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Why do turkeys have dark and white meat?

While pigging out on turkey at Thanksgiving, I gave some thought for my food. I like both dark and white meat, but why do turkeys (and chickens) have both?

Winging It

The white meat comes from the breast, which turkeys use to flap their wings. Wild turkeys fly only as a last resort, in a short burst, to escape predators. White meat consists of fast-twitch muscle fibres that contract quickly, but become exhausted quickly too. They use glycogen, a kind of animal starch that gets broken down into glucose when needed for energy. This process doesn't require much oxygen from the blood.

Breast Choice

Domesticated turkeys have been bred to beef up their breast meat. This extra bulk makes them so heavy they can't fly and have trouble mating, so they tend to be artificially inseminated. Probably not something natural selection would have gone for.

Leg Work

Turkeys spend most of their time on their feet. A critical culinary consequence of turkey lifestyle is that because the thighs carry a heavy load, they need more structure and contain more collagen protein. During cooking, the collagen unwinds and breaks down into the softer gelatin but this takes more time than other proteins. So the thighs and take more time to cook than the breast meat. If you don't cook them enough, they will be tough and chewy.

Twitch, Twitch, Slow

But back to the dark side. The dark meat is in the thighs, which consist of slow-twitch muscle fibres built for the ongoing activity of standing and walking around. Ducks and geese fly long distances and they are all dark meat. Although activity affects the muscle condition, the type of muscle is genetically determined. The u-boot gene isolated in zebrafish is a switch that controls muscle type and may prove to be common to all animals with backbones.

Into the Meat of Darkness

The slow-twitch muscles burn fat for fuel and require lots of oxygen. This muscle is darker because it had lots of myoglobin, an iron-containing protein important for moving oxygen. As well, the tissues have lots of capillaries, to provide a good blood supply. White and dark meat look more different after they are cooked because the myoglobin changes colour with temperature. Above 77C (170F), it becomes metmyoglobin and turns brown-grey.

So the next time you talk turkey, you can also talk science.