When are grapes ripe

There are quite a number of myths surrounding growing and harvesting grapes. The facts are pretty simple. Unlike some fruits, grapes will not improve or ripen after they have been harvested. They must be at peak quality and sweetness when you purchase or pick them.

• Green grapes are the sweetest and best flavored when they're yellow-green in color;
• Red varieties when the grapes are predominately red; and the
• Blue-black varieties when the berries have a full rich color.

Despite the old-wives' tale that persists, frost has nothing to do with the sweetness of the grapes - it is merely a coincidence that the first frost occurs at about the same time of grapes becoming fully ripe, in some areas! Note that last statement!  In many areas the grapes ripen long before the first frost; if you waited, they would be rotten!

There is at least one amateur website claiming that

"There are three grapes that bear the name "frost grapes." Vitis cordifolia, Vitis labrusca, and Vitis riparia all have varieties that sport that name because they are sour until the first frost sweetens them."

This strikes me as patent nonsense.  Again, it is much more likely that the usual ripening date for these varieties merely coincides with the average first frost date. As a side note, I could not find a single authoritative source (a university, a agriculture extension service, etc.) that says that frost improves the flavor or increases the sugar content of grapes.

How do you really know when grapes are ripe? Simple: TASTE ONE! That's really how it's done.  Vintner's have equipment (a hydrometer) to measure the density and sugar content of grapes, but a simple taste test is more than adequate for home growers.  If they taste good, pick them!