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I have two fermenters going with about a cup of slurry of S-04. Pitched Sunday morning at 61 degrees ambient, they were showing activity within 5 hours, vigorous activity within 12 hours. Went to harvest yeast yesterday, and the krausen had already dropped! I increase the chamber temp controller to 64 degrees, and decided to take a hydrometer sample, and it was about 1.029, down from 1.062.

I know airlock activity doesn't necessarily mean fermentation activity, patience is key, etc., however for the benefit of others, is there a rule of thumb for percentage of fermentation that occurs after the krausen has subsided? Could it be that this just premanturely flocculated on me? I have heard that S-04 can do this at lower temps.

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this could have been a straight up stalled ferment...roused the yeast this evening, increased the temp of the ferment chamber, and airlock activity has started up again. Would be an interesting question to answer though, maybe a yeast expert can find an answer. If nobody does in the next few days we can probably close this question. –  Pietro Sep 20 '12 at 2:37
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you know, the airlock activity is more likely the result of the headspace gas increasing pressure with the higher temp, and with co2 coming out of solution. take a gravity sample in a few days to be sure. –  mdma Sep 20 '12 at 10:20
    
the increase wasn't that substantial, though you may be right. Trying to keep in mind that I pitched this 4 days ago. –  Pietro Sep 20 '12 at 14:01
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1 Answer

up vote 3 down vote accepted

I'm not sure how much you can read from a fallen krausen - some yeast strains don't fall at all, while others fall early. When this occurs and how far fermentation has progressed will vary from strain to strain, and maybe even brew to brew due to different wort composition, proteins, oils etc. When the krausen falls, it doesn't always mean that fermentation has stopped, but simply slowed down to a rate that cannot maintain the krausen.

The krausen is the result of vigorous yeast activity, pushing up coagulated proteins, yeast, hop oils and trub. In some brews, the krausen can more or less be permanent. Otherwise, it is maintained by the upward churning of the wort by the yeast - temperature gradients create the churning since wort that is warmed from active fermentation will naturally rise. When it reaches the top, it cools slightly in the headspace and falls down again.

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