In some cases you may find it desirable to stop the fermentation process before it comes to a stop on its own. The most common reason for wishing to stop the fermentation process is that you have found the wine already has the exact amount of sweetness that you prefer and you do not want it to progress any further.
By stopping the fermentation at that point, many winemakers believe that they can preserve the amount of sweetness that the wine has already produced. If you want a really sweet wine, such as a dessert wine, this is certainly understandable. The idea behind stopping the fermentation process is that if you allowed the wine to continue fermenting it would become less sweet as time went on. When the wine became completely dry, the fermentation process would eventually stop on its own without any intervention from you.
As a result, there are several different methods that home winemakers tend to use when attempting to stop the fermentation process in order to preserve the sweetness. None of these methods work very well; however. Let us examine each.
One of those methods is using either Campden Tablets or Sodium Bisulfite. It should be noted that fermentation will not completely stop using these methods. You should also be aware that the chance does exist for some live yeast to be left in the wine, providing the opportunity for the fermentation process to begin again. In fact, it is not unknown for the process to begin again even after you have bottled your wine and stored it. Obviously, that would not be a good situation and would result in some really poor wine.
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Another common option used by some winemakers is Potassium Sorbate. Generally, Potassium Sorbate is used for the purpose of sweetening wine. When it is used for the purpose it is commonly after the fermentation process has already been completed and you are ready to bottle your wine. The Potassium Sorbate is then added with sugar. The purpose of the Potassium Sorbate in this instance is to prevent the yeast from fermenting sugar that has just been added. When added prior to the end of the fermentation cycle; however, Potassium Sorbate will not kill the yeast; it only makes it sterile. This means that it stops producing but it doesn’t stop the fermentation. In other words, it does not prevent the yeast from fermenting the sugar and turning it into alcohol.
If your goal is to preserve the amount of sweetness that is already in the wine, the best way to do this is to actually go ahead and let the fermentation continue on its own until it is completely finished. After the yeast has had an opportunity to settle over a couple of weeks, you will then be able to siphon the wine off and then add some Potassium Sorbate with some sugar.
Keep in mind that it is really imperative to allow the fermentation process to finish before you add anything like Potassium Sorbate or more sugar. If you are not sure whether the fermentation process has finished, you can check it using a hydrometer. Remember that this is the tool that you use to check the alcohol content of the wine. If the process has completed, there should be a reading of no more than 1.000 on the hydrometer.