Safe to warm fermentation?

Hi all,

I made a post yesterday about finishing my fermentation to achieve my desired FG and I got a ton of response about warming up the fermentation, which I've done. It makes sense on a "hunch" level, but I'd really like to understand it more fully.

My OG was 1.072 and my goal FG is 1.012. Currently it's at 1.025 after a week of fermentation solidly at 62F ambient. We started the fermentation at 66F ambient for the first night to let the fermentation get going, then brought it down to 62 for the week. The fermentation's highly-active phase is done, and I was told it's safe to warm it up and I won't get off flavors. Why is that? I understand that yeast are more active when it's warmer, so I get heating it up for the last few gravity points, but why is it safe now?

Also, I read that leaving it at room temperature (66-68F ambient) is good for the first 24 hours so I gave it a shot. Thoughts?

Yes, after a week or two at 65 degrees it is safe to increase the temp to 70+ to help the yeast finish up and get a lower FG.

That is a pretty high gravity brew. I would give it 3-4 weeks in the primary before bottling.

My totally unscientific, over-simplified, anthropomorphized way of looking at it is that you want to keep your yeast happy, but not too happy. You don't want them to be too stressed, but you don't want them to get too excited either. The warmer they are and the more sugar that is available, the happier they are. The first few days is like a feeding frenzy due to the abundance of simple sugars to eat, so you have keep the temp lower to compensate. Later on they're just slowly snacking on the leftover, more complex, harder to eat sugars so the temp can be higher without them getting so excited.

Also I know that at some point they switch gears from creating unwanted by-products to consuming those same by-products, as they run out of other stuff to eat. So you want to try to suppress it when they're creating those by-products, and encourage it when they're consuming them.

I'm sure I'll be corrected if I'm wrong but here's how I see it. During the most active portion of the fermentation the yeast are producing their own heat so the wort can actually get up to 10 degrees warmer than the ambient air temp. So if ambient was 68-70 then the wort could possibly be as high as 80 which is where the off flavors can come in. But as it slows down and there's less activity the temps are going to fall back closer to ambient temps. In your case 62 isn't bad but towards the end it will probably slow their production.

That's the way I understand it to work at least.

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It's safe to warm it up because most of the sugar is gone by this point. The yeast make most of their off flavors (and eat most of the sugar) during active fermentation. I would take an SG reading to see where you are before changing the temp.


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1.024 before putting it in a 19-21c water bath yesterday. 19c at the bottom of the bath, 21c at the top. Still haven't seen any bubbler action, but I know that doesn't mean fermentation is not happening, especially at final leg of fermentation.