For the best batch of organic grapes (Vitis), it is essential to pick the grapes when they are ripe. See below for ways you can test for ripeness.
Tasting is the best test for maturity. If grapes are sweet and flavorful, they are ready to pick.
The next best guide is color. Green varieties, such as ‘Romulus’ and ‘Thompson Seedless,’ turn whitish or yellowish; black and red varieties take on an added depth of color.
If you see birds on the vines, it is a good guide to ripeness. Once grapes have been picked, they will not ripen further. Therefore, wait until your crop is fully mature, then harvest by clipping off the bunches with scissors or a sharp knife. Pick them on a dry day because wet grapes do not keep as well. Discard overripe, withered or diseased grapes soon after picking before they spoil the bunch.
Grapes that are intended for raisins, such as ‘Thompson Seedless,’ should be left on the vine somewhat longer than wine or table varieties in order to increase the sugar content. Grapes to be used for jelly can be picked before they are fully ripe.
Grapes can be stored for several weeks at a temperature of about 32°F (0°C) and a humidity of 90 percent.