Effect of lipid hydroperoxides on pheophytin in the dark

Abstract

Pheophytin was heated with hydroperoxyoleate in the medium of methyl palmitate, methyl oleate or methyl linoleate to 60 "C in inert gas. The rate of pheophytin destruction due mostly to free peroxy radicals generated by hydroperoxide decomposition was proportional to the content of hydroperoxide. Double bonds of fatty acid esters were competitively attacked by free peroxy radicals. In systems containing low levels of oxygen, methyl esters were slowly autoxidized on heating, and pheophytin moderately inhibited their autoxidation. Because of a low concentration of pheophytins the antioxidant activity was more pronounced under conditions of a lower rate of oxidation chain initiation, i.e. in more saturated systems and at a lower content of hydroperoxides. IntroductionChlorophylls and pheophytins belong to minor substances occurring in oilseeds, which pass into crude oils during their processing, especially in the step of solvent extraction. Pheophytins prevail in crude oils due to their better solubility in hydrocarbon solvents used for the plant-scale extraction [I]. The content of chlorophyll pigments in oils depends on the ripeness degree of oilseed and on the processing conditions [2]. The content was found relatively high in zero-erucic acid rapeseed oil [3]. Pheophytins are mostly removed by refining, but traces remain even in refined oils [3,4].The prooxidizing effect of chlorophyll pigments exposed to light has been kuown for several decades [S] ; they act as photosensitizers [6]. Pheophytins produced more hydroperoxides during photosensitized oxidation of methyl oleate and methyl linoleate [7, 81 than chlorophylls under the same conditions. Contrary to pheophytins, chlorophylls are rapidly destroyed during photooxidation [9]. As vegetable oils are mostly stored protected against light, we have investigated the effect of hydroperoxides on pheophytin in the dark, and the effect of pheophytin on lipid oxidation under low partial pressure of oxygen. Experimental MaterialPheophytin was prepared from lettuce leaves after HYNNINEN [lo], and its purity confirmed by spectral characteristics. Methyl palmitate, methyl oleate, and methyl linoleate were prepared by acid-catalyzed 172Die Nahrung 33 (1989) 2 esterification of the respective fatty acids, and distilled under reduced pressure. The substrates were checked for the content of peroxides, and traces present in the substrate were accounted for in the calculations.Methyl hydroperoxyoleate was prepared by autoxidizing methyl oleate at 60 "C in the dark till the peroxide content of 500 mmole . kg-' ; pure concentrate was prepared by liquid-liquid extraction after MERCIER [ll], and fractionated by liquid chromatography on silica gel [12], the least polar (more active) hydroperoxidic fraction being used for the experiments. Analytical methodsThe hydroperoxide content was determined spectrophotometrically in the presence of cadmium acetate [13]: the calibration curve was determined on the basis of the iodometric procedure after IUPAC [14]; pheophytin was determined by s...