Rapid Determination of Hydrocarbons in Edible Oil as an Index of Lipid Peroxidation and Its Application to Determination of the Effect of Chlorophyll Derivatives on Light-Induced Hydrocarbon Formation in Edible Oil

Abstract

A convenient and rapid method for measurement of hydrocarbons formed in edible oil was devised. An oil sample (0.1g), Tween-80 (0.1g) and 8ml of phosphate buffer (pH 7) were mixed well in a reaction flask with a stopper.After addition of ascorbate or cupric sulfate, the flask was kept in a water bath at 37C for 30min with shaking.The headspace gas (2ml) was analyzed by gas chromatography.The greatest amount of hydrocarbons was formed from oil when 0.1mM cupric sulfate and 30mM ascorbic acid were added to the sample mixture. This method allows the analysis of hydrocarbons within 50min, and lipid deterioration can be assessed more sensitively than by the determination of peroxide values. Detection limits of ethane and pentane in oils were 0.1ppm and 0.05ppm, respectively.The major hydrocarbon gas product from soybean oil was ethane or pentane, depending on the contents of linolenic acid and linoleic acid, respectively, in the oil. When edible oil spiked with chlorophyll a, pheophorbide a or pyropheophorbide a was exposed to light, these pigments accelerated the oxidative deterioration of the oil. There was no significant difference in hydrocarbon formation among these pigments up to 6 hr of irradiation, but after 8 hr of irradiation, a slightly higher prooxidant effect was observed with pyropheophorbide a (0.1pmol/g) than with the others.