Unlike many fresh fruits, grapes are harvested fully ripe. After they're picked, they do not become sweeter, so timing is everything.
Determining when grapes are ripe is a real science and both the U.S. Department of Agriculture and California Department of Food and Agriculture are involved in setting and monitoring grape production standards. Sugar content, color, bunch and berry size and uniformity are all measured before harvest begins and the workers who decide which grapes to harvest are trained professionals with years of experience.
Once picked, fresh grapes are easily damaged by rough handling, warm temperatures, excessive moisture and decay-causing organisms. Consequently, grape bunches are carefully inspected and then immediately packed by hand into shipping containers – often right in the field.
Shortly after picking/packing, the field heat is removed from the fruit in cold storage facilities. Grapes are stored at temperatures between 30F and 32F. From this point until they reach their destination – markets throughout the world – the grapes will be maintained in a carefully regulated environment to assure they arrive in just-picked condition.