Abstract

The burnt condition in tuna, known in the Japanese tuna industry as yake, is an abnormal post-capture quality deterioration characterized by muscle discoloration, softening, and off-odors; “burnt” does not indicate cooking or external burning. This exploratory study evaluated whether low-frequency A-mode ultrasound can provide objective, non-destructive information on the burnt condition in frozen yellowfin tuna (Thunnus albacares). Ultrasound waveform characteristics were integrated with ATP-related compounds, K-value, physical properties, and color indices. Ultrasound waveform amplitude and attenuation in the 40–65 µs range differed clearly between fresh and burnt-condition samples. These characteristics also distinguished between two burnt-condition samples of differing severity (NF1 and NF2). Total ATP-related compound content did not differ between groups, whereas the K-value was consistently higher in burnt-condition samples. Physical properties (hardness, cohesiveness, and gumminess) and color indices (a*, b*, and R-value) were also significantly different between fresh and burnt-condition samples. Unlike ultrasound waveform characteristics, however, these parameters varied little between NF1 and NF2. Principal component analysis confirmed that ultrasound waveform amplitude was significantly correlated with the component separating fresh and burnt-condition samples. This variable also clustered with ATP degradation-related indices. These findings suggest that ultrasound waveform characteristics capture internal quality changes related to the degree of the burnt condition, which conventional physicochemical indicators cannot fully represent. This finding supports the potential for non-destructive frozen-state evaluation and sorting of yellowfin tuna.

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