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?