According to USDA (United States Dept. of Agriculture), "red" meat contains more myoglobin than chicken or fish. Myoglobin, is one of the proteins in meat, holds the oxygen in the muscle. The amount of myoglobin in animal muscles determines the color of meat.
(Note: pork is classified a "red" meat because it contains more myoglobin than chicken or fish. When fresh pork is cooked, it becomes lighter in color, but it is still a red meat. Pork is classed as "livestock" along with veal, lamb and beef. All livestock are considered "red meat.")
The white meat of chicken has under 0.05%;
pork and veal have 0.1-0.3%;
young beef has 0.4-1.0%;
and old beef has 1.5-2.0%.
(Iowa State Animal Science, 2009)
Myoglobin 肌红蛋白 (ref)
Myoglobin (Mb) is the primary oxygen-carrying pigment of muscle tissues. It is a single-chain globular protein of amino acids, containing a heme (iron-containing porphyrin) prosthetic group in the center around which the remaining apoprotein folds. It has eight alpha helices and a hydrophobic core.
(Wikipedia)
Myoglobin is related to hemoglobin 血红蛋白, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. Unlike hemoglobin, myoglobin does not exhibit cooperative binding of oxygen, since positive cooperativity is a property of multimeric/oligomeric proteins only. The only time myoglobin is found in the bloodstream is when it is released following muscle injury. It is an abnormal finding, and can be diagnostically relevant when found in blood.
High concentrations of myoglobin in muscle cells allow organisms to hold their breaths longer. Diving mammals such as whales and seals have muscles with particularly high myoglobin abundance. Myoglobin is found in Type I muscle, Type II A and Type II B, but most texts consider myoglobin not to be found in smooth muscle. Skeletal muscle (or "voluntary muscle" is anchored by tendons to bone and is used to effect skeletal movement) is divided into several subtypes:
- Type I - slow oxidative, slow twitch, dense with capillaries and is rich in mitochondria and myoglobin. It can carry more oxygen and sustain aerobic activity.
- Type II A - fast twitch muscle, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.
- Type II B - (also known as type IIx), the fastest muscle type. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acid).
As mentioned above myoglobin is released from damaged muscle tissue. It is filtered by the kidneys but its ferrihemate portion that is dissociated from myoglobin in acidic environments (e.g., acidic urine, lysosomes) is toxic to the renal tubular epithelium and so may cause acute renal failure.
Color of Cooked Meat
The color that meat takes is partly determined by the oxidation states of the iron atom in myoglobin and the oxygen species attached to it. When meat is in its raw state, the iron atom is in the +2 oxidation state, and is bound to a dioxygen molecule (O2).
Meat cooked well done is brown because the iron is now in the +3 state, and is coordinated by a water molecule. If meat has been exposed to nitrites, it will remain pink despite being heated to high temperatures because the iron is bound to NO, nitric oxide (e.g., corned beef or cured hams). Note: pH is also a factor in cooked color. High pH retains pink/red color at high temperatures.
Grilled meats can also take on a pink "smoke ring" that comes from the iron binding to carbon monoxide. Raw meat packed in a carbon monoxide atmosphere also shows this same pink "smoke ring" too. This artificially-induced pink color can persist in the meat for a very long time, reportedly up to one year.
