Making Red Wine
The grape constituents which matter most to the winemaker are (a) sugar content of the juice, and (b) tartness or “total acidity” of the juice.
Sugar content is important because the amount of sugar determines alcoholic content of the finished wine. A sound table wine contains between 10% and 12½% alcohol. The working rule is that 2% sugar yields 1% of alcohol. Example: a sugar content of 22% yields a wine of approximately 11% alcohol.
California grapes normally contain sufficient sugar. Grapes grown elsewhere are often somewhat deficient, and the difference must be made up by adding the appropriate amount of ordinary granulated sugar which promptly converts to grape sugar on contact with the juice.