Troubleshooting Your Wine
The faults and flaws and their analysis and resolution presented below are from my latest book, Modern Home Winemaking, where you can find additional information should you wish to explore these further.
STUCK ALCOHOLIC FERMENTATION (AF)
DESCRIPTION
When yeast prematurely and unexpectedly stops converting sugars into alcohol and the SG/Brix does not change for more than 24 hours, the condition is termed a stuck fermentation.
During what should be an active fermentation, if sugar is above SG 1.005 (1.0 Brix) and you measure a drop of less than 5 SG points (1 Brix), or if approaching dryness and sugar is below SG 1.005 (1.0 Brix) and you measure a drop of less than 2 SG point (0.5 Brix), then you likely have what is called a sluggish fermentation and that a stuck fermentation is imminent, assuming temperature and all other fermentation parameters are favorable. This is where a fermentation progress graph comes in handy.
Here we will treat both conditions as a stuck fermentation for the purpose of implementing a restart protocol. A stuck fermentation can open the door to undesirable microorganisms that can cause undesirable organoleptic deviations or outright spoilage if fermentation is not restarted swiftly.
CAUSES
There can be several reasons, usually very difficult to pinpoint, for a stuck fermentation, and your ability to relaunch the fermentation depends on wine chemistry and conditions. The closer you are to dryness, the more difficult it will be to restart fermentation. You may be unable to restart fermentation when, for example, the SG is 1.000 (0 Brix), and if you decide to bottle it as such without proper stabilization, you may soon find out that yeast can kick in again under seemingly harsh conditions and finish fermenting whatever residual sugar there is, even under high alcohol and nominal SO2.
ASSESSMENT
The first step in trying to restart a stuck fermentation is to consult your winemaking records to make sure you have followed the protocol to the letter, and then to identify the most likely cause and take swift remedial action. First ensure that:
- Your starting SG was less than 1.110 (26 Brix) and that you had chosen a suitable yeast strain that could ferment to dryness within its alcohol tolerance;
- You had properly rehydrated the recommended amount of (dry) yeast with rehydration nutrients, or used the right amount of liquid yeast culture;
- You had supplied a good amount of oxygen at inoculation;
- You had supplied sufficient complex yeast nutrients after inoculation;
- You are fermenting within the yeast’s temperature range and not fermenting whites or rosés excessively cold or reds excessively hot;
- You had added complex yeast nutrients at one-third sugar drop, usually after a SG drop of 30 points (7–8 Brix drop); and
- You had not added excessive amounts of sulfite at crush or in preparing the must. Recheck your calculation.
If a stuck fermentation occurs early on or during the stationary phase, you likely did not observe one of the above instructions.
REMEDIAL ACTIONS
If you have followed the protocol to the letter, first try increasing the temperature, stir the wine, and look for signs of renewed activity within 24 hours. If fermentation does not restart within 24 hours, the wine likely has a significant concentration of fermentation inhibitors and toxic substances (e.g., fatty acids); you need to implement a restart protocol. The priority is to restart fermentation as quickly as possible to reduce the possibility of spoilage microorganisms taking hold, and to ensure that once restarted, it will go to completion. This may involve foregoing the use of your initial choice of yeast and going with a much stronger strain, such as Lalvin K1V-1116 or Uvaferm 43 RESTART, or Premier Cuvée (Red Star), specifically known for restarting stuck fermentations.
Go to the RESTART PROTOCOL.
PREVENTIVE ACTIONS
Follow the recommendations in the ASSESSMENT section above.