General Category > Yeast and Fermentation

Slow Yeast?


I recently fermented a rye stout.  The OG was about 1.065, which is what I was shooting for.  I pitched my yeast and observed vigorous fermentation...but only for a couple days.  The gravity was at about 1.020.  I let it sit for a few more days after attempting to rouse the yeast a bit, but had no such luck.  I even added more yeast and it would not drop below 1.020.  It also did not carbonate very well, but it still did.

I know that my my dextrins were not too high, but I can't for the life of me figure out what happened.  I've never had that problem before and haven't had it there some trick to fermenting dark worts, or is this just some sort of anomaly?

Extract or AG, and what was the grain bill? Yeast strain, quantity, pitching and fermentation temperatures? Just getting the obvious stuff out of the way.

If it carbonated, your yeast were still alive. Is it overcarbonated? If not I don't see any reason to think it wasn't simply finished. 69% attenuation isn't unusual, especially if it was extract.

All grain with the following bill:

9# 2 row pale
1# black patent
1# choc. rye
1.5# flaked barley

Mash 150-152

Ferm @ room temp

Pitched wyeast Irish Ale, and it didn't take(it was fairly old) and then pitched fermentis english ale.  It did not over carbonate.

I checked the gravity after vigorous primary and it was at about 1.024, then I gave it a week in the secondary and it dropped to 1.020.  After another week in the secondary there was no change.

The sad part is, it is a great tasting beer.  It just doesn't scream stout when I poor it into the glass.  It's rather cloying and poorly carbonated.  But, this is the first one I've brewed.  I suppose I'll know for the next time :)

First thought: have you checked your thermometer? See what it reads in freezing and boiling water.

I'm assuming you don't have this problem with lighter beers? Do you treat the water any differently? It's possible that with all the dark grains the mash pH dropped enough that you didn't get full conversion. You could check that with an iodine test next time, or get some strips and measure the mash pH, assuming you don't already.

Does it seem undercarbonated relative to the amount of priming sugar? It sounds like you racked to secondary a little early, so maybe it is a fermentation issue after all.

I would say that the conversion may have been the problem.  I haven't mashed with dark grains before and I've never really felt the need to do a conversion test (and it's worked fine thus far).  The drop in pH could have been the problem.  I will be sure to check for that next time.  With the fermentation, the krausen fell really early and I still gave it a full week in the primary beyond that.  So, given that it carbonated, the temp in the mash was right, and I gave it a good 2-3 weeks of fermentation time, I'm inclined to say you're right about the mash pH being too low.

Although, I did nail my target gravity...


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