Orthophenyl phenol
A synthetic fungicide once applied to citrus fruit peel after harvest to stop mould. No longer a permitted food additive in the UK or EU.
The sodium salt form caused bladder tumours in rodents at high doses. Fetal developmental effects have been recorded in rat, mouse and rabbit studies. The substance is suspected of weak hormone-disrupting activity, though in-vivo evidence remains limited.
What is it?
Orthophenyl phenol (also called 2-phenylphenol or biphenyl-2-ol) is a white crystalline solid produced synthetically. It is a phenolic compound, chemically related to biphenyl. Its sodium salt is E232.
What does it do?
It kills or inhibits mould fungi and bacteria by disrupting cell membranes. When applied as a post-harvest treatment, it prevents Penicillium and other storage moulds from growing on fruit surfaces during transport and storage.
Where you will see it
Historically applied as a surface dip or wax coating to citrus fruits (oranges, lemons, grapefruits, mandarins) and pineapples after picking. When in use, treated fruit had to carry the label 'surface treatment' or the specific additive name. It has also been detected migrating from treated can sealing material into canned beer and soft drinks. It is no longer authorised as a food additive in the UK or EU; any fruit labelled with E231 would be non-compliant.
What the science says
Bladder tumours in rodents from the sodium salt
High-dose dietary feeding of sodium ortho-phenylphenate (the sodium salt, E232) consistently produced bladder tumours in male rats. In one study, 47 of 50 treated rats developed bladder tumours against zero in controls. Female rats and mice showed far lower susceptibility. The mechanism appears to involve urothelial cell irritation, increased cell turnover, and reactive oxidative metabolites rather than direct damage to DNA.
Sodium ortho-phenylphenate produced urinary bladder carcinomas and liver tumours in rodents. IARC classified it as Group 2B, possibly carcinogenic to humans, based on sufficient animal evidence and the absence of human epidemiological data.
Ortho-phenylphenol itself was classified IARC Group 3 (not classifiable as to carcinogenicity in humans) on the basis of limited and inconsistent animal data and no human data.
The carcinogenic mode of action in the bladder is indirect: reactive quinoid metabolites cause cytotoxicity and cell proliferation at neutral or alkaline urinary pH. Evidence for direct DNA binding by OPP or its metabolites is equivocal across more than 130 genotoxicity studies.
Developmental toxicity in animal studies
Animal studies in rats, mice and rabbits found fetal effects including reduced fetal body weight, skeletal developmental delays, and increased resorptions. A 2013 re-evaluation concluded that some of these fetal effects may occur at doses where maternal toxicity is minimal, suggesting a fetal-specific concern that was previously underweighted in regulatory assessments.
Re-evaluation of developmental studies found decreased fetal body weight, increased delayed skeletal ossification and post-implantation losses in rats and mice; increased cleft palate in mice; increased resorptions in rabbits. The authors concluded fetal effects may occur independently of maternal toxicity.
Suspected hormone-disrupting activity
In laboratory cell tests, OPP shows very weak oestrogen-like activity, estimated at one to ten million times weaker than the reference female hormone. This oestrogenic signal has not been confirmed in whole-animal (in vivo) studies. European assessors reviewing OPP in cosmetics flagged it as a suspected endocrine disruptor, though the food-exposure evidence base remains thin given the additive's withdrawal.
In vitro studies identified very weak oestrogenic activity for OPP, but this was not confirmed in vivo. The French agency ANSM flagged OPP as likely to be an endocrine disruptor, prompting a European cosmetics re-evaluation.
Migration into food from packaging
OPP has been detected in canned beers and soft drinks at concentrations up to about 40 micrograms per litre, migrating from treated can-end sealing material rather than from direct food use. Exposures at these levels fall well below the JECFA ADI, but the findings confirm OPP can reach consumers through routes beyond intentional food additive use.
OPP was found in 40 of 60 canned beer samples from multiple countries at concentrations of 1.2 to 40 micrograms per litre, traced to OPP used in can-end sealing compounds.
