Stuck Fermentations: I

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Measuring a beer’s specific gravity can help you determine if a fermentation is stuck or finished.

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.

Comments

  1. Chris Storey says:

    Thanks for the great info Chris, keep it coming. I have listened to you on Basic Brewing for years.

    Thanks and cheers,

    Chris

  2. You avoid all of the “is it really stuck” quetstions by performing a forced ferment test. Example: I made a beer the other day with a significant amount of crystal, a tempermental british yeast that is a strong flocculator. I need to know when this beer is done because the yeast might floc out. The FFT will tell me. I know that at 1.014 is the limit of attenuation with my wort. I reserve about 250 ml of wort, dramatically over pitch probably at factor of 10-20 times the amount of yeast needed, ferment at about 80F, swirl and rouse it at least twice and then give it 48 hours. It’s done. Now I know two days in what my ideal ending gravity is. By doing this, brewers would quickly understand if the limitations in a beer’s attenuation are driven by their fermentation or by their mash.

    • Chris Colby says:

      Yes, a forced fermentation test would tell you what FG your beer should end up with. The Catch-22 there is that most stuck fermentations result from not making a yeast starter; and if you haven’t spent the time to make a yeast starter, you probably aren’t conducting a forced fermentation test, either.

      Forced fermentation tests are easy enough to do with every beer, but they are extra helpful when you try to make a beer with either a high or low degree of fermentability. As you mention, you can get your answer quickly.

      Good comment.

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