The color of meat is much more complex than picking a chicken leg or a breast. Muscle fibers are organized into different categories depending on the amount of myosin, cellular content, and potential to metabolize food and produce ATP. The myoglobin and mitochondrial content of the cells is responsible for the different colors and taste of each tissue. We will focus on two kinds of muscle fibers: red fibers called type I (i.e., slow oxidative or slow‐twitch) muscle fibers and white muscle fibers called type II (i.e., fast glycolytic or fast‐twitch) fibers. There is a third intermediate muscle fiber type, a hybrid of both type I and type II fibers. Type I red muscle fibers use a wide variety of food macromolecules to produce energy in the form of ATP, including carbohydrates, amino acids, and fat. Type I cells require oxygen and additional enzymes to meet the metabolic demand and produce a sustained supply of ATP. These components both add a red color to the tissue and, when cooked, provide several interesting savory flavors to the meat. Type II or fast‐twitch muscle fibers primarily produce ATP from aerobic metabolism of carbohydrates.
These fibers, also called white or glycolytic muscle fibers, are rich in glycogen (i.e., “animal starch”) and the enzymes of the glycolytic pathway. As a result of the high concentration of glycogen in type II muscle fibers, these tissues hold more water and are considered juicier. Thus, endurance muscle groups will need a longer sustained ATP production for muscle contraction and will have a predominantly red color. Slower‐moving or less used muscles require less ATP except for short bursts of activity and will have more type II fibers and appear white.
The red/brown color of meat comes from the predominance of type I muscle fiber rich with metal binding proteins called hemoproteins. Lighter‐colored muscles are primarily composed of type II fibers that lack appreciable levels of these proteins and appear pale or white. Hemoproteins (a type of metalloprotein) bind an iron cation within a carbon cage known as a porphyrin ring. The iron bound within the porphyrin is called heme, and this reddish‐brown molecule is surrounded and held in place by the amino acids of a protein. Together the metal, heme, and protein are known as hemoproteins and have a wide range of interesting chemistry.
Type I muscle fibers have a high mitochondrial content. Mitochondria contain many colorful hemoproteins involved in oxidative metabolism of carbohydrates, amino acids, and fats. A family of hemoproteins in the mitochondria that give some of the dark brown/red tint are the cytochromes. These proteins are responsible for the transfer of electrons ultimately to oxygen in the generation of ATP.
Two additional major hemoproteins found in animals are hemoglobin and myoglobin. Hemoglobin is an iron‐containing protein located in the red blood cells and is responsible for transporting oxygen throughout the circulatory system. Myoglobin also binds oxygen; however myoglobin is limited to the muscle fiber where it stores O2 needed for muscle cell metabolism. Myoglobin is a monomeric (single‐chain) protein made mostly of alpha helices. As a storage compartment of oxygen in muscle, it is critical for the exercise capacity of red muscle.
The more oxygen a muscle fiber can contain, the more metabolism and ATP the cell can produce during long bouts of muscle use. The level of myoglobin can increase with exercise. Deep‐diving animals have such a high myoglobin content, which gives their tissue a dark red color. Both hemoglobin and myoglobin are deep red in color when the porphyrin ring iron is bound to oxygen. However, hemoglobin has almost no impact on the color of meat as most of the hemoglobin is bled from the tissue prior to packing. While there is some hemoglobin in fish post-harvest, myoglobin is the dominant pigment in muscle enriched with type I fiber.
The myoglobins and cytochromes are also excellent sources of bioavailable iron for humans. Iron is an essential mineral for the human diet—since humans also use the heme‐ (and therefore iron)‐containing proteins hemoglobin and myoglobin to transport and store oxygen. It is not well understood, but is well known, that animal sources of iron are more digestible in humans. Plants do contain iron, but we are unable to obtain much of it from eating the plant. The thought is that the heme molecule protects the iron and prevents it from being bound up by indigestible plant fiber.
The color of myoglobin and therefore red meat can change depending on what is bound to the myoglobin. Through the heme iron interaction, myoglobin can bind several small molecules including oxygen, carbon monoxide, and water. Each state of myoglobin (oxygen bound, carbon monoxide bound, and whether the iron is +2 or +3) has a different color absorbance that will significantly impact the appearance of the meat. There are three common forms of myoglobin: (i) oxygen‐bound myoglobin, which has a distinct cherry red color; (ii) deoxymyoglobin, where iron is in the +2 oxidation state and the iron is bound to water instead of oxygen giving myoglobin a purple color; and (iii) metmyoglobin, which takes place when the iron is in the +3 oxidation state and oxygen has been converted to water. Metmyoglobin is a result of the oxidation of iron—that is, the loss of an electron, which takes the iron from a +2 to a +3 cation, and the result is a brown‐colored myoglobin. This reaction occurs slowly over time. The formation of metmyoglobin is increased under conditions of high temperature and with the increase in acidity sometimes seen if an animal is stressed at the time of harvest.
