Eating raw tuna that has been exposed to carbon monoxide gas is another matter. In this case you’re not breathing the gas, and for that matter you’re not even eating it. Gases, of course, are ephemeral, and the carbon monoxide doesn’t hang around on the fish after it’s done its job of brightening its color. The FDA has declared carbon-monoxide-treated tuna to be GRAS, which is “generally regarded as safe” because residual carbon monoxide on the fish is virtually absent. Remember that the carbon monoxide thwarts color changes by replacing the oxygen in the oxymyoglobin molecules. The oxymyoglobin is thus derailed from being oxidized to brown metmyoglobin. So in the end stable color and yummy looking fish.
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