As a recent experiment, I wanted to try something that was unconventional for most homebrewers, and is widely considered to be clown-crap crazy. I wanted to ferment a beer topless, without a lid. I settled on ten gallons of a Belgian Dubbel, split into two batches, one went into a better bottle with bung and airlock, the other half in a bucket without a lid for the first ~48-72 hours. For those who haven't tried this, I'd highly recommend it at least once, as it is amazing, being able to look into the beer closet/fridge and seeing the yeast snap, crackle, and pop while you look down on it during fermentation.

Due to space constraints in the keezer, I could only get one of the two on tap at a time, so I opted for the closed fermentation half first, during which time I cold crashed the other half in the lagering fridge. The closed fermentation tasted very similar to what I would expect for a commercial Dubbel; subdued yeast character, dark fruit, a little clove, hardly any banana, quite malty, a very calm, typical Belgian ale. I just kegged off and drew a sample of the topless fermentation half, and the best way of describing it is: Woah! Completely different flavor and aroma. I get a very profound banana flavor, some clove, some dark fruit, all of it overpowers any malt flavor. It's almost undesirable, it's so intense.

What is the scientific explanation behind open fermentation leading to more ester formation during fermentation? I assume it has something to do with the CO2 pressure blanket over the wort, so I wonder if similar results can be seen if using a carboy with a more confined hole in lieu of a wide open a bucket. Can anyone explain this?

  • Very cool topic for a question.
    – brewchez
    Feb 4, 2014 at 16:19
  • 1
    Great question! Now I know why my second batch(a Hefeweizen) had so much banana flavor. I used open fermentation back then. I've been trying to replicate that without any luck.
    – user2637
    Feb 5, 2014 at 8:42

4 Answers 4


What you're asking is called the Pasteur Effect: the effect of oxygen on yeast fermentation. Pressure's not a factor. Oxygen is.

First, a word about esters. An ester is formed when a molecule of organic acid reacts with a molecule of alcohol. It's a pretty simple reaction and can occur somewhat at room temperature or even below. the smell and taste of the ester are determined by which acid and which alcohol react. That banana smell is isoamyl acetate. Methyl butyrate smells like pineapple. (I'm a retired chemist, so forgive me. Old habits die hard.)

Anyhow, as for the Pasteur effect: Yeast are facultative anaerobes, meaning they can survive either with or without oxygen by using different metabolic pathways. In the presence of air (aerobic fermentation), the abundant oxygen will favor the pathway that produces organic acids, some of which go on to form esters with any alcohol present (and there will always be some).

With no oxygen (anaerobic fermentation), the yeast will use another metabolic pathway that produces mostly alcohol.

As far as I know, all commercial potable alcohol is produced anaerobically. There are some cultures that use aerobic fermentation, but that's is super-primitive technology. Vinegar manufacture uses aerobic fermentation, but I don't think they use the same kind of yeast.

  • 2
    Now we're talkin'! To simplify then, you're stating that by removing the containment of the CO2, and allowing for O2 to come in contact with the yeast during fermentation (in brewer's terms, the krausen), the yeast will use the available O2 to create organic acids which react to alcohol, creating esters. Am I correct in that over-simplification? Additional questions: Is the aeration of the wort pre-fermentation the main cause of the production of esters in closed fermentation? Can over-aerating pre-fermentation increase esters? Can one manipulate which organic acids are generated?
    – Scott
    Feb 4, 2014 at 22:36
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    Nice to have someone in the know comment on these things. Could you say a little how this fits in with the Crabtree effect, where yeast prefer anaerobic fermentation even in the presence of oxygen?
    – mdma
    Feb 8, 2014 at 8:08
  • I'd love to wrap this question up, if only I could get a bit more clarification on my last comment.
    – Scott
    Feb 10, 2014 at 17:48
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    It's not at all true that commercial brewers don't use open fermentation! Anchor Steam for instance is produced that way, as are a great many traditional English ales. See byo.com/stories/item/1211-open-fermentation-tips-from-the-pros May 8, 2014 at 18:09

The answer is sort of hidden in the inverse of your question:
Closed fermentation leads to decreased ester production.

It does this through increase pressure in the vessel. It has been well documented that increased pressure slows fermentation and yeast growth to some extent; both of which also suppress ester production to varying degrees.

This pressure is caused either by an increase in pressure formation in the head of a fermentor or by hydrostatic pressure of a really tall water column pushing down on the beer.

Hydrostatic pressure reducing esters is why many commercial brewers ferment at higher temps than we homebrewers do with the same yeast strains. The tall conicals tend to slow yeast performance more than smaller tanks do (open or otherwise), leading commercial fermentation in the 72F range where a homebrewer may ferment the same wort with the same yeast at 65 to get the same flavor profile.

Hydrostatic pressure though normally isn't an issue in 5, 10 or 15 gallon batches. The "water column" is just to small.

However, relieving the head pressure in two identical fermentors can be significant as you witnessed.

Lastly, I have never seen any literature that actually spells out the biochemical mechanism by which this works. Its just been documented that head pressure does suppress ester formation. I have had this convo with a handful of pro brewers, and had to scale back fermentation temps sometimes when trying to replicate a beer using a recipe from a commercial brewer.

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    Do you know how little PSI worth of pressure is generated via a closed vessel with a blow-off tube submerged in... say 4 inches of water (assuming all that matters when comparing to a simple airlock with a tbsp of water in it)? I can't imagine it's more than 1-2 PSI. I'm not doubting your answer, I'm just shocked that such a minimal amount of pressure could alter the ester profile so dramatically.
    – Scott
    Feb 4, 2014 at 18:47

I think another aspect of all this that got brushed over, is that of temperature. Ester production is very relative to temperature. If you are using ale yeast, you are in or very close to the ester production zone. (17-19C) I tihink open fermenting lends it's self to lagers, because of this ester/temperature issue. Your sub 10C ferment temperature largely attenuates the ester problem. Couch quarterbacking here. Any comments on this.


Im no scientist, but I can tell you that there are wild yeast amongst us! I am reading up on making wild yeast breads and it is quite interesting. I assume that it has more to do with the yeast and less to do with pressure.

What a great experiment for sure. I think I will try it and use bottles for a more direct and immediate side by side comparison that I can perform numerous times and with different groups of friends.

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