Whether you started your brewing journey 10 years ago or just 10 days ago, you have been repeatedly told: cleanliness is the most important factor in ensuring great beer. And part of maintaining cleanliness is keeping things sealed. That’s why we put caps on bottles and stoppers in carboys, and it’s also why the airlock is designed to allow gas to escape without letting air in.
Despite the potential risks of contamination, some brewers insist on fermenting in open containers. Open fermentation is exactly what it sounds like: instead of sealing everything up with an airlock in a carboy, bucket, or cylindroconical fermentor, you leave the top open so that the wort, and eventually the beer, is exposed to the atmosphere.
Why would anyone want to do this?
This was how all beer was fermented at one time or another. And some modern commercial breweries still practice open fermentation. Anchor Brewing of San Francisco, California; Samuel Smith Brewery of Tadcaster, England; and Schneider Weisse Brewery of Kelheim, Bavaria, are just a few of the most famous breweries that engage in open fermentation.
Benefits
Top-cropping, or skimming Kraeusen from the actively fermenting beer, is an excellent way to reuse yeast and ensure its health and vitality: no trub, no hop particles, and no cumbersome yeast-washing procedures. Simply grab a sterilized spoon and mason jar, and collect some yeast.
Esters
Open fermentation is often conducted in wide vessels that are much shallower than the typical cylindroconical or carboy. This shape promotes the formation of fruity esters, which are desirable for many British ales and various Belgian styles. According to Hans-Peter Drexler of Schneider Weisse Brewery in Stan Hieronymus’s “Brewing with Wheat,” exposure to oxygen encourages the production of 4-vinyl-guaiacol, which is responsible for the clove character in Hefeweizen.
Offgassing
Leaving the fermentor open to the air allows sulfur and other unpleasant compounds to escape instead of being reabsorbed into the beer.
Open fermentation is not as simple as just leaving the lid off your fermentation vessel or removing the stopper from your carboy. To get the most out of your fermentation, you’ll want to create a large, shallow vessel.
Because fermentor geometry plays a surprisingly significant role in the character of the finished beer, the overall shape is crucial. The monks of Abbaye d’Orval conducted various tests when they transitioned Orval production from open fermentors to cylindroconical vessels. They made adjustments to keep the new beer as faithful to the old one as possible, and primary fermentation time was reduced from 5 days to 4.
If you want to try open fermentation at home, do it in the cleanest area of your house and avoid drafts that may introduce dust into your open barrel of beer. Brewers with fermentation chambers should thoroughly sanitize the interior before leaving it with an open container of fresh wort. Regardless of how you proceed, it’s important to transfer the beer to another vessel as soon as primary fermentation is complete to prevent unnecessary oxidation and reduce the risk of contamination. Much of the appeal of homebrewing lies in the ability to experiment, and open fermentation provides just another way to explore the wonderful world of beer.
Open fermentation is often performed in large vessels that are much shallower than your typical cylindroconical or carboy. Open fermentation is not as simple as just leaving the lid off your fermentation vessel or removing the stopper from your carboy. If you want to try open fermentation at home, do it in the cleanest place in your home and avoid drafts that may introduce dust into your open barrel of beer. Brewers with fermentation chambers should thoroughly disinfect the interior before leaving it with an open container of fresh wort. Yes, it’s open fermentation, but you don’t want to invite an infection.
