Edward Hellman of Texas A&M University has written the excellent article linked below to help grape growers know when to harvest.
Grape ripeness is an elusive concept for many people and sometimes an elusive achievement for vineyards.
Much of the difficulty with discussions of grape ripeness is that there is often an implied standard, but in reality, ripeness is an entirely subjective judgment. So, there are really two issues to address: 1) how do we define grape ripeness and 2) how do we measure ripeness parameters to assist our harvest decisions.
...Ripeness is really defined by the individual, whether grape grower or winemaker, and it is primarily a function of the intended use for the grapes. Often, an individualâ€™s definition of ripeness is also influenced by what is â€œtypicalâ€� for that variety in their growing region. Some benchmark of ripeness is achieved in one or more seasons and all subsequent crops are compared to that benchmark.
Read the full article for a wealth of information on how grape ripeness impact The Sugar vs Flavor Debate issue.
How to Judge Wine Grape Ripeness Before Harvest