I have rigged up a Raspberry Pi with some DS18B20 temperature probes that measure both the water in the swamp cooler and the beer directly and store the results in a MySQL database.

I brewed a new batch on 7/18, and fermentation began on 7/19, from which I have put varying amounts of frozen water bottles into the swamp cooler on 8 occasions.

Here is the pattern I have currently noticed for adding frozen water bottles during primary fermentation (the patters may well change after the primary fermentation due to the lack of exothermic reaction present):

1) The water in the swamp cooler reaches its minimum temperature in about one hour.
2) The fermenting wort reaches its minimum temperature in about two hours.
3) The wort maintains its minimum temp + 1°F over the next two hours, very slowly working
    its way back up over the course of 5 hours after reaching its minimum
4) Both swamp and wort temperatures tend to equalize in three hours.
5) The temperature drop in the wort is roughly 40-60% of the temperature drop in the swamp

Now my main question is how much of a drop of temperature, in how fast of a time period, can yeast tolerate without dropping out? (Ale in particular, but Lager too if you know the answer). Would starting temperature also factor in--i.e., if starting temp was 75 and minimum temp was 70, would that be different than starting 70 minimum 65?

For example, on my first attempt the wort's temperature was 70.250°F. By placing three frozen water bottles in the swamp cooler, the wort's temperature dropped to 65.861°F in two hours.

On another attempt, the wort was initially 69.350°F, and three frozen water bottles dropped it to 65.412°F in two hours.

On another attempt, the wort was initially 69.125°F, and four frozen water bottles dropped it to 64.850°F in two hours.

On another attempt, the wort was initially 67.212*F, and four frozen water bottles dropped it to 63.162*F in two hours.

On another attempt, the wort was initially 68.100*F, and five frozen water bottles dropped it to 65.600*F in two hours (Interesting, smaller drop this time)

On another attempt, the wort was initially 67.230°F, and five frozen water bottles dropped it to 61.800°F in two hours.

Two hours ago the wort was 67.200°F, and five frozen water bottles have dropped it to 62.967°F.

In the future I will be posting graphs comparing the wort to the swamp with the number of frozen water bottles if anyone else is as interested in this as I am.

  • 1
    It's an admirable experiment, but I see two roadblocks to overcome. First, you'll have to somehow quantify "dropping out". Second, it will be strain dependent and different yeasts will respond differently. Good luck!
    – Denny Conn
    Aug 27, 2013 at 14:53

1 Answer 1


I think that it depends a lot on the yeast you're using, the beer you're using it in and where that beer is in its life-cycle (:D). If you made a particularly large starter of especially hearty yeast and it's at day 2.5 peak-Krausen, it will take a lot more abuse than a beer on it's third week in primary made with 5th generation yeast you underpitched.

If all you really care about is "will this yeast drop" I think shifts of no more than 5º in two hours (and no more than 15º overall) and it should (generally) be okay.

Of course making beer is a mystical search for the divine on par with the most self-torturing of ascetic orders - so even if this okay now, it's not optimal - and (after a 2 second look at your profile) I have a feeling that soon you'll be aiming to minimize temperature swings to within microdegrees during fermentation.

Good luck! Excited to see the graphs...

  • "dropping out" doesn't mean killing the yeast, it means making them to go dormant.
    – mdma
    Aug 27, 2013 at 17:01
  • 1
    does that have the same effect for practical purposes though? unless it's very early on in fermentation, if they went dormant would the resources be there to allow them to 'wake up'?
    – dax
    Aug 27, 2013 at 17:07
  • 1
    Dormant and dead are not the same in practice - you can easily harvest and repitch dormant yeast after primary is done, but not if they are dead, and dead yeast will impart a yeast bite to the beer. I didn't downvote, but I suggest you edit your answer before someone else does! :)
    – mdma
    Aug 27, 2013 at 19:54

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