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The color of the vine also indicates when to harvest Concord grapes. Throughout the growing season, the vine and stems are a light to medium green. When the vine and stems begin to turn to a light then medium brown, it means the grapes are ripening and ready to harvest.
Â· just now. Report Abuse. They will not sweeten by waiting. You can slice them in two and marinade in a simple syrup or you can whip up egg white and dip the dry grape into it and roll or shake sugar over the grapes (I like to do this with small bunches of 3 or 4 grapes).m not sure it will work but the best way to ripen fruits that have been picked already is to put them in a paper bag, close it, and let them sit for a few days at room temp. However not all fruits already picked can be ripened off the vine or tree etc,. Otherwise, frost them with sugar. earswithfeet Â· 8 years ago.