Effect of microwave radiation on lipid oxidation of tilapia fish

This study investigated the effect of microwave cooking on the secondary oxidation of lipid in tilapia fillet treated with various antioxidants (ascorbic acid, alpha-tocopherol and BHT) during four weeks of storage at -10°C. Lipid oxidation in tilapia fillet is thought to initially occur after exposure to microwave radiation. Therefore, fillet were treated with three levels (0.01, 0.10 and 1.0%) of each antioxidant as a primary model to evaluate the optimum level required to protect lipids against oxidation induced by microwave radiation. The optimum level of the three antioxidants was found to be at 0.1% level. Therefore, fillet samples were treated with 0.1% of each antioxidant before frozen storage at -10°C. The experiment was conducted with a two way factorial design (4 x 4), four antioxidant treatments (none, ascorbic acid, alpha-tocopherol and BHT) and four storage periods (1, 2, 3 and 4 weeks) before and after microwave cooking. The lipid oxidation damage in tilapia fillet was determined using fluorimetric thiobarbituric acid (TBA) test. The TBA values of the fillet have significantly increased by either increasing microwave cooking or storage time. Addition of antioxidant protected lipid against oxidation induced by these two factors. Among the antioxidants tested alpha-tocopherol showed a superior effect of inhibition of lipid oxidation to BHT, while ascorbic acid has no significant effect against lipid oxidation in most cases. In addition, ascorbic acid was found to act as a prooxidant rather than antioxidant after microwave cooking, particularly after 4 weeks of storage at -10°C. The result of this study indicated that addition of alpha-tocopherol and BHT was required to reduce lipid oxidation which was induced by either microwave cooking or frozen storage times.