E-numbers / Other additives / E921
also: Cystine · cystine hydrochloride · L-cysteine cystine salts
A sulphur-containing amino acid closely related to E920 (L-Cysteine). It is not a permitted food additive in the UK or EU. L-Cystine does hold a separate Great Britain flavouring authorisation (FL No. 17.006) but this does not grant it E-number food additive status.
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E921 is not authorised for use as a food additive in the UK or EU.
3
of the 43,325 UK supermarket foods we checked declare E921 in their ingredients
That is under 0.1%, counted by its code or its name.
Counted by NutraSafe from UK supermarket ingredient lists, September 2026. How we counted
Why it’s worth knowing. E921 is not authorised for use as a food additive in the UK or EU. If it appears on a label as an additive (E921) it should not be there under current law. Note that L-Cystine is separately authorised as a flavouring substance in Great Britain, which is a different legal category.
L-Cystine is the oxidised, disulphide-bridged dimer of the amino acid L-Cysteine. The two cysteine molecules join via a sulphur-sulphur bond to form cystine. As a naturally occurring amino acid it is present in many protein-rich foods including eggs, meat, dairy and pulses. The additive form has historically been produced from human hair, animal feathers or by fermentation, the same feedstocks used for its close relative E920 (L-Cysteine).
In its natural dietary form cystine is broken down in the gut back to cysteine and contributes to protein synthesis. As a potential food additive its function would be similar to L-Cysteine: acting as a reducing agent to weaken gluten networks in dough, shortening mixing time and improving dough extensibility. Unlike E920, E921 does not hold a UK or EU authorisation for use as a food additive. L-Cystine is separately authorised as a flavouring substance in Great Britain (FL No. 17.006), a different legal category that does not carry an E number.
L-Cystine is a constituent of many protein foods naturally but is not authorised as a food additive in the UK or EU, so it should not appear as an E-number on a UK or EU food label in its additive role. It may appear in an ingredients list as a flavouring substance under its chemical name (without an E number) where used in that permitted capacity. If you see 'E921' listed as an additive on a UK product label, that product may be non-compliant.
E921 does not appear in the UK FSA approved-additives list or in Annex II of the assimilated EU Regulation 1333/2008, which governs permitted food additives. Only additives explicitly listed in that regulation may be used in food sold in Great Britain. Its close relative E920 (L-Cysteine) is authorised as a flour treatment agent but E921 is separately classified and carries no equivalent food additive authorisation. L-Cystine does hold a separate Great Britain flavouring authorisation (FL No. 17.006, CAS 56-89-3) under the FSA regulated products database, authorised from 31 December 2020, but this is a distinct legal category from food additives and does not assign an E number.
E921 (L-Cystine) does not appear on the UK FSA approved-additives list. Only E920 (L-Cysteine) is listed in the 'Others' category for flour treatment.
Annex II of EU Regulation 1333/2008 on food additives lists E920 (L-Cysteine) as a flour treatment agent but does not include E921 (L-Cystine) as an authorised additive.
L-Cystine is authorised as a flavouring substance in England, Scotland and Wales under FL No. 17.006 (CAS 56-89-3), authorised from 31 December 2020. This is a flavouring authorisation, not a food additive authorisation, and carries no E number.
When consumed as part of normal protein-rich food, L-Cystine is an ordinary dietary amino acid. The body reduces it back to cysteine, which contributes to glutathione synthesis, protein structure and other metabolic functions. There is no specific toxicological concern attached to normal dietary exposure from food. The regulatory issue is specifically about its use as an intentionally added functional additive without authorisation.
L-Cystine is a naturally occurring amino acid found in most dietary proteins and is metabolised normally when ingested as part of food.
The commercial additive form of L-Cystine, like L-Cysteine (E920), has traditionally been produced from animal by-products including human hair and poultry feathers, though fermentation-derived versions now exist. This origin is a concern for people following vegan, halal or kosher diets, since the sourcing is not always declared and may involve pork-derived or non-halal material.
Commercial L-Cysteine and L-Cystine have historically been derived from human hair, poultry feathers or pig bristles. Fermentation-derived ('vegan') grades are available but not universally used.
Anyone who eats strictly vegan, halal or kosher should note that if L-Cystine or E921 were to appear in a product, the source material may be animal-derived (feathers, hair or pork bristle) and is unlikely to be declared. Look for 'E921' or 'L-Cystine' in the ingredients list, though its presence in authorised UK food products as an additive (E921) would itself be a regulatory anomaly.
E921 occupies an unusual position: it is a real, naturally occurring amino acid with no intrinsic toxicological concern at normal dietary intakes, yet it holds no authorisation as a food additive in the UK or EU. L-Cystine does hold a separate Great Britain flavouring authorisation (FL No. 17.006) where it may appear by its chemical name, not as an E number. The distinction from its permitted sibling E920 (L-Cysteine) is regulatory, not biological. In practice, shoppers in the UK are extremely unlikely to encounter E921 declared as a food additive on a compliant product label. The concern here is about its food additive regulatory status and, if encountered as E921, likely compliance non-conformity.
E921 is not on the UK FSA approved-additives list, which means it is not authorised for use as a food additive in the UK. It was never formally banned but it was never approved as a food additive either. Additives not on the list cannot lawfully be used as food additives in Great Britain. Note that L-Cystine does hold a separate GB flavouring authorisation (FL No. 17.006) for use as a flavouring substance, which is a different legal category and carries no E number.
E920 is L-Cysteine, a single amino acid molecule. E921 is L-Cystine, its oxidised dimer, formed when two cysteine molecules bond together. E920 is an approved flour treatment agent in the UK and EU. E921 is not approved for use as a food additive in either jurisdiction, though L-Cystine holds a separate Great Britain flavouring authorisation (FL No. 17.006) without an E number.
L-Cystine as a naturally occurring amino acid is found in eggs, meat, dairy, pulses and many protein-rich foods. As an intentionally added food additive (E921), it should not appear in any UK or EU food product because it is not authorised as a food additive. If you see it listed as E921 on a UK label, the product may not comply with UK food law.
Not reliably. The commercial form of L-Cystine has historically been produced from human hair, poultry feathers or pig-derived material. Fermentation-derived versions exist but there is no EU or UK labelling requirement to declare the source. Since E921 is not an authorised UK food additive, encountering it as E921 on a compliant label would be unusual.
Last reviewed: 20 June 2026
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