Hydrogen Sulfide's Reductive Aroma: A Complex Chemistry

Hydrogen sulfide (H2S) is a colorless, flammable, poisonous, and corrosive gas with a pungent rotten egg odor. It is a well-known contaminant in wines with so-called reductive off-odors. H2S is produced by the sulfate reduction sequence (SRS) pathway in S. cerevisiae during alcoholic fermentation. It is toxic to humans and most other animals, inhibiting cellular respiration in a manner similar to hydrogen cyanide.

Hydrogen sulfide is used or produced in a number of industries, including oil and gas refining, and pulp and paper processing. It also occurs naturally in sewers, manure pits, well water, oil and gas wells, and volcanoes. Hydrogen sulfide is heavier than air, so it can collect in low-lying and enclosed spaces, making work in confined spaces potentially very dangerous.

Hydrogen sulfide can be derived from the microbial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers. It is also produced by the human body in small quantities through the bacterial breakdown of sulfur-containing proteins in the intestinal tract.

Hydrogen sulfide is a precursor to other reductive aroma compounds, such as mercaptan, which produces a garlic, burnt rubber, and cooked cabbage aroma in wine. It can also be converted to elemental sulfur, which is not soluble in water.

Hydrogen sulfide is one of the leading causes of workplace gas inhalation deaths in the United States, causing 46 worker deaths between 2011 and 2017.

Hydrogen sulfide's role in wine's reductive aromas

Hydrogen sulfide (H2S) is a volatile sulfur compound (VSC) that is often described as having a "rotten egg" smell. It is the most common VSC in wine and is considered a reductive aroma. H2S is produced by yeast during fermentation when it is placed under stress, such as when the yeast starts to run out of nitrogen. It can also be produced in excess due to the presence of elemental sulfur on grape skins from sulfur sprays, inadequate levels of free α-amino nitrogen (FAN), added SO2, a deficiency of B-complex vitamins, unusually high levels of cysteine in juice, or a high concentration of metal ions.

The detection threshold of H2S in wine is about 1-2 µg/L, and it has been suggested that concentrations below this threshold may play a role in wine complexity. However, higher concentrations of H2S can be undesirable and are considered winemaking faults. H2S can react with other wine compounds to produce larger molecular weight sulfur compounds, so it is best to remove it before it reacts further to produce other unwanted odor-active compounds.

The presence of H2S in wine can be reduced by adding a soluble nitrogen source, such as diammonium phosphate, to the juice. Another method is to use copper sulfate (CuSO4) to react with and remove H2S, although this may also remove desirable volatile thiol aromas. Aeration can also be used to remove H2S, but it must be done promptly to prevent the formation of elemental sulfur, which can then revert back to H2S.

In recent years, there has been a shift in the perception of reductive aromas in wine. While winemaking practices in the late 20th century sought to avoid and undo reduction, there is now an increasing population that welcomes reductive character. Some winemakers believe that reductive aromas can indicate a wine's capacity to age, as a wine with higher reductive strength can absorb more oxygen without oxidizing. Additionally, some appreciate the aesthetic qualities that reductive aromas can bring to a wine.

Hydrogen sulfide's formation during fermentation

Hydrogen sulfide (H2S) is often described as having a "rotten egg" smell and is produced by yeast during fermentation. It is a reductive aroma, and its presence in wine is usually considered a fault. However, some winemakers embrace reductive aromas, and there is an increasing population that welcomes this character in wine.

The amount of H2S produced during fermentation depends on both environmental factors and the genetic background of the yeast. Elemental sulfur, which can be present on grape skins from sulfur sprays, can be converted to H2S by yeast during fermentation. Inadequate levels of free α-amino nitrogen (FAN) and a deficiency of B-complex vitamins can also lead to excess H2S production. H2S can also be produced when yeast cells are placed under stress, such as when they start to run out of nitrogen.

During the winemaking process, hydrogen sulfide production has been linked to various parameters, including the level of solids (turbidity), nitrogen and vitamin concentrations, inadequate mixing and aeration, as well as the size and shape of the vessel employed. The outcome of these variables depends on the juice/must conditions, its composition, and the growth conditions of the yeast.

Yeast strains with the inability to balance the production and incorporation of sulfur compounds can result in the release of volatile sulfur compounds into the wine. The physiological environment of the yeast is controlled by the winemaker, but the genetic predisposition of the yeast and its capacity to produce H2S depending on its environment is more challenging to predict.

