Did you know that bright red tuna isn’t always fresh? Here’s what most people never get told ⬇️ Fresh tuna naturally changes color over time. As it ages, oxygen interacts with myoglobin in the muscle, turning the flesh from deep red to brown or gray - a normal spoilage signal. But instead of discarding that fish… Some suppliers treat tuna with carbon monoxide (CO) or related compounds. Why? Because CO binds tightly to myoglobin and locks in a vivid cherry-red color, even as the fish continues to