Lipid oxidation significantly affects the quality of foods. A simple and fast method for evaluating the oxidative stability of foods during storage is needed to determine its degree of oxidation. Soybean oil accounts for 75% of all vegetable oil consumed in the United States while algal oil is commercially available as a dietary supplement because of its health benefits. The oxidative stability of soybean and algal oil under light storage was evaluated by studying headspace oxygen depletion (HOD) and total volatile compound contents. Results suggest that the degree of oxidation increased as storage time increased in soybean and algal oil under light storage. The contents of 2-octenal, 4-nonenal and 3 unknown compounds in soybean oil and 1-octen-3-ol, 2,4-nonadiene, 2-heptenal, 2-octenal, 2,4-decadienal and 2-ethylfuran in algal oil increased up to a certain point and then decreased during light storage.
Volatile compounds identified in algal oil under light storage were formed from the oxidation of oleic, linoleic and docosahexaenoic acid. Further oxidation of volatile compounds, including 2,4-decadienal and 2,4-heptadienal, and reactions between amine and carbonyl compounds may also lead to the formation of volatile compounds in algal oil under light storage, such as hexanal, 2-pentenal and benzothiazole. Results showed that there was no significant difference in the oxidative stability between soybean and algal oil during storage. However, differences in quantitative changes of volatile compound were observed. Differences in fatty acid composition in the two oils led to the differences in quantitative changes of volatile compound found under light storage. Algal oil was believed to have a lower oxidative stability than soybean oil under light storage becuase it contains large amounts of docosahexaenoic acid; however, the presence of photosensitizers in soybean oil and carotenoids in algal oil may explain why the oxidative stability of soybean oil was not greater than that of algal oil under light storage in this study.
Algal oil is unstable and rapidly becomes rancid with undesirable “fishy” odors when it is stored for short lengths of time. The addition of antioxidants is a practical way to overcome odor problems during storage. The antioxidant and prooxidant activity of tocopherols, ascorbyl palmitate and rosemary extract in algal oil under light storage was investigated and their impact on the oxidative stability of algal oil under light storage was studied. Tocopherols acted as antioxidants at 1000 ppm or below but were prooxidants at 1500 and 2000 ppm in algal oil during light storage. Oxidized tocopherol compounds and radicals formed during tocopherol oxidation acted as prooxidants in algal oil. Ascorbyl palmitate and rosemary extract were found to improve the oxidative stability of algal oil at optimum concentrations under light storage. A ternary mixture of 1000 ppm tocopherols, 200 ppm ascorbyl palmitate and 1500 ppm rosemary extract exhibited maximum antioxidant activity in algal oil under light storage. However, ascorbyl palmitate above 200 ppm and rosemary extract above 1500 ppm were also prooxidants in algal oil under light storage. Caution should be taken with the use of high concentrations of tocopherols, ascorbyl palmitate and rosemary extract.