What gives meat its color?

[Taka]

There are basically two types of meat: dark and white.

Red, or dark meat is made up of muscles with fibers that are called slow-twitch. These muscles are used for extended periods of activity, such as standing or walking, and need a consistent energy source. The protein myoglobin stores oxygen in muscle cells, which use oxygen to extract the energy needed for constant activity. Myoglobin is a richly pigmented protein. The more myoglobin there is in the cells, the redder, or darker, the meat.

When dark meat is cooked, myoglobin's color changes depending on what the meat's interior temperature is. Rare beef is cooked to 140° F, and myoglobin's red color remains unchanged. Above 140° F, myoglobin loses its ability to bind oxygen, and the iron atom at the center of its molecular structure loses an electron. This process forms a tan-colored compound called hemichrome, which gives medium-done meat its color. When the interior of the meat reaches 170° F, hemichrome levels rise, and the myoglobin becomes metmyoglobin, which gives well-done meat its brown-gray shade.

White meat is made up of muscles with fibers that are called fast-twitch. Fast-twitch muscles are used for quick bursts of activity, such as fleeing from danger. These muscles get energy from glycogen, which is also stored in the muscles.

White meat has a translucent "glassy" quality when it is raw. When it's cooked, the proteins denature and recombine, or coagulate, and the meat becomes opaque and whitish.

Cows and pigs are both sources of dark meat, though pig is often called "the other white meat." Pigs' muscles do contain myoglobin, but the concentration is not as heavy as it is in beef. Chickens have a mixture of both dark and white meat, and fish is mainly white meat.

Different types of meat require different cooking times. The best way to determine if meat is done is to use a meat thermometer to check the internal temperature. Beef can be cooked to a variety of temperatures: rare (140° F), medium (160° F), and well-done (170° F). Pork, chicken, and fish have less leeway. It's recommended that pork be cooked to 170° F, chicken to 180° F, and fish to 165° F.

SOURCE: https://www.exploratorium.edu/cooking/meat/INT-what-meat-color.html

[Taka]

As the USDA says, the protein myoglobin is the main cause of the red color of meat; it achieves this color when exposed to oxygen. Red meat (or dark meat) is myoglobin-rich, from "slow-twitch" endurance muscles, while white meat has little myoglobin, and is from "fast-twitch" muscles. So it really is the protein in the meat, as you guessed! But we can understand better than that.

Myoglobin is related to hemoglobin, which is in blood: myoglobin has a heme group while hemoglobin has four heme groups. In both cases the heme groups are bound up in a globular protein (a globin); that surrounding structure is very different in the two proteins. Heme groups are good-sized organic molecules, but the important thing is that at their center, there's an ionized iron atom, which is capable of binding to various other things, allowing things like hemoglobin transporting oxygen in blood, and myoglobin temporarily holding onto oxygen the muscle cells will use soon. Myglobin is most present in slow-twitch muscles, which are used frequently or continuously, and so need the stores of oxygen for endurance - these muscles are what we call red (or dark) meat. Fast-twitch muscles, used for quick burst of energy, don't need these oxygen stores, so they don't have much myoglobin.

The central part of heme looks something like this:

heme (undetermined state)

The gray bonds around the edges are the connections to the rest of the molecule, which we don't care about right now. It's the various things that can bind to the iron atom (that question mark) that are responsible for the color of both blood and red meat. There are several states we see in meat:

When there's no oxygen bound to it, it's bound to water, and is purple/blue. We know this as the color of blood in veins, but you can sometimes see this in vacuum-packed meat, which isn't exposed to oxygen at all. The iron is in the +2 oxidation state (two electrons removed).
With oxygen bound to it, which happens instantly when exposed to air, it's red. This is true for blood which has collected oxygen from the lungs, as well as meat in the state we usually see it. Again, the iron is in the +2 oxidation state.
With the iron ionized further into the +3 state, it can again bind to water, turning brown. This happens after prolonged periods without exposure to much oxygen, for example, meat that's been stored in its packaging for a while. It also happens when the protein is destabilized by acidity or temperature, explaining why cooked meat turns brown. If you cook it slowly at a low temperature, the protein isn't destabilized as much, and some pink color can remain.
With nitric oxide bound to it, which happens in both cured meats and smoked meats, it turns pink. Similarly, carbon monoxide in oxygen-deficient gas or charcoal grills can also turn it pink.

heme in various states

There's a great deal of variety in myoglobin content of various meats, accounting for much of the variation in color. It all depends on the animal and the role of the muscle. Remember, the myoglobin is used for temporary oxygen storage. This means that more powerful, frequently-used muscles need a lot of myoglobin - for example, young cattle aren't using their muscles much yet and have less myoglobin, while whale muscles, which are constantly used and need to last through prolonged dives, have 25 times as much myoglobin and look black. This is actually pretty cool from a nutritional standpoint: the redder the meat is, the more iron it has!

Whew! So that's red meat. White meat, on the other hand, is from myoglobin-poor fast-twitch muscles, so it doesn't get in on all that exciting color-changing stuff. Poultry breasts as well as a lot of fish fall into this category. While some fish (like tuna) have plenty of slow-twitch endurance muscle, most fish don't need a whole lot, because cruising in water is relatively easy. They instead have a lot of fast-twitch muscle for quick turns and sprints, with a layer of slow-twitch muscle near the skin - this is the dark layer you see in some fish!

MORE: http://cooking.stackexchange.com/questions/24208/why-is-meat-red