Prior to harvest look for a whitish coating on the grapes. This is an indicator of ripeness. Fully ripe grapes are usually slightly less firm than unripe fruit. Open one of the grapes and observe the seed color. Seeds of ripe grapes often change from green to brown. Finally, if all other measures indicate the grapes are ripe, do a taste test to make the determination.
Grapes can be stored for up to eight weeks at 32 degrees F with 85 percent relative humidity. They can also be processed into jams and jellies. (Cynthia Domenghini)