By definition, a stuck fermentation is a fermentation that has stopped before all the available sugar in the beer has been converted to alcohol and CO2. If the bubbles in your airlock slow down before your beer has reached its final gravity, you may have a stuck fermentation.
Heat things up. Warming up the carboy is probably the most reliable way to restart a stalled fermentation. Some yeast strains are more temperature sensitive than others and may require some warmth to complete the job.
If too much sugar still appears to be the only problem, you can try adding 3 or 4 packs of Red Star Pasture Champagne Yeast to the batch. This won't be two much wine yeast since the pack you added before multiplied itself to about 150 times the original amount.
A stuck fermentation is one in which fermentation has ceased prematurely or the rate of fermentation is considered too low for practical purposes, leaving a higher residual sugar content than desired in the wines at the end of the fermentation (Bisson, 1999; Henschke, 1997).
The primary reason for fermentation to not start is the health of the yeast, or too little healthy yeast, and this is usually the cause. Perhaps the packet or vial of yeast was old and there was little healthy yeast left to do the job.
If the airlock is not bubbling, it may be due to a poor seal between the lid and the bucket or leaks around the grommet. Fermentation may be taking place but the CO2 is not coming out through the airlock. This can also be caused by adding too much water to the airlock.
Within 24-36 hours, carbon dioxide normally starts bubbling through the airlock, as long as everything is working correctly and if the fermenter is sealed properly. Fermentation can take as little as 3 days if you are using a fast-acting yeast and the temperature is ideal.
How to Tell if You Have a Stuck Fermentation. The easiest way to tell if a wine is stuck is to first taste the wine. If the wine tastes even a little bit sweet you know that there's sugar left in your wine. As this is what the yeast convert into alcohol, fermentation should not end until all the sugar is gone.
When is the Fermentation Over? The fermentation is considered done when you either reach your desired sugar level or go "dry" at 0° Brix. A wine with 0.2% residual sugar contains two grams of sugar in a liter of wine.
Generally speaking, wine can't ferment for too long. The worse that can happen is a “miscommunication” between the sugar and the yeast due to either using the wrong type of yeast or fermenting under the wrong temperature. Even if this happens, you can still salvage most if not all wines.
Another reason both glucose and sucrose seem to be favored by yeast in fermentation could be related to oxygen. Yeast can use oxygen to release the energy from sugar in a process called “respiration.” Thus, the more sugar there is, the more active the yeast will be and the faster its growth.
In general, you do not want to add sugar during fermentation. You will want to add all the sugar to the wine before the fermentation – all at once, upfront.
However, overloading the must with sugar can overwhelm the yeast and make it difficult for fermentation to begin. With small batches (1-gallon recipes), the amount of sugar is small enough that it won't bother the yeast. In these cases, you can add the sugar all at once at the beginning of primary fermentation.
Simply move the fermenter to an area that is room temperature, or 68-70 °F. In most cases, too low a temperature is the cause of a stuck fermentation, and bringing the temp up is enough to get it going again.
If there are still bubbles in the airlock after 14 days let it sit for another few days, or at least until there is no bubbling for at least a minute or two. Once there is no activity in the airlock, fermentation is complete.
Beer, we always recommend that you bottle your beer no later than 24 days in the fermenter. You can go longer but the longer your beer sits the more chance you have to get an infection and get off-flavors in your beer. The 24-day mark has always worked well for us.
It is very possible for a fermentation to stop and start back up again sometime later. This is known as a stuck fermentation. It is usually caused by some environmental change that the wine yeast does not like – temperature being the most common factor.
In general, it's important to protect your wine from exposure to ultraviolet light. It's particularly important when fermenting your wine in a clear glass carboy, because the light can harm the yeasts and interfere with your fermentation.
Realize that any fermentation has smells. There are odors of alcohol, fruit, CO2 gas (carbonation) and even sulfur (lit matches). Don't get these normal smells confused with the bad odors discussed above. These are all aromas of a healthy, vigorous fermentation.
By far, the #1 reason for a wine fermentation to not start bubbling is because of temperature. Wine yeast is very sensitive to temperature… some strains more than others. My recommendation is to keep your fermentation between 70° F.
The Wine Is Still Fermenting
The yeast are still munching compounds in the wine and can create CO2 (Carbon Dioxide) which can stop the wine from clearing. Use a hydrometer and check the specific gravity. If it is over 1.000, it is likely still fermenting.
You can stir the wine if the lees have built up before the malolactic fermentation process. White wine can be racked after the primary fermentation is completed and the sediment has settled (about two days after the fermentation.)
Burping is essentially just opening your bottles ever so slightly during the second fermentation process to release air or “excess pressure” in the bottle. Many brewers recommend burping bottles every day, or every other day while your bottles are fermenting at room temperature.
The airlock is that funny little bit of plastic that affixes to the top of your brew bucket or carboy and bubbles away during fermentation. Except sometimes it doesn't bubble when you think it should, and sometimes it bubbles when you don't expect it to.
The yeast need this oxygen to grow and to produce important cell wall constituents. It is important to introduce enough oxygen into wort at the beginning of fermentation. Shaking the fermenter will, at best, add about half the recommended level of 10 parts per million oxygen into solution.