The Color of Meat
It all comes down to job description and work schedule.
Every single muscle on the bodies of humans and animals, from the tiniest muscle in the inner ear to the massive hamstring that flexes the knee, is designed for a specific task. Each muscle is attached to the skeleton in some unique way on both its ends, and when the muscle contracts it pulls those two bones closer together. The body movement produced is that muscle’s job description. And for both dark red goose breast and bright white chicken breast, that job description is the same. Then why are the colors so starkly different?
In both birds, breasts are the primary flying muscles, and they correspond to the pectoral muscles (or “pecs”) on a human chest. When our pectoral muscles contract, they pull the upper arm bone (the humerus) forward and across the body, as when doing a bench press. When the pectoral muscle in birds contracts, it pulls the humerus in the same way along with the rest of the feathered wing. And because bird bodies are horizontal in flight, forward motion of the wing creates a downstroke, resulting in the primary mechanical force keeping them aloft. The large keel made of bone and cartilage that runs down the middle of their chests like a mountain ridge provides extra surface area for pectoral muscles to attach to. (Whenever the body wants to create more force in a muscle’s contraction, it simply provides more bony surface area for muscles to grip, just like the bony ridge running down the top of a cat’s or dog’s skull.)
While chicken breast and goose breast have the same job description, they are different colors because of another factor: work schedule. Think of dark meat and light meat on different parts of a chicken. The legs have dark-colored muscles, which become dark meat, and these are used for the sustained work of walking around all day, which the chickens do for much of their waking lives. The light-colored muscles on their chests, which become light meat, only do their work in short bursts. Chickens hardly fly, and only flap their wings for brief periods. The same is true of turkey breast muscles: they might flap their wings just long enough to get into a tree’s branches, but they do not fly long distances. And turkeys similarly have lighter breasts than legs. It’s also worth noting that the color difference between breast and leg muscles in domesticated chickens is greatly exaggerated due to intense selective breeding by humans over many years.
A goose’s breast muscles, on the other hand, are marathoners. Geese can migrate thousands of miles in a winter and can even fly over a thousand miles in a single day, weather permitting. The amount of work required of a goose’s pectoral muscles to accomplish this feat is massive, and it must be sustained for hours and hours. Like a chicken’s leg muscles, the sustained work required of these muscles makes the meat a beautiful dark reddish purple. But why?
The dramatic difference between the colors of goose and chicken breast comes down to one colorful protein called myoglobin. It’s a proteinaceous cousin of hemoglobin, and it has a similar function: carrying oxygen. Hemoglobin fills our red blood cells, and when it picks up oxygen in the lungs it turns from a purplish-blue to a bright red. The iron atoms inside every hemoglobin molecule helps grab and hold the oxygen, just as iron grabs and binds to oxygen when metal tools rust.
Once hemoglobin and its oxygen payload arrive at some distant cell of the body, it does something equally important to binding oxygen: it releases it. Oxygen is needed by every cell in our bodies to sustain a burning metabolic fire, just as a burning candle or wood fire requires oxygen to stay lit and quickly becomes snuffed out if the oxygen disappears. Hemoglobin is the perfect tool for carrying oxygen because it is reversible: it grabs and holds oxygen in the lungs but then willingly releases it when it’s in the capillaries. Failure of either task would not be compatible with life.
Myoglobin does something very similar to hemoglobin: it holds oxygen and delivers it to where it is needed in working muscles. It does not, however, participate in the actual contraction of muscle proteins. Muscle cells use fuel like every other cell, though they uniquely use it to contract and generate force. When they are working hard, they burn fuel on an enormous scale, and they need a steady supply of oxygen to do it. This is where the color of muscles comes in.
For muscles that work only intermittently, like the light-colored pectoral muscles of a chicken, not as much myoglobin is needed to feed oxygen into the fire. But for muscles working their asses off for hours and hours at a time, like the pectoral muscles of migrating geese, a massive in-house oxygen store is needed, and this is the job description of myoglobin. As fuel is combusted inside muscles, myoglobin is the bellows stoking the fire with air (or at least air’s active ingredient: oxygen).
Muscles that work in a more sustained fashion have more myoglobin and are therefore darker in color. And myoglobin is similar in color to hemoglobin, which is why that reddish liquid in store-bought packages of meat looks like blood, but isn’t. It’s just myoglobin juice. Myoglobin also has iron in it, which helps to hold the oxygen, and this is where much of the nutritious iron in muscle comes from and the reason that red meat has more iron.
What about pork? Why is it so light in color compared to almost every other kind of mammalian meat? A big reason is the age of slaughter. Pigs are killed at a much younger age (about 5-6 months old) than cattle (16-18 months). And muscle in younger animals tends to have a different types of muscle fibers, and therefore different amounts of myoglobin. Veal is similarly lighter in color than mature beef. It’s worth noting that wild boar meat is redder than store-bought pork, likely because the animals are killed at older ages than domestic pigs are.
What about humans? I asked my colleague a pediatric surgeon about muscle color and young age in humans. He cuts into human bodies from adolescents to babies, and even premature newborns, and he sees their muscles with his own eyes. He said that in general the younger a person is, the lighter their muscles. In some kids the muscles are light pink, and in younger babies it can even be salmon-colored.
The difference between light and dark meat ties in to another muscle dichotomy: that between fast-twitch fibers and slow-twitch fibers. Dark meat has more slow-twitch fibers which are specialized for more prolonged exertion, and white meat has more fast-twitch fibers which are useful for short, intense bursts of activity. Slow-twitch fibers generate substantially more force but can’t keep it up for a long time. Each of our muscles has a mix of fiber types, though this can change with exercise, aging and prolonged weightlessness in space.
When muscle becomes meat, the muscle’s job description and work schedule become important for cooking and eating too. Light-colored muscles burn more carbohydrates in their short bursts of activity, while dark-colored muscles burn more fat. This could be because burning fat releases more energy than burning carbs does, and longer-working muscles churning more continuously need more energy. Whatever the reason, it means there is more fat in dark meat than in light meat. This is very similar to marbling in beef - storing fat inside the muscles makes it more available for burning as a fuel, just like the oxygen held by myoglobin. The presence of more fat is likely a reason that dark meat seems moister than white meat, and why white meat dries out more quickly. And it is also one reason that, in my opinion, dark meat is wildly more delicious than white meat. There are, in fact, few things as delicious as a chicken thigh.