The red juice that often collects in a package of red meat is not blood, as many assume. Most of the blood is removed during processing and any that remains is usually contained within the muscle tissue.
The red liquid, instead, is a mixture of water and a protein called myoglobin, whose purpose is to help ship oxygen to muscle cells. Myoglobin is deeply pigmented, which is why the more myoglobin a meat contains, the darker (or redder) the meat will be.
Red meat is comprised of muscles that are used for extensive activity. Remember, myoglobin’s role is to help bring oxygen to the muscles, and oxygen is required to give muscles energy.
So the more the muscles are used, the more myoglobin they’ll contain (and the redder in color they’ll be). This is why when you prepare “white” meat such as poultry or fish, you won’t find any “blood” in the package – the white meat contains hardly any myoglobin.
Myoglobin Is What Makes Meat ‘White,’ ‘Dark,’ or ‘Red’
The level of myoglobin in meat is what ultimately dictates whether it will be “red,” “dark,” or “white.” The muscles in red meat are used for standing, walking, and other frequent activity, and they’re made up of slow-twitch muscle fibers. Red meats’ high levels of myoglobin make it red or dark in color.
White meat, on the other hand, is made up of fast-twitch muscle fibers and is comprised of muscles used for quick bursts of activity only. They get energy from glycogen and contain little myoglobin.
Some animals, like chickens, contain both white and dark meat, with the dark meat found primarily in their leg muscles. If you’ve ever wondered why wild poultry contain mostly dark meat, it’s because they fly frequently, and the increased muscle usage means the meat contains more myoglobin.
Pigs are often referred to as the “other white meat,” and that’s because, while they contain myoglobin in their muscles, the levels are not as concentrated as they are in cattle (likely because pigs are not as active). Fish, too, are typically considered white meat because most of them are able to float in the water without requiring much muscle use.
Certain types of migratory fish, however, which swim briskly for extended periods, have dark meat, and that is again because of the increased myoglobin (examples would be tuna and shark).