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When I first started working at the State Fruit Experiment Station back in the early 1980s, I remember the Catawba grape growers would taste a couple of berries and say that they were ready to pick for the wineries. Growers were just beginning to routinely take pH and acid readings, along with sugar level at that time, to decide when to pick or not to pick the grapes.
There are several ways you can tell that grapes are getting ripe, but veraison or color change is the signal that the grape clusters are ripening. Veraison occurs when the colored grapes start to change from green to red or blue and the “white” grapes change from an opaque green to a more translucent green or golden color. Once the color has changed and the berries have sized, taste a berry to assess its sweetness and flavor. If you grow grapes for table use, this is really all you need to know in order to decide when to pick. Harvest the grapes for fresh eating when the berries are sweet and taste good. Grapes will not ripen further after harvest.
Grape harvest guidelines for wine making are a bit more involved than those for fresh eating.
White wine grapes are harvested when berry pH reaches between 3.1 and 3.3, and red wine grapes are harvested at a pH between 3.3 and 3.4. The pH in these specific ranges is important so the wine does not spoil. The titratable acidity is also measured and should be 7-9 g/l in table wines.
Sugar levels are also measured, and in general, should be the highest level possible as long as the pH and acid levels are in an acceptable range. The integrity of the cluster is important. Desirable clusters have no rot or other damage. If there is rot in the clusters, the spoiled berries should be removed before the grapes are crushed for wine. Harvest grapes with a grape knife or shears and place in a clean bucket or other receptacle. Grapes with no rot or broken skins can be stored in the refrigerator before use for up to two months.
Dr. Karl Wilker, research enologist and winemaker at the Mountain Grove Cellars, still tastes the grapes to assess the “fruitiness” or varietal character. This subjective observation along with our sugar, pH and titratable acidity levels, as well as conditions such as amount of bird damage, rot and so on, all are considered when making the great grape decision – to pick or not to pick!
If you are a home winemaker, you may want to have a look at the Missouri State bulleting – Making Wine for Home Use available at http://mtngrv.missouristate.edu/Publications/MWFHUpub.htm.
Direct comments or questions concerning this column to Marilyn Odneal via email at MarilynOdneal@missouristate.edu; write to Missouri State Fruit Experiment Station, 9740 Red Spring Road, Mountain Grove, Mo. 65711; or call (417) 547-7500. Visit our Web site at http://mtngrv.missouristate.edu.