Potassium Metabisulfite is an anti-oxidant and is used to preserve both color and freshness, in addition to protecting the wine from unwanted spoilage microbes that can be airborn. It also comes in a Sodium form, although Sodium Metabisulfite is usually used more in the winery during the sterilization of equipment but it can be used in wine also. Most winemakers prefer, however, to not introduce any sodium into their wine and therefore use the Potassium Metabisulfite in the wine. Metabisulfite (of either form) is also commonly referred to as just sulfite (also spelled sulphite) or SO2 (which is sulfur dioxide and is the gas released when either form of Metabisulfite is combined with water). I will use the term sulfite from this part forward.
First, a word of caution, if you add a full dose of sulfite at the same time you add your enzymes, you will inactivate the enzymes. Therefore, I add the sulfite as the very last of the juice is being pumped into the tank. In other words, I add enzymes as the grapes are being crushed and pressed. Then, when the press is finished and I am pumping the last of the juice into the tank, I add the sulfite if I plan to use any.
There are a few schools of thought on sulfite addition at crush and you will hear many arguments. The wine industry has gone back and forth over adding or not adding for all the years I have been making wine. Personally, if I determine that my fruit is fresh and sound with no rot and I will be adding the yeast in a timely manner, I do not use sulfite. I almost never use it on my red wine fermentations. If I add sulfite at crush or press, it is generally anywhere from 20 to 50 ppm (parts per million). A word of caution…if you plan to have your wine go through malo-lactic fermentation later (which I will discuss in detail at another time), also known as secondary fermentation, it is very important to keep the sulfite level low, no more than 20 ppm, as malo-lactic bacteria is very sensitive to sulfite and will not work.
Amateur winemakers often use a product called campden tablets because they are pre-measured. One campden tablet per gallon is equal to 75 ppm metabisulfite. There are two problems with campden tablets, though. First, it is difficult to accurately measure smaller additions of sulfite and second, even when crushing them up and dissolving them in water (which you must do), it seems almost impossible to get it all to dissolve leaving you with some little particles floating in the wine. I would suggest changing over to using the powder form just like the professional winemakers do. Example for home measurement: 25 ppm per gallon of P. metabisulfite = .1601 grams X 5 gallons in carboy = .80 grams = 1/8th teaspoon. Sodium metabisulfite is higher strength so the calculation would be: 25 ppm per gallon of S. metabisulfite is .0931 grams X 5 gallons in carboy = .47 grams = just over 1/16th of a teaspoon.
Sulfite is certainly not unique to the wine industry and you may have noticed this product in the ingredient lists of many foods. Some years ago, the Federal government decided that wineries had to put warning labels on their wine to tell people they had added sulfites to the wines because of allergic reactions to sulfites that had occurred, especially in asthmatics. In truth, this gave both wine and sulfite a bad rap, so to speak. What was happening was that people were having terrible allergic reactions after helping themselves to the offerings at salad bars in restaurants. Every restaurant had a huge salad bar in those days. I bet you thought that person who came out and sprayed the lettuce periodically was just keeping it nice and wet and fresh. In truth, that spray bottle contained a mixture of sulfite and water intended to keep the lettuce from browning. Unfortunately, if you happened to be the first person to take that lettuce off the top, you got a whopping load of sulfite. There were even deaths reported from asthmatics that had severe reactions to it. That, of course, was when the government stepped in and declared that, if any sulfite was added, there needed to be a warning label. To put your mind at ease, though, you should know that wine has a legal limit of 350 ppm and, in fact, generally contains far less as levels this high would not taste very good. In comparison, the offending salad bar lettuce that was tested had 2000 ppm or more. A small amount of sulfite is actually a natural byproduct of the process of fermentation (of anything) and so it is impossible to not have it in a fermented product.
As I mentioned, sulfite is also commonly used in the sterilization of equipment: The most important consideration in making wine is keeping the equipment clean and sterile. To this end, there is a simple solution that can be made up and used to rinse everything that touches your wine – hoses, crusher, press, pumps, fermenters, carboys, clamps, gaskets…everything. In a 5 gallon bucket filled with water, add ½ cup of citric acid and about an ounce of metabisulfite. Most people use the sodium metabisulfite for this solution because it is cheaper. Mix well with a long handled spoon. Then rinse everything with the solution. The usual procedure is to rinse clean equipment with water, then the solution, and then water again. The citric acid does 2 things. It gets all of the equipment to a good (low) pH level, and it provides a good environment for the sulfite to work well. Sulfite needs an acid environment to gain its strength and the higher the acid, the less sulfite you need.