Hydrogen Sulphide and Its Derivatives
By Bill Collings (January 28, 2002, modified April 15, 2007)
Hydrogen Sulfide (H2S) has been known to plague winemakers for centuries, but it needn’t. Its causes are as simple as is its cure, if dealt with soon enough after detection.
Primary Causes
- Residual sulfur on the grapes as the result of a late spray for powdery mildew;
- Some yeasts, such as Montrachet (UCD 522) and some strains of Steinberg, are known to produce higher levels of H2S; and more commonly,
- Low nitrogen levels in the grapes which results in higher levels of H2S being produced by yeast cells (all yeasts produce some H2S that is dissipated during fermentation).
Early Detection and Treatment
Early detection of the rotten egg odour and subsequent racking with deliberate splashing will usually cure the problem, as H2S is highly volatile. However, in order to reduce the risk of H2S formation, it is wise to add yeast nutrient containing diammonium phosphate (DAP) at the rate of 100–200 ppm during the early stages of fermentation.
Progression to Mercaptans and Disulfides
Failure to treat H2S in its early stages will only add to problems later, as H2S, when it interacts with alcohol, produces mono-mercaptans (sulfides) which have a range of odours—garlic, cabbage, onion, rubber, skunky—and are more difficult to remove because they are much less volatile having become bound through interaction with alcohol.
One can successfully remove mercaptans in their early stages by a combination of aeration and passing the wine through a Molly Maid® copper pot scrubber stuffed into a one-inch piece of plastic (PVC) pipe.
If H2S or the mercaptans have not been dealt with, the mercaptans, if the wine has undergone any oxidation (which occurs during barrel ageing), have formed poly-mercaptans (disulfides) which will not react with copper. Disulfide odours have been described as asparagus, corn or molasses.
Treating Advanced Stages
- The addition of USP mineral oil will remove the disulfides because they are more soluble in oil than they are in wine; and
- Treating the wine with ascorbic acid which will break the disulfide back down to sulfide and adding copper sulphate (CuSO₄·5H₂O) solution to remove the sulfide.
Testing Protocol
Testing before treatment is absolutely necessary because it is possible to confuse the off-odour for Brettanomyces, which has a barnyard odour and cannot be eliminated by treating it for mercaptans.
Make two stock solutions:
Copper sulphate solution: Dissolve 4.1 grams in a little water and bring the volume up to one litre with distilled water. (Use 10 ml of this solution with 90 ml of distilled water for the lab test.)
Ascorbic acid solution: Dissolve 10 grams in a little water and bring the volume up to one litre with distilled water.
Put 100 ml of the suspect wine into three glasses. Use the first glass as the control. Put 5 drops of the diluted copper sulphate solution into glass number two. Into glass number three, put 5 drops of the ascorbic acid solution, then after a few minutes add 5 drops of the copper sulphate solution.
Test Results Interpretation
| Glass 2: Copper Sulphate | Glass 3: Ascorbic Acid/Copper Sulphate | Results |
|---|---|---|
| No change in smell | No change in smell | Not a sulfide problem |
| No change in smell | Reduction or elimination of smell | Disulfide |
| Reduction of smell | Elimination of smell | H2S, mercaptan and disulfide |
| Elimination of smell | Elimination of smell | H2S and/or mercaptan |
Treatment Dosage
If disulfide is not present, addition of the copper solution will help; if disulfide is present, both ascorbic acid and copper sulphate must be used. Set up a series of glasses with 100 mls of wine and add 0.05 ml, 0.1 ml, 1.5 ml, etc. of the solution. Select the first one that no longer smells. To treat a 19 litre carboy of wine with 0.1 ppm requires 0.1 ppm x 19 = 1.9 ml of the stock copper solution.
Prior to adding the copper solution, add about 25 ppm of ascorbic acid (about 0.5 grams in a 19 litre carboy). Stir in well and wait at least one day before adding the copper solution. Treatment takes several days before the odour and taste disappear.
Key Takeaway
H₂S (volatile) → mono-mercaptans (becoming bound) → poly-mercaptans (bound). Deal with the problem as soon as it is detected. Research shows that mercaptan formation occurs within two days after the beginning of fermentation and is at its peak at about two months.