Temperature swing of 10+ degrees during fermentation

Question

I'm fermenting a porter, and pitched the yeast at about 75°F (24°C) on Sunday night (4 days ago). Lively fermentation started the next day, and though the wort had cooled to about 68°F (20°C) the first night, the fermentation brought it up to 72-74°F (22-23°C). There was 2 solid days of really active fermentation at this temperature. However, there's been a couple really cold nights, and now the CO2 bubbling out has slowed dramatically. The temperature got down to 65 yesterday, so I put a space heater near it, and it's now up to 76°F (24.5°C) - but the fermentation still seems really slow.

I'm wondering how the swing in temperature is going to affect it - any thoughts? Also, I'm going to be adding coffee and vanilla into secondary fermentation (in about another 4-5 days), how important is the controlling the temperature while in secondary?

Thanks for the help!

Answer 1

Temp swings can cause problems, but temps as high as what you've got can cause even worse problems. Keep in mind that fermentation is an exothermic process. Therefore, you should chill your wort to below the temp you want to ferment at and then let it rise to the proper range. Don't sacrifice beer quality and flavor for a short lag time or fast fermentation. An easy way to control temps is to put the fermenter into a large tub of water. The extra mass helps buffer thermal swings. Then you can put an aquarium heater in the tub to warm it up or ice packs to cool it down. Cheap'n'Easy!

Answer 2

In addition to everyone else's comments, I would add that temperature swings within the first 72 hours are dangerous for the flavor of the beer, but outside of that window, the fermentation is mostly finished, in terms of volume, so it's harder to ruin the flavor. It sounds like you were barely outside that window.

However, you were definitely too warm for almost every strain you could've used for a porter, and as Denny mentioned, that's more worrisome than the delta - your beer might come out too fruity, estery, buttery, or have a number of other off-flavors. To help mitigate this, you should make sure to keep the temperature high now that it's finishing so that the yeast will stay as active as possible. This will be most helpful in cleaning up fermentation byproducts, especially buttery flavors from diacetyl.

To your second question - temperature control during secondary is not critical. If a yeast cake forms, try to keep it from getting too warm (above ~78°F), as this will speed autolysis of the dormant yeast. Otherwise, anything should be fine.

Answer 3

Have you measured the gravity? Temperature swings like that can reduce attenuation, and also increase off flavors that you might not get if you slowly ramp the temperature up and hold it high. Basically, cooling like that causes the yeast to flocculate out before they've had a chance to clean up some of the by-products (such as acetaldehyde) that they produce.

Also, that seems way too warm for fermenting a porter. You may have been better off waiting to pitch until it was at 68 or lower, and not bothering with the space heater.

Answer 4

Temperature swings can create off flavors, but what you're doing doesn't seem too dramatic. Any off flavors that were caused by that temp change will probably be unnoticeable. That being said, temperature control is pretty important all the way through the brewing process, including secondary (but less so, I guess). Anywho, if it's your first stab at a particular style you're probably not going to get it perfect, but it will still probably be a great brew.