Monday, January 21, 2013

Stuck Fermentation

My first brew of the year was a Belgian tripel, which stands in direct contrast to my 2013 brewing resolution to make mostly session beers.  The brew day for the tripel went very well and I hit most of the desired numbers, though being several gravity points below the target.  I put the wort directly onto a fresh yeast cake from a Belgian pale ale I had brewed three weeks earlier.  Fermentation started overnight and proceeded almost violently quickly.  I figured I would hit my target gravity, which was around 1.010, without question.  I was wrong.  I measured 1.028 after 10 days in the fermenter, which is much too high.  I had a stuck fermentation.

Beer can fail to reach a desired final gravity for several reasons (this is called under-attenuation).  The most likely causes for this outcome revolves around yeast health and population size.  If the yeast that is pitched into the wort is old, where many of the cells are dead, then the yeast population has trouble getting active and growing enough cells to churn through the sugars.  Another cause of under-attenuation can be lack of oxygen in the wort.  When wort is boiled and cooled, it contains almost no dissolved oxygen, which is needed by the yeast to build new cell walls and the population multiplies.  So, if the brewer does not add additional oxygen to the wort, the fermentation will be very slow and will likely stop early.  In a similar vein, failure to pitch enough yeast to work through the dissolved sugars will result in a high final gravity (see Jamil's yeast calculator to figure out how much you need to pitch).  Some yeast strains are also susceptible to falling dormant if the wort temperature falls even a few degrees.  For this reason, it is recommended that the temperature be ramped up as fermentation begins to slow.

I have experienced difficulties with under-attenuated beers before, particularly when they are big beers (last year's doppelbock, as an example).  In some cases, I think this is due to under-pitching the yeast population to account for the additional sugars.  However, in this case, I put the tripel wort right on top of a very viable yeast cake from a previous beer and fermentation took off right away.  I remembered to oxygenate so I am sure the yeast population grew adequately.  The most likely cause of the under-attenuation was failing to ramp the temperature as the fermentation progressed, to ensure that it would finish.

Now that I have this situation, what can be done about it?  There are several recommended approaches to getting a fermentation going again:

  • Agitate the wort:  When yeast cells go dormant, they fall to the bottom of the fermenter.  If you can swirl the fermenter up and get the yeast back into suspension, it might start fermenting again.
  • Warm the fermenter:  Given that yeast can fall dormant with dropping temperatures, if you can warm the wort up, fermentation may start again.  Do not be afraid to dramatically increase the temperature at this point, as you are trying to save the batch.  High 70s F is not out of the question.
  • Rack onto a fresh yeast cake:  As a last option, you can rack the wort onto a fresh yeast cake, which had been fermenting very recently.  The yeast added here must be active, and not just sprinkled on dry yeast, as it is going into a hostile environment where the most available food has already been consumed
I intend to try all of these methods and hope to save the batch.  Have you tried any of these methods and which has worked best for you?  We would be interested to hear.




  1. I find my tripels to take much longer than 10 days to finish. I do like to ramp up temps on them, to as high as 76-78 to get them to finish at the very end.

  2. Thanks for your comment, beer crafter. I have been swirling it daily and I have heated it as much as I safely can in a cool basement. It has pulled down a few points, so I am keeping my fingers crossed.

  3. HI Tom,

    By now you have flavors you want from the yeast, in situations like this I often dump a packet of us05 in to finish it off. It's a clean yeast not going to give any flavors and you don't have to worry about exposure with another transfer. Shaking isn't working any longer, as well as the increased temp.

  4. Also a good idea, Peter. I have a friend who has a Belgian yeast cake he will be done with in about a week or so. If my tripel still has not pulled down, I am going to transfer on top of that yeast cake. Thanks for the suggestion.



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