Sour beer is the fastest-growing style in craft brewing, yet it is also one of the oldest. Before Pasteur and the science of controlled fermentation, all beer was to some degree sour, shaped by whatever wild yeasts and bacteria happened to inhabit the brewery. Today, that 'contamination' is embraced as a sophisticated technique, and the results range from gently tart thirst-quenchers to complexly funky wild ales that rival fine wine for complexity.
The Origins of Sour Beer
The Senne river valley in Belgium, in and around Brussels, has been producing spontaneously fermented lambic beer for at least four centuries. Lambic is made by leaving the cooling wort (unfermented beer) exposed to the open air overnight, allowing wild yeasts — primarily Brettanomyces — and bacteria, including Lactobacillus and Pediococcus, to settle and begin fermentation spontaneously. The result is a deeply complex, funky, tartly acidic beer that can age for years.
This was not a choice but a necessity before controlled fermentation was understood. The miracle is that the specific microflora of the Senne valley produce something extraordinary rather than just rotten. Lambic brewers still observe the traditional 'brewing season' of October to April, when the cool air carries less contaminating bacteria and the wild yeasts are at their best.
Wild Fermentation vs. Kettle Souring
Modern craft brewers use two main approaches to create sour beer. Traditional wild fermentation (as in lambic) is slow, unpredictable and produces immense complexity over months or years. Kettle souring is faster and more controlled: bacteria are added directly to the warm, pre-boil wort and allowed to acidify it over 24–72 hours before the wort is boiled to kill the bacteria and fermented normally. Kettle sours are less complex but far more consistent and quicker to produce.
Key Sour Beer Styles
The world of sour beer is extraordinarily diverse:
- Lambic — Spontaneously fermented, aged in oak barrels. Flat (low carbonation), deeply complex, with horse blanket, lemon and earthy funk notes. An acquired taste, but one worth acquiring.
- Gueuze — A blend of young and old lambics, bottle-conditioned for natural carbonation. Known as 'the Champagne of beers'. Effervescent, bone dry and staggeringly complex.
- Flanders Red Ale — A sour ale from West Flanders, fermented and aged in large oak vats. Red-brown, with sour cherry, balsamic and vinegar notes balanced by residual malt sweetness. Rodenbach is the classic.
- Berliner Weisse — A Northern German sour wheat beer, historically very low ABV (2.5–3.5%). Light, tart, refreshing and easy to drink. Often served with sweet syrups.
- Gose — A salt and coriander wheat beer with mild sourness, from Leipzig. Recently enjoying a huge revival, with fruit versions very popular.
- Kettle Sour (including Fruited Sours) — Quick-soured, clean acidic beers, often loaded with fruit purée for vibrant flavour and colour. The most accessible entry point into sour beer.
Pairing Sour Beers
The acidity of sour beer makes it an outstanding food companion — it behaves much like wine or vinegar in the way it cuts through richness and cleanses the palate. Cheese (particularly aged chevre, brie or blue cheese) is an extraordinary match for gueuze. Fruited sours pair beautifully with desserts. Gose with seafood is a revelation. And a Flanders Red Ale alongside a rare steak or charcuterie board is a pairing worth planning a meal around.