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The genetic diversity of yeasts could produce novel flavours

MORE than 7,000 years ago, people living in the Middle East discovered that they could ferment grapes to make wine. The yeast that they unknowingly harnessed for the process can now be found in every vineyard on the planet. As with wine, the processing of coffee beans and cacao, used to make chocolate, also requires some fermentation. But new research shows that coffee and cacao yeasts are far more genetically diverse than wine strains. This opens up the intriguing possibility of imparting entirely new tastes to the terroir of coffee and chocolate.

Cacao originated in the Amazon and was widely cultivated in Central America before Hernán Cortés brought it to the Old World in 1530. Coffee moved in the opposite direction. From Ethiopia it was disseminated throughout the Middle East by Arab traders during the 6th century and ultimately arrived in the New World during the 18th century, where nascent Americans may have seen drinking it as something of a patriotic duty after the Boston Tea Party.

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As Europe’s thirst for coffee and chocolate grew, merchants keen to cash in on the crops started establishing vast plantations wherever the plants could be cultivated. In the first part of the 17th century, Dutch traders transported a Yemeni coffee plant to Holland. Shortly thereafter, they began cultivating its descendants in Sri Lanka and on Java and Réunion. Over the next three centuries, other trading nations completed coffee’s worldwide dissemination and set it up as a mainstay crop of many of the world’s poorest economies. Cacao was treated in much the same way and is now grown in 33 tropical countries.

Given this history, Aimée Dudley of the Pacific Northwest Diabetes Research Institute, in Seattle, and Justin Fay of the University of Washington and their colleagues, wondered if the yeasts associated with cacao and coffee followed these plants from their places of origin just as yeasts had followed wine from the Middle East. To explore this, they collected unroasted cacao beans from 13 countries, including places as disparate as Colombia, Ghana, Madagascar and Papua New Guinea, and unroasted coffee beans from 14 locations, including Ethiopia, Honduras, Indonesia and Yemen. They then set about studying the yeast found on the beans. As a control, the team also studied the yeasts on grapes from diverse locations.

As they report in Current Biology, although all vineyard-yeast strains are extremely similar genetically, there is tremendous diversity among the yeast strains associated with cacao and coffee. More specifically, they discovered that these differences correlated with geography. For example, all cacao beans collected from Venezuela carried closely related strains of yeast that were distinct from those found on Nigerian and Ecuadorean beans. The same was true for the yeasts found on coffee. The differences were so great that the researchers were able to use DNA sequences of the yeast strains alone to determine which country a sample of cacao or coffee came from.

Why cacao and coffee yeasts vary so much is unclear, although human behaviour is likely to play a role. The researchers give several reasons why wine yeasts are so similar. Oak barrels are often exported from an established winemaking region to an area of new cultivation, and these serve as reservoirs of yeasts native to the original location. Winemakers also have a long history of using starter cultures of yeast from places that have traditionally produced wines, which makes it nearly impossible for local species of yeast to compete. In contrast, the use of starter cultures is very rare in the processing of cacao and coffee, where growers tend to rely upon the species of yeast found locally.

This greater diversity of cacao and coffee yeasts means there is the potential to create new flavours by using a strain from one location in another, the researchers reckon. The yeasts of a Hawaiian coffee bean could, for example, be used to ferment beans being grown in Uganda; or the yeasts from Haitian cacao beans could be used with cacao grown in Ghana. No one knows what the resulting coffee and chocolate might taste like, but if Dr Dudley and her colleagues are correct in their hunch, there will be many new flavours for coffee lovers and chocoholics to savour.

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