As the heat of the Texan evening subsides from unbearable to merely stifling, a young woman in towering heels strolls past with a piglet on a leash. Nobody bats an eyelid. After all, she's adhering to the instruction plastered on bumper stickers everywhere to "Keep Austin Weird".
"Right, tuck in," the British man tells the small group of people with him. He takes a forkful of kimchi fries: a plate of chips slathered in grilled onions, Monterey Jack cheese, Korean barbecue meat, Sriracha dressing, a secret "magic sauce", sesame seeds and caramelised kimchi (fermented cabbage).
"You don't get it at first," he says, "but then the heat builds on you. These are really good." "It's a moreish combination," his friend adds. "It ticks all the boxes in terms of flavour." They move down the wall, repeating the ritual with the Seoul burrito, the Korean burger and the bulgogi taco.
These are not hungry tourists exploring Austin's culinary underbelly. The Brit in the flannel shirt is Steve Rowe, the man who runs Marks & Spencer's food division; he is more or less the man who decides what middle England eats. Rowe and his three colleagues - a chef, a buyer and the chain's director of product direction and innovation - are on a trend-hunting trip to find products that they can adapt and sell, and they've invited me to join them. The retailer's food may end up in the fridges of Tunbridge Wells, but its journey starts in places such as this, the car park of Metro Realty. Austin is home to 1,000 food trucks.
"Our role is to pick up on the food trends that people see subliminally," Rowe explains in his London accent. "The British public travels more than ever. What we want to be is one step ahead of them."
Rowe should know what customers want: he has worked at M&S for 25 years, and his father, Joe, also ran the chain's food division, making them the only father-son team outside of the founding families to have held the same board position; his mother, aunt, maternal grandmother and wife also worked there. Between them, the extended Rowes have clocked up more than 100 years' service.
Young Rowe was surrounded by M&S food at home. He recalls eating "steak after steak after steak" when his father was experimenting with new packaging techniques in the 1980s. No family restaurant meal could start before a "forensic discussion about the cut of meat" had taken place. His father, now 67, still offers advice. When Rowe told him he'd got the food job in 2012, his dad congratulated him before telling him the store was cutting its calves' livers wrong.
While eating from roadside shacks might strike some people as an extreme form of market research for one of Britain's more buttoned-up institutions, M&S is playing into a growing trend. "Food today is all about exploration and innovation," says Martin Raymond, co-founder of research body The Future Laboratory. With household budgets under pressure, Brits are spending more on eating in and less on going out. In other words, if a supermarket can sate the public's desire for exotic food they can eat at home, it has cracked a lucrative nut. M&S has some form here: it was the first retailer to sell grapefruits to UK households in the 1930s, and introduced the chicken kiev to the UK in 1979; more recently, a trip to Chicago a few years ago resulted in the "posh dog", a smoky, hot dog-style sausage. Nearly a million units were sold last year.
Outside a Cajun food truck on the University of Texas campus half an hour later, Rowe is perusing the menu. "Can I taste your gumbo?" he asks the shack owner. "I want to try the alligator, but we might have a sourcing issue." Would M&S shoppers really throw crawfish boil into their basket along with their bread, milk and bananas? Absolutely, he claims. About 20% of the store's 6,000-plus products are new every year. This summer, for example, it is selling everything from chocolate-covered wasabi peas to chicken wings coated in tequila and sansho pepper. "We should sell something for everyone," he says. "Too many people in the UK are blinkered about food. There is always something new you can learn."
According to Damian Dixon, the buyer on the trip, the retailer has already taken inspiration from Austin. It is testing a fleet of 14 branded food trucks in the Milton Keynes area, selling lunchtime sandwiches to workers in business parks away from the high street.
But how do you take a product from obscurity into the mainstream? A few years ago, kale, quinoa and rice cakes could be found only in healthfood stores; now you can buy them in some petrol stations. According to food experts, turning an obscure foodstuff into a pantry staple requires - above all else - a total conviction that the product will be a hit with customers, even if those customers don't know it yet.
