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Counting degree days may be a useful exercise when selecting a vineyard location, but it’s less profitable when you’re trying to figure out when to harvest the grapes the site has produced.

"Degree days are not telling you the whole picture," Dr. Mark Greenspan told growers at the annual viticulture conference hosted by the B.C. Wine Grape Council in Penticton, British Columbia, Canada, in July. "Generally speaking, degree days don’t have a lot to do with the ripening process."

While a correlation exists between growing degree days and the time it takes for vines to reach bloom, Greenspan has found degree days less helpful in tracking fruit development between bloom and harvest.

This is particularly true in California, where Greenspan, principal of the consulting firm Advanced Viticulture LLC, is based. Since ripening is more than sugar accumulation, Greenspan told growers that they have to consider how daily fluctuations in weather and temperature affect maturing grapes. Rather than focus on sugar accumulation, they have to take into account the development of compounds such as acids, anthocyanins, tannins, and others that contribute to the ultimate flavor of wine.

Many flavor characteristics aren’t detectable prior to crush, meaning growers have to pay particular attention to other aspects of fruit maturity.

"It’s controversial, but I won’t talk about sugar –accumulation in the same sentence as ripening," he said. "I prefer to think of it as flavor ripening or flavor –maturation."

So, what should growers take into account if degree days aren’t reliable and maturity is more than degrees Brix, the standard measure of sugar content? Berry temperature, Greenspan says, drawing a fine distinction in the mechanics of maturity. While sugar accumulation is dependent on canopy leaf area, its relation to crop load, and other factors outside the grape, the development of the compounds that contribute to flavor characteristics occurs within the grape.

Greenspan therefore focuses on grape temperature, recommending that vine management allow grapes adequate exposure to the sun so they can reach an –optimum temperature for full flavor development.

"Almost everything comes back to berry temperature," Greenspan told growers. "There’s some temperature optimum that’s going to increase the rate of flavor accumulation, flavor development in the fruit."

This is particularly important during the critical ripening period between August 15 and October 15, which explains why a dramatic variation in temperatures between day and night isn’t necessarily a good thing. A chilly night may cool down the grapes too rapidly to allow proper development of flavor compounds, while too much heat during the day risks stimulating aggressive photosynthesis that boosts grapes’ sugar content but does nothing for flavor.

Climate changes

Climate change only complicates matters, Greenspan added. An increase in greenhouse gases—particularly carbon dioxide—boosts the rate of sugar accumulation, potentially putting it further out of step relative to flavor development.

"The higher amount of carbon dioxide in the atmosphere will provide a greater store of carbon for the purposes of the grapevine," Greenspan said. "Maybe it’s part of the reason we’re having a struggle throughout the world to manage elevated sugar accumulation."

How, then, to determine when grapes are ready to harvest?

Since the flavors that show in a wine won’t all be present in the grape at harvest, Greenspan suggests growers as well as winemakers look at maturity characteristics rather than flavor. While sugar content will remain an important measure, Greenspan also pointed to the Quantitative Descriptive Sensory Analysis method put forward by France’s Jacques Rousseau, which assesses the fruit, its pulp, skin, and seeds.

"Brix isn’t going to go away," he said. "[But] what we’re really trying to do is train ourselves to recognize some of the flavors that are important to the wine."