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The discoloration of yellowtail dark muscle begins at the boundary part between dark muscle and ordinary muscle, and progresses toward the body surface side. To gain a deeper understanding of this phenomenon, we examined the processes of glycolysis and lipid oxidation in the two types of muscle. The glycolysis analyses indicated that an increase of lactic acid and decline of pH occurred at the boundary between dark muscle and ordinary muscle. An in vitro experiment using dark muscle extract showed that the decline of pH promoted the oxidation of myoglobin. An in vitro experiment using a mixture of extracted lipid and myoglobin from the dark muscle showed that the oxidation of phospholipid promoted the oxidation of myoglobin. Thus, we conclude that the discoloration, caused by the decline of pH at the boundary between dark muscle and ordinary muscle, progresses toward the body surface with oxidation of the phospholipid.

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We investigated the in‰uence of diet and season on the discoloration of dark muscle of cultured yellowtail Seriola quinqueradiata. The amount of a-tocopherol (aToc) and ascorbic acid (AsA) in the diet aŠected the amount in dark muscle and discoloration was inhibited by the amount of AsA rather than the amount of aToc. Discoloration was suppressed by adding ascorbic acid derivatives to the diet. Discoloration varied depending on the season: the ratio of metmyoglobin to total myoglobin (metMb) was lower from August to October that than from December to March. The amount of AsA from December to March was low, and the pH of dark muscle from August to October was high. Although metMb was correlated with AsA content and pH, there was no correlation with aToc amount. Therefore, the amount of AsA and suppression of pH decrease were considered to be the key to suppressing discoloration.