Yellowtail Seriora quinqueradiata is the most widely cultivated marine fish species in Japan. One of the major problems in the end-product of yellowtail is the speed of the d iscoloration of the dark muscle tissue. Until now , most of the studies have been focused on the effect of supplementation of a specific or few antioxidants on the discoloration of dark muscle. However, fish meal (FM), which is the main feed ingredient, also contains antioxidant. Moreover, the antioxidant capacity of FM can fluctuate depending on many factors including the antioxidant additive level, the country of origin and the batch. Therefore, this study focused on the total antioxidant capacity (TAC) of feed, and its effect on the discoloration of yellowtail dark muscle was examined.
Two feeding tr ials were carried out at the Uragami Experiment Station, Aquaculture Research Institute of Kindai University, Japan. In trial 1, the control diet was designed to fulfi ll yellowtail vitamin E and C requirements (EC1). Three additional diets were formulated to contain 2.5, 5 and 10 times of both vitamins in diet EC1 and referred to as EC2.5, EC5 and EC10, respectively. Fifteen fish (mean weight ca. 460 g) were randomly distributed into each of eight net cages (2×2×2 m3), set in duplicate and fed once daily until apparent satiation for 4 weeks. In trial 2, test diets D1 to D4 were prepared using four types of FM with different antioxidant capacities. The amount of vitamin E and C added was set to a value corresponding to the amount found in the commercial diets. Ten fish (mean weight ca. 1300 g) were stocked into each of eight net cages and fed once daily until apparent satiation for 4 weeks.
TAC of diets EC1, EC2.5, EC5 and EC10 in trial 1 were 5200, 5770, 6730 and 8650 µmol ascorbic acid equivalents per kilo gram of feed, and those of D1, D2, D3 and D4 in trial 2 were 5230, 6450 , 6900 and 9520 µmol, respectively. In trial 1, while the fillet was stored at 4°C for 96 h, the decrease in color value a* (meat redness) and a*/b* (oxidation rate of myoglobin) and the increasing of TBARS (byproduct of lipid peroxidation) of dark muscle was significantly suppressed as the TAC of the feed increased. Trial 2 also displayed similar results. The results of both trials indicate that the difference in the TAC of the feed can be attributed to various factors which can influences the discoloration of dark muscle of yellowtail. This study demonstrates the importance of analyzing dietary TAC , instead of considering the amount of a particular ingredient, prior to use in diets for yellowtail.