Why Some Meat Is Tough and Some Is Tender: Meat Texture Science

A rib-eye and a piece of beef shank come from the same animal. One is prized for how little it takes to make it delicious. The other is practically inedible unless you braise it for hours. The difference isn’t random. It’s biology written in protein structure.

Muscle Fiber Basics

All meat is muscle, and muscle is made of fibers. Each fiber is a single elongated cell, and inside each cell are bundles of myofibrils, the contractile machinery that makes muscles work. Myofibrils are composed of repeating units called sarcomeres, where the proteins actin and myosin slide past each other to generate force.

The diameter and density of these fibers varies between muscles and between animals. Heavily exercised muscles develop larger, denser fibers with stronger connective tissue holding them together. Lightly exercised muscles have thinner fibers and less connective tissue.

That’s the starting point for texture. A lightly exercised muscle (tenderloin) has thin fibers and little connective tissue. You bite through it easily. A heavily exercised muscle (shank) has thick, dense fibers surrounded by collagen-rich connective tissue. Your jaw works much harder.

The Role of Connective Tissue

Connective tissue is the white, shiny material you see in cheaper cuts. It’s mostly collagen, a structural protein that wraps around muscle fibers, groups them into bundles, and connects muscle to bone. The more a muscle works, the more collagen it develops.

Collagen doesn’t melt at normal cooking temperatures. It’s mechanically strong up to around 150°F. So a quickly seared shank is doubly tough: the muscle fibers have tightened from heat, and the collagen hasn’t had time to convert. The only way to tenderize these cuts is long, moist heat that converts collagen to gelatin over several hours. This is the entire basis of braising. Read the full mechanism in collagen to gelatin science.

High-collagen cuts: brisket, chuck, shank, oxtail, short ribs, pork shoulder. Low-collagen cuts: tenderloin, rib-eye, strip, sirloin.

Rigor Mortis and Why Fresh-Killed Meat Is Tough

Immediately after slaughter, muscles go through rigor mortis. During the animal’s life, actin and myosin contract and relax in cycles powered by ATP. When the animal dies, ATP production stops, but the actin-myosin bonds don’t release. The proteins lock in a contracted state and the muscle stiffens.

This is temporary. After a few hours to days (depending on temperature), enzymes in the muscle tissue begin degrading the locked protein structure. The rigor resolves and the meat softens somewhat. But it doesn’t fully return to pre-slaughter softness on its own.

If you cooked meat immediately after slaughter (before rigor fully sets), it would actually be quite tender, because the proteins haven’t contracted yet. Most commercial beef is held for 24-72 hours before processing for this reason.

What Aging Does at the Molecular Level

Both dry and wet aging extend the enzyme-driven breakdown of muscle proteins. The primary enzymes are calpains and cathepsins, which are naturally present in muscle tissue. They degrade proteins within the sarcomeres, particularly the z-lines (the end caps of each sarcomere unit). As z-lines degrade, the myofibril structure weakens and the fibers separate more easily under mechanical force (like chewing).

Dry aging also removes water from the meat’s exterior through evaporation. This concentrates flavor compounds in the remaining meat. It also allows surface mold to form (which is trimmed off before sale) and may produce some enzymatic breakdown of surface proteins that contributes to dry age’s characteristic funky, nutty flavor.

Wet aging (vacuum-sealed in refrigeration) achieves similar protein breakdown but without the moisture loss or flavor development. It’s faster, cheaper, and produces a cleaner beef flavor. Most supermarket beef is wet-aged for 1-2 weeks.

Top dry-aged steaks may be aged 30-90 days. By 60+ days, the flavor has developed significantly. Texture changes plateau earlier, around 21-28 days.

Grain Direction and Why It Matters

“Cutting against the grain” means slicing perpendicular to the direction of the muscle fibers. When you cut along the grain, each bite contains long fiber segments that stretch under the force of chewing. When you cut against the grain, you pre-shorten those fibers before they reach your mouth.

The difference is dramatic on tough cuts. Flank steak and skirt steak have very long, visible fibers running in one direction. Sliced with the grain: chewy and tough. Sliced against the grain into thin strips: noticeably more tender, with the same cook.

This is also why good carving technique matters for brisket and pork shoulder. These cuts often have muscle fibers running in different directions in different parts, and a carver who pays attention to grain direction will produce noticeably better results.

Fast-Twitch vs. Slow-Twitch Muscle

Muscles contain a mix of fast-twitch and slow-twitch fiber types. Slow-twitch fibers (Type I) are used for sustained, low-intensity activity. They’re dense with mitochondria and rich in myoglobin, the oxygen-storing protein that makes meat red. They have more connective tissue and are the source of the dark, rich flavor in active muscles.

Fast-twitch fibers (Type II) are used for quick, high-intensity bursts. They have fewer mitochondria and less myoglobin. Chicken breast is almost entirely fast-twitch, which is why it’s pale and dries out quickly when overcooked: it has little intramuscular fat, low myoglobin content, and its fibers contract hard under heat.

Chicken thigh is mostly slow-twitch. It’s darker, has more fat, and stays moist over a much wider temperature range. The structural differences between leg and breast mean they really do cook differently, and recipes that treat them identically usually do one or the other a disservice.

Why Marbling Changes Everything

Intramuscular fat (marbling) is fat deposited within the muscle tissue between fiber bundles. It doesn’t directly affect collagen content or fiber structure, but it changes the eating experience significantly.

When marbled meat cooks, the fat melts and distributes within the muscle structure. It lubricates the fibers and contributes to a sensation of juiciness and tenderness that lean cuts can’t replicate. A choice rib-eye and a prime rib-eye may have similar collagen content, but the prime grade’s higher marbling produces a noticeably richer, more forgiving texture.

This is why well-marbled cuts are more forgiving of slight overcooking. The fat masks some of the textural damage from excessive actin denaturation. Brining helps lean cuts achieve similar forgiveness through a different mechanism.