Fermentation done in 12 hrs ???!!!

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Dec 25, 2019
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I made a Guinness clone on Monday this week. I wanted to hit an OG of 1.048 ... but my efficiency exceeded my wildest expectation. Instead of getting a standard efficiency I usually get - around 75% - I ended up getting a 83% efficiency and an OG of 1.054... oh well. I cooled the wort down to about 70F and oxygenated with pure O2 for ~1 min (maybe slightly longer). I had a Wyeast 1084 Irish Ale yeast pack - expiry was March 2022 ... really fresh viable yeast. I pitched the yeast at around 9.30 pm at night without any starter - just a single liquid yeast pack.... left my fermenting bucket in the basement with an ambient temperature around 69-70F. This is where things get interesting. The next day - I came down around noon to check if the fermentation had begun... there was no airlock activity at all. I just assumed that the yeast were being "lazy" and gave it another 12 hrs... still no airlock activity. I checked the bucket lid and there was no air leak - when I pushed on the lid - I got bubbles coming from the airlock. It seemed pretty airtight. Finally today I took a flashlight and peaked in through the airlock hole and guess what - krausen had already fallen! Only explanation is that the fermentation had already passed its high activity phase in the first 12 hours after pitching yeast. I havent taken a gravity reading - but is it possible that the fermentation could have been completed in 12 hrs flat??? I have never seen fermentation complete so fast ever before.... Just wanted to know your thoughts and experiences.


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Aug 13, 2021
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Willamette Valley
I’ve had fermentation finish really fast once, but maybe it was 18 hours. I use enzymes and mash low, so I usually 99% attenuation. It also bubbled like crazy.

this does not sound like your situation.