Before it’s a steak or a roast, meat is muscle. And muscles, in a living animal, are engines of motion, structures designed to generate movement by converting chemical energy into mechanical work. Once an animal is harvested and its muscles become food, that purpose ends, but the structure and function of muscle continue to shape everything about how meat looks, feels, tastes, and cooks.
If you want to cook meat well—whether that means a juicy steak, a falling-apart braise, or a perfectly poached fish—it helps to first understand what meat really is, how it’s built, and what happens to it when heat is applied. Without this foundation, it’s easy to miss out on simple techniques that dramatically improve flavor and texture.
Today we will walk you through the basics of meat science, starting with the structure of muscle, moving into the factors that control tenderness, and ending with cooking concepts like resting and searing. Think of this as your groundwork for making better, more informed decisions at the butcher counter and in the kitchen.
Why Muscle Structure Matters
At its core, meat is made up of bundles within bundles of muscle fibers, also known as muscle cells. These bundles of muscle fibers are organized into groups aligned in the same direction—this is what gives meat its grain.
The smallest unit of muscle tissue involved in movement is the sarcomere, made up of the proteins actin and myosin. When an animal moves, myosin pulls actin strands together, shortening the muscle fibers and powering contraction. When we cook meat, those same proteins change shape and texture with heat.
Each fiber is wrapped in connective tissue, mostly collagen. Groups of fibers are bundled together into fascicles, which are wrapped in more collagen. The entire muscle is wrapped in a thick collagen sheath and connected to bone by tendons. While the collagen holds things together, it also has a major influence on meat’s texture, especially tenderness.
If you’ve ever noticed how some cuts of meat feel smoother or rougher to the touch, that’s the result of fascicle size and collagen structure. Tender cuts have finer, more delicate bundles; tougher cuts have coarser, thicker ones. And because collagen is critical to how meat behaves during cooking, knowing where it is and how it changes with heat can make a huge difference in the kitchen.
While fish follow the same structure, they have some differences. To start, the muscles found in fish are less complex because they live an easier life from a mechanical perspective: fish float through water and don’t need elaborate structures to support their weight. Another big difference is that because fish lack elaborate skeletons, their bodies are made largely of muscle. And because most of a fish’s movements are aimed at propelling it forward, nearly all of its muscles are similar in shape and size.
The individual muscles within a fish, called myotomes, are built like the muscles in land animals, with collagen sheaths surrounding the muscle fibers from end to end. Unlike land animals, the fibers don’t simply run parallel to the length of the fish. Instead, each individual muscle is folded into the shape of a W, with one muscle nestled into the next, which is what gives the flesh its zigzag look. Such geometry allows muscle contractions to create the undulatory motions that propel a fish forward so efficiently.
Fish are cold-blooded, which, when it comes to cooking, is more important than you might think. The proteins found in the muscles of most kinds of fish never evolved to tolerate warm, let alone hot, temperatures. One implication is that the proteins can become fully cooked when heated to a relatively cool temperature. At slightly higher temperatures—even those that would leave most meats rare—fish proteins become entirely coagulated, resulting in fish that’s dry and unappealing.
What Makes Meat Tender or Tough?
The biggest single factor in determining whether a piece of meat is tender or tough is what that muscle did in the animal’s life.
Muscles that get a lot of use—like shoulders, legs, and necks—are made to be strong and durable. They contain thicker muscle fibers and stronger collagen, making them tougher but also more flavorful. Muscles that aren’t used as much—like tenderloins or ribeye caps—are more delicate, with thinner fibers and weaker collagen. You can actually feel the difference when you rub the meat with the ball of your thumb: a tender cut feels fine-grained and smooth, whereas a tougher cut is noticeably coarse.
The collagen in meat starts as a tight, elastic triple-helix structure. As meat is cooked, particularly in the presence of moisture and time, collagen unwinds and breaks apart into gelatin, making the meat softer and more succulent. This is the principle behind slow braising or pressure-cooking: you’re tenderizing the meat by transforming its connective tissue into something luscious and rich.
But collagen is tough stuff, especially in older animals. It takes both heat and time to break it down. Cook meat too fast or at too high a temperature, and collagen will shrink forcefully, squeezing out moisture and making the meat dry and chewy. But cook it low and slow, and you unlock a dramatic textural shift. (Hint: This is why we love sous vide so much.)
White Meat vs. Dark Meat
As we mentioned, muscles are specialized based on their job. Some are made for quick bursts of speed (like a chicken’s breast), while others are built for endurance (like a cow’s shoulder or a duck’s legs). This distinction affects not only tenderness but also color and flavor.
Fast-twitch fibers are pale and powerful but tire quickly. They contain little myoglobin, the oxygen-storing protein that gives meat its red hue. These are found in white meats, like turkey breast or veal loin.
Slow-twitch fibers, on the other hand, are built for stamina. They’re rich in myoglobin, giving them a dark red or purple color and a deeper flavor. Muscles used constantly—like those in cattle, lamb, and wild birds—are full of slow-twitch fibers. This is what makes their meat red and often more flavorful.
In some animals, like pigs or chickens, the difference between light and dark meat is obvious. You’ll still find these variations in the muscles of cows, sheep, and wild game, but they’re harder to see. Cuts like the ribeye cap (also referred to as deckle or spinalis dorsi) contain more of these darker fibers and often have more marbling, making them especially tender and rich.
