In our daily life, when we go to the grocery or meat shop, we see that some meat is red whereas some meat is pale. What causes the change in color of meat? Actually, the muscle that are frequently used are red where as those infrequently used are pale in color. We can notice that the leg meat of chicken is darker and the breast meat is white. The different colors of meat reflect the concentration of myoglobin in the muscle tissue.
What is myoglobin?
Myoglobin is the globular protein that functions as an oxygen storage in muscles. Myoglobin is a monomer whereas hemoglobin is a tetramer. That is, myoglobin consists of a single peptide chain and a heme unit, and hemoglobin has four-peptide chains and four heme units. Thus only one oxygen molecule can be carried by a myoglobin molecule. Myoglobin has a higher affinity for oxygen than does hemoglobin. Thus, transfer of oxygen from hemoglobin to myoglobin occurs readily. Oxygen stored in myoglobin molecules serves as a reserve oxygen source for working muscles when their demand for oxygen exceeds that which can be supplied by hemoglobin.
The meat that humans eat is composed primarily of muscle tissue. The major proteins present in the muscle tissue are myosin and actin, which lie in alternating layers and which slide past each other during muscle contraction. Contraction is temporarily maintained through interactions between these two types of proteins.
Structurally, myosin consists of a rod like coil of two alpha helices (fibrous protein) with two globular protein heads. The head portions of myosin interact with the actin.
Structurally, actin has the appearance of two filaments spiraling about one another. Each circle in the structural diagram represents a monomeric unit of actin (called globular actin). The monomeric actin units associate to form a long polymer (called fibrous actin). Each identical monomeric actin unit is a globular protein containing many amino acid residues.
The chemical process associated with muscle contration ( interaction between myosin and actin) requires molecular oxygen. The oxygen storage protein myosin is the oxygen source. The amount of myoglobin present in a muscle is determined by how the muscle is used. Heavily used muscle require larger amount of myoglobin than infrequently used muscles require.
The amount of myoglobin present in muscle tissue is a major determiner of the color of the muscle tissue. Myoglobin molecules have a red color when oxygenated and a purple color when deoxygenated. Thus, heavily worked muscles have a darker color than infrequently used muscles.
The different colors of meat reflect the concentration of myoglobin in the muscle tissue. In turkeys and chickens, which walk around a lot but rarely fly , the leg meat is dark, the breast meat is white. On the other hand, the flying birds have dark breast meat.
Fish have lighter flesh than the land animals and birds because they do not need to support their own weight (supported by water) while moving/swimming. This reduces the need for myoglobin oxygen support. The fish that spend most of their time lying on the bottom of a body of water have lightest meat.
Why meat turns brown in cooking?
Meat, when cooked, turns brown as a result of changes in myoblobin structure caused by the heat; the iron atom in the heme unit of myoglobin becomes oxidized. When meat is salted with preservatives (NaCl, NaNO2 etc.) the myoglobin picks up nitrite ions, and its color changes to pink.