Now that we have discussed getting the wine through various filtrations in general let’s consider how to approach wine that you wish to sweeten.
The usual way to figure sweetening is by percentages, such as 1%, 2%, 3.5% residual sugar, or any variation thereof. In order to find the best balance and sweetness level of the finished wine, you need to do some simple lab trials using several 100 ml samples with different levels of sugar. While I am sure there is a much more complex mathematical formula to do this, I simply start with 100 ml of wine and add the different amounts of sugar to them. An example would be one gram of sugar to the first one, 2 grams to the next one, 2.5 grams to the third one, and so on. This has worked well for me and the important thing line is that you have several samples of increasing sugar from which to choose. So simply make up 3 or 4 samples of sweetened wine, making sure the sugar is well dissolved, and taste each of them, preferably with other people whose opinion you value, and make a decision as to what you like. Once decided, the calculation for addition to your whole batch of wine is also fairly simple. If there will be several tasters involved, you can double your sample.
Let’s say you have decided that 2% residual sugar is the way to go. Your calculation for the full batch is: Gallons X 8.2 (the approximate weight in pounds of a gallon of wine) X 2% equals pounds of sugar needed. An example would be: 550 gallons of wine X 8.2 X 2% equals 90.2 pounds of sugar.
When you add the sugar to the wine, once again, it must be well dissolved. Never just dump the sugar in and mix it. This rarely does the job of dissolving it properly. Depending on the initial volume of wine, several gallons need to be drawn off, or pumped into another container and the sugar mixed in until it is no longer granular. By the way, simple cane or beet sugar can be used, such as what one buys in the grocery store. If larger volumes of wine are involved, such as the 550 gallons mentioned above, a mixing container that holds at least 75 to 100 gallons is necessary. The sugar can also be added in increments, drawing wine into the container, mixing in half the sugar, pumping it back into the original, and then repeating the process. For larger volumes like this, it is nice to have a paddle of some sort that can be sterilized. Some people I know have used a new oar (yes, as in row, row, row your boat) for this process, but there are also large plastic paddles available. While I am talking about sterilizing, don’t forget to clean and sterilize the container into which you are putting the wine.
As you can well imagine, there is always the possibility that the sugar will re-ferment. Potassium sorbate, which is a natural yeast inhibitor, can be added to the wine at the same time as the sugar. I believe strongly in adding potassium sorbate to sweet wines as an insurance policy against re-fermentation. There are those who will argue that it is not necessary if you are doing a sterile bottling. To my mind, however, there is no such thing as a sterile bottling unless the bottling line is actually in what they call a “clean” room, where people are gowned and nobody goes in or out. This rarely occurs. Yeast is rampant in the air of all wineries, at all times. You only need one yeast cell to float into the wine to start a disaster. If the proper amount of potassium sorbate is added, it should not alter the taste of the wine. The calculation for sorbate addition is: Gallons X 3800 X .00025. The original calculation I worked with years ago used a slightly higher factor of .0003 instead of the .00025, but after doing some work with Dr. Jim Gallander, now retired, at the Ohio State Grape and Wine Research Facility, we determined that the amount of sorbate calculated with the .00025 factor was sufficient as long as the sulfite level was in the correct range for the wine.
As far as where sweetening fits into the filtering process, my recommendation is to put the wine through the KDS15 filter pads first, then add the sugar, sorbate, and any sulfite needed. Then filter the sweetened wine through the Steril S80 pads. It is not wise to let sweetened wine sit in the tank for long periods of time, as there is always the concern of re-fermentation if enough ambient yeast gets into it (even if you added potassium sorbate). So always do the last steps of sweetening and filtering just as close to bottling as possible.