Potassium metabisulphite
A sulphite preservative that releases sulphur dioxide to stop wine, cider, dried fruit and processed foods from spoiling. One of the 14 declarable allergens in UK law.
Sulphites can trigger bronchoconstriction, wheezing, urticaria and anaphylaxis in sensitive individuals, particularly those with asthma. EFSA withdrew the safe daily intake limit in 2022 after finding a potential neurotoxicity concern from animal studies. Regular intake also inhibits beneficial gut bacteria at concentrations within the range permitted in food.
What is it?
Potassium metabisulphite is a white crystalline sulphite salt made by reacting sulphur dioxide with potassium carbonate. When dissolved in food or drink, it releases sulphur dioxide (SO2), the active antimicrobial and antioxidant agent. It is produced synthetically with no animal-derived ingredients.
What does it do?
The released sulphur dioxide inhibits the growth of bacteria, yeasts and moulds by disrupting their cellular processes. It also acts as an antioxidant, scavenging oxygen and free radicals to prevent browning and oxidative spoilage. In winemaking it is the primary preservative, added at crush and before bottling.
Where you will see it
Wine, cider, beer and fruit juices are the main uses. Also found in dried fruits (especially apricots, raisins and sultanas), pickled and preserved vegetables, jams, processed potato products such as instant mash and frozen chips, some crustacean preparations, and certain condiments and sauces. On labels it appears as 'potassium metabisulphite', 'E224', or under the collective allergen declaration 'contains sulphites' or 'sulphur dioxide'.
What the science says
Sulphite sensitivity and asthma
In people with asthma, sulphites can trigger bronchoconstriction within minutes of ingestion. Proposed mechanisms include sulphur dioxide forming in the airways and activating a cholinergic reflex, deficiency in the enzyme sulphite oxidase, and histamine or leukotriene release. Reactions range from wheezing and chest tightness to anaphylaxis. A challenge study using oral potassium metabisulphite capsules found a 3.9% reaction rate among asthmatic patients, rising to a higher rate among steroid-dependent asthmatics, who appear most at risk.
An oral challenge with potassium metabisulphite capsules found that 3.9% of asthmatic patients reacted with a greater than 20% fall in FEV1 within 30 minutes. The rate was higher among steroid-dependent asthmatics, who represent the most sensitive group.
Sulphite-induced reactions in asthmatic individuals include bronchoconstriction, wheezing, dyspnea, urticaria, angio-oedema, and in rare cases anaphylactic shock. The mechanism is not fully understood and likely involves multiple pathways.
All asthmatics sensitive to ingested sulphite also developed bronchoconstriction with inhaled sulphite. Sensitivity to inhaled sulphite was more common than sensitivity to ingested sulphite among asthmatic patients.
EFSA 2022 re-evaluation: ADI withdrawn, safety concern flagged
In 2022 EFSA concluded that its toxicity database was inadequate to maintain a firm acceptable daily intake for the sulphite group (E220-E228, which includes E224). The temporary ADI of 0.7 mg SO2 equivalents per kilogram of bodyweight per day set in 2016 was withdrawn. Using a margin of exposure approach, EFSA found that high consumers in most population groups, including children aged 3 to 10 and adults, had margins of exposure below the safety threshold of 80, meaning estimated intakes may exceed the level at which concern begins. Adolescents were the only group where the margin remained acceptable in the refined exposure scenario.
EFSA withdrew the temporary group ADI for sulphites (E220-E228) in 2022 due to an inadequate toxicity database. It adopted a margin of exposure approach, with the critical endpoint being prolonged visual evoked potential latency in rat studies. The benchmark dose lower confidence limit was 38 mg SO2 equivalents per kilogram of bodyweight per day.
Margin of exposure values were below the safety threshold of 80 for high consumers in all population groups except adolescents, indicating a potential safety concern for children (3-10 years) and adults. Children aged 3-10 were potentially up to 12.5% over the threshold at the 95th percentile; adults up to 60%.
In 2016, EFSA had already found that estimated dietary exposure to sulphites exceeded the then-temporary group ADI of 0.7 mg SO2/kg bodyweight/day for all population groups surveyed, and called for additional toxicological studies.
Neurotoxicity signals in animal studies
The primary toxicological endpoint driving EFSA's 2022 concern was prolonged visual evoked potential latency in rats given sulphite in drinking water, which is an early marker of central nervous system dysfunction. Rats deficient in the enzyme sulphite oxidase showed greater effects, reflecting what may happen in humans with lower enzyme activity. The critical reference point used by EFSA was 38 mg SO2 equivalents per kilogram of bodyweight per day. These are animal studies and no equivalent human trials have been conducted.
Sulphite administered to rats in drinking water at 25mg/kg bodyweight caused prolonged latencies in visual evoked potential components and increased lipid peroxidation in brain and retinal tissue. Effects were more pronounced in sulphite oxidase-deficient animals.
Sulphite ingestion in aged rats caused dose-dependent prolongation of visual evoked potential components and changes in brain antioxidant enzyme activity, with sulphite oxidase-deficient animals showing greater effect.
