Winemakers often say that the most critical decision made is when to harvest grapes. Why does this matter?
1) Sugar/Acid Inflection: Heat facilitates ripening, in which sugar increases (generally measured in Brix units), while acidity (generally measured in pH) decreases. Since sugar and acid are inversely correlated, the decision to harvest considers the trade-off between sugar and acid levels so neither is perceived as imbalanced.
2) Phenolic Ripeness: Phenols are molecules (e.g., tannins) that progress in the skin and seeds of a grape. As they develop, they move from bitter green taste profiles to complex maturity that may be perceived as structured tannins and a balanced texture.
3) Flavor Maturity: In theory, each site has a flavor profile at maturity. Harvesting before that site reached its flavor maturity may lead to an underripe perception. For example, if a site typically provides a flavor profile of black cherry and spice, harvesting when the flavor profile is still strawberry and cranberry may not optimize the site’s potential flavor profile.
4) Avoid Damage: If there are imminent threats such as rain or predator damage, a winemaker may make the decision to bring the grapes into the winery to avoid resulting damage such as rot development from wet clusters or water weighted clusters that can seem diluted.
Winemakers need to balance these sometimes competing forces when making the decision of when to harvest, but understanding these factors can increase our understanding of how the end product of wine is impacted.