Effect of long-term storage, ultra-low temperature, and freshness on the quality characteristics of frozen tuna meat.

• Storage at ≤ −45 °C maintained tuna meat quality, irrespective of initial freshness. • MetMb formation was suppressed in all tuna meat samples stored at ≤ −45 °C. • High-pH tuna meat appearance, hardness, and flavor was maintained when stored at −40 °C. • The initial pH of frozen tuna meat affects the suitable storage temperature. • The quality of High- and Low-pH tuna meat can be maintained at ≤ −35 °C and ≤ −45 °C. This study clarified the validity of conventional frozen storage temperature of tuna meat, and investigated the effects of long-term storage and ultra-low temperatures, as well as the effect of the freshness at the time of freezing on the quality of tuna meat. Samples were grouped according to their pH: Low-pH (pH < 6.2) and High-pH (pH > 6.8). The increase in metMb% was higher in the Low-pH samples stored at temperatures higher than −40 °C. An increase in the metMb% was not observed in meat stored at −45 °C or lower, irrespective of the pH. The changes in the drip loss, the microstructure, and sensory scores in the High-pH samples were smaller than those in the Low-pH samples throughout frozen storage. The results demonstrated that the slight differences in freshness affected the quality of tuna meat during frozen and subsequent chilled storage. [ABSTRACT FROM AUTHOR]