Image copyright Leon Neal/Getty

Trade talks, tense affairs at the best of times, often get particularly sticky when it comes to food.

When the UK starts to negotiate new trade deals as it leaves the EU in 2019, food will be one of many areas that will need to be addressed.

The ongoing spat over chlorine chicken highlights how tastes and safety practices around the world can differ hugely.

What might seem normal practice in one country can seem problematic elsewhere.

In the US, it is legal to wash chicken carcasses in chlorinated water to kill germs - but this has been banned in the EU since 1997.

UK Environment Secretary Michael Gove has said the UK should not allow these imports in a post-Brexit trade deal with the US, but Trade Secretary Liam Fox says the practice is "perfectly safe".

Anthony Scaramucci, US president Donald Trump's new communications director, told BBC Newsnight that there would "100%" be a trade deal between his country and the UK - although he confessed he had no idea what was happening about chlorinated chicken.

Here are five occasions when spats over food have made past trade talks tricky.

'Infested' avocados - Mexico v US (1914-97)

Image copyright TOBIAS SCHWARZ Image caption The US wouldn't import Mexican avocados for many years

For more than 80 years, the US refused to import Mexican avocados on the grounds that the fruit was infested with fruit flies and other bugs.

After the signing of the North American Free Trade Agreement (Nafta) in 1994, the US came under pressure to relax its ban, rather than rely on its pricier home-grown avocados.

"Avocados are always used as a pawn in the trading process. Whenever the United States talks to Mexico about opening up other agricultural commodities to US growers... it always comes back to avocados," Jerome Steyhle, who chairs the California Avocado Growers Commission, told the BBC in 2003.

In 1997, the restrictions started to be lifted, and by 2016 the US was importing 1.7 billion avocados across the border each year, according to marketing group Avocados from Mexico.

But the avocado war could be reignited now that President Trump has threatened to renegotiate Nafta - which he described as "the single worst trade deal ever approved [by the United States]".

Earlier this year, there were reports of several Mexican avocado lorries being turned away at the border following an argument about US potato imports.

Beef wars - US v EU (1988-present)

Image copyright Lisa Lake Image caption Some cattle in the US are fed growth hormones

One of the best known food-related trade disputes was over hormone-fed beef.

The use of certain growth hormones in cattle rearing is legal in the US.

But in 1988, the EU banned the use of several major growth promotion hormones, which it said posed a potential risk to human health. This was an effective ban on American beef.

A decade later, the World Trade Organization (WTO) ruled the EU's refusal to import US beef was not based on scientific evidence and violated its members' obligations.

However, the trading bloc still wouldn't buy the meat, leading the US to retaliate by levying higher trade tariffs on some of its EU imports.

"American ranchers raise some of the best beef on the planet, but restrictive European Union policies continue to deny EU consumers access to US beef at affordable prices. For several years we have been asking the EU to fix an agreement that is clearly broken, despite its original promise to provide a favourable market for US beef," US Agriculture Secretary Tom Vilsack said last year.

Stockpiling food - India v US/WTO (2013-14)

Image copyright PRAKASH SINGH Image caption Indian Prime Minister Narendra Modi wanted assurances that the country would still be able to stockpile food

Several years ago, India blocked the implementation of a 2013 global trade agreement it feared would stop it stockpiling food for the poor.

India refused to back the Trade Facilitation Agreement until it was assured proposed limits to farming subsidies would not affect its $12bn (£9.2bn) food-security programme.

It pays farmers over the odds for grain, some of which it sells to poorer households while the rest is set aside in case of shortages.

The WTO trade agreement simplified customs procedures and was designed to add $1tn to the global economy, and benefit developing countries in particular, so India's defiance was strongly criticised by the global community.

India agreed to lift the veto after WTO members agreed that an arrangement known as a "peace clause" - which protects food stockpiling - would remain valid until the WTO could find a permanent solution.

It was due to expire in 2017, but will now effectively continue indefinitely.

Cars for cheese - Japan v EU (2013-17)

Image copyright Sean Gallup Image caption EU negotiators wanted to sell more dairy products to the Japanese, who in turn wanted to sell more cars

Negotiations on a big trade deal between Japan and the EU began in 2013.

Both sides wanted to slash tariffs on a huge range of goods, to boost trade.

This is a sensitive process because domestic producers tend to be wary of foreign competition.

The Japanese side was particularly keen to boost car sales in Europe, while the EU negotiators wanted to sell more dairy products.

Loosening the dairy rules wasn't such a big deal for hard cheeses such as cheddar and gouda, which are not made in Japan.

But Japanese dairy farmers do make softer cheeses, which proved a roadblock in the final stages of the talks, earlier this year.

After some late night haggling, the EU's Agriculture Commissioner, Phil Hogan, secured a compromise.

The EU would have a yearly quota of 31,000 tonnes for soft cheese exports, in exchange for almost complete market access for hard cheese.

A few days later in Brussels, EU leaders and Japanese Prime Minister Shinzo Abe announced the completion of the deal, dubbed "cars for cheese".

Wallonian dairy farmers - EU v Canada (2016)

Image copyright AFP Image caption Some Belgian dairy farmers were worried about the impact of free trade

After years of negotiations, the EU completed its most ambitious free-trade deal to date: the Comprehensive Economic and Trade Agreement (Ceta) with Canada.

But under EU rules, some far-reaching trade agreements require the consent of all 28 EU countries before they can come into force.

To make things even more complicated, in Belgium seven federal, regional and community bodies had to give their approval as well.

Wallonia, the country's French-speaking region, said no.

Politicians in the staunchly socialist region had concerns about the dispute-settlement mechanism in the agreement, along with something else - milk.

Wallonian dairy farmers worried about the impact of free trade on their sales.

A group of them marched outside the European Commission in Brussels to voice their disapproval of Ceta.

Eventually, Belgian political leaders reached a consensus and broke the deadlock, agreeing an addendum to the Canadian deal, which addressed concerns over the rights of farmers and governments.

The European Parliament approved Ceta in February, although it has not come into force yet.