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!

  • 1
    What yeast strain? – baka Dec 16 '10 at 19:06

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!


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.


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.

  • Thanks. The OG on Sunday was 1.069, haven't re-measured since then. I'm going to let it sit for about 4 more days before moving it to secondary and will re-test then. I'm hoping that will help with any of the possible by-products. What temp range should I aim for between now and then, and then once in secondary? – leftend Dec 16 '10 at 21:10
  • At this point, I would just try to keep it stable (within 1 or 2 degrees), so the yeast that are left in suspension can do their work. Most of the off flavors would have been produced in the first 72 hours or so. It may wind up tasting great, but the warmer temperatures will probably give it a fruitiness that you wouldn't have otherwise (depending on the yeast). – baka Dec 16 '10 at 21:18
  • About the 4 days and then secondary...forget about the calendar! every batch will make it's own schedule, so let the beer decide when to do what. Don't be in a hurry to xfer it. As a matter of fact, most of the time you don't need a primary at all. Just give it 3-4 weeks in primary and 90% of the time you'll be good without a secondary. – Denny Conn Dec 17 '10 at 16:57
  • Totally agree Denny, in fact - that's usually what I do. The only reason I'm planning on transferring to secondary this time is because that's when I'm going to add the coffee and vanilla... plus I want to free up my glass carboy for another batch. – leftend Dec 17 '10 at 22:07
  • I'll just add that every time I've forced a beer to conform to my schedule instead of going with what the beer was telling me, I've been disappointed in the results. – Denny Conn Dec 18 '10 at 16:33

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.

  • Thanks PMV, not my first porter, but it is my first batch at this time of year... thus the colder temperatures. I hope you're right about not noticing it... I'm just looking forward to tasting it to find out! This is only my 6th or 7th batch, and my first attempt at a secondary fermentation - and adding flavors during it, so I'm a little nervous but excited too. Appreciate your comment. – leftend Dec 16 '10 at 21:18

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