Tuna quickly turns an unappetizing brown (or chocolate, as it is called in the industry), whether it is fresh or conventionally frozen and thawed. Carbon monoxide, a gas that is also a component of wood smoke, prevents the flesh from discoloring.

Is Brown tuna bad?

The color

Checking the color before you eat your canned tuna can assure you that your food is still good to eat. However, do not eat your tuna if there are any spots of discoloration. These could be dark brown, green, or black. Discolored tuna is definitely not safe to eat so you shouldn't even think about it.

Is Brown tuna safe to eat?

Strange Color

Otherwise, tuna turns a visually unappealing brown shade. Tuna in cans is already a beige to brown color. Watch for dark brown discoloration around the bone, in tuna steaks. Dark brown, black or green colors -- in fresh or canned tuna -- let you know that the tuna is unsafe to eat.