Have you ever ordered a steak done medium — or even well — and then reeled from shock when it came back with red liquid pooling underneath it? Surprise — contrary to popular belief, this liquid is not actually blood!
Most of the blood in meat is removed during processing, and the little that remains is located inside the meat itself. The red liquid is actually a mixture of water and myoglobin, a protein that transports oxygen to muscles. During exercise, the oxygen from myoglobin supplements the oxygen carried by hemoglobin in the blood, allowing the muscle to continue its activity.
The red liquid is actually a mixture of water and myoglobin, a protein that transports oxygen to muscles.
Because myoglobin provides the additional oxygen needed for prolonged exertion, higher levels of it are found in those muscles that are used more often. These muscles appear red, since myoglobin itself is reddish in color. This explains the distinction between red and white meat. The muscles in red meat are used constantly, such as muscles involved in walking or swimming. White meat, such as breast meat, is composed of more inactive muscles.
The white and red meat composition also differs based on the animal. Chickens, for example, contain both white and red meat. Cows, on the other hand, supply red meat almost exclusively due to their more active nature. Pigs do contain myoglobin in their muscles but less than cows, perhaps due to their more inactive nature. Most fish are considered to be white meat, but those that swim for long distances (e.g. during migration), like tuna and shark, contain substantial amounts of red meat as well, usually in the fin and tail regions.
Myoglobin itself is a protein containing a porphyrin ring, a cyclic structure with alternating double bonds that give it an intense color. At the center of the ring is an iron atom, which is bound to an oxygen molecule and has an oxidation state of +2. The oxidation state refers to the number of electrons that are removed (positive oxidation state) or added (negative oxidation state) to the atom. The +2 oxidation state means that iron lost two electrons, and gives raw red meat its red color. When the meat is cooked, the iron loses an electron and goes to an oxidation state of +3, a change that also turns the meat brown. Oxidation can also occur naturally when the meat is exposed to air.
Although the red color of meat is often associated with freshness, this metric can be misleading. Oxidation, the loss of electrons, can be prevented by artificial means. Carbon monoxide, for example, binds to the iron in myoglobin and prevents it from interacting with oxygen. This stabilizes the red color. Nitrites, which are used in the preparation of bacon and other preserved meats, also allow it to keep its red color. So just as you might be fooled into thinking that the red liquid in your steak is blood, you might also be fooled into thinking that your meat is fresh by the color!