Predicting Harvest Date

Although we don't have any grapes this year, having just planted our vines in May, I was very interested in an article in the May, 2013 issue of Practical Winery & Vineyard called Predicting harvest date using berry sugar accumulation.¹ From the U.C. Davis online course, I am aware that berry sampling should be done as part of the harvesting decision.² I found one of the most useful graphs on grape ripening in the July/August 2002 issue of Practical Winery and Vineyard Journal called Understanding Grape Berry Development.³

The May 2013 article explained a concept called "berry sugar loading" which occurs after véraison, the onset of fruit maturation. Berry sugar loading leads to berry softening as sucrose is being transported to the berry and hydrolyzed into it's components, glucose and fructose, allowing those sugars to be accumulated in the berry, creating a concentration gradient. The berry sugar loading reaches a plateau and the duration of this plateau can be used to determine the level of ripeness in red cultivars.

Linda Bisson wrote: One of the most important decisions made by the winemaker is the time of harvesting of the fruit. The chemical composition of the berry at harvest will largely dictate the chemical composition of the finished wine.² This statement corroborates the often repeated "The wine is made in the vineyard".
References:
1. Alain Deloire, Predicting harvest date using berry sugar accumulation, Practical Winery & Vineyard, May, 2013, pg. 58-61.
2. Linda Bisson, Lecture 3: Viticulture from the Winemaker’s Perspective, VEN124.
3. James Kennedy, Understanding Grape Berry Development, Practical Winery and Vineyard Journal, July-August, 2002, Illustration done by Jordan Koutroumanidis of Winetitles.