Wine is (or should be) the product of the juice of the grape by fermentation. When so prepared, it is an alcoholic solution varying in strength from 6 to 25 volumes per cent, and containing flavouring and other substances. The chief chemical constituents of the juice are sugar, for fermentation of alcohol; organic acids or their salts, chiefly of tartaric, citric, or malic acid; and albuminous substances. The stones or seeds furnish essential oils, which are largely responsible for the bouquet of wines, and the skins and stones furnish pigments and tannin. The quality of wine depends on the amount of sugar and albumin in the juice. In the process of fermentation the yeast germs split up the sugar, with the formation of alcohol. If there is a small amount of sugar and a large amount of albumin in the juice, fermentation goes on till all the sugar is split up. This furnishes a dry wine with a slightly acid taste, e.g. hock. If, on the other hand, there is a large amount of sugar and much smaller amount of albumin, the fermentation is less complete, some sugar is left in the wine, as in "sweet wines." When as a result of fermentation the proportion of alcohol has risen to 15 per cent, by volume, the process of fermentation is arrested by the alcohol, hence a natural wine never contains more than 15 per cent, alcohol. Wines are often fortified by the addition of spirit. This has the twofold effect of adding to their strength and preserving them from further fermentation, and so preventing the production of acetic and other acids.
Dark grapes are used in the making of red wines, the skins and stones being left to ferment with the pulp, to which they yield tannin, pigment, and extractives. The average composition of red wines may be taken as follows: -
8 to 11 per cent.
.5 " 65 "
•5 per cent.
Tannin and pigment
.1 to .2 "
White Wines are usually made from white grapes. They are produced in great variety, and they have as a rule a rather higher percentage of alcohol than red wines.