Hydrogen Sulfide

Hydrogen Sulfide

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.

There are three primary causes:

1. Residual sulfur on the grapes as the result of a late spray for powdery mildew;

2. Some yeasts, such as Montrachet(UCD 522) and some strains of Steinberg, are known to produce higher levels of H2S; and more commonly,

3. 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 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.  Do not add DAP at the beginning of fermentation, as it will overpower the yeast which has not yet had enough time to multiply to full activity.

Failure to treat H2S in its early stages will only add to your 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, unlike H2S, they are much less volatile having become bound through interaction with alcohol.  Even at this stage it is possible to treat the wine and remove the offensive odour, but it is more difficult to do so. One can successfully remove mercaptans in their early stages by a combination of aeration and passing the wine through a Molly MaidÒ copper pot scrubber stuffed into a one-inch piece of plastic (PVC) pipe.  It is absolutely essential that the wine be exposed to as much of the copper surface as possible and that the copper be free from contamination resulting from handling.

If you have not dealt with either the H2S or the mercaptans, then you are in trouble big time because now 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.  Simply dropping a piece of copper into the wine and swishing it about may affect the mono-mercaptans but it will not have any effect on the poly-mercaptans; and it may work only if the copper is highly polished and clean of any contamination.  If you think the copper eliminates all the odours, you are being swayed by the power of suggestion.  The only way to deal with this problem is to reduce the disulfides back to the mercaptan stage, and there are two ways to do this:

1.     The addition of USP mineral oil will remove the disulfides (see Jackish) because they are more soluble in oil than they are in wine (And then you have to remove the oil.); and

2.     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 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.

To test, it is necessary to first make two stock solutions:

One of copper sulphate which is done by dissolving 4.1 grams in a little water and bringing the volume up to one litre with distilled water.  (Use 10 ml of this solution with 90 ml of distilled water to make 100 ml total for the lab test.)

One of ascorbic acid which is done by dissolving 10 grams in a little water and bringing the volume up to one litre with distilled water.

Next, 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 and stir well.  Into glass number three, put 5 drops of the ascorbic acid solution, stir well and, after a few minutes, add 5 drops of the copper sulphate solution and stir well.  The following table illustrates the possible results.

After Yair Margalit: Winery Technology & Operations.

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.  To determine the amount of the copper solution to use, 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.  Check the smell of each glass and select the first one that no longer smells.  The addition of the copper solution used is the equivalent in parts per million of copper sulphate addition.  Thus 0.1 ml = 0.1 ppm.  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, or about 0.5 grams in a 19 litre carboy.  Stir in well and wait at least one day before adding the copper solution.

Ascorbic acid in conjunction with copper sulphate works very well, but it is not instantaneous; it takes several days before the odour and taste disappear.  Do not exceed the recommended dosage of copper sulphate or you may induce a copper haze which will be difficult to remove. 

Remember,H2S (volatile) à mono-mercaptans (becoming bound) à poly-mercaptans (bound), so deal with the problem as soon as it is detected.  This process is not discrete: that is, while H2S is present, it is likely that mono-mercaptans are forming; and poly-mercaptans may be forming before the H2S in its volatile form disappears.  Research shows that mercaptan formation occurs within two days after the beginning of fermentation and is at its peak at about two months after which the poly-mercaptans become dominant.  Since most winemakers barrel-age their wines for much longer periods, if H2S has been detected and removed in the early stages, constant checking for mercaptan odours is critical since the barrel is where the mercaptans are formed, and they will continue to develop in the bottle.