Spirits usually contain about fifty per cent. The alcohol in all cases results from the breaking up of the sugar in the fermenting liquid.
Fruit sugar exists in all the sub-acid fruits as grapes, currants, apples, peaches, etc.
When these are dried, it changes to grape sugar forming the whitish grains which are seen on the outside of prunes, raisins, etc. Grape sugar is found to a limited extent in fruits associated with fruit sugar. Cane sugar is readily changed by the action of acids or ferments into fruit sugar, and the latter into grape sugar, but the process cannot be reversed. Grape sugar is the only fermentable variety, the others becoming changed into it before fermentation. Hence one of our most important sources of alcohol is to be found in the starch of barley, corn, wheat, potatoes, etc.