Fresh, uncooked red meats like steak typically have a bright red color that changes to brown-gray once the meat is cooked to a certain temperature. This change in color is so sensitive to temperature that it is often used in place of a thermometer to tell when meat is done, as described by bis above. Both the fresh, red color and cooked, brown color are due to a protein pigment called myoglobin. Myoglobin is present in muscle tissues and is responsible for shuttling oxygen to these tissues. Muscles that are used more frequently, such as chicken legs, contain higher levels of myoglobin and are designated “dark meat” while muscles used less often, such as chicken breasts, contain lower levels and are labeled “white meat”.
Myoglobin has a porphyrin structure that looks like a ring with an iron molecule suspended in the center of the ring (check out the heme writeup for a similar structure). This iron is responsible for binding oxygen. When the iron molecule is in its ferrous state (Fe2+) it can bind oxygen, forming a complex called oxymyoglobin. This complex is responsible for giving fresh, raw steak its bright red color. When the iron is oxidized to its ferric state (Fe3+) myoglobin can no longer bind oxygen. Two different compounds can form from myoglobin in this state that both give meat a brown-gray color.
When freshly slaughtered meat is cut into steaks, the muscle tissue comes into contact with oxygen in the air. The myoglobin in the meat binds this oxygen, forming oxymyoglobin and giving the meat a red color. However, if fresh meat sits for a period of time, generally over the course of several days, the structure of the myoglobin changes. The iron molecule in the middle is oxidized from its ferrous to ferric form and a different complex is formed called metmyoglobin. This compound turns the raw meat a brown color. The meat is usually still safe to eat when cooked, but the brown, unappealing color turns off most consumers. To avoid having your fresh meat turn brown, use it as soon as possible after purchasing it.
Myoglobin is also responsible for changing meat from a red color to a brown one when it is cooked. This is because the structure of myoglobin is sensitive to temperature. At temperatures lower than 140 ° F the protein is stable and the meat stays red. However, between 140 and 160 ° F the iron molecule in the center loses an electron and is oxidized to its ferric state and myoglobin can no longer bind oxygen. This creates a compound called hemichrome, which has a different structure than metmyoglobin but also has a brown-gray color. When a steak is cooked the outside will heat up first, turning the edge of the steak brown. As the temperature increases the inside of the steak heats up, more myoglobin is changed into hemichrome, and more of the steak changes color from red to brown. The inside of the steak is the coolest part of the meat and will stay red until the steak is cooked to the well done stage (above 170 ° F). At this temperature there is enough hemichrome in the meat to turn it a light brown-gray color all the way through.
Occasionally red meat will not change color from red to brown when cooked. There are several reasons why this change doesn’t occur. One is that the meat has a higher pH than normal. Higher pH conditions hinder the conversion of myoglobin to hemichrome and therefore it takes longer cooking times or higher temperatures to turn the meat brown. Another reason is that the meat contains more myoglobin pigment. Because of these two conditions it is possible that meat cooked to safe temperatures will still be red. To be sure meats are thoroughly cooked, it’s best to use an instant-read thermometer in the thickest part of the meat instead of judging by color alone.