Cooking science

Muscle fibres contracting

When meat heats, its muscle fibres contract — first along their length, then across their width — and squeeze out the water held between and within them. That is why a steak shrinks, why a pan fills with liquid, and why overcooked meat is dry.

Two contractions, at different temperatures

Muscle fibres are bundles of long protein filaments held in a matrix of water. As they heat, the proteins denature and the structure tightens — first the fibres shorten lengthwise, and then, at a higher temperature, they contract sideways.

The sideways contraction is the one that matters most. It squeezes the water out of the spaces between the fibres, and that water leaves the meat. The pan fills, the meat shrinks, and it feels drier in the mouth even though it has not been dried by heat as such.

This is why doneness and juiciness are the same axis. Every additional degree contracts the fibres further and expels more liquid, and there is no technique that reverses it.

What changes the outcome

Fat

Melted intramuscular fat lubricates the fibres and makes the meat feel juicier at the same level of contraction. This is why a marbled cut is more forgiving.

Collagen

In a slow-cooked cut, collagen converting to gelatine adds moisture-holding material as the fibres contract. That is why a braise can be tender and moist while a lean cut at the same temperature is dry.

Salt

Salt dissolves some muscle proteins and lets the meat hold more water. That is the entire mechanism behind brining.

Speed

Faster cooking gives a steeper gradient and a wider band of heavily contracted meat. Slower cooking contracts more evenly.

Resting

The fibres relax slightly as the temperature falls, and some expelled liquid is reabsorbed.

Mince

Enormous surface area and disrupted fibres, so it weeps far more readily than a whole cut.

Where you see it

• A steak shrinking visibly in the pan as it cooks.
• Liquid appearing under chicken breasts in a roasting tin.
• A burger curling into a dome as the fibres on one face contract faster.
• Mince flooding the pan and steaming instead of browning.
• A pool of juice under an unrested steak on the board.
• Sausages splitting, where the contracting meat pushes against the casing.
• A joint losing a quarter of its weight during roasting, nearly all of it water.

Working with it rather than against it

1. Brine or dry-brine lean cuts. Salt genuinely increases how much water the muscle holds.
2. Choose fattier or more collagen-rich cuts if dryness keeps happening. Chuck, shoulder, thigh and belly are all more forgiving than loin and breast.
3. Cook to the doneness you want and stop. Every extra degree costs liquid.
4. Slow down for thick pieces so the contraction is more even and the overcooked band narrower.
5. Rest the meat, which allows partial reabsorption.
6. Slice across the grain, which shortens the fibres you have to chew through and makes the same meat eat more tender.
7. Keep the juices. What ends up on the board or in the tin is concentrated and belongs in the sauce.

Common questions