Color is used by consumers to determine if meat is fresh and safe to eat. It is the single most important driving factor in a consumer’s decision to purchase meat. Myoglobin is the heme iron containing protein that gives meat its color, and it is a great source of dietary iron. Myoglobin stores oxygen in muscle cells and is similar to hemoglobin that stores oxygen in blood cells.
The more myoglobin content meat contains the darker red it will appear in color. Myoglobin content is higher in beef and lower in poultry with lamb and pork having intermediate amounts. The age of an animal will also impact the myoglobin content of the muscles with older animals having more myoglobin and darker meat. Muscles that are used for movement also have more myoglobin content than muscles used for support. Along with water from muscle, myoglobin is what is found in meat packages that leaks out of the muscles during storage and most people think is blood. Almost all of blood is removed from muscle at the time of slaughter.
Myoglobin has three natural colors depending on its exposure to oxygen and the chemical state of the iron. If no oxygen is present, the meat appears purple red, like in vacuum packaged meat, and is in the deoxymyoglobin state. Meat is bright red when exposed to air and is typical of meat in retail display. Bright red color indicates oxymyoglobin is present. Meat appears tan or brown when only very small amounts of oxygen are present such as when two bright red pieces of meat are stacked on each other excluding the oxygen. Meat can also appear brown when the meats color life is exhausted late in display when the iron in the pigment becomes oxidized. Metmyoglobin is the state when the iron has oxidized and is tan or brown in color.
Although brownish-red colored meat can indicate spoilage, it doesn’t always mean that meat is spoiled. Purchasing meat that has been discounted at the retail counter because of discoloration can still be safe to consume if it is properly stored and prepared according to Michigan State University Extension.