglyerols existed in the system, thesignificant darkening of oils were observed because of the weak intermolecular force. Therefore,the possible mechanism of color reversion is that γ-tocopherol and triayclglycerols are oxidizedto γ-tocopherol quinine and oxidized trialcylglycerols and the dark color of the assembly ofthem result apparent color reversion of oils. α-tocopherol could resist color reversion significantly by inhibiting the production of γ-tocopherol quinone. Under light or heat, Thecolor reversion compounds are unstable, which may result in color lightening.Finally, the safety of the color reversion compounds was researched. It was found fromcytotoxicity test that color reversion compounds did not inhibit the production and propagationof HepG2, HEK-293, Hela and TF-1. This proved that color reversion compounds showed nocytotoxicity and did not affect safety of vegetable oils.