Why does egg white turn solid and opaque when you cook it (or add acid)?
Egg white is mostly protein folded into a precise 3-D shape held by relatively weak bonds (hydrogen bonds, disulfide and ionic links).
Egg white is mostly protein folded into a precise 3-D shape held by relatively weak bonds (hydrogen bonds, disulfide and ionic links). Heat or acid disrupts those bonds, so the protein unfolds and its chains tangle and clump together — denaturation. The clumped proteins scatter light (opaque) and lock into a solid gel. The amino-acid sequence is unchanged; only the folded shape is destroyed, which is why it's irreversible here.
Key point
Heat/acid denatures protein — the folded shape unravels and the chains coagulate.
Common mistake
thinking cooking changes the protein's amino-acid sequence.
Memory tip
denaturation breaks the folding, not the peptide backbone.
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