Scientists have unmasked a culprit responsible for contaminating untold bottles of wine with the musty, corky odor generally known as taint.
More than 20 years after the isolation of MDMP, a compound that can turn even the finest wine into plonk, the identity of a microbe that churns out the stuff is now in hand, researchers report online November 8 in the
Journal of Agricultural and Food Chemistry
. The next step is figuring out where the critter thrives and when in the journey from vine to bottle contamination is most likely.
“This is a valuable step,” says flavor chemist Mark Sefton of the University of Adelaide in Australia. “Once you have identified and characterized it, you can look for it in the environment, understand its metabolism and understand under what conditions does this bug grow and get into our food. The more you know the easier it is to control.”
Recent advances in understanding the sources of TCA, another molecule that can foul even the finest wine with a similar “corked” flavor, have allowed winemakers to take steps to avoid that contaminant’s production.
Depending on the quantity of contamination, 2-methoxy-3,5-dimethylpyrazine, or MDMP, may just make a wine smell “off,” says Pascal Chatonnet of Excell Laboratory in Merignac, France, who led the new work.
“The first impression may be something is not correct — not that the wine is contaminated — but it is bad,” Chatonnet says. “You say ‘I am in front of a bad wine.’”
In larger quantities, MDMP smells like “an old damp dishcloth that’s gone moldy with slightly coffee, slightly nutty overtones,” Sefton says. The compound had been characterized in 1984 by chemists looking at off odors in machine shops, but those researchers couldn’t ID the responsible microbe. Sefton and colleagues then isolated MDMP from wine corks in 2004. Chatonnet and his colleagues went back to the machine-shop isolate, cultured the microbe in a dish and extracted its DNA. A phylogenetic analysis revealed that the MDMP maker was a previously unknown species of Rhizobium, which the scientists have named R. excellensis (not because it excels at making MDMP, but after their laboratory).
The bug could be getting into wine via oak chips, which many wine-makers who don’t use oak barrels use to impart tannins and flavor, Chatonnet and his colleagues report.
Producers who employ oak chips should be sure to toast their chips: 10 minutes of toasting at 220° Celsius eliminated 93 percent of MDMP, the team found.
The team also found that about 40 percent of untreated, natural corks sampled had detectable levels of MDMP. “No more untreated natural corks,” says Chatonnet, “We are taking too many risks.”
Though Chatonnet and his colleagues have successfully identified the MDMP-making microbe and shown ways MDMP may get into wine, the definitive step will be to isolate the microbe itself from cork or other places, says Susan Ebeler, a chemist in the viticulture and enology department at the University of California, Davis. Then researchers can figure out where the microbe thrives and how best to eliminate it.
The compound best known for imparting a moldy “corked” taste is 2,4,6-trichloroanisole, or TCA, and its chemical siblings. But reports of TCA contamination have declined in last decade or so, as research focused the efforts of winemakers and cork producers to eliminate the microbes that make TCA or at least stop its production. Corks are no longer rinsed with chlorinated water, for example, and fungicides containing chlorine are avoided, as the element is an important ingredient in TCA. Similar efforts may do the same for MDMP.
