I always prefer to taste akami to judge and it is a matter of contains myoglobin, a pigmented protein that stores oxygen in the muscle tissue. The amount of myoglobin in a tuna’s muscle determines its color. The more myoglobin the redder the flesh.
The amount of myoglobin is a function of a tuna’s age, physical activity and species. When the flesh of tuna is exposed to air particularly when it is cut into pieces, an iron ion in the myoglobin molecule will start to oxidize. The dark, purplish red color of freshly cut tuna is due to deoxymyoglobin, which in air changes first to bright red oxymyoglobin and then to brown metmyoglobin.