Véraison [vay-ray-ZON] is a wine term which refers to the onset of ripening. It is French word, but is used throughout the wine world. The official definition of véraison is "change of color of the grape berries." Véraison represents the transition from berry growth to berry ripening. In the northern hemisphere, véraison typically occurs anywhere from late June to mid August, depending on the climate.
Here is a short video from Adopt a Grape, discussing the importance of véraison in the vineyard.
Veraison '08 at Adopt A Grape from Adopt A Grape 2009 on Vimeo.
Following fruit set, grape berries are green and hard. They have very little sugar and are high in tartaric and malic acids. They begin to grow to about half their final size when they enter véraison. During this phase the colors of the grape change from green to red or yellow, depending on the variety. For those big on science, this color change is due to the chlorophyll in the berry skin being replaced by anthocyanins (red wine grapes) or carotenoids (white wine grapes). The berries start to soften as they build up sugars. Within six days of the start of veraison, the berries begin to grow dramatically as they accumulate glucose and fructose and acids begin to fall. If you were to look at a graph of acid versus sugar levels, you would see that véraison occurs when the decrease in acid crosses over the increase in sugar. The acids are actually being "burnt up" by the respiration of the grape berry. If you want to get into great detail on the actual science, check out "Wine Science Principles and Applications" by Ronald S. Jackson, or "Knowing and Making Wine" by Emile Peynaud. Both books are very technical.
Some wine growers will perform a green harvest, where less ripe bunches are removed. The idea is help concentrate all the energy into the remaining berries, which in turn, should produce more concentrated wines. There is a bit of a gamble however, namely if the remaining time between véraison and harvest experiences severe drought or excessive heat, the result could be dry berries, which produce unbalanced wine. So, when you visit the vineyard this time of year, you'll see growers checking their crop. They know that once véraison starts, they are only about 45 to 50 days from harvest. And that can be 45 to 50 days of holding their breath!
Let the countdown to harvest begin!