Troubleshooting Your Wine
The faults and flaws and their analysis and resolution presented below are from my latest book, Modern Home Winemaking, where you can find additional information should you wish to explore these further.
HYDROGEN SULFIDE (H2S)
DESCRIPTION
Hydrogen sulfide (H2S) is the most common of a very broad class of volatile sulfur compounds, or VSCs, or mercaptans. It is the reduced form of sulfur — “reduced” means the opposite of oxidized — hence why it is said to impart a reductive character in certain cases where it might be expected, for example, in wine bottled under screw cap, which allows H2S to manifest itself due to the low-oxygen environment.
H2S imparts a familiar smell of rotten eggs, perhaps even sewage, cooked cabbage, struck flint or rubber. It can be detected at levels as low as 1 µg/L (ppb).
CAUSES
There can be various causes for H2S in wine. It can be due to low nitrogen supply during the AF, wine aging on the gross lees for too long, or from metabolism of such substances as sulfur-containing amino acids, inorganic sulfur, SO2 and sulfate.
Yeast cells can become overly stressed and produce excessive amounts of H2S if the amount of nitrogen found in nutrients needed for a healthy fermentation is too low or also too high, or if fermentation temperature is too low or too high. It can occur if juice is highly turbid with a large amount of soluble solids, and therefore, as red winemaking involves maceration of wine with grape solids, H2S is a much more common occurrence in red wines.
But the most common cause of H2S in home winemaking is the degradation of sulfur-containing compounds from autolysis when wine is left too long on the gross lees.
The presence and concentration of naturally occurring or yeast-derived sulfur-containing amino acids in the juice is another significant source of H2S. During yeast fermentation, yeast cells generate sulfur to synthesize sulfur-containing amino acids as part of their sulfur metabolism; however, if these amino acids cannot be synthesized, sulfur can turn into sulfides.
Other causes include: the degradation by yeast of inorganic sulfate from elemental sulfur from the overuse of sulfur-based vineyard sprays to control mildew; a harvest that was too close to spraying; and from spent wicks accidentally dropped at the bottom of barrels during sulfuring.
H2S can also result from the excessive use of SO2 and sulfate from exogenous additions of KMS and copper sulfate (CuSO4), respectively.
During AF, yeast produces small amounts of H2S (as well as SO2), but because of its volatility, much of that H2S produced early in winemaking is lost through entrainment in carbon dioxide (CO2). And different yeast strains produce different amounts of H2S and SO2, the latter becoming a substrate for H2S production.
And in a lesser-known cause, H2S and other sulfides that had become bound — and therefore non-volatile — can become released during aging. This factor can be particularly problematic as it may resurface well after an initial problem with H2S had been resolved.
If there is excessive H2S or if left untreated, H2S can react with ethanol to form ethanethiol, another VSC and which imparts a smell of raw onions, rubber or natural gas, or a skunk smell. Ethanethiol can then become esterified to the sulfurous-smelling ethyl thioacetate or oxidized into diethyl disulfide, two compounds that impart unpleasant odors of onions. These compounds are easier to smell and detect but can be almost impossible to remove.
ASSESSMENT
There are no tools to quantify H2S in home winemaking, and therefore, you must rely on your sense of smell. H2S smells of rotten eggs or burnt rubber; it should never be detectable in sound wine.
REMEDIAL ACTIONS
Very mild cases of H2S can be treated through volatilization by sparging the wine with inert gas, e.g., nitrogen, or, for red wine only, by aerating via, for example, a vigorous racking. Affected white and rosé wines should not be aerated, or you’ll risk oxidizing the wines.
For more serious cases, treat with a 1% CuSO4 solution. Run bench trials first to determine how much CuSO4 is needed to treat the entire batch. High doses of residual copper can become a source of oxidation problems and, more important, pose a health hazard. Treated wine will form a copper sulfide (CuS) precipitate; rack and filter in one week.
Some amateurs resort to the use of copper piping for treating H2S because of its simplicity. This practice is not recommended — it is in fact very dangerous as the amount of copper transferred into the wine cannot be controlled and easily becomes excessive and toxic. For reference, legal limits in commercial wines are typically in the 0.2–1 mg/L range, 1.0 mg/L in the United States.
As copper can possibly react with other VSCs, the copper sulfate treatment should be weighed against the likelihood of reducing desirable volatile thiol aromas in, for example, Sauvignon Blanc and Chenin Blanc. Copper is non-selective on thiols — in addition to reacting with other volatile thiols, it can react with non-VSCs, such as the naturally occurring antioxidant glutathione (GSH), and the wine could become more prone to spoilage.
For bench trials and to treat the affected batch, use a commercially available 1% CuSO4 solution, such as Sulfidex, or make one by dissolving precisely one gram of cupric sulfate pentahydrate (CuSO4•5H2O) crystals in approximately 75 mL of distilled water in a 100-mL volumetric flask or good 100-mL graduated cylinder, then bring to volume with more distilled water.
There are excellent alternative commercial products for treating H2S. Reduless comprises inactivated yeast with “biologically bound copper” and some bentonite, and Kupzit, a relatively new product that uses copper citrate to remove H2S; it too contains bentonite.
Following are instructions on how to run proper bench trials and to treat a batch using a 1% CuSO4 solution. You can also download the CONDUCTING CuSO4 BENCH TRIALS tool to guide you through the process.
1.
Label two wine glasses, one as control and one as copper. Pour 100 mL of the affected wine into each glass. With a 1-mL pipette or syringe, transfer one drop of 1% CuSO4 solution into the copper glass. Swirl each glass, let them stand for about five minutes, then smell each, but do not taste. If the copper glass still has the stinky smell, no safe amounts of CuSO4 can eliminate the H2S smell. The single drop represents roughly 1 mg/L of copper, and therefore just at the acceptable limit used in many winemaking regions of the world.
If the single drop does neutralize the smell, continue the trial to determine how much CuSO4 you need to treat your entire batch. Here, 1 L (¼ gal) of wine in a 1000-mL graduated cylinder works best.
2.
Using a 1-mL pipette or syringe, transfer a single drop of CuSO4 solution into the wine in the 1000-mL graduated cylinder. Stir the wine to thoroughly dissolve the CuSO4. Pour some wine into the copper glass (not in the control glass), swirl both the control and copper glasses, let stand for about five minutes, then smell each.
If the smell of H2S persists, return the sample from the copper glass back to the graduated cylinder (you don’t want to change your 1000-mL reference volume), add another drop of CuSO4 solution, and repeat the process until the H2S smell disappears, without exceeding 0.4 mL to stay within the safe maximum of 1 mg/L.
3.
Once the smell is neutralized, scale up the volume used in the 1-mL pipette or syringe for the entire batch. For example, if you used 0.1 mL to rid of the H2S smell in the 1000-mL sample and you need to treat a 23-L (6-gal) batch, then you need to add 0.1 X 23 or 2.3 mL of 1% CuSO4 solution.
4.
Once the batch is treated, let settle for one week, then rack and coarse filter to remove the black powder-like copper sulfide precipitate. Complete removal of copper sulfide by racking or filtration may not always be possible.
PREVENTIVE ACTIONS
There are many preventive actions to mitigate the possibility of H2S forming in wine:
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2.
Add complex nitrogen-containing nutrients to must prior to inoculating for AF and add more complex nutrients when SG/Brix has dropped by one-third.
3.
Avoid yeast strains (e.g., Red Star Premier Classique, formerly Montrachet) known to produce large amounts of H2S, or consider using a so-called “H2S-preventing” yeast (e.g., Renaissance).
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7.
Be careful not to spill burning sulfur wicks in barrels during sulfuring.