Dr. Dario Cantu, an assistant professor at the University of California at Davis, is a longtime devotee of sweet wines, and it's little wonder: For the past 11 years, he's made it his business to study Botrytis cinerea, the fungal infection that can elevate grapes to honey-rich wines by producing so-called noble rot—or rapidly devastate acres with gray rot. Now Cantu and his team at the viticulture and enology department have discovered how noble rot impacts grape metabolism, and their findings could have a larger impact on flavor development in winemaking.
The three-year study, recently published in Plant Physiology, first took shape in November 2012 when Cantu visited winemaker Greg Allen, who produces sweet wines in the style of Bordeaux Sauternes at his Napa Valley Dolce winery. As Dolce's Sémillon grapes began to show the effects of noble rot, Cantu noted that the changing color of the berries from yellow to pink looked similar to red grapes in the midst of veraison, the period when grapes change color as they ripen.
This was thought to be chemically impossible: White grapes are white because, as Cantu explains, "They are mutants." White grapes possess a genetic mutation that prevents them from producing anthocyanins, the color compounds responsible for the red, purple and blue hues in grape varieties such as Cabernet Sauvignon and Pinot Noir.
Vintners have long known that grapes with noble rot change from yellow to pink to purple, but scientists could shed little light on why. Cantu and his team found that the noble rot changed the grapes' metabolism and activated the biosynthesis of color development.
Noble rot, simply put, is a fungal infection that feeds off the dead matter of grapes. As the fungus grows on the berries, it puts stress on the grapes’ metabolic pathways, effectively speeding up ripening and eventual death. Like an ornery teenager, the grapes' hormones go into overdrive, rapidly enhancing the development of color compounds, as well as flavor and aroma compounds. Add warm, wet weather, followed by drier weather, a meticulous winemaker and a bit of luck, and a rich sweet wine is the final result. (Learn more about how botrytized and other sweet wines are made.)
As Cantu explains, the study offers potential insight into more than just noble rot. "We're showing that we now have the tools to study flavor development in the vineyards," said Cantu. "If we can control this change of metabolism in the absence of noble rot, we could tweak the system."
If scientists can manipulate the metabolism—and thereby the color and flavor—of grapes, just as botrytis does, they could better control ripening. Winegrape production could thrive in areas previously considered less than optimal for winemaking. That could be even more relevant as climates change.
"Temperature is critical. Understanding how these environmental factors change, and their development on ripening, is critical." Cantu stressed that changes in grapes' metabolism occur in response to any environmental condition; noble rot is just one example. "I don't want to use the word terroir, but it all relates back to the impact of the environment."
Plant scientists have previously added hormones to plants to dictate the development of growth, but a more detailed understanding of altering metabolisms could lead the way to new substances, such as liquids or sprays, that could direct flavor and color development. The sky is the limit, but for the time being, Cantu says, "What we now want to control is ripening."