Different yeast strains have different alcohol tolerances, meaning that at some point there is too much alcohol in their environment that the cells start to die due to alcohol toxicity. Once this point is reached it is unlikely that fermentation will start up again (unless a more alcohol tolerant yeast or bacteria is introduced) however, it is possible for some fermentation to continue unless you stabilize your wine. If you want to assure that no corks start popping out of bottles down the road, or you would like to “back sweeten” your wine, stabilizing is an important step to finishing your wine.
What is happening?
Stabilizing consists of two steps; stopping the current fermentation, then preventing further fermentation. This is done with wine additives, starting with a sulfite, usually either sodium or potassium metabisulfite (Campden). This produces sulfur dioxide gas that will kill yeast and bacteria (although a small amount of yeast will go dormant to survive the hostile environment). Use Potassium Sorbate in conjunction with Campden. Sorbate will work better with sulfites present, and the combination works better than sulfites alone. Sorbate will stop the reproductive cycle of yeast, so any cells that may have survived both the alcohol and the sulfur dioxide have no real chance of fermenting anything.
How to do it:
Crush 1 Campden tablet along with ½ teaspoon of Sorbate, per gallon of wine, and stir up into roughly a cup of warm water or your wine. Make sure any clumps are dissolved and then add cup to your batch of wine. Stir in well. You can now sweeten to taste and bottle, or wait a week or two to allow any of the dead and dying yeast to fall out of suspension. This will make for a clearer wine in your bottle.
Sweetening your wine to taste, or “back sweetening / back blending” is the process of adding juice or sugar back into your wine to give it more pronounced flavor, body, sweetness, or all three. Since many wine yeasts will eat enough sugar to “dry” out your beverage, you may want to add back to it. You can only do this if you stabilize first. Table sugar is common, but it is recommended to use whatever you fermented (strawberry juice for strawberry wine, grape juice for grape wine, apple juice for apple wine or cider, honey for mead, etc.) This will be to taste, so it is hard to give an amount. It is whatever YOU think tastes good. Just add a little bit to your stabilized batch of wine, taste, and repeat till you get it right. White grape juice or apple juice can be used as an all purpose sweetener and is recommended in most cases over table sugar.
Carbonating stabilized wine:
Because active, live yeast is necessary to bottle condition (carbonate naturally), once you have stabilized your wine, cider, or mead you can no longer carbonate unless you keg and force carbonate with CO2.
Next, how dry/sweet do you want it?
The commonly accepted final gravities for each level of dry/sweetness are:
Dry: 0.990 – 1.006
Medium: 1.006 – 1.015
Sweet: 1.012 – 1.020