Véraison is a viticulture term representing "the onset of ripening". It is originally French, but has been adopted into English (veraision). The official definition of veraision is "change of color of the grape berries." Veraison represents the transition from berry growth to berry ripening, and many changes in berry development occur at this time.
The initial phase of berry growth, now behind us, is a result of cell division and expansion. As berry growth slows the lag phase takes over. The lag phase is not a physiological stage, but a designation between the two periods of berry development. This is when veraision occurs.
Post-veraision, or when the fruit is entirely black, fruit acidity decreases with the degradation of malic acid. The degradation of malic acid during ripening elevates tartaric acid as the predominant acid; grape berries possess a small amount of citric acid. This is also an important flavor component.
At the same time acidity decreases, sugars accumulate. This accumulation of sugars (glucose,fructose) reduces the volume of water entering the berry relative to the volume of sugar, resulting in an increase in sugar concentration. Further increases in sugar concentration are due to dehydration of the fruit. The amount of sugar developing in the berry depends on the level of leaf photosynthesis.
As ripening continues, the fruit becomes attractive to animals due to changes in aroma from acidic to sweet and fruity. As ripening occurs, herbaceous aromas are reduced.
This is our biggest concern right now. Birds are keeping close eye, as are raccoons, our arch nemeses. We have turned on our "electric fence". These are two rows of wires around each of our over 50 rows, delivering 8000 volts of electricity at low amps for any curious critter.
We lost both our 2010 and 2011 Estate crops to raccoons. Thanks to the creative mind of Mike Mcquire, then 18 years old and working for the Sonoma County Wildlife Rescue organization, we installed our experimental electrical solution. This was after several years of trapping and killing over a dozen with no marked effect on the damage. We were outmanned and outgunned.
With our electical chargers clicking in cadence, notifying us of their diligent work, veraision is again a celebratory time. The vines are full and the berries multicolored.
The trigger of veraison is unknown, like so many things in agriculture.