At every Thanksgiving dinner table, there are those who decree, “Save the dark meat for me,” while others remain white-meat loyalists. I, for one, am part of the latter faction. But whichever side of the meat-color line you stand on, you have at one point probably wondered: What’s the difference between the two? What’s the difference between white meat and red meat, for that matter? The answers lie in the science of muscle tissue. Vegetarians beware; this column is about to delve deep into meat.

First, let’s take a closer look at skeletal muscle in general, which is, for the most part, the type of muscle we call “meat,” and enjoy on our plates and in between buns. Skeletal muscle is made up of muscle fibers, which can be divided into two types: slow-twitch muscle fibers, which are also known as red fibers, and fast-twitch muscle fibers, which are also known as white fibers. Given the nomenclature, you may already have an idea of where this is going.

So, what’s the difference between the two types of fibers, and what does the presence of one over the other mean? Slow-twitch fibers are high in myoglobin ”“ an oxygen-binding protein similar to its better-known cousin hemoglobin ”“ and derive their energy aerobically, or with oxygen. Myoglobin also contains iron-rich red pigments called hemes, which give these fibers their deep-red color. (The hemes present in hemoglobin also give blood its red color.) Fast-twitch fibers, on the other hand, are anaerobic, contain less myoglobin and are lighter in color.

A muscle fiber’s type determines its functionality. Fast-twitch fibers, as their name suggests, can contract quicker than slow-twitch ones, but they also fatigue quicker, while slow-twitch fibers contract slower but do not fatigue as easily. Thus, fast-twitch muscle fibers are better suited for short-term, high-powered anaerobic activities, such as sprinting, while slow-twitch muscle fibers are better suited for long-term, low-powered, aerobic activities, such as walking or running slowly.

Human athletes can be used as an example to illustrate the key differences between the two types of skeletal muscle. “Successful long-distance runners typically have a greater percentage of red fibers than white fibers. Short-distance runners usually have a greater percentage of white fibers than red fibers,” according to Kaplan’s MCAT Biology Review textbook.

The same concepts apply in animals. In fact, gradations of redness in meat are directly related to the concentration of myoglobin in the muscle fibers. Texas A&M’s Department of Animal Science website provides a chart, characterizing pork with a myoglobin concentration of 2 mg/g as “pink,” lamb with a myoglobin concentration of 6 mg/g as “light red” and beef with a myoglobin concentration of 8 mg/g as “cherry red.” Furthermore, if you’ve ever wondered why a veal chop is lighter in color than a steak, this is because as an animal’s age increases, its myoglobin content goes up, leading to darker-colored, or redder, meat. This happens because, over time, myoglobin loses its affinity for oxygen, so more is needed.

Returning to the notion of the different jobs both types of muscle fiber carry out, this correlation makes perfect sense. Cattle rarely move very fast, for example, spending most of their time grazing; therefore, they are largely made up of the myoglobin-rich red fibers. Pigs’ muscles do contain myoglobin, although not nearly as much as cattle, so pork ”“ despite being called “the other white meat” ”“ falls somewhere in the middle of the spectrum. Similarly, in fish, species that are larger and more migratory tend to be red meat, while smaller fish that must move in quick, darting patterns to avoid predators tend to be white meat.

Meat from chicken and turkeys serves as an interesting illustration of how different muscle groups within a single organism carry out different functions. A turkey’s thigh muscles, for example, are used for standing and walking, while a turkey’s pectoral muscles, or breasts, are used to flap its wings, propelling it into short bursts of flight. Hence, from these birds we get dark meat and white meat, respectively. Ducks fly more frequently and for longer distances than chickens or turkeys, and this behavior explains why duck meat is red.

Lastly, if you’re wondering why heat changes the color of meat, to put things simply, when red meat is cooked, the structure of its myoglobin is altered, leading to a color change ”“ from red to brown. And when white meat is cooked, its proteins denature, causing the pink-to-white transition we’re familiar with.

Alas for a white-meat lover like myself, there seemed to be far more dark meat available than white meat in the center of my family’s dinner table on Thanksgiving. I suppose we sprung for a long-distance runner this year.

— Angelo J. Verzoni is a freelance writer for the Journal Tribune.

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