Let’s start with the basics. When your wine is properly fermented, there is no residual sugar. In other words, all of the sugar, whether it’s from the fruit or the stuff you added, it’s all gone. Every bit of it has been turned into alcohol, carbon dioxide, and some other stuff we won’t discuss.
So your juice is now wine. It has been sitting in a carboy for a couple of months and it is beautifully clear. Time to bottle. Only you want a wine that is sweet, not dry. I believe it is a common misconception that you somehow stop fermentation with some sugar left over. Nope.
Here’s the secret. We add more sugar. Yep, that’s it. You absolutely must also add something to stop the yeast from getting reactivated. The common chemical for that is Potassium Sorbate. A 6-gallon batch of wine needs only one tablespoon to inhibit the yeast from partying again once the sugar is added.
So how do you decide how much sugar to add? It’s basically trial and error. What you do is make a syrup of 2 cups of sugar to 1 cup of water, heat it until it’s clear, then let it cool a bit (this gives you 500 ml of solution).
Now draw off a 50 ml portion of your wine and start adding sugar solution by the drop. I do this rather crudely, but basically I’ll add 5 drops and try it, then another 5 and try it again. You end up with less and less liquid, so once you think you got it right, you might want to try again. When the taste is where you like it, calculate how much sugar solution you need. The formula for this is: (# drops) x (45) = the number of ml of solution for a 6 gallon batch of wine.
I rack the wine to a bottling bucket (a 7 gallon plastic bucket with a stopcock) and mix in the Sorbate and the calculated sugar. Stir the sugar in well, then bottle. Give the wine a couple of weeks to recover from bottle shock and give it a try.
Now that you know this, if you are drinking some wine and you think it would be better if it was a bit sweeter, add a little table sugar. That’s all the winery is doing. Cheers.