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Winning the wine war

March 18, 2005 By Emma Marris This article courtesy of Nature News.

Could DNA help the fight to keep bottle labels honest?

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Wine enchants because of its complexity, but that very trait makes it difficult to regulate. That bottle full of aromatic red liquid with hints of cherry may be genuine Pinot Noir from the California coast, or it may be a New Jersey Merlot, diluted with water and tarted up with sugar or sophisticated synthetic flavourings.

In the arms race between the adulterators and the regulators, detection systems have become ever more sophisticated, as have the cheaters. But at least one common ruse - claiming that the wine is one variety, when it is actually entirely or partly another - may come to a sudden stop if DNA can be successfully extracted from wine on the shelf.

Norbert Christoph of the Bavarian Health and Food Safety Authority works on one of today's typical wine authentication programmes: he uses stable isotope analysis. Carbon, hydrogen and oxygen atoms all occasionally show up in versions with slightly different mass. The amounts of such isotopes vary from region to region, so when they are incorporated into grapes, they tell a tale about where the wine was made.

The snag is that they also vary with local weather conditions, so samples of wine from each region have to be taken each year for comparison and entered in the European Wine Data Bank. For example, says Christoph, "2003 wines were collected before the late rains, so there were values for oxygen that were more like Southern Italy."

Rocky ratios

To prove which grapes were used to make wine, experts can also look at particular components of the wine with chromatography. This creates a graph showing which wavelengths of light are absorbed, and software types wines according to absorption pattern with decent accuracy. Things fall apart a bit, however, with blends.

In Germany, the world's largest wine importer, regulators often compare the ratios of two forms of anthocyanin, the molecules that make wine red. This ratio is determined by the genes of the vine, and was thought to be an unalterable marker that could tell you what variety of wine you were drinking.

But Ulrich Fischer, a chemist in the viticulture department of the Neustadt Teaching and Research Institute, Germany, has shown that some processing techniques can change this ratio quite significantly. Techniques can involve long fermentation, high temperatures and adding enzymes.

He says that many producers of Cabernet Sauvignon have had their wine unfairly rejected by the German government and been forced to sell it at half price. He suggests using more kinds of anthocyanins to type wine.

Definitive answers

The argument could be avoided if Patrice This, a grape geneticist at the University of Montpellier, France, has his way. He and his team are trying to perfect the extraction and purification of grape-skin DNA from bottles of wine.

They are also working to identify a set of genetic markers to differentiate the 2,500 or so varieties of grape in existence. Short sections that repeat a different numbers of times in different varieties seem like a good bet, This says. He described his project on 16 March at the American Chemical Society meeting in San Diego.

This is confident that if forensic scientists can get DNA from ancient skeletons, he can do the same for a bottle of Pinot Gris. "Our goal is to do it in two years," he says. "If it doesn't work by then, it isn't going to work."

Fischer is less sanguine. "I am very sceptical that they can purify [the DNA]," he says. "Wine is so processed."

All this fuss stems not just from a passion for authentic tastes, but from economic motives. "What other food has such a big price range?" asks Fischer. "It goes from $2 to $200 for the same size bottle."

Still, a certain appreciation of wine makes the work more pleasant. Fischer's favourite variety is Riesling, he says, because it can be very crisp or "lushly sweet". This prefers Syrah: "I am from that region and I am used to it now."

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