Changes in ultrastructure and cytochemical Mg2+-ATPase activity in the dark muscle of cultured yellowtail Seriola quinqueradiata stored in pure oxygen

We recently reported that refrigerating yellowtail Seriola quinqueradiata fillets in oxygen-filled packs slowed down the decrease in redness while suppressing browning in dark muscle as compared to refrigerat-ing the fillets in air-filled packs. To better understand this phenomenon, we stored cultured yellowtail mus-cles packed in oxygen and air at 10 & DEG;C, and compared changes in the color variables (L*, a* and b*), ultra -structure and cytochemical Mg2+-adenosine triphosphatase (Mg2+-ATPase) of the dorsal dark muscle. The dark muscle stored in oxygen exhibited significantly higher a* values (redness) than those stored in air. No significant difference was observed in the collapse of Z lines and A and I bands between the oxygen and air -stored muscles. Ultrastructural disruptions of sarcoplasmic reticula and mitochondria were less pronounced when muscles were stored in oxygen than in air. Mg2+-ATPase activity was observed in the sarcoplasmic re-ticula, mitochondria and nuclear membranes of the dark muscles immediately after death. On day 2, faint Mg2+-ATPase activity was detected in the sarcoplasmic reticula and mitochondria of muscle stored in oxygen but was undetectable in those stored in air. These findings suggest that the maintenance of mitochondrial functional activity and preservation of the ultrastructure of the sarcoplasmic reticula and mitochondria are much better when muscles are stored in oxygen, consequently delaying the color deterioration of the dark muscle.