L-Cysteine
202114-66-7 · 52-90-4 · 56-89-3 · (2R)-2-amino-3-mercapto-propionic acid;hydrochloride · (2R)-2-amino-3-mercaptopropanoate · (2R)-2-amino-3-mercaptopropanoic acid · (2R)-2-amino-3-sulfanyl-propanoic acid · (2R)-2-amino-3-sulfanylpropanoate · (2R)-2-amino-3-sulfanylpropanoic acid · (2R)-2-azaniumyl-3-sulfanylpropanoate · (R)-(+)-Cysteine · (R)-2-Amino-3-mercaptopropanoic acid · (R)-2-Amino-3-mercaptopropionic acid · (R)-2-Amino-3-sulfanyl-propanoic acid · (R)-Cysteine · 2-amino-3-mercaptopropanoate · 2-Amino-3-mercaptopropanoic acid · 2-Amino-3-mercaptopropanoic acid, (R)- · 2-Amino-3-mercaptopropionic acid · 2-amino-3-sulfanylpropanoic acid · 2-azaniumyl-3-sulfanylpropanoate · [R-(R*,R*)]-3,3'-dithiobis [2-aminopropanoic acid] · alpha-Amino-beta-mercaptopropanoic acid, L- · alpha-Amino-beta-mercaptopropionic acid, L- · alpha-Amino-beta-thiolpropionic acid · alpha-Amino-beta-thiolpropionic acid, L- · beta-Mercaptoalanine · beta-Mercaptoalanine, L- · bis(β-amino-β-carboxyethyl) disulfide · CCRIS 912 · cisteina · Cisteinum · Cisteinum (Latin) · Cys · CYSTE · Cystein · cysteine · Cysteine, L- · Cysteinum · Dicysteine · E 920 · FEMA No. 3263 · FREE CYSTEINE · Gelucystine · H-Cys-OH · Half cystine · Half-cystine · HSDB 2109 · L Cysteine · L-(+)-Cysteine · L-2-Amino-3-mercaptopropanoic acid · L-2-Amino-3-mercaptopropionic acid · L-Alanine, 3-mercapto- · L-Cys · l-cystAine · L-Cystein · L-cysteine · L-Cysteine 7403 · L-Cysteine from non-animal source · L-Cysteine High · L-Cysteine High 7404 · l-cysteinum · L-cystine · L-Zystein · Propanoic acid, 2-amino-3-mercapto-, (R)- · Thioserine · α-diamino-beta-dithiolactic acid · β,β'-diamino-β,β'-dicarboxydiethyl disulfide · β,β'-dithiodialanine · E920 · E921 · (2R)-2-amino-3-[[(2R)-2-amino-2-carboxyethyl]disulfanyl]propanoic acid · (2R)-2-amino-3-sulfanylpropanoic acid · L-CysteineE number E920CAS 52-90-4
L-Cysteine is a dough conditioner that relaxes gluten, cutting the mixing and proving time of industrial bread. It is the additive most often raised as a dietary question, because one historic production route was hydrolysis of human hair and poultry feathers.
Where it is and isn’t permitted
| Jurisdiction | Status | Scope and limits | Citation |
|---|---|---|---|
| United States | Permitted | 21 CFR 184.1271retrieved 2026-09-07 | |
| Codex Alimentarius | Not listedDoes not appear on this jurisdiction’s permitted list. That is not the same as being prohibited. |
What it is made from
Now produced chiefly by bacterial fermentation, and also by extraction from keratin — duck and chicken feathers, hog bristle, and historically human hair. Manufacturers need not declare which route was used.
- Vegan status depends on the source
- Vegetarian status depends on the source
- May raise a halal question
- May raise a kosher question
This is not an omission. What this additive is made from is not fixed — depending on the manufacturer it may be plant, animal or microbial in origin, nobody is required to say which, and the same product can change source between production runs. Ask the manufacturer if it matters to you; a label cannot answer it.
- CAS registry number
- 52-90-4
- InChIKey
- XUJNEKJLAYXESH-REOHCLBHSA-N
- PubChem
- CID 5862
Every name it appears under
74 recorded names. A label may use any of them.
E number
- E920
- E921
CAS registry number
- 202114-66-7
- 52-90-4
- 56-89-3
As it appears in regulation
- L-Cysteine
Other names
- (2R)-2-amino-3-mercapto-propionic acid;hydrochloride
- (2R)-2-amino-3-mercaptopropanoate
- (2R)-2-amino-3-mercaptopropanoic acid
- (2R)-2-amino-3-sulfanyl-propanoic acid
- (2R)-2-amino-3-sulfanylpropanoate
- (2R)-2-amino-3-sulfanylpropanoic acid
- (2R)-2-azaniumyl-3-sulfanylpropanoate
- (R)-(+)-Cysteine
- (R)-2-Amino-3-mercaptopropanoic acid
- (R)-2-Amino-3-mercaptopropionic acid
- (R)-2-Amino-3-sulfanyl-propanoic acid
- (R)-Cysteine
- 2-amino-3-mercaptopropanoate
- 2-Amino-3-mercaptopropanoic acid
- 2-Amino-3-mercaptopropanoic acid, (R)-
- 2-Amino-3-mercaptopropionic acid
- 2-amino-3-sulfanylpropanoic acid
- 2-azaniumyl-3-sulfanylpropanoate
- [R-(R*,R*)]-3,3'-dithiobis [2-aminopropanoic acid]
- alpha-Amino-beta-mercaptopropanoic acid, L-
- alpha-Amino-beta-mercaptopropionic acid, L-
- alpha-Amino-beta-thiolpropionic acid
- alpha-Amino-beta-thiolpropionic acid, L-
- beta-Mercaptoalanine
- beta-Mercaptoalanine, L-
- bis(β-amino-β-carboxyethyl) disulfide
- CCRIS 912
- cisteina
- Cisteinum
- Cisteinum (Latin)
- Cys
- CYSTE
- Cystein
- cysteine
- Cysteine, L-
- Cysteinum
- Dicysteine
- E 920
- FEMA No. 3263
- FREE CYSTEINE
- Gelucystine
- H-Cys-OH
- Half cystine
- Half-cystine
- HSDB 2109
- L Cysteine
- L-(+)-Cysteine
- L-2-Amino-3-mercaptopropanoic acid
- L-2-Amino-3-mercaptopropionic acid
- L-Alanine, 3-mercapto-
- L-Cys
- l-cystAine
- L-Cystein
- L-cysteine
- L-Cysteine 7403
- L-Cysteine from non-animal source
- L-Cysteine High
- L-Cysteine High 7404
- l-cysteinum
- L-cystine
- L-Zystein
- Propanoic acid, 2-amino-3-mercapto-, (R)-
- Thioserine
- α-diamino-beta-dithiolactic acid
- β,β'-diamino-β,β'-dicarboxydiethyl disulfide
- β,β'-dithiodialanine
Systematic names
- (2R)-2-amino-3-[[(2R)-2-amino-2-carboxyethyl]disulfanyl]propanoic acid
- (2R)-2-amino-3-sulfanylpropanoic acid
Hazard reference values
No reference value for this substance in the sources we hold.
Evidence and how bodies read it
Studies, and the differing conclusions regulators drew from them, are curated by hand rather than machine-read, and none has been written for this substance yet.
Sources for this page
- ecfr retrieved 2026-09-07
Full licence terms and publisher details on Sources. How this page is built and updated: Method.
Machine-readable:/api/v1/additives/l-cysteine.json