John Czrny has asked several questions about the use of copper sulfate to treat wines that have a problem with hydrogen sufide (H2S), which is the smell of rotten eggs, and the treatment of mercaptans, which is what is created when hydrogen sulfide is not dealt with early on. Mercaptans can have the smell of rubber, skunk, and cabbage as well as other aromas. Needless to say, very unpleasant stuff, indeed.
These problems occur mainly for one of two reasons. The first would be spraying the grapes with sulfur too close to harvest, thus having residue on them which then has a reaction while fermenting. The other would be not racking the wine off of sediment in a timely manner, meaning as soon as fermentation is finished. The yeast autolyses, or basically decomposes, back into the wine creating nasty aromas.
In order to treat a wine exhibiting these aromas, you need to first run a copper sulfate trial. This is not complicated but several things need to be done. The copper sulfate you buy from your supplier should be a 1% solution. Make sure to ask. To run a trial you have to make a .004% solution. You can do this is by taking one milliliter of 1% solution and putting it into a flask that will hold more than 250 milliliters. Add distilled water to the 250 ml measurement (in other words, 249 ml of water). It is important to always use distilled water in lab tests because tap water or other types of bottled water can skew the test due to impurities or mineral content etc. Mix well.
It is recommended that you pull five 100 milliliter samples of wine and put them in wine glasses (of exactly the same style), numbering them 1 through 5. The first glass should be unaltered (your base wine). To the next four, add .5 ml, 1.0 ml, 1.5 ml, and 2.0 ml of the .004% solution respectively and mix them gently. Cover these glasses with something non-porous, like sheets of paper or small plates, and leave them for several hours or even overnight. Then uncover the glasses and smell each. The first one that shows the disappearance of the aroma is the one you will use for your calculations. DO NOT TASTE THE WINE. COPPER SULFATE IS NOT GOOD FOR YOU. I am sure you are thinking, ‘how can I put this in my wine then?’ You can later use bentonite or Sparkolloid (as discussed in a previous question posted on this site for clearing pear wine) to remove some of the copper sulfate. If you are using the upper limits of this product, it may be a good idea to do a yeast fining to remove the copper. See information on doing a yeast fining below. Filtering, however, is highly recommended.
In order to figure out how much 1% copper sulfate solution to add to your wine, recall the 4 trials that you have done. They equate to .05 ppm (parts per million), .10 ppm, .15 ppm and .20 ppm. In case you were wondering, .20 ppm is the legal limit in commercial wine. You can see these are in .05 increments. For each .05 ppm, you need to add .075 ml of 1% copper sulfate per gallon. So, for example, if the first glass where the aroma disappeared was the one with .15 ppm you would need to add 3 X .075 ml = .225 ml of 1% copper sulfate per gallon
As you can see this stuff is very potent. It is very important that you measure precisely. If you are dealing with such a small enough batch of wine that this is difficult to measure, you can dilute the copper sulfate with distilled water. For example if you dilute it 50-50, making it ½% strength instead of 1%, you would double the volume you are putting into the wine. This should make it easier to measure for, say, a 5 gallon carboy.
To do a yeast fining, you would mix wine yeast at about .68 grams per gallon into boiling water to kill it. Let this sit overnight and then mix it into your wine. If you have some older yeast, that is perfect. The idea is to make it no longer viable. The packages of yeast for home winemakers are 5 grams, so for 5 gallons, this would be about ¾ of that package. If you end up putting a bit more in, no harm done.
If none of these trials works and you still have these aromas in the wine, you probably have a compound such as dimethyl disulfide that has formed. This compound does not react to copper. If this is the case the wine must be treated with ascorbic acid first. This will convert the dimethyl disulfide back to methyl mercaptan. The addition of ascorbic acid is a very small amount – 190 milligrams per gallon is all you need (keep in mind that 1000 milligrams = 1 gram). One gram of ascorbic acid is equal to ¼ teaspoon. Once again, a little more ascorbic acid won’t hurt anything. This is, after all, the acid used for canning fruit. After mixing the ascorbic acid into the wine, wait a few days and proceed with the copper sulfate trials above.