By Harold McGee
The principal pigment in meat is the oxygen-storing protein myoglobin, which can assume several different forms and hues depending on its chemical environment. Myoglobin consists of two connected structures: a kind of molecular cage with an iron atom at the center, and an attached protein. When the iron is holding onto a molecule of oxygen, myoglobin is bright red. When the oxygen is pulled away by enzymes in the muscle cell that need it, the myoglobin becomes dark purple. (Similarly, hemoglobin is red in our arteries because it’s fresh from our lungs, and blue in our veins because it has unloaded oxygen into our cells.) And when oxygen manages to rob the iron atom of an electron and then escape, the iron atom loses its ability to hold oxygen at all, has to settle for a water molecule, and the myoglobin becomes brown.