L-cysteine (E920)
An amino acid used to soften dough. It can be synthetic, fermented, or extracted from feathers or hair, and bread labels do not say which.
Also labelled as: Cysteine, L-cysteine hydrochloride, Dough conditioner, E920.
What it is
L-cysteine is an amino acid used in commercial baking as a dough conditioner. It relaxes gluten so dough can be machined faster, which is why it appears in pizza bases, burger buns, bagels, and long-life pastries. It also has non-food uses in pharmaceuticals and cosmetics.
Why the source matters
L-cysteine is produced in three quite different ways: chemical synthesis, microbial fermentation, and extraction by hydrolysis from keratin such as duck and chicken feathers or human hair. All three yield the same molecule. Feather-derived and hair-derived L-cysteine raise questions for halal and kosher observance that a synthetic or fermented version does not, and the label never distinguishes them.
L-cysteine by dietary profile
| Profile | Verdict | Why |
|---|---|---|
| Halal | Depends on the source | Synthetic and fermentation-derived L-cysteine are generally accepted. Feather-derived material depends on the bird and its slaughter, and human-hair-derived material is widely regarded as not permissible. Certification is the only practical resolution. |
| Kosher | Depends on the source | Source determines status, and certification is required to know it. Human-hair-derived material is not acceptable to kosher authorities. |
| Vegetarian | Depends on the source | Synthetic and fermented versions are suitable. Feather-derived material is animal-derived. |
| Vegan | Depends on the source | Only synthetic or microbially fermented L-cysteine is suitable, and the label will not tell you which was used. |
| Pescatarian | Depends on the source | Same position as vegetarian, resting on the production route. |
| Hindu vegetarian | Depends on the source | Suitable only if synthetic or fermented. Keratin-derived material is not. |
How to spot it on a label
- L-cysteine, cysteine, or E920 in a bread, bun, or pastry ingredient list.
- Dough conditioner as a category name may cover it without naming it.
- A halal or kosher mark on baked goods usually implies an approved source.
- Home baking flour rarely contains it; industrial bakery products are where it appears.
What PlatePal cannot determine
Known limits for l-cysteine
- Which of the three production routes was used.
- Whether feather-derived material came from an animal slaughtered to halal requirements.
- Whether an unlabelled dough conditioner blend contains it at all.
When any of these apply, PlatePal returns Insufficient data rather than a guess. A scan that says it does not know is doing its job.
Sources
Regulations change and authorities differ. This page is general information, not a religious or medical ruling. Last reviewed 2026-09-12.
Related ingredients
- Gelatin: A collagen protein set from animal hide and bone. Whether it fits your diet depends entirely on which animal it came from, and labels rarely say.
- Carmine: A red colour made from cochineal insects. It must be named on US labels, which makes it one of the easier additives to spot.
- Rennet: The enzyme that curdles milk into cheese. It can be animal, microbial, or fermentation-produced, and Enzymes on a label tells you nothing about which.
Back to the ingredient guide, or see how PlatePal checks a whole product with the halal, kosher, and allergen scanners.