Synergistic effects of longan (Dimocarpus longan) peel extracts and food additives on oxidative stability of tuna oil

Longan peel, which is fruit by-product, is known for its antioxidant activities regarding the presence of phenolics. To minimize the use of synthetic antioxidants in food, natural antioxidants including phenolics from plant extracts have received much interest. This study screened combinatorial pretreatments between longan peel extract (LP) and food additives including vitamin E (E) and ascorbyl palmitate (AP) for application in refined tuna oil. Our results demonstrated synergistic effects of LP/E/AP pretreatment against lipid oxidation in the oil. Under Rancimat accelerated oxidation at 100 °C, the LP/E/AP combination exhibited a synergy by significantly increasing induction period in comparison to the blank control. Similarly, under accelerated storage at 50 °C, this combination significantly decreased peroxide value, conjugated dienes and thiobarbituric acid reactive substances. Gas chromatography analysis demonstrated that the LP/E/AP combination maintained the levels of polyunsaturated fatty acids, which was comparable with that of a synthetic antioxidant i.e. butylated hydroxytoluene (BHT). In addition, the combined pretreatment resulted in synergistic decrease of free radicals in the oil, which might contribute to the antioxidative effects. Collectively, our results have demonstrated the potential of LP used in combination with particular food additives to enhance protective effects against lipid oxidation.