Rusting of Foods : Red Pigments Formed by Prostaglandin-Like Structure Substances

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Properties and structures of the red pigments formed by the reaction of prostaglandin-like conjugated carbonyls with amino acids were studied. After oxidation of polyunsaturated lipids in the dark at 40^ﾟC, the carbonyls, 0.05-0.1 % maximum production of applied lipids, were prepared successively by gel chromatography on Sephadex LH-20, column chromatography on Silica gel 60 and High Performance Chromatography (HPLC) on mu-Porasil. In the preparatory experiment, a new carbonyl of methyl 5-[2-(2-formylvinyl)-3-hydroxy-5-oxocyclopentyl] pentanoate and it's precursor were found in autooxidized gamma-linolenate. Using purified 3-(2-ethyl-5-hydroxy-3-oxo-cyclopentyl)-2-propenal and Gly, effects of pH and reaction temperature on the formation of red pigments were investigated : at pH 7 the maximum production was obtained ; at acidic pH the least reaction rate was observed ; at alkaline pH or in high temperature the pigments were unstable and changed into fluorescent substances. Using a HPLC-equipment with UV, VIS, and fluorescence detectors, each of the pigment mixtures obtained by the reaction with various amino acids was analyzed. One or more of pigment peaks (A_<515nm> peculiar to the applied amino acids were observed without fluorescence. The results suggested that the whole molecules of amino acids were incorporated without Schiff base condensation into red pigments. Structure analyses of red pigments are in progress by mass spectrometry and NMR-spectrometry.