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.
What the For wine of 10% by For wine of 12% by saccharometer shows volume, add volume, add Ounces of sugar per gallon
10 11.8 16.2 11 10.1 14.8 12 8.9 13.3 13 7.4 11.9 14 5.9 10.4 15 4.6 8.9 16 3.0 7.5 17 1.5 6.0 18 4.3 19 2.9 20 1.4
_Note_: The result is not precise, yield of alcohol varying under the conditions of fermentation.—
Adapted from _Grapes Into Wine_ by Philip M. Wagner.
In using non-California grapes, you need to test the sugar content in advance. That is done by a simple little instrument called a saccharometer, obtainable at any winemakers’ shop. This is floated in a sample of the juice, and a direct reading of sugar content is taken from the scale. The correct amount of sugar to add, in ounces per gallon of juice, is then determined by reference to the sugar table.
[Illustration: Saccharometer and hydrometer jar. Instrument floats at zero in plain water. It floats higher according to sugar content of grape juice.]
If _total acidity_, or tartness, is too high and not corrected, the resulting wine will be too tart to be agreeable. Again, California grapes are usually within a satisfactory range of total acidity. Grapes grown elsewhere are often too tart, and acidity of the juice should be reduced.
In commercial winemaking this is done with precision.
The home winemaker rarely makes the chemical test for total acidity but uses a rule of thumb. He corrects the assumed excess of acidity with a sugar solution consisting of 2 pounds of sugar to 1 gallon of water—adding 1 gallon of the sugar solution for every estimated 4 gallons of juice. This sugar solution is in addition to the sugar required to adjust sugar content of the juice itself.