Basic Science

Eggs are primarily made up of protein. When an egg is raw, the protein strands are bound in discrete little bundles, kind of like those nests of angel hair pasta you see in the store. Because these bundles are separate from each other, they allow light to pass between them, which is why egg whites are transparent when raw.

When heat is applied to these protein strands, they begin to unwind. Eventually the now-loosened protein strands run into each other and reattach in a pattern more like a spider web. This web is dense enough to prevent light from passing through, which is why cooked egg whites are white in color. The web also traps moisture (added or innate to the egg), which is why eggs cooked to just this point are moist and tender.

If you keep cooking the eggs, though, the protein strands once again become unstuck from each other, the web dissolves, and the moisture comes flowing out. That's why, for example, if you overcook scrambled eggs, they begin to "weep," or if you bake a custard (which is thickened by eggs) too long, you reach a point where the custard "breaks" and gives off liquid. The idea, then, is to cook the eggs just long enough to make them tender and solid, but not so long that they lose their moisture and become tough.

Egg whites and egg yolks coagulate—turn from a fluid into a thickened mass—at different temperatures. Egg whites begin to set at 144 degrees; yolks, because they contain fat that delays coagulation, begin to set at 149 degrees. This explains why with fried and poached eggs, it's possible to have fully set whites but fluid, runny yolks.