Temperature milestones where proteins unwind
Proteins unfold and lose their original structure when heat disrupts the weak bonds holding their coils and folds together. Egg white, mostly albumin, begins setting visibly at 62 degC and becomes fully opaque by 65 degC. Egg yolk, with different protein ratios, stays runny until 68 degC because its proteins require higher temperature to denature.
Collagen, the tough fibrous protein in connective tissue and tendons, has the highest threshold: it begins breaking down into gelatin around 70 degC. Below this temperature, low-and-slow cooking doesn't tenderize tough cuts; above it, collagen melts and dissolves over hours, turning gelatinous.
Why precise cooking temperatures matter
The narrow windows between raw and overcooked exist because protein denaturation happens in bands, not all at once. Steak cooked to medium-rare reaches 135 degF (57 degC), enough to denature most surface proteins while keeping myoglobin in its pink form. Fish flakes at lower temperatures because its collagen denatures faster than land animals.
Overcooking doesn't just make food tough, it also drives out moisture. Once proteins fully denature, they tighten and squeeze out water, which is why a well-done steak appears grayer and drier than medium-rare, despite being hotter.