L-Cysteine
An amino acid used in industrial bread-making to soften dough. Often sourced from duck feathers or animal hair, making it unsuitable for vegans, vegetarians, and some religious diets.
E920 is frequently derived from duck feathers, hog bristles, or goose feathers. Vegan, vegetarian, halal, and kosher consumers cannot assume it is suitable without contacting the manufacturer. Fermentation-derived versions exist but are not always used.
What is it?
L-Cysteine is a naturally occurring sulphur-containing amino acid found in proteins. As a food additive it is produced commercially by one of two routes: chemical extraction from animal by-products (poultry feathers, hog bristles) or bacterial fermentation using plant-based starch. The human-hair extraction route is explicitly prohibited in the EU and UK.
What does it do?
L-Cysteine is a reducing agent. In dough, it breaks the disulphide bonds that cross-link gluten proteins, making the network more extensible. This shortens mixing time, reduces the energy needed to develop dough, and produces a softer, more elastic dough that is easier to machine at high speed. In very large-scale industrial baking, this is the primary reason for its use.
Where you will see it
Found mainly in mass-produced sliced bread, rolls, pizza bases, crackers, and pre-mixed bread improvers. Also used in some cereal-based snack products and processed savoury pastries. On a UK label it appears as 'flour treatment agent (E920)', 'L-cysteine hydrochloride', or simply 'E920'. It can also be declared as a processing aid, in which case it may not appear on the label at all.
What the science says
Sourcing from animal by-products
The traditional commercial route for producing L-Cysteine is acid hydrolysis of keratin-rich animal material, primarily duck and goose feathers and hog bristles. The resulting product is chemically identical to L-Cysteine from any other source, but its animal origin makes it incompatible with vegan, vegetarian, halal, and kosher diets. A fermentation route using plant starch exists and is growing in use, but manufacturers are not required to state the production method on the label.
L-Cysteine sold into the food industry is predominantly sourced from poultry feathers and animal hair by acid hydrolysis. Fermentation-based production using corn starch exists as an alternative.
Human hair as a raw material for L-Cysteine production is explicitly not permitted under EU and UK food additive legislation.
Health profile of L-Cysteine as a nutrient
L-Cysteine is one of the 20 standard amino acids and is present naturally in meat, eggs, dairy, and many plant foods. As a food additive the quantities ingested via bread are very small. No numerical acceptable daily intake has been set by EFSA, reflecting the view that it functions as a normal dietary constituent at the levels used. No carcinogenicity, genotoxicity, or reproductive concern has been identified at food-use levels.
EFSA's food additives panel noted that L-Cysteine is a naturally occurring amino acid and set no numerical ADI, considering the additive acceptable at the levels used as a flour treatment agent.
Use as a flour treatment agent and labelling gaps
Under UK and EU food law, flour treatment agents can in some circumstances be classified as processing aids rather than additives if they are technologically inert in the final product. When classified as a processing aid, E920 does not have to appear on the ingredient label. This means consumers who need to avoid it for dietary or religious reasons cannot always detect its presence from the label alone.
E920 is permitted as a flour treatment agent under UK assimilated food additive legislation and may be used without label declaration when it meets the processing aid criteria.
Where it stands with the regulators
Who should be careful
Vegans, vegetarians, and those following halal or kosher diets should be aware that E920 is commonly derived from animal by-products (poultry feathers, hog bristles). The label will not always state the source. Contact the manufacturer directly to establish the production route. Look for 'E920', 'L-cysteine', or 'flour treatment agent' on ingredient lists.
The honest read
There is no toxicological debate about L-Cysteine at food-use levels; it is a normal dietary amino acid and regulators have not identified a health concern at the amounts present in bread. The live issue is the sourcing: the majority of commercially produced L-Cysteine still comes from animal keratin, and the labelling system does not require manufacturers to state the origin. Fermentation-derived vegan versions exist, but without manufacturer confirmation there is no way to know which route was used. For people whose diet or faith excludes animal-derived ingredients, the uncertainty is real and cannot be resolved from the label alone.
Related additives
Common questions
Is E920 banned in the UK?
No. E920 is an approved food additive in the UK, permitted for use as a flour treatment agent under assimilated EU Regulation 1333/2008. The only restriction is that the human-hair extraction route is not permitted; poultry feather and fermentation sources are both allowed.
Is E920 derived from human hair?
Not in the UK or EU. Human hair as a raw material for E920 is explicitly prohibited under UK and EU food law. However, it may be derived from duck or goose feathers, hog bristles, or produced by bacterial fermentation. Manufacturers are not required to state which route was used.
What foods contain E920?
E920 is used mainly in mass-produced sliced bread, rolls, pizza bases, crackers, and bread improver blends. It may also appear in some savoury pastries and cereal snacks. It can sometimes be used as a processing aid, in which case it will not appear on the ingredient label.
Is E920 vegan?
Not reliably. When produced by fermentation using plant starch it is vegan, but the traditional and still widely used route involves acid hydrolysis of duck or goose feathers or hog bristles. The label rarely specifies the source, so vegans and vegetarians need to contact the manufacturer to confirm.
Sources
- UK FSA: Approved additives and E numbers
- Vegetarian Resource Group: L-Cysteine Update 2025
- Vegetarian Resource Group: L-Cysteine in Bread Products Still Mostly Sourced from Feathers and Hair (2011)
- Real Bread Campaign: Industrial loaf additives
- BAKERpedia: L-Cysteine
- Food-Info.net: E920 L-Cysteine
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