In the vineyard, the antepenultimate event is the harvest in which the grapes are removed from the vine and transported to the winery to begin the wine making process. In the Northern Hemisphere this is generally between September and October while in the Southern Hemisphere it is generally between February and April. The time of harvest depends on a variety of factors-most notably the subjective determination of ripeness. As the grape ripens on the vines, sugars and pH increase as acids (such as malic acid) decrease. Tannins and other phenolics also develop which can affect the flavors and aromas in the resulting wine. The threat of detrimental weather and vine diseases (such as grey rot) can also play a role in the time table. The balance of all these factors contributes to when a winemaker or vineyard manager decides that it is time to harvest.[1]
Look at the skin on the grapes. The best grapes will have a healthy shade to the skin. Ripe red grapes should have a dark purple hue, as any signs of green indicate that red grapes are not yet ripe. Ripe black grapes should have a full, rich shade of bluish-black, and green grapes should have a yellowish hue.[3]
In addition to skin tone, healthy grapes should also have a light silver-colored dusting on the skin called bloom.
Bloom is commonly mistaken for dirt, but it's actually the plant's way of protecting the fruit from drying out or rotting.[4]