What is stuck fermentation?
The first year I made homemade wine, I made a batch of swill. There’s no other way to put it and I later found out that the culprit was a stuck fermentation that I never un-stuck! The wine was flat, sweet with no real body…not exactly the award winner for which I hoped. If you’re a home wine maker and you think your fermentation may be stuck, there are three things you should know.
- It’s probably your fault.
- Hope is not lost.
- There is probably an easy way to “un-stick” it!
Stuck fermentation takes place when yeast fails to convert all of the sugars present into alcohol. This leaves residual, undesirable sugar after fermentation. So what, you ask? Turns out there are some real problems with stuck fermentation. First, your wine will taste far too sweet if you bottle it without completing fermentation. Unless your objective is a port or dessert wine, you’re probably going to be dissatisfied with that outcome. More importantly, you will have a lower alcohol content in your wine, which can leave it flat and insipid. In addition, remaining sugars can make the wine susceptible to a host of other problems during aging. Stuck fermentations need to be remedied in order to craft the wine of your dreams.
But how do I know if it’s really stuck?
The good news is, you can remedy almost any situation and get things moving again so that your fermentation completes the task at hand. Before you do, you’re probably wondering, how do I know if the fermentation is stuck? There are some telltale indications and visual signs that fermentation has ended. When your wine no longer bubbles, when the temperature of the must drops or when the “cap” starts to fall, you can be pretty sure that things have slowed down or stopped. But the surefire way to know is to test with a hydrometer.
Your goal is to convert all of the available sugar to alcohol. You’ve been measuring specific gravity, watching it drop steadily throughout primary fermentation. When you reach 1.000 or less, your fermentation is complete. If you measure the specific gravity of the wine and are above the 1.000 target, then you have some residual sugar in your wine that hasn’t converted. In other words, your fermentation is stuck!
What causes stuck fermentation?
Here’s the deal, yeast works well in very specific conditions and will do its job well if happy. Unfortunately, these conditions don’t always exist and when the yeast doesn’t like the environment, it takes a break or quits working altogether.
There are probably 15 – 20 possible causes, but the most common culprits are:
- Temperature Too Hot or Cold
- Deficiency in the Yeast
- Excessive CO2 or Insufficient O2
- Insufficient Nutrients for Yeast
- Unsanitary Equipment
- Excessive SO2
Great, so how do I fix it?
First of all, you should do your best to prevent stuck fermentation by giving the yeast the environment it demands. Watch the temperature, use the right yeast for your wine type, keep your equipment cleaned and sanitized. Be very cautious about the addition of sulfites. Excessive SO2 inhibits fermentation. If you have too much SO2, it can be quite difficult to restart your fermentation. Even if you have done everything as planned, you can still encounter stuck fermentation and you should be able to restart it by taking a few key steps.
If you know what caused the problem, then you can target your solution. During fermentation, the ideal temperature is around 70° F. You should keep things in the 55° – 85° range. When I made wine my first year, it was particularly hot, about 100° most days. I didn’t create a system to lower the temperature of the garage or the temperature of the must and primary fermentation took about 4 days – much too fast. If you know that temperature is your issue (and you should because homemade winemakers should ALWAYS measure and document their steps), then you will take some steps to lower the temperature of the wine within the optimal range. This alone may restart your fermentation.
If your yeast itself is the problem, then you can re-innoculate with a strain that is more likely to work. Like most homemade winemakers, I’ve always used Lalvin EC-1118 and even used it to restart a stuck fermentation with success. It’s particularly good at fighting off other strains of yeast that may also be inhibiting the process. Add the yeast to a mixture of juice & water at room temperature. Let it stand for 5-10 minutes and mix back into the must.
Excessive CO2 or Insufficient O2
Yeast needs an oxygenated environment to properly do the job. If you have too much CO2 or not enough O2, then the fermentation will stall. This is an easy solve, simple give the must a thorough stir to let out the CO2 and expose the must to oxygen.
Yeast need food to survive. Yeast nutrients are rich in amino acids and nitrogen, critical for the function of yeast during fermentation. Adding a yeast nutrient will give the yeast something to work with and allow for the fermentation process to continue until all of the sugar is converted
If you have taken the right steps, then fermentation should restart. You may not see obvious visible signs, but you should see the brix drop to zero and fermentation complete. Not only will you learn some techniques for getting un-stuck, but you’ll learn a valuable lesson for your future years of winemaking.