Denaturation

Denaturation is what happens when a protein loses its natural folded shape. Proteins in food start as tightly folded chains. Heat, acid, salt, alcohol, air and even whisking can make them unfold. Once unfolded, they often stick to each other. That is why a clear egg white turns white and firm, raw fish turns opaque in lime juice, and meat shrinks and firms as it cooks.

Why it works

Unfolded proteins expose parts that were hidden inside the fold. These parts bond with nearby proteins and form a network that traps water. A loose network gives a soft set, as in custard or a coddled egg. A tight network squeezes water out, as in a rubbery overcooked egg, dry meat or curdled sauce.

Different proteins unfold at different temperatures. Egg white starts to set at about 62°C (144°F) and is firm by about 80°C (176°F). Yolk thickens from about 65°C (149°F) and sets around 70°C (158°F). Muscle proteins in meat start changing from about 50°C (122°F), which is why a steak changes texture so much between rare and well done.

How to use it

• Heat: cook eggs gently for a tender set, and pull meat at the right internal temperature.
• Acid: in ceviche, lime juice denatures the outer protein of fish so it turns opaque and firm. It looks cooked but has not been heated, so it does not kill bacteria or parasites the way cooking does.
• Salt: a brine partly unfolds muscle proteins so they hold more water.
• Whisking: beating egg whites unfolds their proteins at the surface of air bubbles, which builds a stable foam that can reach stiff peaks.

Common mistakes

• Thinking denaturation and food safety are the same thing. Acid or salt changes texture but does not make raw food safe.
• Cooking eggs or lean meat too hot, so the proteins squeeze out their water.
• Adding acid or salt to milk proteins carelessly, which can make them clump.

In recipes

Recipes rarely use the word. You see its effects in "cook until the egg whites are set", "until the shrimp turn pink and opaque" and "until the fish flakes".