Myoglobin
The oxygen-storing protein in muscle tissue that gives steak its red color — and whose denaturation determines visible doneness.
Myoglobin is a globular protein found in muscle tissue that stores oxygen for aerobic metabolism. It contains a heme group (an iron-containing molecule) that binds oxygen and gives meat its characteristic color. Myoglobin is the reason raw meat is red, cooked meat is brown, and "steak juice" is red — none of which involves blood.
Color changes with temperature: Myoglobin exists in several states depending on its chemical environment. In fresh raw meat exposed to air, it's bright red (oxymyoglobin). In vacuum-sealed meat, it's purple-red (deoxymyoglobin). As temperature rises, the protein denatures and the heme group's configuration changes: - Below 120°F: red/purple (native state) - 120–140°F: pink to light pink (partial denaturation) - Above 150°F: gray-brown (fully denatured, hemichrome form)
The "blood" myth: The red liquid that drains from a steak is not blood. Blood is removed during slaughter. The red liquid is water with dissolved myoglobin. Myoglobin's heme group gives it a similar red appearance to hemoglobin (the oxygen-carrying protein in blood), but they're structurally and functionally different proteins.
Muscle activity and myoglobin concentration: Active muscles need more oxygen storage, so they contain more myoglobin and appear darker. This is why beef (from a large, active animal) is redder than pork or chicken, and why leg muscles are darker than loin muscles.