(Sarah of the Clue Crew points out a grape vine in the vineyard in Napa Valley, CA.) Because American root stock and Europe vines resist different pests, growers combine them using this technique
(Sarah of the Clue Crew tests a beaker of wine in an aging cellar in Napa Valley, CA.) If this level in wine drops below about 3.0, the wine will be probably be a trifle sharp