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 syn- thetic reducing agent
the significance of cysteine is sulfhydryl group is that two of them different proteins can be oxidized to one molecule of cystine, with the creation of a disulfide bond between them. When gluten molecules become linked 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
to yeast, cause sensitivity reactions in some people, and require special label declarations if used in the U.S. at levels in the finished product above 10 parts per million.
Ascorbic acid (vitamin C) is used as a reducing agent only in certain closed con- tinuous mix applications. In the presence of oxygen it functions as an oxidizing agent, but in the absence of oxygen, as a reducing agent. It can be used in coated form to de- lay its reaction until the desired part of the process.
Other acids that have been suggested as reducing agents, but are not commonly used, include sorbic acid and fumaric acid. They are part of a group of "activated double-bond compounds" and are more commonly used as preservatives. These acids are inhibitory to yeast and less eco- nomical than other synthetic reducing agents.
Proteases are used to decrease mix time and increase elasticity, but are not reducing agents. They are natural enzymes that break down gluten irreversibly so must be used
with careful attention to dose, time, and temperature to avoid over conversion