Essential Science: Why does wine smell bad?

Posted Dec 17, 2018 by Tim Sandle
With many wines, laying them down for a long period improves them and this is a factor in good wines becoming great wines. However, sometimes wines go off and develop a bad smell. New research reveals why.
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Sometimes a wine has been in storage, with the expectation that the bottle of vino, when opened, will produce something special to the palate and to the nose. To create the delightful bouquet, within wine there are volatile and non-volatile compounds that contribute to the makeup of a wine's aroma.
Occasionally a bottle is removed from the cellar and, when opened, there is a highly unpleasant smell. The main cause of this off-odor is hydrogen sulfide, which delivers to the affected wine an aroma of sewage or rotten eggs.
The taste of wine is affected by wine glass design.
The taste of wine is affected by wine glass design.
Hydrogen sulfide
Hydrogen sulfide is the chemical compound and presented as a colorless chalcogen hydride gas. At certain levels it is very poisonous, corrosive, and flammable. Hydrogen sulfide is typically generated as a result of the microbial breakdown of organic matter in the absence of oxygen (as would be found in a sewer). With microorganisms, this is a form of anaerobic digestion undertaken by sulfate-reducing microorganisms.
Sulfate-reducing microorganisms can be traced back to 3.5 billion years ago and are considered to be among the oldest forms of microbes, having contributed to the sulfur cycle soon after life emerged on Earth.
Why wine goes off?
Although the chemical that causes off-wine has been established, the specific causes have always been uncertain. Now, with hydrogen sulfide, scientists have identified some potential sources of this stinky compound.
According to Laboratory Manager magazine, hydrogen sulfide is produced naturally during fermentation. However, the bulk of the gas disappears or is removed in subsequent winemaking steps. Why it sometimes re-emerges after bottling has been a puzzle.
A scrumptious white wine from the Rhone valley area  tasting even better in Lyon.
A scrumptious white wine from the Rhone valley area, tasting even better in Lyon.
One theory, with a touch of irony, is that it might derive from polysulfanes and other sulfur byproducts created during the actual act of hydrogen sulfide removal.
Research process
For the study, the scientists developed a model wine that was composed of a mixture of polysulfanes. Taking this wine, the researchers treated it with antioxidants like sulfur dioxide and ascorbic acid. These additives are introduced to many wines as preservatives during bottling.
Once this was prepared, the researchers identified and calculated the concentration of a several sulfur compounds in the wine after six months of storage. It was discovered that polysulfanes containing four or more linked sulfur atoms per molecule were most likely to decompose during wine storage. This reaction correlated with elevated levels of hydrogen sulfide.
It was further found that the polysulfane decomposition and hydrogen sulfide release occurred more frequently in the wine treated with sulfur dioxide compared with untreated wine or wine only treated with ascorbic acid.
The inference from this discovery is that wines with polysulfane additives are most likely to experience re-emergent hydrogen sulfide. This finding is set to be tested out on a bigger scale.
The study was funded by Wine Australia and the Australian government.
Research paper
The new research has been published in the Journal of Agricultural and Food Chemistry. The research paper is titled: “Liberation of Hydrogen Sulfide from Dicysteinyl Polysulfanes in Model Wine.”
Essential Science
This article is part of Digital Journal's regular Essential Science columns. Each week Tim Sandle explores a topical and important scientific issue. Last week we looked at a new drug combination, which is effective against the skin cancer, and which could save millions of lives each year.
The week before we looked at new research affecting Parkinson’s disease. This was centered on a new drug that aims to freeze Parkinson's disease in its tracks. The drug targets neuroinflammation, and researchers think this could halt further neuronal loss in Parkinson’s disease.