It is late July, and we are enjoying beautiful summer weather here in the Napa Valley. Daytime high temperatures will generally range between 75 to 95 degrees (24-35 C) and the nights will cool down the high 50’s (14 C). Daytime high temperatures will vary widely throughout the valley and hills, which gives our area and its sub-appellations such unique characteristics. Today, for example, there is a 25 degree (14 C) difference in afternoon high temperatures between the Carneros and Pope Valley viticultural areas, which are both located within the broader Napa Valley.
This is an exciting part of the growing season when grapes start to change color. The color change is called veraison, which is a transitional period between berry growth and ripening. The onset of veraison marks a significant change in the fruit’s chemistry that will occur over the next 4 to 8 weeks.
The two main chemical changes that occur during ripening are sugar accumulation and acid conversions. At this point in the growing season, the primary sugar in grapes is sucrose, which is formed through photosynthesis. Sucrose is the same chemical as table sugar but it is not fermentable by yeast. As the fruit begins to ripen, enzymes will convert the sucrose to glucose and fructose. In the early stages of this sugar conversion, the berries contain only about 5% sugar, and there will be about 5 times more glucose than fructose. When the berries are physiologically ripe, which is about 40 to 60 days after veraison is complete, the fruit contains about 25% sugar with fairly equal amounts of glucose and fructose.
Grapes contain many types of acid molecules, but most of the acidity in grapes, which we call titratable acidity, is derived from malic and tartaric acid. When the grapes begin veraison, they contain approximately 75% malic acid and 25% tartaric acid. Over the next six to eight weeks, the fruit will use malic acid as an energy source and will consume about 85% of the malic acid. The tartaric acid levels in the grape remain mostly unchanged throughout the ripening period. When the fruit is finally ripe, the total amount of titratable acid will be cut in half and will be composed of about 70% tartaric acid, 27% malic acid and 3% citric acid.
Over the next two months I will start walking the vineyard rows to sample and taste berries, and conduct analysis on the sugar and acid levels. As the fruit becomes physiologically ripe, based on taste and analysis, the final judgment for making harvest decisions will ultimately come down to a careful balance between sugar and acid levels, flavors and textures. We anticipate a slightly earlier harvest this year, but all the pieces are in place for a great vintage!