Protein Denaturation
The unfolding and structural change of proteins caused by heat — the molecular process that transforms raw steak into cooked steak.
Protein denaturation is the process by which a protein's three-dimensional structure unfolds in response to heat (or acid, or mechanical force). In cooking, heat-induced denaturation is the fundamental mechanism that transforms raw meat into cooked meat — changing its texture, color, and ability to retain moisture.
Muscle tissue contains several major proteins, each denaturing at different temperatures:
- **Myosin** (motor protein): begins denaturing around 122°F, roughly 50% denatured at 131°F. This is the primary protein responsible for the firmness change from rare to medium-rare.
- **Collagen** (connective tissue): begins denaturing around 150°F, converts to gelatin at sustained temperatures above 160°F. Important for tough cuts but minimal in tender steaks.
- **Actin** (structural protein): begins denaturing around 140°F. Actin denaturation causes significant muscle fiber contraction, squeezing out moisture. This is the turning point where steaks become noticeably drier.
The doneness connection: What we call "doneness" is really a measure of how much protein denaturation has occurred. Rare (120–125°F) = minimal denaturation. Medium-rare (129–134°F) = moderate myosin denaturation. Medium (138–145°F) = significant myosin + beginning actin denaturation. Well-done (160°F+) = near-complete denaturation of all major proteins.
Why temperature matters more than time for steaks: Denaturation is temperature-dependent. At a given temperature, it happens relatively quickly and then stops. This is why sous vide works — holding a steak at 131°F causes myosin to denature but actin remains intact, regardless of how long you hold it (within reason).