Julian Metcalfe, co-founder of Pret a Manger and founder of the Asian-inspired Itsu chain, says you have to be brave. "It's a question of having the guts to bring the product over. To import it, to pack it and to push it, rather than play safe. Take rice cakes with chocolate on them - we now sell 55,000 a week. No one had ever heard of them a few years ago."
So where does the inspiration come from? "A love and respect of food, and quite a lot of travelling," Metcalfe says. "Look at coconut water. I sat with people in Brazil day after day at lunch and all they drank was coconut water, whereas in Thailand I believe they used to throw it away. I'd never seen it before. That was eight years ago."
Henry Dimbleby, co-founder of the Leon chain of "healthy fast food" outlets, agrees that conviction is required to back an obscure foodstuff. A process of gradual osmosis can then turn it from tryout to trend. "The way these things happen is that an individual or company believes that something's absolutely delicious and they put it in a context where people in the mainstream begin to understand it and get over their fears. Then other people do it, the papers write about it and it becomes a thing."
Dimbleby says foodies such as himself are permanent magpies who are "constantly travelling, eating, finding kernels of ideas and morphing them".
Dimbleby cites Leon's quinoa-heavy superfood salad as a dish that came from somewhere niche: "When I was growing up, my mum [food writer Josceline Dimbleby] used to make salads that were quite substantial. They had vegetables in them, and were often dressed with bits of flavour running through them. Mum used to call them sumptuous salads. John [Vincent, Leon co-founder] came up with the name superfood salad in 2004, we Googled it and it didn't exist. We played with recipes. Our version had peas and greens and broccoli and sprouts, and it was delicious. The whole package added up to something that people understood. And it took off." Google "superfood salad" today and you'll get more than two million results.
This is how all big consumer trends start, be they about music, books, fashion or food. Someone with foresight, money and enthusiasm co-opts something obscure and makes it part of life's fabric. It's why in hipster-land you can't move for kale. It's why Nick Clegg owns a onesie. According to Metcalfe, there is one golden rule worth remembering when trend-hunting: strange is a relative term. "The thing is not weird in the place where you find it," he says. "What's weird here is not weird there."
For M&S, the trick when introducing a new foodstuff is not to scare its customers. Rowe says the retailer's role is not to try to educate them, either. Rather, it must gently encourage them to try new things. When M&S started selling sushi in the 1980s, it bombed. Customers weren't ready for another 20 years. Rowe's key lieutenant on this trip is Cathy Chapman, who is in charge of innovation. Softly-spoken Chapman, an M&S old-timer, is a reluctant legend in food circles. It was she who ushered in Britain's ready-meal revolution in the 1970s, as an M&S technologist. She also oversaw the introduction of mass-produced chicken kievs.
Awaiting a plate of fried lobster at a pavement table as the first tendrils of a summer tornado whip the street - a tornado that just one hour later will prise roofs from buildings - Chapman explains how she persuades their customers to try something new. "It can take quite a long time to take off because you've got to almost take the customer away from what she's been buying for a few years, like Indian or Chinese," she says. "People might want tikka masala on a Friday night. You won't get them off that because it's been their ritual for several years. We can introduce them to smaller pieces [of new food] and sharing plate sizes. It's not such a risk and they can buy them on top of what they love, and gradually you can take them with you."
Dishes from this trip will be presented and rigorously debated back at the London HQ. James Newton-Brown, the brand's chef and a veteran of most of London's high-end hotel kitchens, will work out if the flavours and cooking processes can be replicated, whether it's possible to make them at a good price, and also consider what he calls "the practicality of eat". "It may taste and look good, but if people work in the City and have ties on, is it practical?" he says of the sauce-heavy kimchi fries. Test versions will be made and products could hit the shelves by next summer.
