Metmyoglobin is the oxidised form of the oxygen-carrying protein myoglobin. Metmyoglobin is the cause of the characteristic brown colouration of meat that occurs as it ages.

In living muscle, the concentration of metmyoglobin is vanishingly small, due to the presence of the enzyme metmyoglobin reductase, which, in the presence of the cofactor NADH and the coenzyme cytochrome b4 converts the Fe 3+ in the haem prosthetic group of metmyoglobin back to the Fe2+ of normal myoglobin. In meat, which is dead muscle, the normal processes of removing metmyoglobin are prevented from effecting this repair, or alternatively the rate of metmyoglobin formation exceeds their capacity, so that there is a nett accumulation of metmyoglobin as the meat ages.

This has important consequences for the meat packing industry, as brown meat is less aesthetically appealing, and in order to be sold, it is usually downgraded and minced, at a reduced price, resulting in a substantial economic loss