Cysteine is unique among the protein amino acids because it has a sulfhydryl group at the end of the molecule. Cysteine is important to dough reduction chemistry because it occurs in the gluten protein from flour, in the tripeptide glutathione from yeast, and in free amino acid form as a synthetic reducing agent.
The significance of cysteine's sulfhydryl group is that two of them from different proteins can be oxidized to one molecule of cystine, with the creation of a disulfide bond between them. When gluten molecules become linked (oxidized) during bread making, the dough strength increases but its extensibility decreases. During mixing these linkages are broken mechanically to provide the extensibility needed for moulding. The process is reversible, and the gluten matrix reforms during the later stages of proofing and baking.
The disulfide bonds in gluten that are broken mechanically during mixing can also be broken chemically by a series of reactions with cysteine or glutathione