An old wine industry adage says "....the best way to restart a stuck fermentation is to bottle the wine." Most of us have had the sad experience of bottling wine and then a few weeks later discovering fermentation had started in the bottles. Even small amounts of sugar can cause problems, so any wine containing more then 0.2% residual sugar cannot be considered stable. Reworking is seldom feasible, so fermentation in newly bottled wines is usually a complete fiasco. Several ways of preventing refermentation in sweet wines are listed below.
(A) Ferment the wine until it is completely dry. Just before bottling time, when the wine is clear and stable, add 250 milligrams per liter of potassium sorbate. Raise the molecular sulfur dioxide level to 0.8 milligrams per liter and add the desired amount of sugar or sweet reserve. Potassium sorbate stops yeast cells from multiplying. It does not stop fermentation, so sorbate is only effective when added to clear wines (containing only a few yeast cells).
(B)Increase the alcohol content until yeast cannot survive. The traditional method is to add high proof brandy until the alcohol content is about 18% or higher. A second method relies on renewing fermentation by adding small quantities of sugar each time the Brix drops to zero. Here, sugar is added slowly and the yeast has time to acclimate to higher and higher alcohol levels. Even so, yeast will succumb when the alcohol reaches about 16 to 18 percent. This method is fine for dessert wines but not for table wines.
(C)Raise the wine temperature to 160 degrees at bottling time. This method was called "hot bottling," and it was commonly used before sterile filtration was perfected. A large microwave oven is convenient, but a large water bath on top of the stove will work. This is an effective way of stabilizing dessert style wines, but unfortunately, the quality of table wines often suffers from the heat required for pasteurization.
(D)Stop fermentation by chilling the wine. Keep the wine cold and allow the yeast to settle. Remove most of the yeast by racking or filtration. Warm the wine to room temperature, and restart fermentation. Repeat this process several times until fermentation cannot be restarted. Each new generation of yeast consumes nutrients from the wine, and after several generations, the nutrients are so depleted the yeast cannot reproduce. This method effective stabilizes sweet wines, and it maintains wine quality. Asti Spumante wine is stabilized this way, but this method requires time and work.
(E)At bottling time, add sugar or sweet reserve and put the wine through a sterile filter. The bottles, corks and all of the equipment contacting wine after the filter must also be sterile. Getting wine sterile is easy with membrane filters, but getting and keeping equipment, bottles and corks sterile can be difficult.
(F)Do not bother with wine stabilization. Ferment and bottle the wine dry. When a bottle is opened, add sugar syrup and sweeten the wine just before serving. The dry wine can be safely stored with no danger of refermentation, and wine sweetness can be easily adjusted by changing how much syrup is added. This is the easiest method.