Grapes are gathered and then squashed to release their juice, which is then fermented. see post Throughout fermentation, yeast takes in the sugar in the grape juice and transforms it into alcohol.
How many pounds of grapes are required to make one gallon of red wine? More sugar in the grape juice suggests more alcohol in the white wine.
Wine-making is both a science and an art, so grape-to-wine ratios are not always precise. However as a general rule of thumb, it takes around eight pounds of grapes to make one gallon of white wine.