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I am fermenting an alt in my cellar using WLP036 (Düsseldorf Alt). Pitched a 2 litre starter into 21 litre 1.051 wort. Fermentation was extremely vigorous at 16 C ambient temperature for 36 hours, then dropped fast. Ambient temperature has dropped somewhat, to 14 C.

White Labs list the optimum range as 65-69 F (18-21 C), but it is supposedly common knowledge that the best fermentation temperature is cooler.

My question: should I move the fermentor to a warmer place, or is it fine where it is?

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14C is 57F or so. That awfully cool for an ale yeast. Even 16C is only 60F. That's still below their recommended range. While it may be recommended to ferment at lower temperatures, 4C/8F lower than WL's recommended minimum will definitely slow, and possibly halt, fermentation.

You said that initial fermentation was "extremely vigorous" so this drop in fermentation activity may be due to finishing fermentation rather than due to temperature. 1.051 wort isn't particularly high gravity, so it's possible that with that large of a starter fermentation may be complete.

I suggest taking a gravity reading and comparing that to your expected FG. That's the only definitive way to tell what is truly going on.

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  • Thanks! SG is now 1.018 (target is around 1.013), and it is still fermenting, albeit slowly. I have never used this yeast and was amazed at the speed of primary fermentation. Ambient temperature is now 16 C after I moved it to a different location, but I am considering moving it to room temperature.
    – Nemis L.
    Nov 7, 2014 at 9:19
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    The 2L starter surely helped kick off a great fermentation. Fermenting out 5 gravity points shouldn't take that much longer, even at cooler temperatures. You could move it to warmer temperatures, but I bet even at 16C you'll reach target, or close to it, in another 5-7 days.
    – rjbergen
    Nov 7, 2014 at 18:37
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The OG value is not the problem. You have to understand that Altbier is fermented at lower temperatures. That's what the style requires. Deviations are not allowed. Altbier is a mixture of ale and lager and you absolutely need 16°C or less to get the appropriate flavour. It is one of the most difficult styles of all, as everything has to be balanced.

I wonder about this: normally an Altbier yeast like WLP036 Düsseldorf should also work in its low temperature range; 18-21°C is for Ale, but normally a yeast like W165 from Weihenstephan is intended for 16-18°C.

Maybe White Labs forgot the additional specification on it? I have sent a message to WLP but have not received an answer yet.

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    Required? Deviations not allowed? Eh, homebrewers don't care...
    – Robert
    Dec 29, 2021 at 5:07
  • Oh my god, you're talking nonsense. There is a difference between a good beer and a very good beer.
    – Stubby Hobbs
    Jan 15, 2022 at 16:30

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