This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Wine made from the expressed juice of different varieties of the grape consists of an alcoholic solution varying in strength from 6 to 25 volumes per cent, and containing flavouring and other substances.
The maximum normal percentage of alcohol which fermenting grapes are capable of developing is not above 15 per cent, but alcohol is often added to equal 18 or 25 per cent or more.
The pulp of the grape furnishes sugar for fermentation of alcohol, and also organic acids or their salts, such as citrates, malates, and tartrates. The stones or seeds furnish essential oils, some of which give the "bouquet" of volatile ethers, and the skins and stems furnish pigments and tannin. The latter is preservative; it precipitates albuminous substances and prevents mouldiness. The chief pigment of wines is primarily of a blue colour, but it is reddened, like litmus, - by free acid, more or less of which is always present.
The fermentation of wines is caused by germs, which exist upon the stems or skins of the grapes.
Besides water, sugar, alcohol, volatile ethers, and carbonic acid are added in the manufacture of the different kinds of wine.
Leoser gives the following list of minor substances, traces of which are more or less constantly present in wines: "Gelatin, gum, fat, wax, albumin, gluten, tartaric acid, potassic tartrate, racemic acid, malic acid, calcic malate, oxide of manganese, oxide of iron, potassium sulphate, sodium chloride, calcium phosphate, magnesia, silicic acid, tannic acid".
The composition of the subtle substances which impart the flavour and aroma or bouquet to different wines is unknown. Most of these substances develop during fermentation, but a few grapes, such as the Muscatel, yield their own aroma to the wine.
According to Konig, the average percentage composition of grape must is water, 74.49; sugar, 19.71; nitrogenous material, 0.28; non-nitrogenous material, 4.48; ash, 0.40; acid, 0.64.
The sugar fluctuates sometimes as much as 24 per cent and the acid 1.2 per cent, and these substances usually stand in inverse proportion to each other.
The perfected wine has the properties of colour, "body" or substance, and flavour, aroma, bouquet, or "fruitiness." Different wines are valued for their astringency (tannin), sweetness (grape sugar), strength (alcohol), acidity (organic acids), colour or sparkle, flavour, and ability to stimulate the appetite and digestion.
Wines are both naturally and artificially fermented, and many varieties are re-enforced by flavouring extracts and fortified by addition of alcohol. The latter method has the twofold effect of adding to their strength and of preserving them from further fermentation, and hence is used especially for the wines of the Cape, Madeira, and Portugal. In the cooler climate of France, Germany, and Hungary fermentation proceeds more slowly and fortification is less necessary, for the wines of these countries are drier, less fruity, and require less time to mature. They also have stronger bouquet, because they possess more acid, which combines with alcohol to furnish the aroma (Pavy).
The quantity of sugar present in wines varies considerably. It is sometimes almost completely eliminated by fermentation into alcohol, or it may be added in excess to make the natural flavour sweeter.
Some Greek wines have a peculiar flavour, the liking for which is an acquired taste. This flavour is derived from rosin contained in the wood of casks used for preserving the wines, which induces chemical changes.
The Hungarian wines have agreeable fruitiness, but they are not so completely ripened as French wines, and are hence likely to turn sour when transported.
Both glucose and saccharose can be oxidised into acids instead of fermenting to alcohol, and when this process occurs extensively in a wine it becomes sour, like vinegar, and is unfit for consumption.
In general the flavour and bouquet of a wine depend upon (1) the nature of the soil in which the grape is grown, (2) the climate and temperature, (3) the quality of the grape, (4) the use of but a single variety of grape for a given wine, (5) the ripeness of the grape, (6) the duration of fermentation, (7) the addition or subtraction of material by the art of the wine grower, (8) the age of the wine.
In vineyards where the best wines are made the grapes are tested from day to day to determine the right time to pick the vines, for the quantity of sugar and acid present depend upon the degree of ripening and influence the flavour of the wine.