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