Homebrewing has changed a lot since I first started, but some things never change. One problem that homebrewers seem to constantly face is stuck fermentations. A stuck fermentation is when the fermentation stops prematurely, leaving the beer at a higher-than-desired final gravity (FG). This can happen to any beer, but occurs more often in big beers.
Avoiding This Situation
Before addressing how to remedy a stuck fermentation, know that the best way to fix a stuck fermentation is to prevent it from happening in the first place. If you pitch enough yeast, aerate your wort well and keep the fermentation temperature in right range, you will likely never encounter a stuck fermentation. For homebrewers, a big part of this is making a yeast starter, when needed, to achieve the right pitching rate; since I began making yeast starters, I have never encountered a single stuck fermentation.
Is It Really Stuck?
But let’s say that you suspect you have a stuck fermentation. What do you do? First off, don’t panic. Instead, determine if your fermentation is really stuck . . . and don’t even think about pouring the batch down the drain until you are sure there is a problem, and if it is fixable.
Many homebrewers, myself included, use the bubbling rate in the airlock as an indication of active fermentation taking place. But sometimes your fermentation can be fine, but no bubbles appear in the airlock. Check to see if your stopper is fit snugly into your carboy and the lid of your bucket is sealed before assuming something is wrong with your fermentation. Also, consider how long your fermentation has been going on; they have to stop sometime.
The next thing to check is whether the fermentation has reached a reasonable final gravity (FG). Brewers who have never made a barleywine or other big beer may have never seen a beer finish up around 1.030. However, depending on the original gravity, a high FG like this may be reasonable. A quick way to check is to go to the yeast manufacturer’s website and check the yeast strain’s attenuation range. Take the lower percentage of the range and convert it to a proportion. Multiply this by your starting gravity (either in °Plato or as the first three digits following the decimal point in specific gravity). Subtract this number from your original gravity. If your beer is below this, the fermentation is not stuck, it’s done.
For example, let’s say you made a barley wine with an OG of 1.092. Your yeast strain’s attenuation range is 72–80% and your beer has stopped fermenting at SG 1.028. Is the fermentation stuck, or finished? Multiply 0.72 times 92 to get 66 (rounding off the numbers trailing the decimal point). Subtract 66 from 92 to get 32, which corresponds to 1.032. Since your beer stopped fermenting at 1.028, it is within the yeast’s attenuation range. In other words, your fermentation is not stuck.
One final thing to check is if your hydrometer is calibrated. Incorrect high readings may lead you to believe your fermentation is stuck, when in reality everything is OK.
Tomorrow, I’ll discuss what to do if your fermentation really is stuck.