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In Sicily, a Treasure Trove of Wine Yeast
6 March 2012 4:10 pm
Sicilian winemakers don't stomp grapes with their feet anymore. But in the south of the Italian island, many vintners still follow many of the old ways, making wine by mixing mashed grapes in old stone and concrete troughs and then letting fermenting microbes native to that same vineyard convert sugars into alcohol. Researchers have now taken a census of these local wine yeasts, showing that many can compete with, or even outstrip, widely sold industrial yeasts used to make wines.
In Italy and elsewhere, modern wineries shun dirt, says Daniele Oliva, a molecular biologist at the Istituto Regionale della Vite e del Vino in Palermo, Italy. Big vintners, for instance, usually clean their equipment between uses and rely on commercially purchased yeast strains to ferment their grapes. Consumers "want the standards of a hospital in these little places where they make a little wine," Oliva says.
But not in much of southern Sicily. Here, vintners make wine much as their ancestors did centuries ago, which means that yeast hiding beneath grubby fingernails ride to and from farmland, while others cake the bottoms of poorly scrubbed tubs. Those cuticle-carried microbes include wild strains of Saccharomyces cerevisiae,the same yeast species that's grown and sold commercially for winemaking.
Oliva says he suspected that Sicily's family vineyards created isolated environments where never-before-seen strains of yeast evolved to fit local conditions. To dig into this hidden diversity, he and colleagues scraped yeast directly out of the antique-style fermentation basins in southern Sicily wineries. This survey found 209 unique yeast strains spread across seven sites, the group reported last week in PLoS ONE.
The team also put the local yeast to the test by making wine. Oliva and colleagues used two local strains to ferment juice from the Nero d'Avola and Frappato grapes, both also Sicilian. The local strains consistently produced wines between 12% and 12.5% alcohol by volume. The team members saw very similar alcohol profiles when they mixed a common type of industrial yeast into those same musts. One of the Sicilian yeast strains also churned out high levels of esters, sought-after byproducts of fermentation that give wine its fruity aroma.
Lots of esters, however, don't necessarily make a good wine, so Oliva and colleagues hosted blind taste tests for Sicilian wine experts. Sure enough, the oenologists preferred wines made from local strains, noting that they seemed to preserve the character of the Sicilian grapes. In the case of the Nero d'Avola, that meant wine that maintained the fruit's slight cherry flavor.
The local yeast's prowess makes a good case for preserving microbial diversity on vineyards in Sicily and elsewhere, Oliva argues. His team has already marketed one of the island yeast strains to winemakers looking to produce fruity wines low in alcohol. But connoisseur's tastes are also constantly evolving, he says, so it's useful for winemakers to have a range of yeast capable of making equally wide arrays of libations—say, varieties heavy on the booze and light on cherry undertones—at their fingertips.
Lucy Joseph, a curator at the University of California, Davis, Wine Microbe Collection, agrees, saying that yeast catalogs provide scientists and vintners alike with a bank "that can be drawn on in the future." Other researchers have discovered similar microbial treasure troves in winemaking regions across the globe, but Oliva's team was thorough in characterizing the yeast isolates, she says.
Isak Pretorius, yeast biologist at the University of South Australia in Adelaide adds that scientists have described less than 1% of the estimated 700,000 strains of wine yeast worldwide. That means that yeast wranglers such as Oliva have a lot more work on their hands: “There is still a lot of untapped potential that can be mined.”