Clearly, anything that interferes with the successful production and marketing of wine presents an obstacle to be removed.
On one level, making the stuff may appear extraordinarily simple: grape juice plus yeast equals wine. But fermentation is a complex process encompassing a great many chemical reactions.
Most wine drinkers have experienced the unpleasant experience of opening a bottle only to be greeted by a revolting aroma, often described as resembling rotten eggs or sewage.
One of the main causes of this stink is hydrogen sulfide (H2S), a volatile sulfur compound that is produced naturally during fermentation. Now, a team of researchers from Australia, Britain and the US have identified some potential sources of this stinky compound.
Their report appears in the Journal of Agricultural and Food Chemistry, published by the American Chemical Society, with funding from Wine Australia and the Australian federal government.
Led by Marlize Bekker, from the Australian Wine Research Institute, and David Jeffery, from the University of Adelaide in South Australia, the scientists created a model wine and then exposed it to an array of experiments, including adding a mixture of polysulfanes, sulfur-based chemical compounds, and then treating it with antioxidants such as sulfur dioxide and ascorbic acid, which are often added as preservatives.
They looked into not only the chemical processes that occur in wine fermentation, but also chemical reactions that take place as a result of common winemaking practices.
For example, the report says, hydrogen sulfide is usually removed by the winemaker by aeration, or by the addition of copper sulfide (Cu2) in a process known as copper fining.
“But hydrogen sulfide and other volatile sulfur compounds such as methanethiol can often return after bottling, when reductive conditions are resumed,” the researchers note.
Also, they add, “it had been assumed that Cu2 fining caused hydrogen sulfide to form the highly insoluble copper sulfide (Cu2S) that is easily removed by filtration, but this is not the case”.
Bekker and colleagues began to suspect the cause of the bad smells might be polysulfanes and other sulfur byproducts created during hydrogen sulfide removal.
The scientists identified and measured the concentration of a variety of sulfur compounds in the wine during six months of storage. They found that one class of polysulfanes tended to decompose during wine storage, correlating with a rise in hydrogen sulfide. In addition, the polysulfane decomposition and H2S release occurred more frequently in the wine treated with sulfur dioxide than in untreated wine or wine treated with ascorbic acid.
The findings provide strong evidence that polysulfanes were the source of re-emergent hydrogen sulfide, though this conclusion will need to be confirmed in real wines, the researchers say.
Confirming such a role for polysulfanes could help identify practical ways to manage the re-emergence of stinky sulfur compounds, one of the main faults in bottled commercial wine.