Potassium bromate
A flour-improving chemical banned in the UK and EU because residues that remain after baking are classified as a possible carcinogen.
Potassium bromate causes kidney and thyroid tumours in animal studies and damages DNA through oxidative injury. It is banned in the UK and EU; if you see it on a label, the product was likely made outside these markets.
What is it?
Potassium bromate is an inorganic salt of bromic acid (KBrO3). As a flour treatment agent it acts as a strong oxidising agent. It was historically used in bread-making to strengthen gluten networks, producing a higher rise and finer crumb. It does not survive complete baking at high temperatures in theory, but measurable residues have been detected in finished bread when baking conditions are not tightly controlled.
What does it do?
During dough mixing and fermentation, bromate ions oxidise the sulfhydryl groups in gluten proteins, forming disulfide bonds that tighten and strengthen the gluten network. This makes dough more elastic, improves oven spring and produces a lighter, whiter crumb. The reaction is slower than many alternative improvers, which suited long fermentation times. Residual bromate that is not consumed during baking remains in the finished product as the unreacted salt.
Where you will see it
Historically used in white bread, bread rolls, and other yeasted baked goods. Since the EU and UK ban it is not permitted in any food sold in these markets. It remains legal in the United States, parts of Asia, and some other countries, so it can appear in imported baked goods. On a UK or EU label it would appear as 'potassium bromate' or 'E924a', but such products would be non-compliant with UK food law.
What the science says
Animal carcinogenicity and IARC Group 2B classification
In rat studies, potassium bromate caused renal tubular adenomas and carcinomas, thyroid follicular tumours, and peritoneal mesotheliomas in males. Mice showed a low incidence of kidney tumours in males, and hamsters showed a marginally increased incidence of renal tubular tumours. IARC reviewed this evidence in 1999 and classified potassium bromate as Group 2B, possibly carcinogenic to humans, based on sufficient evidence in animals but inadequate evidence in humans (no human epidemiological data were available).
Rats given potassium bromate in drinking water developed renal tubular adenomas and carcinomas, thyroid follicular tumours, and peritoneal mesotheliomas in males; the evidence in experimental animals was judged sufficient for carcinogenicity.
IARC classified potassium bromate as Group 2B, possibly carcinogenic to humans, noting inadequate evidence in humans but sufficient evidence in experimental animals.
Genotoxicity and oxidative DNA damage
Potassium bromate is genotoxic in multiple laboratory assays. It is clastogenic in hamster and rat cells in vitro, triggers micronucleus formation in rodents in vivo, and is genotoxic in experimental systems in vivo. Results in bacterial mutagenicity assays were equivocal. The primary mechanism appears to be oxidative: bromate generates reactive oxygen species in kidney tissue, producing oxidative DNA lesions including 8-hydroxydeoxyguanosine. This direct DNA damage mechanism is why regulators treat residues in food as unacceptable regardless of dose.
Potassium bromate was clastogenic in Chinese hamster and rat cells in vitro and induced micronucleus formation in mice in vivo; bacterial mutagenicity results were equivocal, with no firm conclusion drawn for mutagenicity in bacteria.
Potassium bromate produces lipid peroxidation and oxidative DNA damage in rat kidney, including induction of 8-hydroxydeoxyguanosine, indicating the carcinogenic mechanism involves oxidative DNA injury.
Residues in finished bread
The food industry argument for bromate was that it converts completely to bromide during baking and leaves no residue. Studies have shown this is not reliably the case. Measurable potassium bromate residues have been detected in commercially baked bread, particularly when oven temperatures are lower than ideal or loaves are denser. This unpredictability of residue elimination underpinned the UK and EU decisions to ban it.
Potassium bromate residues have been detected in finished baked bread products, demonstrating that complete conversion to bromide during baking cannot be guaranteed under real commercial conditions.
Where it stands with the regulators
Who should be careful
Everyone in the UK and EU is already protected by the ban, so the practical concern is imported bread or baked goods from countries where it remains legal, particularly from the United States. If buying imported bread products, check the ingredients list for 'potassium bromate' or 'bromated flour'. Anyone with kidney disease may have particular reason to be alert, given the kidney as the primary target organ in animal studies.
The honest read
The science on potassium bromate is more settled than for many other flagged additives. The animal carcinogenicity data is robust across multiple species and tumour types, the genotoxicity evidence is consistent across multiple assay systems, and the oxidative DNA damage mechanism is well characterised. The gap is human epidemiology: no population studies have directly measured cancer risk from dietary potassium bromate exposure in humans, partly because the ban removed the exposure route in most countries before large studies could be run. IARC's Group 2B reflects that gap, not absence of concern. The UK and EU regulators took a precautionary position in 1990 and the subsequent IARC review reinforced it. The debate that remains is mainly in the United States, where the FDA has not acted despite acknowledging the carcinogenicity data. The science is not settled on the human risk magnitude, but the direction of the signal is clear.
Related additives
Common questions
Is E924a banned in the UK?
Yes. The UK banned potassium bromate as a flour improver under SI 1990/399, which came into force on 1 April 1990. It is not on the UK FSA list of approved food additives and cannot legally be used in food sold in the UK.
Why was potassium bromate banned in flour?
Two reasons: first, animal studies showed it caused kidney and thyroid tumours; second, residues were found in finished bread, proving the additive did not always convert fully to harmless bromide during baking as claimed. The combination of carcinogenicity data and unpredictable residues led the UK to ban it in 1990, with the EU following the same position.
What foods contain E924a?
None legally sold in the UK or EU. It was historically used in white bread and bread rolls as a dough improver. It can still appear in bread and baked goods imported from countries where it remains permitted, most notably the United States, where it is still legal. Check the label on any imported bread products.
Is E924a vegan?
Potassium bromate is a synthetic inorganic salt with no animal-derived ingredients, so it is vegan in that narrow sense. However, it is banned in the UK and EU, so the vegan question is of limited practical relevance for shoppers here.
Sources
- IARC Monographs Volume 73: Some Chemicals that Cause Tumours of the Kidney or Urinary Bladder in Rodents and Some Other Substances (Potassium bromate)
- IARC Monographs Summary and Evaluation, Volume 73 (inchem.org)
- UK FSA Approved Additives and E Numbers
- WHO/SDE/WSH/05.08/78 Bromate in Drinking-water (WHO background document)
- The Potassium Bromate (Prohibition as a Flour Improver) Regulations 1990 (SI 1990/399)
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