the vintage, i.e. on the weather conditions preceding and during the gathering of the grapes and the subsequent fermentation. Of all these factors, that of the nature of the soil on which the vine is grown is perhaps the most important The same vine, exposed to practically identical conditions of climate, will produce markedly different wines if planted in different soils. On the other hand, different varieties of the vine, provided they are otherwise not unsuitable, may, if planted in the same soil, after a time produce wines which may not differ seriously in character. Thus the planting of French and German vines in other countries (e.g. Australia, the Cape) has not led to the production of directly comparable wines, although there may at first have been some general resemblance in character. On the other hand, the replanting of some of the French vineyards (after the ravages due to the phylloxera) with American vines, or, as was more generally the case, the grafting of the old French stock on the hardy American roots, resulted, after a time, in many cases, in the production of wines practically indistinguishable from those formerly made.
Wine-making.—The art of wine-making is, compared with the manufacture of beer or spirits, both in principle and in practice a relatively simple operation. When the grapes have attained to maturity they are collected by hand and then transferred in baskets or carts to the press house. After the stalks have been removed either by hand or by a simple apparatus the juice is expressed either—as is still the case in many quarters—by trampling under foot or by means of a simple lever or screw press or by rollers. In the case of red wines the skins are not removed, inasmuch as it is from the latter that the colour of the wine is derived. The must, as the expressed juice of the grape is termed, is now exposed to the process of fermentation, which consists essentially in the conversion of the sugar of the must into alcohol and various subsidiary products. The fermenting operations in wine-making differ radically from those obtaining in the case of beer or of spirits in that (if we except certain special cases) no yeast is added from without. Fermentation is induced spontaneously by the yeast cells which are always present in large numbers in the grape itself. The result is that—as compared with beer or spirits—the fermentation at first is relatively slow, but it rapidly increases in intensity and continues until practically the whole of the sugar is converted. In the case of the production of certain sweet wines (such as the sweet Sauternes, Port and Tokay) the fermentation only proceeds up to a certain extent. It then either stops naturally, owing to the fact that the yeast cells will not work rapidly in a liquid containing more than a certain percentage of alcohol, or it is stopped artificially either by the addition of spirit or by other means which will be referred to below. As the character of a wine depends to a considerable extent on the nature of the yeast (see Fermentation), many attempts have been made of late years to improve the character of inferior wines by adding to the unfermented must a pure culture of yeast derived from a superior wine. If pure yeast is added in this manner in relatively large quantities, it will tend to predominate, inasmuch as the number of yeast cells derived from the grapes is at the commencement of fermentation relatively small. In this way, by making pure cultures derived from some of the finest French and German wines it has been possible to lend something of their character to the inferior growths of, for instance, California and Australia. It is not possible, however, by this method to entirely reproduce the character of the wine from which the yeast is derived inasmuch as this depends on other factors as well, particularly the constitution of the grape juice, conditions of climate, &c. The other micro-organisms naturally present in the must which is pitched with the pure culture are not without their influence on the result. If it were possible to sterilize the must prior to pitching with pure yeast no doubt better results might be obtained, but this appears to be out of the question inasmuch as the heating of the must which sterilization involves is not a practicable operation. After the main fermentation is finished, the young wine is transferred to casks or vats. The general method followed is to fill the casks to the bung-hole and to keep them full by an occasional addition of wine. The secondary fermentation proceeds slowly and the carbonic acid formed is allowed to escape by way of the bung-hole, which in order to prevent undue access of air is kept lightly covered or is fitted with a water seal, which permits gas to pass out of the cask, but prevents any return flow of air. During this secondary fermentation the wine gradually throws down a deposit which forms a coherent crust, known as argol or lees. This consists chiefly of cream of tartar (bitartrate of potash), tartrate of lime, yeast cells and of albuminous and colouring matters. At the end of some four to five months this primary deposition is practically finished and the wine more or less bright. At this stage it receives its first racking. Racking consists merely in separating the bright wine from the deposit. The wine is racked into clean casks, and this operation is repeated at intervals of some months, in all three to four times. As a general rule, it is not possible by racking alone to obtain the wine in an absolutely bright condition. In order to bring this about, a further operation, namely that of fining, is necessary. This consists, in most cases, in adding to the wine proteid matter in a finely divided state. For this purpose is in glass, gelatin or, in the case of high-class red wines, while of egg is employed. The proteid matter combines with a part of the tannin in the wine, forming an insoluble tannate, and this gradually subsides to the bottom of the cask, dragging with it the mechanically suspended matters which are the main cause of the wine's turbidity. In some cases purely mechanical means such as the use of Spanish clay or filtration are employed for fining purposes. Some wines, particularly those which lack acid or tannin, are very difficult to fine. The greatest care is necessary to ensure the cleanliness and asepticity of the casks in which wine is stored or into which it is racked. The most common method of ensuring cask cleanliness is the operation known as “sulphuring.” This consists in burning a portion of a sulphur “match” (i.e. a flat wick which has been steeped in melted sulphur, or simply a stick of melted sulphur) in the interior of the cask. The sulphurous acid evolved destroys such micro-organisms as may be in the cask, and in addition, as it reduces the supply of oxygen, renders the wine less prone to acidulous fermentation. Sweet wines, which are liable to fret, are more highly and frequently sulphured than dry wines. After the wine has been sufficiently racked and fined, and when it has reached a certain stage of maturation—varying according to the type of wine from, as a rule, two to four years—the wine is ready for bottling. Certain wines, however, such as some of the varieties of port, are not bottled, but are kept in the wood, at any rate for a considerable number of years. Wines so preserved, however, develop an entirely different character from those placed in bottle.
Chemistry of Wine
Maturation of the Grape.—The processes which take place in the grape during its growth and maturation are of considerable interest. E. Mach has made some interesting observations on this point. At first—i.e. at the beginning of July when the berries have attained to an appreciable size—the specific gravity of the juice is very low; it contains very little sugar, but a good deal of acid, chiefly free tartaric acid and malic acid. The juice at this period contains an appreciable amount of tannin. As the berry grows the amount of sugar gradually increases, and the same up to a certain point applies to the acidity. The character of the acidity, however, changes, the free tartaric acid gradually disappearing, forming bitartrate of potash and being otherwise broken up. On the other hand, the free malic acid increases and the tannin decreases. When the grape is ripe, the sugar has attained to a maximum and the acidity is very much reduced; the tannin has entirely disappeared.
The following figures obtained by Mach afford an interesting illustration of these processes:—
At first the sugar in the juice consists entirely of dextrose, but later fructose (laevulose) is formed. The sugar in ripe grape juice is practically invert sugar, i.e. consists of practically equal parts of dextrose and fructose. The proportion of sugar present in the juice of ripe grapes varies considerably according to the type of grape, the locality and the harvest. In temperate climates it varies as a rule between 15 and 20%, but in the case of hot climates or where the grapes are treated in a special manner, it may rise as high as 35% and more.
Fermentation.—The fermentation of grape juice, i.e. the must, is, as we have seen, a relatively simple operation, consisting as it does in exposing it to the spontaneous action of the micro-organisms contained in it. The main products formed are, as in all cases of
