L-Cysteine (E 920)

Last reviewed October 19, 2026

In plain words: L-cysteine (E 920) is an amino acid used in small amounts to make some doughs easier to work with. It can be made by fermentation or from animal sources such as feathers, and labels usually don't say which.

What the evidence review says

L-cysteine is an amino acid used in small amounts to make some doughs easier to work with. Safety reviews have not found a problem at these levels. It can be made by fermentation or from animal sources such as feathers, and labels usually don't say which; EU rules do not allow human hair as a source.

At the dough dose, toxicology reviews have not found a safety problem: it is a normal amino acid used at up to 0.009 part per 100 parts of flour in the US rule. The consumer issue is source. EU specifications state that human hair may not be used as a source. Feathers and other animal keratin can still be raw materials, and labels usually do not say which route was used. Fermentation-derived material is widely available. That is a transparency and dietary-practice issue, not evidence of toxicity of the purified amino acid.

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This page is for general education. It is not medical advice and does not replace advice from your doctor, pediatrician, allergist or registered dietitian. Research changes; we review these pages when new evidence or rules come out. Last reviewed October 19, 2026.