What happens to grapes when they are left on the vine for extra ripening?

When grapes are left on the vine for an extended period, they undergo a process called over-ripening, which increases their sugar content significantly. As the ripening continues, water content in the grapes decreases due to evaporation and dehydration. This leads to the sugar concentration rising, and in some cases, the grapes can start to shrivel and transform into raisins if they are left on the vine long enough. This phenomenon is utilized to produce certain styles of wine, particularly sweet wines, as the high sugar content enhances the richness and complexity of the final product.

The other options do not accurately describe the effects of prolonged ripening on the vine. While grapes do lose acidity as they ripen, the focus on sourness in the second option is misleading since the primary change is the increase in sugar levels. The idea that grapes would stay the same or merely shrivel without increasing their sugar content does not reflect the importance of the ripening process, where sugar accumulation is a key outcome.