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Open Fermentation – Everything You Need To Know

Written by Josh Charig

19 September 2022

Virtually every book, instruction manual, YouTuber, brewer will tell you to never look inside a fermenting beer. Opening the fermenter will cause a host of hungry bacteria and yeast to raid the beer leaving nothing but sourness and burnt rubber. Sure, for the majority of brews this is sage advice, but for every rule there is an exception.

For most of human history, beers have been fermented in containers without lids. Beer outdates civilisation itself, and it was only with Louis Pasteur in the 1800s that we really started to learn about the microbiology of beer. So for ~150 years out of >8,000 we’ve adopted this closed fermenter attitude.

Sure, comparing pre-industrial era beer to what we drink now is comparing apples to goats, but considering a lot of yeasts we use now come from this era perhaps open fermentation can be beneficial?

Some commercial breweries still do open fermentations. Most famously, in Belgium the lambic producing breweries will have open fermentations to “let the wild bugs in”, and some British breweries make use of the Yorkshire square to open ferment their brews – a way of brewing which is starting to catch on in some American breweries. Speaking of America, Anchor Brewery in California has been doing open ferments throughout its long history.

It’s also possible to open ferment at home whilst minimising risk to the beer going bad. It does take some diligence though.

Why Open Ferment?

Fermentations create a fair amount of carbon dioxide, and this creates pressure in the fermenter whilst pushing out oxygen. A lot of that pressure gets released through the airlock, but even an airlock can create enough back pressure to start subduing some yeasts’ ester and/or phenolic production. Whilst this is desirable in a clean lager or IPA, yeast forward styles like saisons, Belgian wits, and some English ale yeasts will benefit from a pressureless environment.

Increased surface area helps as well, as it means more oxygen can get in contact with the beer whilst releasing more CO2. At certain stages of fermentation, oxygen helps the yeast develop more esters. Hence the Yorkshire square, a wide open square, like a small swimming pool, which allows for a large surface area of beer to come into contact with the air.

Some yeasts are better off harvested from the krausen on top, and open ferments make it a lot easier.

How to Open Ferment Homebrew

Open fermentation isn’t as simple as just taking the lid off a fermenter, there is an increased risk of infection but through the right procedure it can be mitigated. Something to note is even in commercial breweries they don’t just ferment open to the elements, the environment is still controlled. Often the fermentation rooms will have positive pressure, so when someone opens the door it blasts air outwards stopping micro-organisms blowing into the room, and the staff are very careful with what they traipse in!

The fermenter needs to be in a very clean place. If you have a fermentation chamber make sure it’s clean before using it for open ferments. It’s recommended to cover the fermenter with cling film with holes punched in it, or to even rest the lid on top with the airlock removed. This will help stop bugs getting in whilst allowing the vessel to remain unpressurised. If you do have a fermenter with a wide opening, use this to increase the surface area.

The fermenter will need a lid which can seal the vessel. Once krausen dies down, less CO2 will be produced and the beer is less protected from spoilage microorganisms. Therefore at this stage the lid needs to be fixed on. Whilst there is less CO2 produced, there will still be some produced so fix a lid with an airlock to vent excess gas.

As always, anything coming into contact with the wort or beer needs to be sanitised with Chemsan.

What Styles Benefit the Most From Open Ferments?

As mentioned above, yeasts that produce a high amount of esters or phenols. If you’re using an English style yeast and those yeasty flavours are desirable, open fermentations can help increase that flavour. Similarly, as well as Saisons, wits and Belgian styles, mixed fermentation brews like Lambics, oud bruins, Flanders red ales etc. can get increased yeast flavours through open fermentations.

Kveiks can also benefit from open fermentations. They are very flavourful yeasts with unique characteristics. Most varieties require top cropping (harvesting from the krausen) so will benefit from this technique. Just remember that kveiks ferment very fast so it might be a day or two, or even a matter of hours, before having to seal the fermenter up.

Let us know how you got on with your open fermentations, and what differences you perceived!

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