Prevention of thaw-rigor during frozen storage of bigeye tuna Thunnus obesus and meat quality evaluation

Thaw-rigor is often found in frozen meat of bigeye tuna Thunnus obesus. Excessive amounts of drip loss and stiffness greatly lower the commercial value of tuna meat. In order to prevent thaw-rigor in meat stored at −60°C post-capture, we adapted a temperature shift technique that stores the meat at −7°C for 1 day or −10°C for 7 days before thawing. Biochemical changes in muscle of bigeye tuna before and after the temperature shift to −7 or −10°C were characterized. Contents of ATP, NAD⁺, glycogen, and creatine phosphate decreased after the temperature shift. NAD⁺ levels decreased faster than ATP levels and were highly correlated with the rigor index. Thaw-rigor occurred in muscle containing NAD⁺ at 1 μmol/g and ATP at 7 μmol/g. On the other hand, the meat color of tuna during frozen storage changed to brown depending on the storage temperature and reflected the rate of metmyoglobin (met-Mb) formation. Met-Mb formation increase was dependent on the decrease in NADH levels during the frozen storage. A temperature shift technique with storage at −7°C for 1 day or −10°C for 7 days before thawing prevented thaw-rigor and met-Mb formation.