Thursday, December 24, 2009

When Your Yeast Quits

This is an often experienced occurance in the homebrew world and has happened to me a couple of times. In fact, I presently have a German hibiscus/wheat beer that has stopped dead at 1.020 when I expected it to finish at about 1.012, the original gravity was 1.050. This is a German style hefe weizen with the addition of hibiscus flower. I used the Safeale S-33 Belgian ale yeast from MoreBeer to ferment. There are a number of reasons that stuck fermentations happen. Yeast viability, lack of adequate aeration, high mash temperatures, low fermentation temperatures, you could probably name a few others. The point is that the beer has not fermented completely and you've got five or ten gallons of under-attenuated beer sitting in the fermenter looking up at you with an expression of defeat.

An under-attenuated beer can be problematic for a couple of reasons. The main reason is that the taste will be sweeter than you may want considering that you hopped with an expectation of a drier beer and the mouthfeel fuller as a result of the remaining sugars in solution. The second problem effects those that are bottling the beer. Normally you would add approximately half a cup of bottling sugar for a five gallon batch (48-12oz. bottles) to achieve the carbonation that you want in the finished beer but with all that residual sugar in solution to begin with, you may just be creating bottle bombs caused by the over carbonation of the additional sugar.

In the case of this beer, my expectation was for a final gravity of around 1.012 and so I am a little dissapointed but determined to get the rest of those points out of it one way or another. My belief is that the temperature got too cold and I may not have oxygenated the wort well enough causing the yeast to drop out of solution assuming a dormant state. My solution?

1.)pitch a fresh pack of US05 ale yeast
2.)gently stir up the yeast cake with a sanitized spoon to get it back into solution
3.)warm the beer up to 75f. with a 60watt light bulb placed strategically near the fermenter

Not real dignified but I have high hopes for success here, I have to.

In the past, US05 has been successful in getting a couple more points out of the gravity and I've read of others using Champane yeast with good results. The other part of this equation is patience. It may take an extra week or even two for the yeast to slowly do their job. I have to remind myself of this regularly as I tend to want the beer done quickly and into a keg or bottle to prevent any additional exposure to the elements since I basically 'open ferment'.

Finally, if the beer doesn't ferment any further after all these efforts then I will reduce the amount of bottling sugar I use. Additionally, I will plan to increase the amount of hibiscus flower that I add because it causes a distinct sense of dryness to the mouthfeel in this recipe.

If you've got more solutions to this common problem, please leave a comment.


Kristján Þór Finnsson said...

you could try to add some oxigen.
take a sample from the fermentation, and shake it well. if the sample goes down to 1012 then it could be oxigen problem. i'm going through a similar thing with my bock.
cheers, kristfin said...

thanks for the advise, I will try that and see.

Scott Schluter said...

I had a similar issue with my Tripel. But I guessed it was because my mash temperature was off due to a wonky thermometer. Got me thinking about this alot. Is there a way to tell if your yeast just quit or if it did the job on the available fermentable sugars. I mean when you take specific gravity, it doesn't measure the sugars that will ferment out, right? How can you predict fermentation rate in a case like that?

Anyways, I, like you tried to get it started again. I put some additional table sugar in, roused the yeasties, and warmed it up. I got fermentation going again and dropped the few extra points I added and a very few additional ones. I did the process a few more times with the same result. I believe the yeasts did their job. The resulting beer is nice an alcoholish like a tripel should be, but there is quite a bit of body left, more like a quad. said...

You should get a pretty good idea of the attenuation from the discription of the yeast provided by the mfg. they will provide a % rate of attenuation. If you make the assumption that it's possible to ferment out ALL of the sugars you can predict what the finished gravity will be loosly based on what the yeast normally ferments to before it quits. For instance I can expect 80% fermentation from the Safeale us-05 and have found that this is pretty consistent. In the case above, I didn't have the stats for s-33 and if I had read about other brewers experience (go to link below)

I would not have used it. My final gravity for
the above beer turned out to be 70% attenuation. Too low for my tastes, but apparently typical for S-33 In any case, I'm hoping that with the addition of the tartness from the hibiscus that I will add at bottling, that the beer will be o.k.

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