| Photooxidation and photosensitized oxidation on the formation of volatile compounds in linoleic acid, milk, and lard were studied by a combination of solid-phase microextraction (SPME)-gas chromatography (GC)-mass spectrometry (MS) and headspace oxygen content.; Photooxidation is the oxidation under light in the absence of photosensitizers such as chlorophyll and riboflavin. Photosensitized oxidation is the oxidation under light in the presence of photosensitizers. Total volatile compounds in linoleic acid without added chlorophyll under light and in the dark did not increase for 48 hr at 4°C. Total volatile compounds in linoleic acid with added 5 ppm chlorophyll under light at 4°C for 0, 6, 12, 24, and 48 hr, increased from 8.9 to 11.6, 21.7, 26.1, 29.3 (1 × 104 ) in electronic counts, respectively. 2-Pentylfuran, an undesirable reversion flavor compound in soybean oil, 2-octene-1-ol, 2-heptenal, and 1-octene-3-ol were formed by photosensitized oxidation only. Light excited photosensitizers like chlorophyll can generate singlet oxygen from ordinary triplet oxygen. 2-Pentylfuran, 2-heptenal, and 1-octene-3-ol can come from C10, C12, and C10 hydroperoxide of linoleic acid, respectively, which can be formed by singlet oxygen oxidation but not by triplet oxygen oxidation on linoleic acid. The singlet oxygen oxidation mechanisms for 2-pentylfuran, 2-heptenal, 1-octene-3-ol, and 2-octene-1-ol from linoleic acid were proposed.; Milk with or without added riboflavin, ascorbic acid, sodium azide, and butylated hydroxyanisol (BHA) was stored at 4°C under light and in the dark. Pentanal, dimethyl disulfides, hexanal, and heptanal were formed only in the light stored milk and increased significantly as the added riboflavin concentration increased from 5 to 10, 50 ppm (P < 0.05). As fat content in milk increased from 0.5 to 1.0, 2.0, and 3.4%, pentanal, hexanal, and heptanal increased significantly (P < 0.05) but dimethyl disulfide concentration did not change. BHA and ascorbic acid, hydrogen donating free radical scavengers, reduced hexanal and heptanal formation. Sodium azide, a singlet oxygen quencher, prevented dimethyl disulfide formation. Formation of pentanal is different from that of hexanal and heptanal in milk. Singlet oxygen and free radicals play important roles in the formation of volatile compounds in photosensitized milk.; Volatile compounds and headspace oxygen content from lard containing 0 and 5 ppm chlorophyll in air-tight 10-mL bottles were studied at 55°C under light and in the dark. Total volatile compounds in lard with 5 ppm chlorophyll under light were 19 times more than that in samples with 0 ppm chlorophyll for 48 hr. Headspace oxygen content in lard with 5 ppm chlorophyll under light changed from 21 to 15% for 48 hr but that in lard with 0 ppm chlorophyll did not change significantly (P > 0.05). Photosensitizer-free lipid model system using lard was developed. Photooxidation mechanism seems to be a free radical chain reaction like autoxidation.; Photosensitizers like chlorophyll and riboflavin play important roles in the formation of volatile compounds. Singlet oxygen, which can be generated from triplet oxygen by light excited photosensitizers, is involved in the formation of volatile compounds in linoleic acid, milk, and lard. Photosensitizer content in food should be minimized and food products should be packed with light impermeable package to prevent the photosensitized oxidation on lipids. |
