With winter weather arriving later each year, wineries in the region are benefitting from giving late-maturing grape varieties like Cabernet Sauvignon increased ripening time on the vine. The challenge, however, is that hanging grapes later into the growing season can often bring them toe-to-toe with frost.
Although the grapes themselves can survive a light frost, researchers at Brock’s Cool Climate Oenology and Viticulture Institute (CCOVI) have discovered that frozen leaves and petioles, often referred to as MOG (materials other than grapes), can impact the final wine quality.
“As more wineries in the region opt to hang their late-maturing varieties into the late fall, you then contend with the addition of those frozen leaves and petioles in with the fruit,” CCOVI’s Andrew Reynolds explained. “When mixed with the fruit, the MOG increases the concentrations of compounds most likely responsible for an unfavourable floral or green aroma, decreased colour intensity and a bitter taste in Cabernet Franc and Cabernet Sauvignon wines.”
Reynolds and his team began studying the impact of frozen MOG after local winemakers asked them to investigate the origin of undesirable floral characteristics appearing in red wines they’d harvested after a frost in 2015. Emily Aubie (OEVC ’13), a winemaker with a PhD in chemistry, was invited back to Brock as a post-doctoral fellow to help tackle the problem.
After preliminary research, they discovered that wines containing the highest concentration of frozen leaves and petioles (the stem that joins the leaf to the cane) also had the highest concentration of a variety of odor-active terpene compounds.
“When we did the sensory on the commercial wines it was clear the wines identified as having high floral characteristics were the ones associated with these terpenes,” Reynolds said. “MOG in general — and particularly the petioles — most definitely increases the concentration of multiple odor-active compounds that are most likely responsible for floral taint in Cabernet Sauvignon and Cabernet Franc.”
Armed with these findings, the team is now expanding its research to strengthen the results and examine different methods for combating the problem, both in the vineyard and the winery.
Using varying levels of MOG additives, they are fermenting fruit that has been harvested using a variety of methods and strategies. These include hand-harvesting, using an optical sorter that separates all of the leaves, petioles and rotten berries as it harvests in the field, sorters that remove the MOG in the winery on a sorting table, and a technique where a leaf-blower removes the leaves before the harvest machine picks the fruit.
They are also experimenting with different yeast strains to see how effective they are in mitigating the floral aroma after the fruit is already picked and the wine begins fermenting.
Jiaming Wang, a wine chemistry specialist who is working with Reynolds on the project said the research is especially important for cool climate regions like Ontario, and will be of great benefit to local grape growers and wine makers.
“The most interesting thing for me is studying how different viticulture practices can actually impact the aroma of the wine,” he said. “This research will also help us understand how we can optimize the harvest strategy in terms of which yeast strain winemakers should choose, what kind of harvest method they need to use and whether they need to introduce more advanced machinery, like an optical harvester, into their practices.”