Harvesting grapes is an extremely critical part of the winemaking process. In fact, in quality wine, when to pick is perhaps the most important decision a winemaker is faced with. Ideally, grapes should be harvested at the peak of their growth when sugar and acid levels are perfectly balanced.
Of course, it is not usually entirely up to the winemaker. Weather plays a pivotal role in the ripening of wine grapes and their condition at the time of harvest. If there is too much moisture, many grape varieties develop fungal disease. These grapes can be left on the vine a few extra days if further ripening is needed. If rain is forecast, however, the fungus can become a larger problem, so the winemaker has little choice but to pick the slightly unripe grapes. Rain can also be an issue even for heathy grapes. Again, if grapes are not yet ripe, but rain is expected, the winemaker must choose to either pick prematurely to avoid diluted juice or wait a few days and hope for some dry, warm weather following the rain to allow the grapes to ripen.
Once the winemaker has chosen to pick the grapes, he or she must decide what time of day to pick. In warm climates, harvesting usually takes place at night or in the cool hours of the morning in order to deliver the grapes as fresh and cool as possible to the winery. Although machines are increasingly used to harvest grapes, all of the best wines around the world are made with hand-harvested grapes. Hand picking the grapes insures a level of gentleness and discrimination that a machine cannot possibly achieve. Also, machine harvesters are unable to pick whole bunches of grapes at once.