Food safety is a major stumbling block in EU-US talks aimed at creating the world's biggest free trade zone.
The Transatlantic Trade and Investment Partnership (TTIP) would dwarf previous free trade deals. The European Commission estimates that by 2027 it could boost the size of the EU economy by €120bn (£94bn; $152bn) - equal to 0.5% of GDP - and the US economy by €95bn (0.4% of GDP).
European opponents argue that TTIP risks watering down EU regulations in the drive to remove trade barriers.
There are tensions over TTIP in the European Parliament, whose draft recommendations proved too controversial for MEPs to vote on it this week. What divides the EU and US on food safety?
The transatlantic dispute over genetically modified organisms (GMOs) has been raging for years.
American and European biotech firms are frustrated that the EU currently allows cultivation of just one GM plant - MON810 maize. It is grown only in five EU countries, on just 1.5% of the total land area devoted to maize crops.
The European Commission - the EU's top regulator - has given EU member states the power to ban GMO cultivation for reasons beyond environmental or public health concerns. It has proposed a similar freedom to ban sales of GM foods and animal feed - basically leaving it up to national governments to decide.
There is widespread opposition among Europeans to GMO cultivation, amid fears that finely balanced ecosystems could be harmed by artificially engineered crops.
Nevertheless, most animal feed in the EU is imported, with soya the main component. In 2013 nearly all of that came from the US and South America, where soya is overwhelmingly GM. The US provided 16% of those imports - and in the US 93% of soybean cultivation is GM.
US frustration with EU restrictions on GMOs is explicit in a 2014 report by US Trade Representative Michael Froman, which says "not once in over 12 years has an EU regulatory committee accepted a proposed measure to approve a new GE [GM] product".
The US complained to the World Trade Organization (WTO) and won in 2006, when the WTO ruled that the EU had imposed "undue delays" over GMO applications.
Biotech firms argue that GMOs can have great advantages for farmers, by proving more resistant to certain pests or drought, so boosting the yield.
Authorisation of chemicals faces a big hurdle in the EU - the so-called "precautionary principle". It demands that politicians base a risk assessment on more factors than existing scientific data, in cases where there is scientific uncertainty.
The US approach puts the onus on authorities to prove that a particular chemical is hazardous before imposing a ban. Where no such scientific data exists, the US argues, companies should be free to sell their product.
These contrasting positions mean that the EU and US disagree over maximum residue levels (MRLs) - the measure of pesticide residues allowed in food.
The US trade representative's report complains that "frequently... no EU MRL has been established, or the EU MRL is set at a level that is too low".
But in Europe there are fears that TTIP might bypass the European Food Safety Authority (Efsa), by obliging the EU and US to follow higher chemical residue levels set by the Codex Alimentarius Commission, an international food regulator.
An EU text on food safety, proposed as part of TTIP, suggests tighter deadlines for authorising pesticides:
"The Parties shall ensure that tolerances and maximum residue levels adopted by the Codex Alimentarius Commission after the entry into force of this Agreement will be applied by each Party without undue delay, unless the importing Party had signalled a reservation in the Codex Alimentarius Commission. Such tolerances and maximum residue levels shall apply between the Parties within 12 months after their adoption."
Bart Staes, a Belgian Green MEP and negotiator on TTIP, says Efsa has opposed up to half of the Codex standards in recent years.
The European Parliament's environment committee reports that 82 pesticides used in the US are banned in Europe.
The EU took nearly 10 years to hammer out the REACH system for registering chemicals - a system that met with "fundamental and sustained opposition" from the US, the committee says.
A big row has erupted over endocrine-disrupting chemicals (EDCs), which interfere with the body's highly sensitive hormone system. Studies point to EDCs causing reproductive problems and they have been linked to certain cancers.
Even tiny amounts of EDCs pose particular risks to unborn children and infants, experts warn. But much research remains to be done.
Controversially the European Commission has delayed setting EU criteria for defining EDCs, until 2017 at the earliest. Sweden has reacted by taking the Commission to court, accusing it of breaking a pledge to issue the criteria by the end of 2013.
The use of growth-promoting hormones in cattle is standard practice in the US, but banned in the EU.
After years of dispute the EU agreed to import high-quality US beef from cattle free of the added hormones. But most US beef is still excluded.
Meanwhile, EU beef exporters have been blocked by a US ban imposed because of Europe's 1990s Bovine Spongiform Encephalopathy (BSE) crisis - known as mad cow disease.
In January the US announced that the ban would be lifted - following years of EU action to purge BSE from the food chain. Ireland - the biggest EU beef exporter to the US before the ban - will be first back in.
Another dispute centres on ractopamine, a non-hormonal chemical used in the US to accelerate weight gain in cattle and pigs. It was originally a drug to treat asthma, and is banned in EU agriculture.
But in a controversial, narrow vote the Codex Alimentarius classified the drug as safe.
Any EU acceptance of US standards in livestock production would risk putting European farmers at a cost disadvantage, a European Parliament policy paper says.
But after years of argument the European Commission has allowed the washing of beef carcasses with lactic acid - something the US had urged it to do.
It is among various meat-washing processes used in the US to kill bacteria - called Pathogen Reduction Treatments (PRTs). With the exception of lactic acid the EU only tolerates hot water as a meat-washing treatment.
PRTs are an issue with chicken too - chemical washing is prohibited for poultry in the EU. The US practice is to use chlorinated water.
The US trade representative's report says various EU agencies have concluded that "these PRTs do not pose a health risk to consumers".
Efsa is currently considering whether to allow peroxyacetic acid as a poultry rinse.
Food safety lobbyists in Europe warn that US animal welfare standards are generally lower than those in Europe.
The US regulations apply mainly to slaughterhouses - at the federal level America does not have the EU's "farm-to-fork" strategy of tightly regulating each link in the food chain.
But the picture is complicated. California for example bans small cages for hens, applying stricter welfare standards than many other US states or, indeed, than some EU countries.
And Europe's 2013 horsemeat scandal highlighted deficiencies in EU food safety monitoring.