Anyone who works with brewer’s yeast knows that they need to provide it with a comfortable working environment in order to maintain its function. When yeast cells are unhappy, such as when they are unhealthy or lacking nutrients or help from other cells, they stop doing their job. If you have experienced this, you know how frustrating it can be to identify the cause and find a solution.
What is a Stuck Fermentation?
A stuck fermentation occurs when your yeast simply stops fermenting before reaching the desired final gravity, resulting in a stalled or stuck fermentation. You can identify a stalled fermentation within the first 24 hours if you notice that the pH levels are not decreasing rapidly. After this initial period, you should only worry if the gravity reading has remained unchanged for 48 to 72 hours. At this point, you will need to restart the fermentation in order to achieve the desired beer.
To do so, you first need to determine the cause of the problem.
What Causes Stuck Fermentation?
As mentioned earlier, there are several factors that can trigger a stuck fermentation. These include:
- Dead or unhealthy yeast cells
- Insufficient amount of yeast pitched
- Excessive yeast pitched leading to excessive krausen and loss of healthy yeast
- Inadequate nutrients in the wort to support yeast activity
- Rapid flocculation of yeast (clumping together and settling)
- Low temperatures that slow down yeast activity and eventually put it into dormant state
- Excessively high temperatures that kill the yeast (also known as “yeast autolysis”)
ACE Micro Brewery Fermentation Tanks
How to Prevent a Stuck Fermentation
Proper yeast management is crucial in preventing a stuck fermentation. Assuming you have obtained or propagated yeast of good quality, it is important to keep it viable and healthy. Store the yeast slurry in a sanitized airtight container at temperatures preferably below 42 degrees but above freezing, and handle it with proper hygiene to avoid contamination.
Furthermore, use a hemocytometer to count the number of cells in the slurry.
Pitch the yeast and ferment at the correct temperature, avoiding sudden fluctuations. While you can usually obtain specific temperature recommendations from the vendor or manufacturer for your particular yeast strain, ale yeast generally thrives in wort below 80 degrees, preferably around 68 degrees, while most lager yeast prefers temperatures between 45 to 55 degrees.
How to Restart a Stuck Fermentation
First, check the temperature. Then, check it again. Is it too hot or too cold? Adjust accordingly.
If the fermentation temperature is properly controlled, you may have heard that aerating the wort at this point can stimulate yeast respiration and reproduction. However, this is mostly an urban myth and a risky one at that. Aeration will not actually promote yeast reproduction, and the potential risks of introducing oxygen into the fermentation outweigh the benefits. To avoid off-flavors that can result from attempting to revive the yeast in this manner, it is much better to follow the step below.
If you determine that the yeast itself is the cause of the stalled fermentation, either due to low vitality, flocculation tendencies, or low density/pitch rate, you can restart the fermentation by adding vigorously fermenting wort to compensate for the inactive yeast and jump-start the fermentation. Typically, a successful addition of vigorously fermenting wort should account for 10% to 20% of the volume of the wort in the tank.