OPP was also detected in canned soft drinks sampled in the United States and Germany, confirming migration from can materials is a widespread rather than isolated phenomenon.
Where it stands with the regulators
Who should be careful
Because E231 is no longer authorised as a food additive in the UK or EU, consumers should not encounter it on food labels. Anyone buying imported citrus fruit from countries where OPP remains in use as a post-harvest treatment should look for 'surface treatment', 'orthophenyl phenol', '2-phenylphenol', or 'E231' on the label and wash or peel fruit before eating.
The honest read
The science on OPP is layered. The substance causes bladder tumours in male rats at very high dietary doses, and its sodium salt is an IARC Group 2B carcinogen based on animal data alone; there are no human epidemiological studies. The mechanism involves tissue irritation and cell turnover rather than direct DNA damage, which is why JMPR concluded in its 1999 evaluation that OPP is 'unlikely to represent a carcinogenic risk to humans' at realistic food exposures. The developmental toxicity picture is more contested: a 2013 re-evaluation argued that older regulatory assessments underweighted fetal effects seen in rats and mice. The endocrine-disruption question remains unresolved, with weak in-vitro signals and no confirmed in-vivo effect. The additive's withdrawal from food-additive legislation in the UK and EU means the consumer-exposure question is now moot for fruit bought in UK shops, though low-level migration from can coatings continues to be documented.
Related additives
Common questions
Is E231 banned in the UK?
E231 is not on the UK FSA approved-additives list and cannot legally be used as a food additive in Great Britain or Northern Ireland. OPP was also withdrawn as an approved plant-protection active substance in Great Britain in 2025 under GB COPR, confirmed by the Pesticide Properties DataBase. It is not 'banned' in the sense of an active prohibition order, but it has no current authorisation, which means it cannot be used.
Why was E231 removed from the EU approved additives list?
The precise legislative step that removed E231 from EU food-additive authorisation is not fully documented in publicly available primary sources. The compound is absent from the current consolidated text of EU Regulation 1333/2008. Contributing factors in the scientific record include: IARC Group 2B classification for the sodium salt in 1999, evidence of bladder tumours in rodents at high doses, and concerns raised about developmental toxicity and suspected endocrine-disrupting activity.
What foods contain E231?
In the UK and EU, no food should legally carry E231. Historically it was applied as a surface wax or dip to citrus fruits (oranges, lemons, grapefruits, mandarins) and pineapples after harvest. OPP has also been detected migrating from can-sealing compounds into canned beer and soft drinks, though this route of exposure was not covered by the E231 food-additive authorisation.
Is E231 vegan?
Orthophenyl phenol is a synthetic organic compound with no animal-derived ingredients. Its use is no longer authorised in UK or EU food, so the question is largely academic for UK consumers.
Sources
- Approved additives and E numbers | Food Standards Agency
- EU Regulation (EC) No 1333/2008 on food additives, consolidated text June 2024 | EUR-Lex
- ortho-Phenylphenol and its Sodium Salt | IARC Monographs Volume 73 | NCBI Bookshelf
- ortho-Phenylphenol and its Sodium Salt | IARC Volume 30, 1983 | INCHEM
- Sodium ortho-Phenylphenate | IARC Supplement 7 | INCHEM
- o-Phenylphenol JECFA database | WHO Food Additives
- Brusick D. Analysis of genotoxicity and the carcinogenic mode of action for ortho-phenylphenol | Environmental and Molecular Mutagenesis 2005
- Kwok ESC and Silva MH. Re-evaluation of Developmental and Reproductive Toxicity of OPP and SOPP | Cell and Developmental Biology 2013
- SCCS revision of opinion on o-Phenylphenol in cosmetic products | PubMed
- Review of existing maximum residue levels for 2-phenylphenol | EFSA Journal 2017
- 2-Phenylphenol | Pesticide Properties DataBase | University of Hertfordshire
- Determination and Levels of ortho-Phenylphenol in Canned Beers | Journal of Agricultural and Food Chemistry 2006
- 2-Phenylphenol and its Sodium Salt | JMPR Toxicological Evaluations 1999 | INCHEM
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