Stuck fermentation

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Stuck or stalled fermentation is a commonly reported problem among new brewers. However, actual stuck fermentations are relatively uncommon and usually it's a simple misunderstanding.

This article will help find the source of the problem, and offer ways to correct it if needed.

Definitions

Stuck fermentation

A fermentation that has unintentionally stopped before the yeast have consumed all the fermentable sugar.

Specific gravity

This is a measure of density. In our case we use density to monitor fermentation progress since dissolved sugar increases fluid density whereas alcohol decreases density. Also referred to as sample "gravity" or s.g.

Original gravity

OG is the specific gravity at the beginning when the yeast is pitched.

Final gravity

FG is the specific gravity at the end of fermention.

Attenuation

This is the degree to which the yeast have consumed the sugar. For simplicity's sake, brewers generally refer to "apparent attenuation" (AA), which assumes 1.000 would be the final gravity if all the sugar were consumed.

Troubleshooting

1. Did you use a regular glass hydrometer?

   If not, you need to get one and use it. See hydrometer for more info.

   • Refractometers do not accurately measure the density of solutions with alcohol.
   • Tilt™ hydrometers (and other electronic hydrometers) can be wildly inaccurate as well. You need to verify the reading with a reliable glass hydrometer.
2. Have you measured the gravity over a period of several days?

   This is necessary to confirm that fermentation is actually stopped. Bubble activity in the airlock is not a valid way to monitor fermentation. If brewing beer, consider using a fast fermentation test to determine when the fermentation is complete.

3. What was the original gravity and what is the current gravity?
   • Calculate the apparent attenuation and compare that to what is expected for your particular strain of yeast. The listed attenuation is just an approximate value. If yours is reasonably close, the fermentation is probably just completed. Note that attenuation only applies to beer (not wine, cider, mead, etc).
   • Calculate the current ABV and compare it to the alcohol tolerance for your particular strain of yeast. Yeast will be unable to complete fermentation if the alcohol potential is above the listed alcohol tolerance. If the alcohol potential is too high for the yeast you pitched, you need to pitch a yeast with a higher alcohol tolerance appropriate for the fermentation. Do NOT pitch wine/champagne yeast into beer!
4. What was the yeast strain and fermentation temperature?
   • Certain yeast strains (notably English ale yeast like SafAle™ S-04) are prone to early flocculation. In other words, they can settle out of the beer/cider before finishing fermentation (those lazy bums).

     The solution to this is simple: Warm up the beer/fermenter and then give it a good swirl to re-suspend (AKA "rouse") the yeast, and then give it a few more days. Repeat if needed.

   • Too low or too high fermention temperature can cause it to stall. If it's too cold, warm it up to the preferred temperature range of your yeast and then rouse the yeast by swirling the fermenter. If it's too hot (generally above 85°F [29°C]), is may be necessary to cool it down and then pitch new yeast. Ideally you should you a method for controlling fermentation temperature to within the listed range for the particular yeast strain you are using.
5. Did you add lactose or maltodextrin?

   These sugars are not fermentable by normal brewers yeast, and therefore they will increase the final gravity and decrease the apparent attenuation.

6. Is this an all-grain beer?

   Let's make sure nothing went wrong during the mash...

   1. What was the mash temperature and duration?
      • Mash temperature and duration both affect attenuation. Generally speaking, higher mash temperature and/or shorter mash duration will result in a higher FG (less fermentable wort).
      • Have you calibrated your thermometer(s)? If not, you should. See thermometer for more info.
   2. How was the mash temperature controlled?

      If you apply heat during the mash, this could be the problem. Heating the mash may create areas of high heat which degrades the enzymes necessary for creating a fermentable wort, especially if the mash is relatively thick.

      Instead of applying heat, consider using a strategy to insulate the mash tun. A few degrees of heat loss during an hour long mash is nothing to worry about. More advanced systems exist for improved temperature control if desired (like RIMS or HERMS).

   3. Was mash pH predicted and checked?

      Mash pH can affect the action of the enzymes and ultimately the attenuation. Controlling the mash and boil pH can improve the consistency and quality of your beer. However most of the time an uncontrolled mash pH won't cause a huge difference in attenuation (i.e. "stuck" fermentation), though it might be a contributing factor.

   4. Was anything besides grain added to the mash?

      If an unusual ingredient pushed the pH out of the desirable range, it could have affected the enzymes and subsequently the attenuation.

7. Was this a partial boil beer?

   When topping up with water, the combination can be difficult to fully mix. Verify that your original gravity makes sense mathematically for the amount of malt extract used, or the amount of grain.

8. Is this a sour beer?
   • The lactic acid in sour beer increases the final gravity and decreases the apparent attenuation. This is normal.
   • If the mash was excessively acidic, e.g. by adding excessive acidulated malt, the inappropriate mash pH probably affected the mash enzymes. Use a good sour beer method next time.
   • If the beer was pre-soured (e.g. a kettle sour), the yeast may be struggling. Consider pitching more yeast. Use a good sour beer method next time.
9. Is this a hard seltzer or mead?
   • It needs nutrients. See hard seltzer or mead.
   • Keep in mind that the sugar or honey needs to be fully, completely, and totally mixed with the water to obtain an accurate original gravity. Honey and sugar are not easy to dissolve. Verify that the reading makes sense mathematically.

Unsticking the Fermentation

Here's a summary of options for unsticking a fermentation, including some more advanced techniques. Make sure to read the Troubleshooting section to verify that the fermentation is actually "stuck", or at least "stopped" at a higher FG that you would prefer.

These solutions are ranked (subjectively) from most reasonable to more extreme. They usually should be used in combination:

1. If the fermentation temperature is on the cool side, warm it up by at least 5-10°F, but not to more than 85°F.
2. Swirl, shake, or stir the fermenter to rouse the yeast.
3. If OG was too high, consider dilution (e.g. add deoxygenated water).
4. Select an appropriate yeast, make a starter with nutrients, and pitch it at high kräusen.
5. For beer with too much unfermentable sugar, consider adding an enzyme to break down the dextrins into sugars the yeast can ferment. Maybe you'll want to confirm that an overabundance of unfermentable sugar is in fact the problem by conducting a FFT first.

   Amylase is the less powerful enzyme option and glucoamylase (AKA amyloglucosidase) is the more powerful option.

6. Consider pitching a diastaticus variant Saccharomyces and/or Brettanomyces. Brettanomyces can generally consume starch and dextrins (which includes any maltodextrin you may have added), and some strains of Brettanomyces can also metabolize lactose.

See Also

• "Preventing (& Fixing) Stuck Fermentations" at BYO
• Scott Labs has products to help restart stuck wine fermentations.
• Yeast hulls