Hydrogen sulfide's removal from wine

Hydrogen sulfide (H2S) is a common volatile sulfur compound (VSC) in wine, often referred to as "rotten egg gas". It is produced by yeast cells when they are stressed, for example, due to a lack of nitrogen. H2S can also be produced in excess by yeast during fermentation due to the presence of elemental sulfur on grape skins, inadequate levels of free α-amino nitrogen (FAN), added SO2, a deficiency of B-complex vitamins, unusually high levels of cysteine in juice, or a high concentration of metal ions. The production of H2S is also yeast strain-dependent.

There are several methods to remove or prevent the formation of H2S in wine:

• Preventing vineyard sprays of elemental sulfur too late in the season
• Ensuring adequate levels of nitrogen in the grape must
• Using a yeast strain that does not produce large amounts of H2S
• Avoiding yeast stress by maintaining recommended fermentation temperatures
• Aerating red wines during alcoholic fermentation
• Racking wine off the gross lees within 2-3 days of completing alcoholic fermentation
• Being careful not to spill burning sulfur wicks in barrels during sulfuring
• Using copper sulfate (CuSO4) to react with and remove H2S, forming copper sulfide (CuS)
• Using alternative commercial products such as Reduless or Kupzit to remove H2S
• Using polyvinylimidazole/polyvinylpyrrolidone (PVI/PVP) or bentonite to remove copper from wine

It is important to note that the presence of reductive aromas in wine may not always be a fault. In some cases, these aromas can add complexity or indicate a higher reductive strength, resulting in increased longevity and aging potential.

Hydrogen sulfide's toxicity

Hydrogen sulfide is a highly flammable, toxic, colorless gas with a strong odor of rotten eggs. It is commonly found in sewage, swamps, manure gas, hot springs, geysers, and volcanoes. It is also used in some industries, such as oil refining and sewage treatment, and can be generated as a by-product.

Toxicity

Toxicity from hydrogen sulfide usually occurs through inhalation, though it can also be absorbed through the skin. It is heavier than air and tends to accumulate near the floors of poorly ventilated rooms. Exposure to hydrogen sulfide can cause irritation to the eyes and mucous membranes, headache, insomnia, dizziness, fatigue, cough, dyspnea, nausea, vomiting, vertigo, bradycardia, agitation, cyanosis, and acute lung injury. Very high-level exposure can lead to myocardial infarction, rapid loss of consciousness, seizures, and cardiopulmonary arrest.

Hydrogen sulfide inhibits cellular respiration by binding to the ferric moiety of mitochondrial cytochrome c oxidase, thereby blocking ATP synthesis. It also reacts with iron to enhance the Fenton reaction, leading to free radical damage.

Treatment

There is no antidote for hydrogen sulfide toxicity, and treatment is mainly supportive. Removal from the source of exposure is critical, as is the use of personal protective equipment for rescuers. Supplemental oxygen should be provided, and decontamination with water is necessary for ocular or skin irritation. Treatment also includes administering crystalloids, vasopressors, sodium bicarbonate, and, in some cases, sodium nitrite or hydroxocobalamin.

Hydrogen sulfide's role in the sulfur cycle

Hydrogen sulfide (H2S) is a central participant in the sulfur cycle, the biogeochemical cycle of sulfur on Earth. The sulfur cycle involves eight electron oxidation/reduction reactions between the most reduced H2S (−2) to the most oxidized SO42− (+6).

Hydrogen sulfide is produced by the microbial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers. It is also produced by sulfate-reducing microorganisms, which derive energy by using sulfates to oxidize organic compounds or hydrogen. This process produces hydrogen sulfide as a waste product.

Hydrogen sulfide can be oxidized by microbes to form elemental sulfur, which can be stored as polysulfides. Elemental sulfur can also be produced by the oxidation of sulfide by sulfur oxidizers. Elemental sulfur can be further oxidized to form sulfate.

Hydrogen sulfide can also be produced by the reduction of sulfate by sulfate reducers. This process is known as dissimilatory sulfate reduction and occurs in both bacterial and inorganic processes.

Hydrogen sulfide is toxic to humans and most other animals, but some highly specialized species thrive in habitats that are rich in this compound.

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