Many people believe that the red color in meat comes from blood. It doesn’t. Blood is mostly drained at slaughter. The color you see in raw meat comes from myoglobin. The more endurance a muscle needs, the more myoglobin it contains, and the darker the meat becomes.
The hue can shift depending on whether myoglobin is oxygenated (red) or deoxygenated (purple). That’s why vacuum-sealed meat often appears darker or purplish until exposed to air.
Red meats—from animals like cows, sheep, or wild game—are rich in myoglobin and slow-twitch fibers. White meats—like chicken breast or pork loin—have much less.
When it comes to fish, they mostly have light-colored muscle. These powerful fast-twitch muscle fibers allow fish to make quick getaways when faced with imminent danger. The steady swimming they do the rest of the time is powered by a much smaller amount of dark, high-endurance fibers running along their flanks near the midline of the backbone and just beneath the skin. Thus the texture and flavor of a piece of fish vary because the balance between light and dark muscle fibers differs, and this in turn depends on where the piece came from within the body of the fish.
Cooking Collagen
Tough cuts like brisket, chuck, oxtail, or shank are collagen-rich and often underused in modern kitchens. But if you know how to cook them, they can be the most rewarding.
When meat is cooked above 50 °C / 122 °F, collagen converts into gelatin, which is sometimes referred to as denaturing or hydrolysis. When meat is heated in the presence of moisture, the elastic mesh of collagen unwinds, fragments, and loosens. The result is that the formerly strong network of collagen molecules becomes deliciously soft gelatin. A second chemical reaction also occurs when meat is cooked: collagen shrinkage. Collagen shrinkage compresses bundles of muscle fibers, squeezing moisture out. This makes meat less juicy, more dense, and tougher to bite through.
This reaction kicks off when the temperature reaches 58 °C / 135 °F. By 65 °C / 149 °F more than half the collagen in a piece of meat will have shrunk, and by 85 °C / 185 °F nearly all of it will. Shrinkage happens quickly—much faster than the conversion of collagen into gelatin. If you quickly bring meat to high temperatures, it undergoes rapid collagen shrinkage and will be at its toughest. Then, slowly, the collagen-to-gelatin conversion takes place, turning the meat tender.
The trick for cooking tough cuts of meat is to ensure the collagen transforms in gelatin without drying out the meat. This usually means cooking at moderate temperatures (70–82 °C / 160–180 °F) for several hours, with plenty of moisture. Sous vide is especially useful here.
High temperatures cook faster, but they also force juices out and make the meat denser and less appealing. Low temperatures preserve moisture but require time. That’s the trade-off: time for tenderness. For us, the additional time is worth it. We like to cook tough cuts sous vide, including our Braised Short Rib recipe on page 229 of Modernist Cuisine at Home.
With tender cuts, like filet or rib eye, collagen is weak and minimal, so your main concern is not overcooking. These cuts are best cooked hot and fast or with highly controlled methods like sous vide.
The collagen in fish differs from that of terrestrial animals in two ways. First, collagen in fish muscles is a lot weaker than its counterparts on land. Because they float in water, they don’t need elaborate skeletons nor strong collagen to support their weight. Fish collagen doesn’t have a lot of stabilizing cross-links to slow its degradation, which is one reason why fish fillets or steaks that aren’t fresh are prone to falling apart. Also, fish collagen melts quickly with just a bit of heat. Normally this tenderness is a good thing; it can become off-putting if taken too far.
Fish collagen differs in other ways as well. You may have noticed distinct lines zigzagging along a fish fillet. These are collagen sheaths, called myocommata (or myosepta), that divide individual muscles. They run at an angle from the backbone to the skin and are what make swimming possible. These sheaths grow farther apart as the fish matures and each muscle gets bigger.
During cooking, the large gaps between collagen sheaths make the flesh of the fish fall apart more easily, sometimes so much as to be a problem. This is why cooked fish often comes apart in flakes. Each flake is an individual muscle, or myotome. Myotomes separate as flakes because their collagen largely dissolve during cooking.
Bringing It All Together in the Kitchen
Understanding the biology of meat is a practical guide to better cooking. Overall, we recommend:
- Choosing cuts based on use. Tough cuts for long, slow cooking; tender cuts for quick, precise heat.
- Recognizing muscle grain and collagen structure to anticipate texture and to plan cooking methods.
- Using temperature wisely. Too hot and you lose moisture; too cool and collagen won’t break down.
- Looking for darker, myoglobin-rich cuts for more flavor in terrestrial animals.
Cooks who understand the fundamental nature of muscle, how it is transformed into meat, and what happens as the temperature increases will be able to select the best strategies for preparing meat and avoid common mistakes. Your technique will improve, further empowering you to experiment and innovate.
FURTHER READING
- How Muscles Work—Modernist Cuisine vol. 3:6
- Tender Cuts—Modernist Cuisine vol. 5:3
- Great Grilling with Three Tricks to Tender, Tasty Beef
- Best Bets for Cooking Tough Cuts—Modernist Cuisine at Home, p. 228
- Braised Short Ribs—Modernist Cuisine at Home, p. 229
- Hanukkah Short Ribs
- All About Cooking Beef
- 72-Hour Braised Short Ribs
- Sous Vide Rare Beef Jus
- Cooking the Perfect Steak