A young Bordeaux scientist has discovered the cause of geosmin, a compound that gives a musty flavour that has bedevilled winemaking for several years.

The smell is often described as ‘turnip’, ‘musty’, ‘rotten’, or like wet earth warmed by the sun. It differs from the classic musty flavour of TCA or cork taint, which does not have a vegetal edge.

Geosmin has been known about for decades, but was not common knowledge amongst winemakers when it seriously infested Beaujolais in the 2000 and 2002 vintages. It had also affected red wines from various regions including Bordeaux, Burgundy and the Loire since at least 2000.

Like brettanomyces, which also gives a musty odour, it was often put down to ‘terroir’.

‘2002 was a year I won’t forget. A lot of wines had a musty taste and nobody knew why,’ said Etienne Akar of Beaujolais négociant Paul Beaudet.

Bordeaux scientist Stéphane Laguerche has now discovered the cause of geosmin.

Laguerche’s PhD thesis concentrated on why certain botrytis-affected vines were not also contaminated by geosmin. He found that geosmin needed both the botrytis fungus and a mould called Penicillium Expansum (commonly found on vegetables about to go off) for it to contaminate the grapes.

Although the geosmin problem is quick to appear on vines, its contamination of the wine can last several years before it disintegrates. Long-term barrel maturation can break it down – over two years for Pinot Noir – but this is of little comfort to producers who sell their wines en primeur.

‘Even though a cluster looks fine, its smell indicates the presence of geosmin. Two to five clusters in a hundred are enough to contaminate a wine’, said Philippe Darriet, teacher and researcher at Bordeaux University.

In Beaujolais in 2000 and 2002 it was common for winemakers to add oil or milk – or in some cases charcoal – to wines to get rid of the smell. These were all illegal additives but it is understood the the Direction de la Concurrence et des Fraudes (the Fraud office) turned a blind eye to their use.

French authorities have now reacted to the problem and for the 2004 vintage winemakers will be allowed to use officially forbidden oenological filtering methods on the contaminated wines. Both milk and oil contain proteins that attract geosmine without destroying the wine’s flavours.

Although this authorization comes as a relief for many winemakers, there is a body of opinion that suggests better sorting during harvest could solve the problem.

‘In 2004, those who diligently sorted the clusters in their vineyards haven’t encountered this geosmin problem,’ said Jean-Luc Berger, chairman of the Technical Institute of Wine and Vine (ITV).

Laguerche has now been given funding by the Burgundy local government in order to study the geosmin problem there.

Written by Florence Kennel