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How much wiggle room is there on fermentation temps?

OK, so I’ve been having a problem with what I think is diacetyl taste in several different brews. One of the possible reasons is my fermentation temps, so I’m trying to keep a closer eye on that.

My fermentation room is in my basement, where the temps are pretty consistently 62-66 at this time of year. I am working on some kind of heating system to keep it more consistent and a little higher.

I’ve got two kits I really want to brew. Problem is, the instructions for one say the ideal temp is 64-70 (caribou slobber, Danstar Windsor yeast), and for the other 70-85 (saison, Wyeast 3724). Which seems really high to me, but… I could actually ferment that upstairs, maybe.

So, questions: how much wiggle room do I have? If I brew that first one (a saison) and the temps dip below 64 now and then, or even daily, is that going to hurt me?

Second, these temps refer only to external temperature, is that right? The internal temperature of the fermenting beer is already accounted for?

And third, if I continue to have this same diacetyl problem, should I just give up and become a hippie mead-maker?


Sounds like your basement is perfect for ale temps. You could move your saison upstairs if you want to increase the esters. But it will ferment fine in the basement also just get a cleaner taste. The recipe probably calls for a warmer temp to speed the fermentation for the impatient new brewer who tends to rush to bottle. Once you brew awhile you won’t pay much attention to the directions

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Plus one to Brew-Cat’s input.

As for your diacetyl problem, maybe give your beer more time? It can get cleaned up by yeast post fermentation. The concept of a D-rest for lagers also works for ale yeasts. When you taste a sample, if you detect diacetyl, give it a little more time before racking.

If it’s 66 degrees ambient temp in your basement your saison and any other beers fermenting there will rise into the 70s from the heat produced during fermentation. It would be better to focus on controlling the temperature of your ales with a swamp cooler than trying to warm the room IMHO.

Not sure I understand your question about what temperatures refer to? The target temperature on the yeast package is referring to the temperature of the wort, NOT ambient room temp. A stick on fermometer on your vessel will give you the reading you need.

If you’re fermenting warm and have no temperature control it’s quite possible you are producing diacetyl. The best bet is to ferment ales on the low end of the optimal temperature range as stated on the package or manufacturers website, then warm it by about 8 degrees as fermentation is winding down. This helps the yeast to clean up diacetyl and other unwanted side effects of fermentation.

Yeast performs the best when the temperature of the fermenting beer is stable. The fermentation temperature is independent of the ambient temperature since the fermentation is exothermic. The activity of the yeast produces heat. Fermentation at the low end of the yeasts range may produce more esters. At the high end produce more phenols. This is just a basic generalization though. The best information is on the manufacturers web site.

I ferment all my beers in a water bath, a.k.a. swamp cooler. Used as a swamp cooler it limits the maximum fermentation temperature during the active fermentation. Maximum temperature is what I prefer for the yeast being used. My swamp cooler also has an aquarium heater controlled with a STC-1000. I can warm the water bath for higher initial fermentation temperatures or use the water bath to maintain a higher temperature than the ambient after the active fermentation ceases to maintain a stable temperature.

Diacetyl rests are usually associated with lagering. In lagering the fermentation is usually very cool. Near the end of the fermentation the temperature is increased to ale fermentation temperatures so the yeast can clean up the off flavor. I am now maintaining the highest fermentation temperature with all my ales up to a couple of days before bottling.

In your situation I would recommend some method of controlling the active fermentation temperature and the temperature up to bottling time. I’ve purchased the STC-100 for $13.63 on Amazon. A work box to put in for $11.00 at Menards. A basic outlet to plug into for heating and cooling is a couple of dollars.

A picture of my first unit.

I’ve since bought two more to have multiple brews going at the same time at different temperatures.

In the corner of the picture is my water bath tray. It’s a restaurant bussing tray. Two for $12 at Sam’s Club.

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Thanks for the replies. Uberculture, let me ask you about that “more time” thing. My usual timeline has been 2 weeks in the fermenter, then bottle. If there’s diacetyl a couple weeks later, shouldn’t it get better over time? Because it doesn’t seem to. Maybe racking to a secondary before bottling is the way to go, then.

flars, does that work like a thermostat, so it would get down to/up to the desired temperature, turn itself off/down, then turn back on as the temperature changes?

Two weeks is what the instructions say. Yeast can’t tell time though, so a lot of us give it more time. Gravity readings are the only way to tell for sure, but yeast metabolizes some of the compounds produced during fermentation. Try another week in primary. The problem with racking off the yeast cake is that there are then far fewer yeast cells to clean up. I had diacetyl in the bottle once, and it took months to clear up. In primary, I feel like it would have cleared up faster.

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