The necessity to stop fermentation before all sugar has not processed into alcohol or wine has not yet reached its maximum potency (natural causes of stopping) is often caused by a desire to speed up the preparation process or to keep the current characteristics of the beverage (sweetness and strength). In home winemaking three methods of fermentation stopping suitable for all types of wine (grape, apple, cherry, etc.) are used:
1. Securing with alcohol. This is the most simple and effective method which facilitates prolonged shelf life of wine. Wine yeasts stop working when the potency is higher than 14-16% (some artificially bred strains are active at a concentration of alcohol up to 18%, but these are rare cases).
In order to stop the fermentation you have to remove the wine from the sediment and secure it with alcohol (preferably with grape distillate) or with vodka until concentration reaches 16%. If the initial sugar content of the material is unknown, you won’t be able to calculate the unstrained potency acquired through fermentation. In this case 10-15% of alcohol is added to the wine.
Flaws: the wine becomes very strong, its taste changes, vodka might add an unpleasant smell.
2. Stopping the fermentation by cooling. That is the only method which has no influence on aroma, taste, sweetness, and potency of the wine. At a temperature of 50F° / 10°C yeast fungi go into a state of anabiosis (hibernation) and precipitate out at the bottom.
In order to stop the fermentation you should leave the wine in a cold room with -a temperature of 36-50F° / 2-10°C (necessarily above freezing point) for 3-5 days until the full stop of the fermentation, precipitation, and at least a partly must clarification. Then you should remove the wine from the sediment at a temperature not more than 61F° / 16°C.
Flaws: there’s no guarantee that all of the yeasts will get filtered with the sediment. With temperature rising the fermentation might start again. To stop this from happening you should add sulfur trioxide (0.1-0.14 oz / 3-4 grams per 2.6 gal / 10 liters) or sorbic acid (according to the instruction). But adding preservatives is not the best solution for home winemaking.
3. Thermal processing. Wine yeasts die at temperatures exceeding 104F° / 40°C. To stop the wine fermentation it is sufficient to heat the wine above the yeasts surviving point.
In case of classic thermal processing the wine removed from the sediment is heated to 131-158F° / 55-70°C (pasteurized) in order to eliminate not only yeasts but also other hazardous organisms (mold fungi, viruses, etc.) which can survive in more extreme conditions. The stronger the beverage, the lower the temperature is. Then the wine is pasteurized for 10-20 minutes, cooled down to 50-61F° / 10-16°C air tight (it’s hard to achieve at home), bottle it for storing and hermetically seal it.
Flaws: the taste and aroma of wine is getting worse during thermal processing. In order to prevent subsequent contamination you have to secure the wine against contacting the outside environment. If you don’t do it, the pasteurization becomes less effective.