Ask the Experts: Addressing a Stuck Fermentation

Homebrew expert Brad Smith, author of the Beersmith homebrewing software and the voice behind the Beersmith podcast, offers some practical advice for when your fermentation becomes stuck when brewing with a low-attenuating yeast.

Brad Smith Sep 27, 2017 - 4 min read

Ask the Experts: Addressing a Stuck Fermentation Primary Image

A Craft Beer & Brewing Magazine® reader recently asked us the following question:

I brewed an English pale ale and purposely added a high percentage of crystal malt and also mashed at a high temperature (158°F/70°C). Now the fermentation seems to be stuck at 1.025. Is this normal?

While a finishing gravity of 1.025 is quite high, the combination of a high mash temperature and high percentage of crystal malt can lead to very low yeast attenuation, particularly if you use a low-attenuating English yeast such as White Labs WLP002 English Ale. You could consider pitching more yeast or, if that fails, blending the beer with a lighter bodied beer. Short of blending, there is not much you can do to correct the beer at this point. It will probably be quite malty when it finishes—perhaps more like an English cask ale.

It is worthwhile to take a look at the contributing factors in the recipe that led to the very high finishing gravity. First, you probably do not need a large percentage of crystal malts in an English ale. Crystal malts add body because they have a high percentage of non-fermentable complex sugars. However, adjuncts of any kind in your grain bill are really used to accent certain flavors and should make up a small portion of the grain bill. Most adjuncts have low fermentability, and if you use an excessive percentage of adjuncts, it will throw off the malt-hops balance of your beer. The vast majority of your grain bill should be pale malt or another base malt, and adjuncts in most cases should be 10 percent or less. I’ve had discussions with several yeast manufacturers who tell me that the percent of adjuncts used has the largest impact on overall yeast attenuation, so this factor alone is probably the driving factor in your high finishing gravity.

Next, consider the mash temperature you used. Yeast attenuation goes down as mash temperature goes up, so if you mash at the low end of the range (around 148°F/64°C), you will get higher attenuation than mashing at the high temperature range (156°F/69°C). This is due to the active temperature range for the major mash enzymes, which produce more fermentable sugar chains at the lower temperature. So mashing at a lower temperature would have reduced your final gravity by at least a few points.

The final significant contributor is the yeast strain you used. While you did not mention which strain you used, you can look at the yeast data sheets for a variety of English ale yeasts and see that there is quite a range of possible attenuations. For example White Labs WLP002 English Ale yeast has an attenuation range of 63–70 percent. White Labs WLP005 British Ale yeast, a similar strain, has a much higher attenuation of 67–74 percent. So if you are making a high-gravity, full-bodied English pale ale, it might be better to use a higher attenuating yeast than you might use for a low-gravity bitter.