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The Magic of Mixed Fermentation

The term “mixed-ferm” is thrown around casually by many beer enthusiasts, as well as beer writers, who typically use it to talk about sour and barrel aged beer. The truth is that “mixed-ferm” can apply to something that is simply fermented with more than a single strain of yeast, a technique used by brewers to create different flavours; from fruity and ester-y, to tart and funky, to wild, sour and beyond. In the guide to mixed-fermentation Matt Curtis will unpack this terminology and explain how and where it is most relevant in the modern beer vernacular. Perhaps most importantly, it will explain that “mixed-ferm” doesn’t simply equal “sour”.

Matthew Curtis

Matthew is an award-winning writer and photographer based in Manchester and is the co-founder of Pellicle Magazine as well as having written for several publications. He is the author of Modern British Beer due to be published August 2021 by CAMRA Books.

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Emma Inch

A CAMRA member since 1997, John is volunteer with a keen interest in the technicalities of beer dispense, a GBBF bar manager, editor of Manchester’s Beer Buzz magazine and sits on CAMRA’s Technical Advisory Group.

Wild Beer Co. Barrels

In its simplest form, wort—the sugary liquid made by steeping malt in hot water, before boiling with the addition of hops—undergoes fermentation and becomes beer thanks to a single strain of yeast.

If that beer is an ale it will most likely have been fermented with a strain of Saccharomyces Cerevisiae, sometimes referred to as warm, or top-fermenting yeast. If that beer is a lager then it will probably have been produced using Saccharomyces Pastoranious, a bottom, or cold-fermenting strain. While in the world of modern brewing neither of these things are universal (many contemporary ales are made using bottom fermenting yeast strains, and vice versa) using a single strain of yeast is fundamental to how much of the beer we enjoy today is produced.

Not all beers are made this way however. A prime example would be the Belgian Trappist ale, Orval. While this magnificent beer does indeed undergo its primary fermentation using a traditional ale yeast strain, it then undergoes a secondary stage of fermentation in the bottle using another strain of yeast known as Brettanomyces Bruxellensis

Wild Beer Co. Barrels

In its simplest form, wort—the sugary liquid made by steeping malt in hot water, before boiling with the addition of hops—undergoes fermentation and becomes beer thanks to a single strain of yeast.

If that beer is an ale it will most likely have been fermented with a strain of Saccharomyces Cerevisiae, sometimes referred to as warm, or top-fermenting yeast. If that beer is a lager then it will probably have been produced using Saccharomyces Pastoranious, a bottom, or cold-fermenting strain. While in the world of modern brewing neither of these things are universal (many contemporary ales are made using bottom fermenting yeast strains, and vice versa) using a single strain of yeast is fundamental to how much of the beer we enjoy today is produced.  

Not all beers are made this way however. A prime example would be the Belgian Trappist ale, Orval. While this magnificent beer does indeed undergo its primary fermentation using a traditional ale yeast strain, it then undergoes a secondary stage of fermentation in the bottle using another strain of yeast known as Brettanomyces Bruxellensis.

“This process of using more than a single yeast strain to ferment a beer in order to produce a different range of flavours is referred to by brewers as “mixed fermentation.””

— Matt Curtis

“You get soft, almost wine like aromatics, refeshing… acidity and apple driven, when you try Kent and Eastern style ciders”

— Alison Taffs

If you are lucky enough to try Orval when it is fresh and young at less than three months of age, it will still predominantly taste of sweet malt and peppery hops. At six months these characteristics begin to fade, the beer gradually taking on a drier, more complex quality. While at 18 months the snap of hops is all but a whisper, its sweetness replaced by a woody, fruity (often described as “funky”) note created by Brettanomyces as it slowly consumes the remaining sugars present in the beer. This also gives older bottles of Orval an incredibly dry finish and high levels of carbonation.

This process of using more than a single yeast strain to ferment a beer in order to produce a different range of flavours is referred to by brewers as “mixed fermentation.” Orval is but one window through which to experience how this process can change the inherent flavours of a particular beer. But the world of mixed fermentation beer is as broad as it is delicious, artfully using a range of different yeasts and bacteria to produce beers that range from tart and fruity, to acidic and mouth-puckeringly sour.

WILD BEER CO, Barrels
If you are lucky enough to try Orval when it is fresh and young at less than three months of age, it will still predominantly taste of sweet malt and peppery hops. At six months these characteristics begin to fade, the beer gradually taking on a drier, more complex quality. While at 18 months the snap of hops is all but a whisper, its sweetness replaced by a woody, fruity (often described as “funky”) note created by Brettanomyces as it slowly consumes the remaining sugars present in the beer. This also gives older bottles of Orval an incredibly dry finish and high levels of carbonation.
WILD BEER CO, Barrels

This process of using more than a single yeast strain to ferment a beer in order to produce a different range of flavours is referred to by brewers as “mixed fermentation.” Orval is but one window through which to experience how this process can change the inherent flavours of a particular beer. But the world of mixed fermentation beer is as broad as it is delicious, artfully using a range of different yeasts and bacteria to produce beers that range from tart and fruity, to acidic and mouth-puckeringly sour.

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