Influence of muscle glycometabolism and lipid oxidation on the discoloration mechanism of dark muscle of cultured yellowtail Seriola quinqueradiata

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.