Your jaw tightens, your tongue tingles, and your cheeks pucker like you've downed a shot of vinegar.
A first taste of sour beer often comes as a rather sharp shock. But it can soon prove addictive stuff: the perfect crisp, refreshing pint to sup in the sunshine.
With a zesty acidity closer to a very dry cider or white wine, sour beer also doesn't match expectations of what a normal pint of beer should taste like.
And yet sour is soaring: it's popularity is on the rise not only with beer geeks, but also with casual boozers looking for a summery alternative to hoppy beers or overly sweet ciders. While this style may have its roots in Belgium and Germany, and has been re-popularised by the craft beer scene in the US, British brewers are now barrelling in.
Huddersfield brewery Magic Rock's take on a Gose-style sour proved a runaway hit, becoming one of their core beers and sold in colourful pink cans across the country. Thornbridge Brewery in Derbyshire just netted both gold and silver medals in the prestigious World Beer Cup sour beer category. Brewdog recently announced that they're investing heavily in a new sour beer brewing facility in Aberdeen, while London-based Beavertown are launching a whole new programme of barrel-aged beers, including sours.
What exactly is a sour beer, then? There are many styles, the speediest being Germanic kettle-soured wheat beers, such as a Berliner Weisse, or a Gose (from Leipzig). These often have fruit or spices added. Magic Rock made their first Berliner Weisse in 2013. Their brewer Richard Burhouse describes process as similar to that of milk going off.
"You add Lactobacillus, which is the culture of bacteria, to the wort, the pre-beer, and it'll sour over about ten to 12 hours," he explains. "You measure the PH and once it's at the acidity you'd like, you boil the beer."
Then there's the Belgian school of sour: Lambic and Geuze beers. The method is slower, and altogether more mysterious. Traditionally, the wort (from barley and wheat) is left open to the atmosphere, for spontaneous fermentation by wild yeasts and bacteria floating on the air. It's then stored in wooden barrels, with fermentation continuing for up to five years before being blended back together.
Burhouse—currently working on a cherry barrel-fermented beer—explains that the barrel method is a more difficult to control.
"It's a much longer process but ultimately it produces much better results," he says. "It just takes a lot of dedication, space, and time."
Beavertown knows all about that. I visited their brewery on an industrial site in Tottenham, where they've dedicated a whole unit to barrel-aged beers, including sours (although they add specific wild yeasts, rather than leaving it up to nature). With over 200 barrels, their warehouse teeters with anticipation.
Brewer Tiago Falcone taps a barrel for me to try: it's still fermenting, and has a funky acidity. I also sample Sour Power, a cherry Lambic that's been aging in red-wine barrels for two years. It's delicious: tart, but with a rounded fruitiness and dangerously gluggable compared to some teeth-stripping, one-sip-at-a-time sours.
What's the appeal of sour beer, to a brewer, given the process is so time-consuming, space-consuming and unpredictable?
"You find a whole new range of flavours—there's a lot going on there," says Falcone. "There are so many different types of microorganisms, they produce complex flavours."
Sour beers also often find unlikely fans among beer refuseniks—myself included. I can't stand the taste of hops: no matter how many IPAs I try, I find the flavour throat-closing and nose-shrivelling. I'm always the miserable one trying to order a cider in a pub full of hop-heads. Having always loved acidic flavours—from pisco sours to pickled eggs—sour beers were a revelation.
Still, it seems I'm depressingly gender-typical in this regard.
"A lot of women really like it," says Burhouse of their Gose, Salty Kiss. "My mother-in-law loves it actually, and she doesn't want to drink any of our other beers! That really surprised us—it wasn't why we made it, but it's had real mass appeal.
Sour beers often play well with wine and cider-drinkers, too.
"In my experience, some people try sour beer and just love it, and some people absolutely cannot stand it," says Kieran Hawkins, who runs Beavertown's taproom.
But he agrees there's a rapidly growing market for Berliner Weiss and Gose, with Beavertown's own Phantom series proving super-popular.
"They're quite easy-drinking, and low alcohol [around 5 percent]," he says. "They appeal to real beer geeks, and to people trying them for the first time, because it's clean and refreshing, like a sorbet."
Beavertown and Magic Rock are both newish breweries but sour beer isn't just for hipster-baiting young-guns. Established in 1795, Elgoods in Cambridgeshire is one of England's oldest breweries and has been experimenting with a traditional, Belgian approach that sees wort sit in coolships (big shallow tanks) open to the air overnight. The allows wild yeast to ferment.
Brewer Alan Pateman explains they had the old tanks on-site anyway, and were using them to cool ordinary beer before refrigeration. A visiting American importer spotted them and said they'd make perfect "coolships"—the name they've now adopted for their version of a Lambic.
"It ticks lots of boxes for authenticity and it's really caught the imagination," says Pateman.
Although Elgoods began making sour beer to appeal for export to the American market, he says there has been a growth in interest across the UK too.
Pateman was sceptical about sour beer at first, but the distinctive taste of their brew ("a lemony citrus note, a background of oak") has grown on him. Does the tartness still surprise first-time drinkers?
"Surprise is putting it mildly—shock and awe would be the term!" he says. "People of my generation screw their face up and say, 'What the hell is that?' I would have said the same thing three years ago, but you're never too old to learn."
I'll raise a glass to that. And once you've developed a taste for the tart, you'll approach a sour beer only with sweet, sweet anticipation.