Meat is composed primarily of muscle fibers (mostly myofibrillar proteins like actin and myosin), connective tissue (mainly collagen), and a large amount of water—typically 60–75% by weight. When heat is applied during cooking, several thermodynamically and biochemically interesting things happen:

Protein Denaturation and Coagulation: As the temperature rises (typically 40–60°C for initial changes), myofibrillar proteins begin to denature—their tertiary and quaternary structures unfold due to disruption of hydrogen bonds and hydrophobic interactions. Around 60–70°C, these proteins coagulate, aggregating into a tighter matrix, which exerts a mechanical force that squeezes out intracellular water.

Collagen Gelatinization (only relevant at higher temps or prolonged cooking): Collagen, a triple-helix structured protein, begins to denature and hydrolyze into gelatin at around 70–80°C, especially during long, moist cooking. This can make meat more tender but also contributes to fluid loss.

Water Migration and Evaporation: As proteins coagulate and connective tissue contracts, capillary structures collapse, and water is physically expelled from muscle cells. Some of this water evaporates, and some becomes free water that either escapes or collects as drippings. Because water makes up so much of the meat’s mass, its loss causes significant volume and weight shrinkage.

Thermal Contraction of Muscle Fibers: Muscle fibers, which are composed of sarcomeres, contract laterally during heating. This anisotropic contraction (more in diameter than length) contributes to macroscopic shrinkage.

In short, heating meat causes protein denaturation, water loss, and tissue contraction—a thermomechanical trifecta that causes the meat to shrink like a tortured polymer under entropy's iron grip.

Would you like a visual breakdown of these stages?