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I am brewing a stout and using Safale US-05. Here in middle Georgia the weather is already plenty warm so I'm using a water bath to keep my wort at a stable temperature. I pitched this yeast at a temp of 60°F and planned to let the temp gradually grow 1-2 degrees per day until primary was finished. I'm on my fourth day of fermentation. Things seemed to be still going well until I checked my wort temp this morning and it was 69°F.

I was worried about the temperature rising too much while at work, so I chucked a frozen gallon jug in the water bath to neutralize the yeast activity, but I came home on my lunch break to find the wort temp at around 62°F. Obviously, I'm worried about the yeasties going down for a nap prematurely.

I'm curious how resilient is Safale US-05 to temperature drops. I know some yeast strains can handle it, but 7 degrees feels like a lot to me. OG was 1.076 and fermentation didn't reach high krausen until yesterday morning.

If my fears are realized and fermentation stalls, should I raise the temp to 75°F and try to stir them up or keep the temp at 70°F and stir them up? What are my options?

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Brewing a stout using safale05 is a novel concept but no reason why it will not work. I think the wish to keep a particular temperature is reasonable but not overly necessary for a stout. I have brewed many stouts at ranges from 14C to 22C and the only thing I can say is that the fermentation takes longer at the lower temperatures. The taste for similar brews did not noticeably vary from batch to batch. One might say that stout is such a highly flavoured brew that minor variations are hardly noticeable. If one was brewing a black lager then the need to keep cool might be more pressing - but even then I did not find a variation of 5C over the course of the day/night made a great deal of difference.

If the fermentation "stalls" then rousing the yeast with a good stir can help. There is IMHO not need to get overly worried about exposing an active ferment to air. Use a clean paddle, take the top off the fermenting vessel and stir vigorously for 10s or so, aiming to get settled yeast fully distributed again. However the yeast should not "stall" if it is correctly chosen for attenuation in the planned brew and added in sufficient quantity and the wort was correctly aerated to start with. If the yeast really has "stalled" then warming up to near 20C is usually the best idea and if that does not work it is also possible to add a new starter batch of suitable yeast that has been shown to be active (ie the starter batch is actively bubbling). It is also possible to mix yeasts influence flavour and final ABV.

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  • I'm not so worried about the temperature fluctuation as I am about the sudden large drop. It's been 24 hours since then. In that time, I've allowed the temp to slowly climb back up to 66F as of now and fermentation rate still seems typical for five days in. Just removed my blow off tube and did a gravity check before putting on airlock. Gravity right now is 1.032. Do you think I'm in the clear? If stalling were to happen, generally how fast might it be? I've never had fermentation stall. – thekolnik Mar 4 '17 at 1:08
  • The temperature drop reported is not particularly great and I don't suppose it would have been very sudden, so I see no problem due to temperature. Being colder IMHO generally means it all takes longer to ferment. The SG reading proves the yeast is active and that there is a little way to go before the fermentation is over. I would leave the brew for 14 days in total and then take another SG reading. If the SG was still the same after (say) 18 days then I would prime and bottle, whatever. Taste will improve by letting the stout condition in the bottle for several months. – barking.pete Mar 4 '17 at 20:25

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