In general, you do not want to add sugar during fermentation. You will want to add all the sugar to the wine before the fermentation – all at once, upfront. There is no real advantage to spreading the sugar throughout the primary fermentation, just as long as you are shooting for a reasonable level of alcohol (10% to 14%). Any wine yeast you choose to use will be able to readily ferment to this level of alcohol, even when all the sugar is added to the wine must before the fermentation.
The biggest reason you’ll want to add all the table sugar all at once, besides the fact it’s less work, is that it makes it easier to calculate your wine’s finished alcohol.
Sugar is what turns into alcohol during the fermentation. This is fermentation 101. To know how much alcohol the fermentation is making, you have to know how much sugar has been fermented. This requires you to know how much sugar the fermentation started with and how much sugar the fermentation ended with. The difference is what was fermented into alcohol. Both of these things can be easily determined with a hydrometer by taking a reading before and after the fermentation and comparing the two.
If you add sugar to the wine during the fermentation, additional hydrometer readings will need to be logged to eventually know how much alcohol is in the wine. These additional calculations can be annoying and even hard to remember to do. It requires you to pull out the hydrometer each time you want to add more sugar and take a specific gravity reading both before and after the addition of the additional sugar.
The only possible time you would want to add sugar fermentation is if you intend to make a high-alcohol wine. In this case you would want to start out the fermentation with enough sugar to reach 13% or 14% alcohol. Then as the fermentation runs out of sugar – which is determined with hydrometer readings – you will want to start feeding sugar to the fermentation in intervals.
The goal is to end up with a wine that is high in alcohol but not too sweet to drink. The fermentation will come to a point where the wine yeast can do no more. Exactly when that will be is not a certainty. It varies from one fermentation to the next, depending on a number of variables. When this happens you want little to no remaining sugar in the wine. This is the reason why you would feed the sugar to the fermentation as it progresses beyond 14%.
So in the end I guess the answer to the question: “can I add sugar during fermentation?”, is yes you can. With the only side note being “but it only makes sense if you are making a high alcohol wine”. For any normal wine making situation, it is only creating more work to do so.
Ed Kraus is a 3rd generation home brewer/winemaker and has been an owner of E. C. Kraus since 1999. He has been helping individuals make better wine and beer for over 25 years.