I Didn’t Know That. . .

Q: Why do eggs turn hard when you boil them?

A: The hardening is a chemical reaction induced by heat and made possible by the fact that eggs have a high protein content. Proteins are long chains of amino acids that are folded on themselves and held in place by weak bonds. Heating causes these bonds to break, allowing the proteins to unfold. Additional heat then causes the proteins to form strong bonds from one protein molecule to another, creating a solid megamolecule. Cooking is best done with moderate heat because high heat causes the protein to become tough and rubbery. High heat also causes a chemical reaction between the yolk and the white that leaves a green film of iron sulfide around the yolk. Heat is not necessary to harden the white. Vodka or vinegar poured on the white will produce the same effect.