Farming for aromas
Practices that encourage cluster exposure can improve grape aromas.
Growers know that viticultural practices influence a whole host of things in the vineyard, such as yield, berry size, disease incidence, and grape color. But farming for aromas and flavors?
An international team of viticulture and enology experts helped Washington State's wine industry better understand what can be done to encourage desired aromas in the grapes. The aroma management session was part of the Washington Association of Wine Grape Growers annual meeting.
The half-day session was unique in that it was presented a week earlier in California at the Unified Wine and Grape Symposium, originating as a priority of the National Grape and Wine Initiative. NGWI is a nationwide coalition that focuses on research and extension to strengthen the competitiveness of America's grape and grape product industries.
Wine grape growers don't typically think of themselves as aroma farmers, but aroma compounds play an important role in wine quality. Today's scientists have unlocked the complexities of grape aroma components and can claim understanding of nearly all the most important odorants of wine, Dr. Vincente Ferreira of the University of Zaragoza in Spain told the packed-room audience. Researchers have identified 47 compounds that are regarded as positive potential contributors to wine aroma nuances and more than 30 compounds that can negatively affect wine quality, he said, adding that technology allows scientists to take molecules out of wine and put them back as they deconstruct and reconstruct wine aromas.
"The aroma of wine is not static," Ferreira said, explaining that it can be considered as a "living system in which aroma precursors act as 'pumps' to deliver a continuous flow of odor chemicals until its exhaustion (death)." The grape plays a key role in the aroma of wines by conditioning yeast by-products, delivering odor chemicals that are released by the yeast, and delivering the pool of precursors. Many of these compounds that are released during fermentation are important because they provide the aroma strength needed for wines that are aged.
Ferreira said that bad or off-flavor aroma compounds are those that induce the suppression of a positive aroma nuance. The "baddies" can be aromas that are too strong, reminiscence of a bad or unexpected odor.
In terms of aromas, wines can be characterized as simple, having only one or two impact compounds, or they can be complex, the result of the complex interaction of many chemicals. He noted that for some aroma molecules, just one part per trillion can change the wine's aroma.
His Spanish team of scientists, in analyzing nearly 100 wines from different parts of the world, found there are aroma compound differences from the same varietals grown in different world regions. For example, Sauvignon Blanc wines from New Zealand have higher levels of the compound 4M4MP (4-methyl-4-mercaptopentan-2-one) compared to French and Spanish Sauvignon Blanc wines.
In summarizing the role of aromas in wine, Ferreira likens wine quality to an orchestra. "Quality is given by harmony and it is easy to discover who is out of tune. Complexity of sound increases with the size of the orchestra—the higher the complexity, the smaller the role of individual compounds."
Ontario, Canada's Dr. Andrew Reynolds of Brock University, shared several viticultural practices that can have direct impact on wine quality and wine aromas.
The odor impact compound methoxypyrazine is responsible for the vegetal aroma (bell pepper, green bean, asparagus) and can be present at very low concentrations—five to ten parts per trillion—and at very low odor-active thresholds. "It appears that there are no enzymes that break them down," Reynolds said, pointing out that strong sunlight causes their breakdown.
Viticultural practices that encourage cluster exposure, such as basal leaf removal and vertical trellis systems that keep fruit in the sunlight will reduce methoxypyrazine concentration levels in the grapes, he said, while practices that lead to excessive shading, like excessive irrigation or high planting density, can increase concentration levels.
Reynolds said that researchers in southern France found they could substantially increase concentration levels of some norisoprenoids (volatile thiol precursors responsible for mint, eucalyptus, and floral aromas) in grapes by using reflective mulch material under the vines.
Much of Reynolds' work has focused on improving the concentration of terpenes that are responsible for many wine aromas. Based on years of research, he has developed four aroma management practices that can improve the concentration of terpenes and other important active-odor compounds in cool-climate grapes:
• Keep fruit exposed to sunlight
• Make sure leaves receive sunlight
• Achieve vine balance
• Minimize water stress
He shared results of studies that showed removing basal leaves in vigorous vineyard sites as well as in Riesling, Muscat, and Viognier vineyards had significant impact on increasing free or potentially volatile terpenes or improving floral, apricot, mango, and Muscat aromas in the wine.
Reducing shoot density encouraged development of higher levels of fruit aromas compared to the more green aromas from wines grown under high shoot density. Vine spacing also had an impact, he said, adding that with deep soils, there was a linear increase in the levels of potentially volatile terpenes as the vine spacing increased. Using trellis systems like the Scott Henry or Geneva double curtain, both which divide the canopy, also increased the terpene levels.