Tuesday, 26 February 2019

The microbiology behind wine going bad

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.

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 sulphide, which delivers to the affected wine an aroma of sewage or rotten eggs.

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 sulphide, scientists have identified some potential sources of this stinky compound.

According to Laboratory Manager magazine, hydrogen sulphide 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.

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 sulphide 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 sulphide.

It was further found that the polysulfane decomposition and hydrogen sulphide 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 sulphide. 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.”

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology

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