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by The Beverage People, Copyright 2025
What follows are simple instructions on treating this common cider flaw. Volatile reduced sulfur (VRS) can generally be treated easily and successfully, as long as you don't wait too long to treat it.
How did the cider end up smelling like rotten eggs instead of apples? No, nobody snuck into your house and dropped a rotten egg into your fermentor. But if you smell it when you rack your cider, you have a problem. “Rotten eggs” (peuw!) is one of many unpleasant descriptors applied to the volatile reduced sulfur (VRS) compounds than can occur during the fermentation and aging of cider. Much easier to prevent than correct, these compounds interact with each other, and the cider, in very complex ways. Simply stated, if you detect this kind of aroma, fix it quick!
The simplest, and generally first, VRS to appear is Hydrogen Sulfide, H2S. This is the compound commonly described as smelling like rotten eggs. Since humans can detect the smell when the concentration is only one or two parts per billion, it doesn’t take much to make the cider very unpleasant. In cider, by far the most frequent cause is lack of nutrients—primary amino nitrogen or certain vitamins—during primary fermentation. You can address prevention of that problem by analyzing your juice nutrient level (YANC), boosting the initial nitrogen level with diammonium phosphate (DAP), and then using a complex, vitamin-containing nutrient like Fermaid K during the active fermentation. Apple juice typically contains less nitrogen, particularly, than the yeast require. If you catch a egg whiff early in the primary fermentation, the nitrogen is insufficient. Most often, when the aroma is caught before the fermentation is finished, you can simply add some DAP, which will correct the nitrogen deficiency, and the H2S aroma will be driven off with the CO2 production during the remainder of the fermentation.
Learn more about making cider yeast nutrient additions for a healthy fermentation.
But let’s suppose the odor shows up anyway (which it may). The most conservative treatment is to aerate the cider during racking—splash it into the receiving vessel (but be sure your free SO2 level is up where it should be prior to the splash racking—otherwise you may oxidize your cider, turning it brown and producing a bruised-apple aroma). A more effective solution is to treat with copper. When exposed to copper, the sulfide combines with the copper to make copper sulfide, which is not soluble in cider. While some books will tell you to just run the wine over a sheet of copper, this technique is not the most effective. Instead, the direct addition of a small amount of 1% copper sulfate solution is usually quite effective.
Copper Sulfate Treatment Directions
If you have not promptly removed H2S, your cider may go on to develop more complex VRS compounds. Next in line are the mercaptans: methyl mercaptan smells like burnt rubber or rotten cabbage and ethyl mercaptan smells like burnt matches or dirty ashtrays. These are not volatile enough to remove by aeration, but copper (just as for H2S) still works. To check for possible effectiveness, clean a copper penny in a mild acid solution (a little citric or tartaric in some water). Place your now-bright penny in a glass, add cider, and swirl. Let it stand for a minute or two, and the bad smell should go away if you have a copper treatable problem.
If your problem goes on even longer without treatment, you may get into the disulfides. Dimethyl sulfide resembles cooked asparagus or canned corn, diethyl sulfide is reminiscent of new tires, dimethyl disulfide brings the delights of onion aroma to your cider, and diethyl disulfide brings garlic aroma. (A related compound, methyl ethyl sulfide, is used to give the familiar warning aroma to natural gas.) Now you are really in trouble. Old Italian country winemaking treated disulfides by extracting the wine with a portion of olive oil, then skimming off and discarding the oil. Since the olive oil that remains in the wine may go rancid, drug-store mineral oil would be a more modern choice. But don’t expect a miracle: if you are trying to clean up your wine with oil it has gone very bad, indeed.
So let’s go back to the top: To avoid VRS in the first place, make sure to use adequate nutrients in the fermentor. If VRS appears anyway, aerate (with SO2 present) to try to drive off the obnoxious gas. If that doesn't get rid of it, treat with copper. If necessary, repeat the copper and remove it with yeast hulls.
