The scent of a vineyard
The small size of terpene compounds allows them to volatize, giving off distinctive smells for different wine grape varieties.
Dr. Kerry Ringer's new role as Washington State University Extension enologist and assistant food scientist will take her sniffing around vineyards and wineries as she studies the scent of grapes.
Ringer, an Ellensburg, Washington, native who majored in biology at Western Washington University and received her doctorate in molecular plant sciences at WSU, joined the viticulture and enology research and Extension team in August. Ringer has a broad background in the wine and food science industries. She developed a wine quality program at Covey Run Winery and was co-founder of a plant and process development company where she helped invent and patent fruit dehydration technology and methods to determine total antioxidant activity in wines, foods, and other compounds.
Ringer gave grape growers a short course on the major aroma compounds found in grapes and wine during the Washington State Grape Society's annual meeting held in Grandview.
What's in a vineyard scent?
The different smells of wine grape varieties come from complex blends of aroma compounds. Of the hundreds of aroma compounds in grapes, esters are found in both Concord juice and wine grapes, while terpenes are found only in wine grapes, she explained.
"In Concord grapes, there is really only one major compound associated with Concords, and that is methylanthranilate," Ringer said, adding that it was identified in 1922. The compound is used to flavor cough syrups and as a bird repellent. Methylanthranilate is what gives the foxy odor in Concords.
In today's research laboratories, molecular biologists are identifying the genes involved in making aroma compounds and the enzymes involved. For example, alcohol acyltransferase is the enzyme that makes methylanthranilate. Researchers have also learned when the grape begins making the Concord enzyme (about ten weeks after flowering until ripening is complete) and where it is found (in the pulp near the grape skin).
In the eight years that terpenes have been studied in grapes, more than 40,000 different terpenes have been identified, she noted. Terpenes have important functions in plants, such as attracting pollinaters and helping to defend the plant. Terpenes are found in some pharmaceuticals, in essential oils, and in many spices, including rosemary, basil, and black pepper. Taxol, a plant terpene, is now used as an anticancer agent for breast cancer.
Ringer said that the three important aroma groups of terpenes in wine are monoterpenes, norisoprenoids, and sesquiterpenes. "The compounds are very small—that's the reason why you can smell them," she said, adding that their small size enables them to volatize and give off odors.
Monoterpenes are responsible for the floral, fruity smell of white wines. Norisoprenoids are more complex, giving off honey, fruity, and kerosene smells in red and white wines. Sesquiterpenes are found in white wines. Glycosylated terpenes are developed late in wine grape maturity and are no longer volatile, which is why they have no smell or taste.
Researchers know that terpenes are made in the skin, pulp, and seeds and that very small quantities can have an effect on wine. Researchers around the world are now focused on terpenes as they try to identify new ones and how the terpenes are made in the grape. Australian scientists are trying to identify the compound in Syrah grapes that gives the wine its characteristic "peppery" smell. Ringer will be working with terpenes in her research, trying to understand how and when the aroma compounds are biosynthesized in the wine grape.