Whatever street food comes from this trip, it could be subject to certain limitations, commentators say. Due to the scale of what they do, big chains can struggle to recreate the intricacies of a dish. Kevin Gould, Guardian food and travel writer and now a branding expert, says food that is "shot through with nuance and wit and subtlety" cannot easily translate into "mass-manufactured, factory-made food". There is a profit motive to some of these trends. The nation's obsession with trendy kale is a godsend for chains such as M&S, Gould says. "Kale is cheap - it's basically slutty cabbage - so it's the best thing that could ever happen to them," he says. "Stick a bit of kale in a juice, and you can sell it for £2.50 as opposed to £1.99."
The last food trucks of the tour provide the lightbulb moments for the trend-hunters. At Turf N' Surf Po'Boy - an operation run by the bear-like Ralph Gilmore - a taco containing pulled pork and "mango tango" dressing proves a hit.
Gilmore, who started opening trucks because they were cheaper than opening restaurants in "fancy buildings", is a leading light on the Austin food scene, with a weekly turnover of more than $50,000 (£29,000). But it is a plate of his avocado deep-fried in sesame seeds, almonds, chilli flakes and cornflakes from a van called The Mighty Cone that gets the team most excited.
"Oh my God, it's fantastic," Chapman says. "It's one of those things you'll remember all your life." Rowe takes a bite and almost leaps from the rickety picnic table with excitement: "That is on my list of the best. To do that in the UK would be an absolute game-changer."
There follows a detailed discussion about how they could get unfrozen avocado through the flash-frying process needed to create such a dish without damaging the produce. "You have to apply technical cooking abilities that would stretch everyone," Rowe says. They all nod eagerly. They may be 5,000 miles from Tunbridge Wells, but here under the Texan sun they think they've struck gold.
From avocado to kimchi: a short history of fashionable food
Avocado
When small green fruits called avocado pears first appeared on supermarket shelves in the 1960s, no one quite knew what to do with them - in Manchester one woman stewed them and served them with custard. Her letter of complaint to M&S saw the retailer rebrand them and start selling them with an explanatory leaflet. Fast-forward 40 years and you can't move for guacamole and avocado smoothies.
Sushi
M&S's attempts to sell it in the 1980s fell flat as consumers balked at the idea of eating raw fish. It took off in the 1990s thanks to the expansion of the Yo! Sushi chain and the opening of Nobu in London. Brits now spend about £40m on the stuff every year.
Popcorn
Available in Britain for almost 100 years, but recently given a makeover. No hip party is now complete without a stall festooned with bunting and faux-Victorian signage offering popcorn in flavours such as cheese and tomato, wasabi, sea salt and olive oil, and even gin and tonic. Gourmet popcorn is so hot right now that the fact that many of these products taste like a wedge of cardboard that has absorbed the moisture at the bottom of the fridge is moot.
Coconut water
No longer the bit you throw away. This milky-coloured drink is high in potassium and is a big hit with gym bunnies and models, but it has now reached corner stores and kids' lunchboxes: 39 varieties are now sold in the UK, and sales are predicted to rise to £250m by 2017.
Kale
Kale - chewy and bland, but high in fibre and iron - is nothing new: it featured prominently in the Dig For Victory campaign during the second world war, but fell from favour soon after. Partly thanks to Oberon Sinclair, a canny British PR working for the American Kale Association, it has become the modern superfood of choice. Peak kale arrived when Beyoncé put a picture of herself on Tumblr wearing a "Kale" sweatshirt with 'Kale' written on it.
Kimchi
This Korean fermented cabbage is part of the vogue for "partially decayed" foods that includes pickles and kombucha, a slightly fizzy fermented tea drink that is never far from Gwyneth Paltrow's fridge.
One of 1,000 food trucks in Austin, Texas: 'Can I try your gumbo? I want to try the alligator, but we might have a sourcing issue back in the UK.' Photograph: Sandy Carson for the Guardian