When we talk about fermentation temperature, what exactly do we mean?

Sorry about the ridiculous question, but I have been curious about this for a while. There are tons of threads that talk about fermenting at this and that temperature, but is everyone talking about the temperature of the ambient environment (e.g. the thermostat on the keezer, the water bath temperature, the temp. of the room that the fermenter is in), or the actual temperature of the fermenting beer? I only ask since the beer itself can easily be 10 degrees higher than ambient when fermentation is really active...

They are talking about the temp of the fermenting beer.

Temp of beer in the bucket/ carboy/ better bottle etc.

Some people measure and control the temperature directly with an immersion well, some stick a sensor to the surface of the carboy, some immerse the carboy in water and control the water temperature. I did some experiments and found that the surface of the carboy sitting in air can be as much as 5 degF lower than what is actually happening inside. But I also found that the temperature will never be different by more than 1 degF for a carboy immersed in water. So, I use the temperature controlled water bath technique.

When I talk about fermentation temps I mean the temperature of the fermenting wort/beer. And yes it can be 10 degrees higher than the ambient temperature. I have a storeroom where a glass water setting on the floor will be about 60. My ale fermentations run about 68 - 72 depending on the strain of yeast.

Almost always the beer temperature--when someone says ferment @ 65F, they mean that's the beer temp. If they mean ambient, it's usually obvious--they'll say something like "pitch the saison @70F and let it free rise" or something.

Thanks for the replies. It's what I was expecting to hear, but now I' confused.

If the temp diff. between a carboy in water, and the fermentation happening inside is only 1-2 degrees, I've got some funny fermentation happening.

I've got a cider going with s-04 in a 3 gallon glass carboy immersed in an ice cooled water bath (pic below).



Depending on which thermometer I read, the water is between 48 degrees, and 54 degrees... I've had active bubbles in the airlock (1 every 2-3 seconds) for almost a week now, but the yeast's fermentation range starts at 59 degrees. Can yeast go below their ranges?

I've also got a wet shirt on the carboy and a fan going at all times... I've been assuming that the carboy must be hotter inside than the water given that I took the temp all the way to 45 degrees in the beginning and it was still fermenting at that temperature... I've let it get to the mid 50's because of laziness (I don't like to change the ice more than twice a day).

scone said:

Thanks for the replies. It's what I was expecting to hear, but now I' confused.

If the temp diff. between a carboy in water, and the fermentation happening inside is only 1-2 degrees, I've got some funny fermentation happening.

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Immersed in water, 1-2 degrees is common but I could see more with a particularly aggressive fermentation.

I've got a cider going with s-04

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What was the OG? The higher it was, the greater the fermentation heat (so a bigger delta, too).

in a 3 gallon glass carboy immersed in an ice cooled water bath (pic below).

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The smaller the fermenter, the higher the surface area to volume ratio, so the lower the temperature differential.

Depending on which thermometer I read, the water is between 48 degrees, and 54 degrees... I've had active bubbles in the airlock (1 every 2-3 seconds) for almost a week now, but the yeast's fermentation range starts at 59 degrees. Can yeast go below their ranges?

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Yes, most yeast are pretty happy to ferment in a pretty wide band, sometimes 10-20 or more degrees outside their range.

The flavor profile will be different--going above the range is generally pretty bad taste-wise unless you know what you're doing and want something very estery. Going below it may not be that big a deal if you're going for a relatively clean taste, though complete fermentation may take a long time and attenuation may be lower than desired.

jmansfield said:

But I also found that the temperature will never be different by more than 1 degF for a carboy immersed in water. So, I use the temperature controlled water bath technique.

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That's good to hear. This is the method I use and it has not required a blow off tube since going to a water bath. I was curious about the temp difference between water and wort.

SumnerH said:

What was the OG? The higher it was, the greater the fermentation heat (so a bigger delta, too).

Yes, most yeast are pretty happy to ferment in a pretty wide band, sometimes 10-20 or more degrees outside their range.

The flavor profile will be different--going above the range is generally pretty bad taste-wise unless you know what you're doing and want something very estery. Going below it may not be that big a deal if you're going for a relatively clean taste, though complete fermentation may take a long time and attenuation may be lower than desired.

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I'm not sure about the OG (I never measured it). It's a pretty standard 2.5g pasteurized apple cider + 22 oz. clover honey. I didn't know that about yeast, but I'm glad to hear it. Here i've been thinking my water bath technique is failing me.

As far as flavor profile, I'd guess the cleaner the better since it's cider. My thoughts were to ferment cold so very active co2 production doesn't drive off aroma components of the cider as much. I plan to bring it to room temp (78) and leave it for a month once fermentation looks about complete before priming with more apple cider and bottling. I figure this will take care of any potential bottle bombs due to a high FG.