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.