To preserve the red color of fresh meat, some meat producers use carbon monoxide, a gas that, when present at the time of packing, will replace oxygen and water in the +2 iron/myoglobin to produce a deep red myoglobin and meat color that lasts twice as long as the red color of untreated meat. Use of CO to preserve the color of red meat has been used in several countries including Canada and Norway. In the United States the Food and Drug Administration will allow a maximum concentration of 4.5% CO during processing and packing. Because actual spoilage of meat is not about the color but rather contamination by microbes or the oxidation of fats, color is a poor indicator of meat spoilage. The amount of CO in the meat is harmless and tasteless, but this process, called modified atmosphere packaging (MAP), is still controversial.
Consumers are concerned about the masking of spoiled meat that looks attractive and red. However, others argue that CO‐enriched packaging is preferable to other MAP processes where meat is packaged in a high‐oxygen environment. High O2 MAP will provide a more “natural” red‐colored meat, but the presence of high concentrations of oxygen gas will also support harmful oxygen‐requiring (aerobic) bacteria. Addition of CO inhibits such dangerous oxygen‐requiring bacteria providing an additional safety factor in meat production.
Muscles are considered red or white as a result of the fiber type and amount of myoglobin found in the tissue. However, the distribution of type I and II muscle fiber varies within and between organisms. Chickens possess 10% red fiber in the white breast muscle, while the migratory duck, goose, and quail have 75–85% red muscle fiber in their breast muscle. There is very little muscle where all fiber is type I or type II, and most muscle is a mixture of the two. Pork, while considered the “other white meat,” is actually made of 15% or more red muscle fibers than white chicken muscle. Domestic pork is “white” because of the mostly sedentary lifestyle that limits the development of the red fibers. Fine differences are observed with the darker pork leg. A muscle supporting a bone that requires more stamina is darker, where the outer muscle is made of more of the glycolytic type II muscle fiber. Beef, while a red meat, mostly made of intermediate red fast‐twitch fiber type IIA (a hybrid of type I and II muscle cells) still has about 10% type I and 15% type II fiber.
Fish and shellfish are very different in red and white muscle makeup. Fish, with smaller muscle segments or myotomes, have mostly white aerobic muscle tissue arraigned for fast bursts of speed. Red muscle tissue is located just under the skin, particularly along the middle of the fish that powers the slower steady swim movements. Bottom‐dwelling fish (demersal) drift along with current; most of them are not active swimmers and do not have as much red fiber.
Active swimming top current fish (pelagic) have more red muscle fiber and will have a rich taste to the meat. Tuna, salmon, and shark, fish that swim long distances often times at high speed, are rich in mitochondria, myoglobin, and type I fibers and have darker or red‐colored flesh. Because of the endurance exercise needed by these fish, the meat is highly marbled with fat, providing energy directly to produce ATP needed for actin and myosin contraction. Shellfish, however, are mostly made of white meat as they require short bursts of energy to close a shell or scuttle to a hiding location.
As we have seen, myoglobin content differs between red and white muscle fibers, but myoglobin content in the muscle tissue also differs by species and the age of the animal. These factors also contribute to meat color. Younger animals have less myoglobin in their muscles than older animals. For example, veal is a very pale brownish pink (veal has 2 mg of myoglobin per gram of meat), while young beef is a cherry red color (8 mg of myoglobin per gram of meat).
Across species, beef has the highest myoglobin content, followed by lamb and then pork. Fish and fowl (chicken and turkey) have even less myoglobin. This trend also follows the level of physical activity we might expect from these animals. For example, farm‐raised pork is from a sedentary pig, while beef is from the slightly more active cow. Game animals—wild turkey, venison, and so on—have more myoglobin in their muscles compared with their domesticated counterparts due to their more physically active lifestyles.
By Joseph J. Provost, Keri L. Colabroy, Brenda S. Kelly and Mark A. Warlet in "The Science of Cooking - Understanding the Biology and Chemistry Behind Food and Cooking", John Wiley & Sons, USA, 2016, excerpts pp. 283-288. Digitized, adapted and illustrated to be posted by Leopoldo Costa.