The grapes at the HRC are just hitting véraison. This is also known as the “onset of ripening” in grapes, and it’s a exciting point in the physiology of the grapevine. At this point in time, the vines start diverting all their energy from photosynthesis into the grape. It’s also exciting for winemakers, because we can start to see that the date of harvest isn’t too far off. Historically, at least for V. vinifera varieties, harvest is about 45 days after 50% of the grapes have changed color. Good vineyard management ensures that the grapes will accumulate sugar from photosynthesis and burn up some of the acids that were stored during the vegetative growth phase.
During this second period of growth, grapes become a sink for nutrients. Glucose and fructose are the main sugars present in grapes. In green (unripe) grapes, glucose represents nearly 85% of the sugars. The ratio being about 5:1 glucose to fructose. At véraison, this ratio decreases to 2:1, and at harvest the ratio is closer to 1:1.
The second most important phenomenon for winemakers is the fact that malic acid (malate) that has accumulated in the grape is now “burned up” through the vine’s respiration. Malate is biologically a very important compound in various biochemical pathways. At véraison, the vine increases respiration, and malic acid takes on the role of energy vector for the vine. At night, or at low temperatures, excess imported malic acid is transformed to glucose by gluconeogenesis. This is one of the reasons why regions with high diurnal temperature variation produce higher quality grapes. At harvest, tartaric to malic acid ratio should be around 2:1. This ratio varies according to grape variety and seasonal differences, however malic acid has been observed to be between 40 and 60% of the total acidity in most wine regions. Overripe grapes contain little malic acid (this is normally not a problem in northern climates).
Potassium is one of the few minerals that continues to accumulate in the grape berry after véraison. It plays an important role in the pH of the juice/wine, so this is why monitoring the pH as you approach harvest is important. While pH rises in part due to the decrease in total acidity, as potassium uptake continues you will see the pH increase at a more disproportional rate to your total acidity. In warm climates where fruit is typically overripe, high pH is usually a problem.
Other important compounds such as nitrogen uptake, and the evolution of grape phenols and aromas are also an important part of the maturation process from a winemaking standpoint. Nitrogen is consumed by yeast during fermentation. Juice should contain a minimum of 160 mg/L of yeast available nitrogen (YAN) for a proper fermentation (it can be supplemented with a maximum 300 mg/L Nitrogen if deficient).
As sugar and acid levels begin to look like they are reaching the right levels for harvest, it is important to begin evaluating the tannins and aromatic development of the grapes by tasting them. Chew on the skins and seeds to get an idea of the phenolic ripeness.