Myoglobin is a protein found in animal muscle tissues. It is primarily involved in oxygen management within muscle cells. Similar to hemoglobin, which transports oxygen in blood, myoglobin’s function is specific to muscle tissue. Its presence and chemical state are central to understanding meat’s appearance.
Myoglobin’s Distinct Color
Myoglobin has a purplish-red color. This hue originates from a molecular component called the heme group. The heme group contains a single iron atom at its core, responsible for binding oxygen and the protein’s color. The specific shade depends on the iron atom’s chemical state and what it is bound to.
When the iron atom is in its ferrous (Fe2+) state and not bound to oxygen, myoglobin appears purple-red; this is known as deoxymyoglobin. Upon exposure to oxygen, the iron atom reversibly binds to an oxygen molecule, forming oxymyoglobin, which gives meat a bright red color. If the iron atom becomes oxidized to the ferric (Fe3+) state, it forms metmyoglobin, which results in a brown color.
Myoglobin’s Function in Muscles
Myoglobin primarily functions as an oxygen-storing protein within muscle cells. It acts as an intracellular reservoir, ensuring oxygen for muscle activity, especially during periods of high demand or intense exercise. When muscles are actively working, myoglobin releases its stored oxygen to support cellular respiration and energy production.
Myoglobin also aids in the diffusion of oxygen from the cell membrane to the mitochondria, where energy is generated. This role is important in muscles that undergo sustained activity, like those in diving mammals, which have high concentrations of myoglobin for prolonged breath-holding. While hemoglobin transports oxygen in red blood cells, myoglobin’s higher affinity for oxygen allows it to efficiently extract and retain oxygen specifically within muscle tissues.
How Myoglobin Influences Meat Color
Myoglobin’s various chemical states directly influence the visual appearance of raw meat. When meat is freshly cut and has limited exposure to oxygen, myoglobin is in the deoxymyoglobin state, giving it a purplish-red hue, commonly seen in vacuum-packed products. As meat is exposed to air, myoglobin binds with oxygen to form oxymyoglobin, which creates the bright cherry-red color typically associated with fresh meat displays. This process is known as “blooming”.
Over time, or with prolonged oxygen exposure, the iron in oxymyoglobin can oxidize from the ferrous (Fe2+) to the ferric (Fe3+) state, forming metmyoglobin. Metmyoglobin imparts an undesirable brown color to meat. Cooking meat also alters myoglobin; heat denatures the protein, causing the iron to oxidize and form brown pigments, which explains why well-done meat appears brown or gray. Packaging, storage conditions, and the animal’s age or muscle activity also affect myoglobin’s state and the meat’s color.