Maintenance of pH and Quality of Thawed Bigeye Tuna Meat by Temperature Control Before Thawing

Abstract

Frozen fish meat incurs thaw rigor if frozen immediately after sacrifice. In this study, we examined whether controlling the freezing storage temperature at which frozen fish meat is stored before thawing can help maintaining the pH and quality of thawed meat. We investigated the changes in the pH and quality of thawed bigeye tuna (Thunnus obesus) meat following storage under different temperatures. During thawing, storage temperature was increased from approximately −50°C to −10°C and maintained for several days. This thawing method has been suggested to be effective not only in the suppression of thaw rigor and drip loss, but also in maintaining high pH in thawed meat. In the present study, frozen bigeye tuna meat was stored at −10°C for 0 to 10 days and then thawed rapidly. Changes in the contents of NAD and ATP before and after thawing were examined, and the conditions necessary for the maintenance of pH were investigated. As a result, the content of NAD decreased below 0.2 µmol/ g when stored at −10 C for more than 2 days. After thawing, meat pH remained high (>6.5) during storage at 3°C for 7 days. The observed maintenance in pH would likely result in a reduction in anaerobic glycolysis and subsequent suppression of ATP degradation and lactic acid synthesis because of NAD reduction. These phenomena could contribute to the maintenance of meat pH after thawing. The suppressive effect of pH decrease was observed regardless of thawing speed. Furthermore, our result from sensory evaluation suggested that thawed tuna meat with high pH improved texture as Sashimi.