The freeze–thaw process used for beef muscle accelerated color deterioration during cold storage. The oxidation of Mb was not related to lipid oxidation. It is possible that the damage to cellular and sub-cellular compartments produced from physical disruption by ice crystals during freezing and thawing could release enzymes into sarcoplasm, and the enzymes could potentially be involved in MetMb formation in the final thawed meat. The MetMb reductase activity in mitochondria intermembrane space was decreased by freeze–thaw process.