When you sniff a glass of red wine, fruity and floral aromas should come to mind—not green beans and bell peppers. New research shows that certain viticultural practices can influence the concentration levels of methoxypyrazine, the aroma compounds that give the bell pepper and green bean smell to wines.
At low levels, methoxypyrazine compounds can add complexity to wines, said Dr. Justine Vanden Heuvel, viticulturist at Cornell University. But as the concentration increases, the green bell pepper aroma takes over and suppresses the fruity and floral smells.
"You can’t smell the fruity aroma as much, because it becomes overwhelmed by the green pepper aromas," she said during an aroma session at the convention of the Washington Association of Wine Grape Growers held in Kennewick, Washington. "In Sauvignon Blanc, the compound is a good thing, but when the consumer comes across the green pepper smell in Cabernet Sauvignon or Franc, they think it means unripe, green fruit."
The class of compounds responsible for the green pepper smell are found in wines at very low concentration levels—from 0 to 50 parts per trillion, she explained, adding that in red wines, the detection threshold is between 8 to 15 ppt.
In studying viticultural practices that influence concentration levels of methoxypyrazines on Cabernet Franc, a widely-planted cultivar in New York, Cornell University scientists have learned that growers can impact the levels in their grapes. Vanden Heuvel said that while the research was conducted on Cabernet Franc, it should be applicable to Cabernet Sauvignon grapes.
Higher levels of methoxypyrazines have been associated with shaded clusters compared to clusters exposed to sunlight, she said.
"These differences were stated very early in the growing season, as early as five days after bloom," Vanden Heuvel said, noting that while concentration levels vary, the degradation rates were similar for all levels.
"We really need to reduce the size of the peak concentration level, because that’s what is determining the level of concentration. If we can keep the peak lower, we will end up with a lower concentration at harvest."
Leaf removal studies with Merlot and Cabernet Franc were conducted in 2007 and 2008 to learn how field practices could reduce the methoxypyrazine levels. Leaves were removed at berry set, 30 days and 50 days after berry set, with two levels of removal severity—50 and 100 percent of the leaves removed from the first 5 nodes. She said that while the 100 percent removal might sound extreme, it is the standard leaf stripping practice of growers in the Long Island area. Berries were sampled five days after the treatment was implemented and at harvest.
She reported that the untreated control had the highest levels of methoxypyrazines at harvest and the lowest levels were treatments with leaves removed at berry set. "We had a big impact on how high the methoxypyrazine peak was by the leaf removal treatment, with the smallest peak at the 100 percent leaf removal at berry set." In 2007, a year with low methoxypyrazine levels, the 100 percent leaf removal at berry set in Cabernet Franc resulted in .5 ppt, while in 2008, the level was 2.5 ppt. However, Vanden Heuvel noted that in 2008, significant differences could not be detected between treatments.
"In summary, leaf removal in one year had no impact, but generally, leaf removal tended to reduce the methoxypyrazine levels, and it appears that removal at berry set or 30 days past berry set resulted in the greatest reduction," she said. Growers must consider their individual site conditions before removing leaves at berry set because in some locations, like New York, fruit set can be reduced if leaves are removed at berry set.
Cornell scientists are also studying a range of vineyard sites and practices to further look for practices that influence methoxypyrazine levels. In a multivariate trial, a host of vineyard characteristics and practices in four New York grape producing regions are being analyzed, from cultivar, training system, light, temperature, and rainfall to vine vigor, vine water status, and yield.
Preliminary data shows positive correlation between basal shoot diameter and methoxypyrazine levels. "The bigger the shoots, the higher the methoxypyrazine levels," she said. Shoot length was also positively correlated with methoxypyrazine.
Vanden Heuvel noted that they’ve found no correlation with Brix or yield, but grapes from warmer growing temperatures had lower methoxypyrazine levels. And while the cluster exposure component showed that the more light, the less methoxypyrazine, she said that light interception didn’t explain as much as they had hoped.
She concluded that preverasion levels largely determine the final methoxypyrazine concentration at harvest.
"If you’re looking for practices to reduce methoxypyrazine, you need to act early in the growing season, just after berry set," she said. "You need to prevent the peak at verasion from getting too high."
Data showed that the more light into clusters, the less methoxypyrazine; the bigger the shoots, the more methoxypyrazine, Vanden Heuvel said. "So again, it comes down to having a balanced vine. If you have a balanced vine, then hopefully you don’t have bull wood, but shoots that are around the diameter of a pencil."
She recommended that growers implement management strategies to improve light exposure of clusters, such as leaf removal, shoot thinning, reducing shoot vigor and using the appropriate training system for their site if they want to reduce the green vegetable aromas in their grapes.