Photo-oxidation of lipids impregnated on the surface of dried seaweed (Porphyra yezoensis Ueda). Characterization of volatiles

Volatiles generated by the photo-oxidation of lipids applied to the surface of dried seaweed (Porphyra yezoensis Ueda) previously exposed to visible light were studied. The surface of dried seaweed was impregnated with eicosapentaenoic acid (EPA) ethyl ester or linoleic acid (LA) methyl ester. The sample was then divided into two parts, and each part was sealed in a 50-mL crimp-top vial with a PTFE silicone-lined cap. One vial was exposed to light from a 100-W tungsten bulb (6000 lux) in an oven at 45°C. The other vial was covered with aluminum foil to shield the seaweed from light and kept in the oven as a control. Volatile compounds in the headspace of the vials were collected by a solid-phase microextraction technique and analyzed by GC-FID and GC-MS. The numbers of peaks as well as the peak areas of volatiles in the light-exposed sample were much greater than those in the control in GC profiles obtained in the same oxidation period. The peak areas of volatiles changed with the prolongation of oxidation time, and the formation rates were different between volatiles. Approximately 28 volatiles were identified in the control kept in the dark as well as in the light-exposed sample impregnated with EPA ethyl ester. The relative amounts of propanal, 2-propenal, 1-penten-3-one, 1-penten-3-ol, 2-butenal, heptanoic acid, and 2-pentenal in the headspace of the light-exposed vials were significantly higher than those in the control, whereas the relative amounts of 3,5-octadien-2-one, ethyl butyrate, and 2,4-heptadienal in the control were significantly higher than those in the headspace of the light-exposed vial. Approximately 35 volatiles were identified from the dried seaweed impregnated with LA methyl ester. The relative amounts of hexanal, 2-heptenal, 2-octenal, octanoic acid methyl ester, and hexanoic acid in the headspace of the light-exposed vial were significantly higher than those in the control, and the relative amounts of 2-decenal, 2,4-nonadienal, and 2,4-decadienal in the control were significantly higher than those in the headspace of the light-exposed vial. We proposed the formation mechanisms of some volatiles to be the well-accepted homolytic-heterolytic cleavage of hydroperoxides that were generated by oxidation of the unsaturated lipids.