How to resurrect yeast and keep fermentation alive
Most brewers have had the unhappy experience where fermentation is proceeding along nicely, then quite suddenly and unexpectedly stops shy of the target gravity. In a panic, the fermenter lid is cracked to look inside, and one half expects to see a nasty scum growing on the surface. But no, everything looks happy and sanitary. If fermentation is not complete, the beer can't be bottled for risk of beer bombs going off in the cupboard, and repitching with new yeast is a pain, especially if the ingredient store is not nearby.
So what causes yeast to stick? There a number of reasons (other than infections or bad yeast) why this might happen. A couple of these include overactive initial fermentation or poorly prepared yeast, or a sudden temperature change causing the yeast to go frigid. The above may explain why fermentation has stopped, but neither imply that the batch of beer is necessarily ruined. However, fermentation does need to be completed before the beer can be kegged/bottled.
One thing that can be attempted that often works in getting yeast back on the job, and is easy to do, is to simply add warm (not hot) water into the jacket of the 3-in-1. This is accomplished in two simple steps:
As the ETC senses the temperature is lower than target, it sends a signal opening the TCV. Warm water enters the jacket of the 3-in-1 at the bottom, flows up through the jacket transferring heat into the vessel, warming the wort/beer. It continues to flow until the temperature hits target at which point the TCV is closed. As the water warms the wort/beer at the bottom, the warmed wort rises up (hot water rises) carrying yeast that may have settled to the bottom, back up into the body of the fermenter. As the yeast is warmed, its metabolism is reactivated/sped up and it is introduced to sugars.
This may be all it takes to get fermentation going again and complete attenuation.
This method can also be very useful with highly flocculant yeast strains that want to drop out of suspension. Periodic warming and the ensuing currents created inside the fermenter may be all it takes to keep fermentation happily bubbling along.
This method was used with great success during the brewing of the Belgian Saison with the notoriously difficult (but amazing) Wyeast 3724. (See video here.)