What is a sour beer?
It's International Sour Beer Day, and we can't think of a better way to mark it than a short history lesson. Sour beer is kind of our thing, after all.
A sour beer is a beer made tart on purpose, by letting bacteria ferment it alongside the yeast. The bacteria produce acid, usually lactic acid, and that acid is what gives the beer its bite. Think lemonade or cider rather than a pint of bitter. The sourness is the point, not a mistake, and it is not a craft beer trend either. Sour beer is the oldest beer there is.
Brewing goes back almost 6,000 years, and for nearly all of it, beer was tart whether you wanted it or not. Open wooden vessels, no refrigeration, no idea that yeast was a living thing. Then science ruined the party. Louis Pasteur proved in 1876 that fermentation was the work of microorganisms, Emil Hansen isolated a single pure yeast strain at Carlsberg in 1883, and within a generation sourness went from unavoidable to unacceptable. For about a hundred years a sour beer meant somebody had got it wrong. Belgium ignored the memo entirely, which is the only reason lambic, gueuze and Flanders red survived for the rest of us to rediscover.
There are two ways to make one now. Kettle souring drops bacteria straight into the kettle, acidifies the wort in a day or two, then boils it off and ferments normally. Fast, clean, and it gives you a straightforward tartness. Mixed fermentation puts yeast and bacteria in the same tank at the same time and lets them compete. It takes longer and it is harder to control, but that competition builds acidity with layers in it instead of a single note. We do the second one.
At the centre of everything we brew is our house mixed culture, and it has been in every sour beer we have made since 2018. It began in May of that year, when we were trading yeast samples by post and waiting to see what turned up. One of them was a domesticated Lithuanian strain, and it was exactly the foundation we needed for fresh fruit flavours. Sitting alongside it is Kveik, the Norwegian farmhouse yeast whose name started life as a verb meaning "to start". Kveik was kept alive for generations on kveik rings: wooden rings left in the beer to catch yeast cells, then dried by the fire until the next brew. Both of these yeasts come from the same corner of brewing history. While the rest of the industry was standardising around single pure strains, farmhouse brewers across the Baltic and Nordic countries quietly kept their own going, and the result is yeast that is absurdly hardy and full of character. Kveik will ferment close to freezing if it has to. We run our culture at almost 40 degrees for 72 hours instead, because heat is where esters come from, and that is why the base beer already tastes of juicy pear and sherbet before a single piece of adjunct goes near it.
Then there is the bacteria. Alongside the yeast sit several strains of Lactobacillus, doing the actual souring. It is lactic acid bacteria, the same family that ferments yoghurt, cheese, sauerkraut, kimchi and cider, and lactic acid is the clean one. Acetic acid reads as nail polish. Malic reads as unripe plums. Lactic reads as lemon sherbet, which is exactly what we want. Several varieties compete in the tank alongside the yeast, and between them they build the base. All they ask for in return is a constant supply of fresh wort, which seems like a fair deal. Andy Gibson wrote the full version of this back in 2020, in Yeasty Business.
Then the fruit goes in, sometimes 2,000kg in a single beer, most of it from Scottish farms up the road in Perthshire. Every label carries our Sour Scale, 1 to 5, so you know what you are opening before you open it. If you are starting out, start low. Peach & Lemon Table Sour is juicy and sessionable. Cloudy Lemonade, Citra & Nelson Sauvin , built on real lemon juice and zest, and the one that turns lager drinkers. Triple Fruited Mango is 4.8%, a triple dose of Alphonso mango, and a 3 on the scale. Raspberry Sour is full of Scottish raspberries and no apologies. When you are ready to stop being sensible, the Marvellous Liquids series is waiting. Face Froster is a mouth-freezing sour with blueberry and honeyberry. Numbing Nectar puts lime and orange up against the tingle of sansho berry and Szechuan peppercorn. And Brainmelting Fluid, lemon, lime and raspberry, is the only beer we have ever put a 6 on.
No we don't want to get too sentimental about bacteria, but we do owe them quite a lot. Happy International Sour Beer Day, and we hope you're enjoying a glass of the sour stuff.