Open-air Fermentation and a Recipe

Open-air fermentation?  You mean fermentation with no lid?  NO AIRLOCK!?  Yes.  Its time to take the top off.  I know what you’re thinking: the beer will get infected.  You’re also thinking: that’s crazy, why would you even bother with such non-sense?  This is the 21st century, not the dark ages; we use lids in century 21, sir!

Why Ferment Open

Open fermentation does two things: it increases access to oxygen throughout the fermentation and decreases head pressure.  Yeast use oxygen primarily to divide and multiply, a separate metabolic pathway than the anaerobic process which produces alcohol.  This means you will have yeast with very strong cell membranes, ideal for top-cropping and reuse in another batch.  As a side product of the aerobic pathway, you will also get increased ester and phenol production.  Depending on the specific strain of the yeast, this means more fruity flavors (like those in English strains), more banana and clove (like from a hefe yeast), and more of that classic Belgian flavor from Belgian yeasts (which encompass all of the previous flavors and then some).  Even Chico strains tend to take on a more British character.

Flip Your Lid

Flip Your Lid

Decreasing the head pressure effectively decreases the amount of CO2 which can be in solution.  Ethanol and CO2 are both waste products of yeast, and are hence toxic to the yeast.  The water making up the beer can only hold so much CO2 in solution, more when its cooler or the pressure is higher.  By keeping the lid off (or even loose), the fermentor will be kept at atmospheric pressure.  When bubbling through an airlock or blow-off bucket, the pressure needs to be enough to push the fluid out of its way so a bubble can escape.  Thus, its at a higher pressure.  Though realistically, I think this is of little consequence for home brewers.

Many commercial brewers employing open fermentation also use very shallow vessels to decrease the pressure of the beer itself pressing down on the yeast cake.  For every 33 ft/ 10 m of a water column, it will push down with an additional force equal to another atmospheric pressure (about 15 psi).  The danger in this case is over-compaction of the yeast cake, especially in conical fermentors, which can lead to autolysis (yeast death).  In the home brewing world, this is not a problem; a shallow fermentation vessel would serve only to increase the surface area exposed to oxygen.

I like to think of this process as yet another tool in your tool belt.  It need not be something you do all the time (I do open ferments only rarely), but its another control knob you can turn to fine tune the character of your beer.  Like many brewing techniques, it could be appropriate for some styles, but be completely out of place for others.  If you want to draw out a bit stronger yeast character in your hefeweizen or Belgian blonde, this might be a technique to try.  If you are shooting for a clean lager, lids on.

Something to keep in mind is that the aerobic metabolic pathway tends to generate less alcohol and more water, so your beer may marginally lower alcohol than you might expect.

Avoiding Infection

You may have heard about people doing wild fermentations by simply setting a bucket of wort out to collect wild yeast and other microbes that float around us all the time.  Its an interesting technique, but tends to create wildly varied results.  So it makes sense to do a reality check and ask, “why doesn’t that happen with open fermentations?”

The key is to pitch an adequate amount of active yeast.  Brewer’s yeast is one of the only microorganisms that can survive an alcohol solution greater than a small fraction of a percent, and at such a low pH.  And the yeast know it.  The first thing brewer’s yeast does in solution is quickly drop the pH while it is replicating to make it toxic to other organisms, then start producing alcohol to really make it an inhospitable place.  Of course it can become unhealthy for them, too, but they are much better able to cope with such stresses.

Another protection afforded your fair bucket is the production of CO2 during fermentation.  While I have serious doubts about its ability to form a blanket like most people claim (the law of partial pressures and the speed at which gas molecules diffuse prevents blanketing), it will at least make a decent updraft and prevent any microbe-covered dust particles floating around from falling into the beer.

What does form an actual blanket is a true top-cropping ale yeast during high krausen.  Thus, only some yeasts are good for this technique.  Those that tend not to collect on the surface remove one layer of protection, and are unable to take as much advantage of the additional oxygen anyways.  Depending on the yeast, you may find it necessary to rouse them with a (sanitized) spoon a few times during fermentation- they tend to get a bit sluggish in the presence of the additional oxygen.

You may notice that none of the above protections apply prior to the start of fermentation.  Before the fermentation starts to krausen, and after it has started to die off, you’ll probably want to set the lid on top of the fermentor.  It need not be actually attached, it just needs to keep dust and such out.  Choosing the proper environment is also important for this reason.  You’ll want somewhere with relatively still and clean air, away from where something can fall into it.  And if your pets are of the curious persuasion, be sure to keep them away too- it would be quite a contest for most angry between you and your cat if your cat found itself in a bucket of beer, and hops can be toxic to dogs.

For more information, check out BrewingTV episode 4.


Batch Size: 5 gal/ 18.9 L

Malt/ Fermentables:
12 lb/ 5.44 kg   Golden promise malt
2 lb/ 910 g        Munich malt
1 lb/ 450 g        Aromatic malt
6 oz/ 170 g       Special B malt
2 lb/ 910 g        home-made dark candy syrup

1.6 oz/ 45 g     Tettnang, 6.1%, 60 min, 27.7 IBU
1 oz/ 28 g         Tettnang, 6.1%, 30 min, 14.1 IBU
Total IBU:       41.8 (Tinseth)

2L starter         WLP575 Belgian Style Ale Blend

Target CO2:     3 vol

OG:                   1.085 (56% mash eff; target 1.087, 58%)
FG:                    1.013
ABV:                10.5% after conditioning

Mash temp:          142.5F/ 61.3C (target 143F/ 61.7C)
Mash thickness:   1.75 qt/lb/ 3.65 L/kg
Single infusion mash, single (batch) sparge
Boil time:              90 min

Fermentation: up to 79F/ 25.5C, lamp on after ferment tapered to keep above 77F

Tasting Notes:
Appearance: small fast-dissipating head which laced slightly; beer is dull brown in color

Aroma: strong fruit and spice

Taste: yeast contribution very reminiscent of Chimay blue; hops provide enough bitterness to balance without contributing any flavor or real character; not enough malt character; candi syrup contribution could be more delicate (it was probably a touch scorched, leading to a very slight background harshness as the other flavors fade; this went away after a few months in the bottle); no alcohol character present whatsoever- even at over 10% ABV, it gives no indication it is a high alcohol beer

Mouthfeel: pretty thin, relatively high carbonation; more watery than I would like- could probably use a bit more character malt and a higher mash temp

Overall: This beer was a first for me on two fronts: the first to use a home-made candy syrup, and the first to use open fermentation.  The candy syrup was maybe a bit over the top, but my favorite part of this beer was the contribution from the yeast.  The open ferment performed wonderfully, perhaps because this was a mix of yeasts.  The 2L starter was actually under-pitched by half, but I still got 85% attenuation.  After a year in the bottle there is still no sign of any infection from my open fermentor.  The only real change I would make to this recipe next time would be to raise the mouth feel and malt contribution by raising the mash temperature a bit.  As it is, its sort of stuck between styles and is kind of a dark trippel.  This would also help the incredibly poor efficiency I got on this brew, as barley starch doesn’t fully gelatinize until 150F/ 65.6C or higher.  By raising the temperature in the last 10-15 minutes of the mash, your efficiency will greatly improve with little impact on the fermentables profile.

– Dennis, Life Fermented


About Dennis
Home brewer, home chef, garage tinkerer. Author of Life Fermented blog.

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