Gut microbiome effects
At concentrations within the legally permitted range for food, sulphites inhibit the growth of four species of beneficial gut bacteria in laboratory experiments. The researchers found both growth-inhibiting and bacteria-killing effects at concentrations within the regulated food-safe range. No human trials have tested whether this effect occurs under real dietary conditions.
Sulphites inhibited the growth of Lactobacillus casei, Lactobacillus plantarum, Lactobacillus rhamnosus and Streptococcus thermophilus. Growth was inhibited at 250-500 ppm and bacteria were killed at 1000-3780 ppm, concentrations within the range permitted in food.
Sulphites and vitamin B1 destruction
Sulphite ions chemically cleave the thiamine (vitamin B1) molecule, rendering it nutritionally inactive. This is why sulphites are not permitted as preservatives in foods that are major sources of vitamin B1. For typical uses such as wine, where thiamine is not a meaningful dietary source, this is not a nutritional concern for consumers, but it is an established chemical effect.
Sulphite ions cleave thiamine at the methylene bridge, separating its two ring components and rendering the vitamin nutritionally inactive. The reaction is accelerated under acidic conditions.
Where it stands with the regulators
Who should be careful
People with asthma should be particularly vigilant, as sulphites can trigger bronchoconstriction ranging from mild wheezing to life-threatening bronchospasm, especially those with steroid-dependent or poorly controlled asthma. Anyone with known sulphite sensitivity should avoid foods and drinks carrying 'contains sulphites', 'sulphur dioxide', or any E220-E228 number in the ingredients list. The highest dietary exposures come from wine, dried fruit, and processed potato products. Reactions typically occur within 15 to 30 minutes of ingestion.
The honest read
E224 and its sulphite relatives are among the most widely studied food preservatives, used extensively since ancient times. The well-established real-world risk is sulphite sensitivity in people with asthma, which is documented, declarable under UK law, and occasionally severe. EFSA's 2022 decision to withdraw the safe daily intake limit, because it could not be justified by the available data, is a meaningful regulatory signal. The neurotoxicity signals that drove that decision come from animal studies; the gap between those animal doses and typical human dietary intake is not yet bridged by human evidence. Whether regular sulphite intake at typical food concentrations affects the gut microbiome or nervous system function in the general population is an open question the current evidence cannot settle.
Related additives
Common questions
Is E224 banned in the UK?
No. E224 potassium metabisulphite is approved for use in the UK under retained food additives legislation. It must be declared on labels as an allergen whenever total sulphur dioxide exceeds 10mg/kg or 10mg/litre in the final product.
Why did EFSA withdraw the acceptable daily intake for sulphites in 2022?
EFSA concluded in its 2022 follow-up that the available toxicity data were insufficient to maintain a firm safe daily intake. Animal studies identified neurotoxic effects at higher doses, and dietary exposure surveys showed that high consumers in most population groups were already exceeding the previous temporary limit. EFSA flagged a potential safety concern and called for more human data, but did not recommend a ban.
What foods contain E224?
Wine and cider are the most common sources. Other significant sources include dried fruits such as apricots and sultanas, processed potato products such as frozen chips and instant mash, beer and fruit juices, jams and pickled vegetables, and some crustacean preparations. On the label, look for 'potassium metabisulphite', 'E224', or the collective declaration 'contains sulphites'.
Is E224 vegan?
Yes. Potassium metabisulphite is a synthetically produced inorganic salt made from sulphur dioxide and potassium carbonate. No animal-derived ingredients or processing methods are involved in its manufacture.
Sources
- EFSA Panel on Food Additives and Flavourings: Follow-up of the re-evaluation of sulphur dioxide (E220) and sulphites (E221-E228) as food additives, EFSA Journal 2022;20(12):7594
- EFSA Scientific Opinion on the re-evaluation of sulphur dioxide (E220) and sulphites (E221-E228) as food additives, EFSA Journal 2016;14(4):4438
- UK FSA: Potassium metabisulphite (E224) regulated product entry
- UK Food Standards Agency: Approved additives and E numbers
- Anaphylaxis UK: Sulphites fact sheet
- Prevalence of sensitivity to sulfiting agents in asthmatic patients, American Journal of Medicine 1986, PMID 3535492
- Irwin et al. (2017): Sulfites inhibit the growth of four species of beneficial gut bacteria, PLOS ONE, doi:10.1371/journal.pone.0186629
- Yargicoglu et al. (2005): Visual evoked potentials in normal and sulphite oxidase deficient rats exposed to ingested sulphite, Neurotoxicology, PMID 16150492
- Ozsoy et al. (2016): The effect of ingested sulphite on visual evoked potentials, lipid peroxidation and antioxidant status of brain in normal and sulphite oxidase-deficient aged rats, Toxicology and Industrial Health 32(7):1197-1207, PMID 25342669
- Considerations for the diagnosis and management of sulphite sensitivity, PMC4017445
- UK FSA: Food allergen labelling and information requirements guidance
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