Cystine

Cystine is a white crystalline amino acid made of two cysteines joined as an oxidized dimer through a disulfide bond. Healthy people synthesize it from methionine or homocysteine, so it is not counted as essential. It stores much of the sulphur in proteins such as keratin, gluten and egg white, and these cross-links lend dough its elasticity and egg gels their firmness. Heat and reducing agents in processing can break and re-form the bonds, reshaping texture in baked and egg-based foods.