What Is Spontaneously Fermented Beer?
Spontaneously fermented beer is beer fermented by naturally occurring yeast and bacteria, without the brewer deliberately adding a fermentation culture. These microorganisms turn sugars in the unfermented beer, known as wort, into alcohol, acids and other compounds that shape its flavour.
At Crossover Blendery in North Hertfordshire, we begin with overnight cooling in open vessels called coolships. From there, our beer ferments and matures in oak barrels for one to four years before blending.
For the drinker, the appeal is in the glass: acidity, fruit, dryness and flavours that develop through a long fermentation.
What does spontaneously fermented beer taste like?
There is no single flavour shared by every bottle. Fermentation, maturation, blending and any added fruit all contribute to the finished beer.
You might find citrus-like tartness, earthy aromas or a dry finish. Fruit can bring its own acidity, fragrance and texture. Some beers are bright and delicate; others have a deeper or more pronounced wild character.
Our Discovery Case shows how those elements can come together. Onyx is refermented on Kent-grown peaches. Damascene uses damsons from Worcestershire. Bramble, Thorn and Briar brings raspberry and blackberry flavours, with bright acidity and a clean, dry finish.
If you are choosing a bottle, look beyond the fruit on the label. The tasting description will help you understand its acidity, barrel character and finish.
How does spontaneous fermentation work?
Most beer is fermented with a yeast culture selected and added by the brewer. In spontaneous fermentation, the wort picks up microorganisms from its surroundings.
At Crossover, hot wort cools overnight in our coolships, exposed to the surrounding environment. It then goes into oak barrels, where fermentation and maturation continue.
We add no cultured yeast, including at bottle conditioning. A small amount of priming sugar provides the sugars needed for natural carbonation before the bottles rest until release.
Why does it spend years in oak?
Fermentation and maturation take time. During that time, acidity, aroma and texture develop, and each barrel changes at its own pace.
Blending lets us bring those different characters together. We taste the barrels for acidity, depth, fruit, oak, softness and finish, then select the combination for a release.
A blend can contain beer of several ages. Onyx and Damascene, for example, combine one-, two- and three-year-old golden ales before refermentation on fruit.
Is spontaneously fermented beer the same as sour beer?
The terms overlap, but they explain different things.
Spontaneously fermented
describes how fermentation begins.
Sour
describes a tart or acidic flavour.
Sour beer can be made through other methods, including kettle souring, where bacteria acidify the wort before it is boiled and fermented with added yeast. Wild beer is also a broader term: a brewer can deliberately add wild yeast or a mixed culture without using spontaneous fermentation.
That is why the production method matters alongside the tasting notes. Two beers described as sour can offer quite different drinking experiences.
Is it the same as lambic?
Lambic is the Belgian brewing tradition most closely associated with spontaneous fermentation. Traditional lambic producers work in the Pajottenland and Zenne Valley around Brussels.
Crossover makes British wild beer in Hertfordshire. Spontaneous fermentation, maturation in oak and blending give lambic drinkers familiar points of reference, but our beers have their own ingredients and character.
If you enjoy lambic, those shared methods may give you a useful starting point when exploring our bottles.
Does adding fruit make the beer sweet?
Fruit flavour does not necessarily mean sweetness.
Fruit sugars can ferment, while the fruit’s aroma, acidity, colour and tannin still contribute to the beer. A beer can smell vividly of peaches or berries and finish dry.
We use British fruit for the character it brings, including cherries, raspberries, damsons and grapes. Some beers are refermented on fruit; others develop without additions or spend further time in spirit barrels.
Explore our spontaneously fermented beers
Browse our current releases, from fruit-led wild ales to beers shaped by further barrel ageing. Each bottle’s description explains its ingredients, barrel treatment and flavour, helping you choose what appeals to you.
Shop spontaneously fermented beer
Not sure which bottles to choose? The Discovery Case brings together three fruit beers in 37.5cl bottles to taste and compare.
