Page:EB1911 - Volume 28.djvu/739

VINE DISEASES] alcoholic fermentation, ethylic alcohol, water and carbonic acid. At the same time various subsidiary products such as glycerin, succinic acid, small quantities of higher alcohols, volatile acids and compound esters are produced. In the case of red wines colouring matter is dissolved from the skins and a certain amount of mineral matter and tannin is extracted. It is to these subsidiary matters that the flavour and bouquet in wine are particularly due, at any rate in the first stages of maturation, although some of the substances originally present in the grape, such as ready-formed esters, essential oils, fat and so on, also play a role in this regard. In view of the fact that fresh grape juice contains innumerable bacteria and moulds, in addition to the yeast cells which bring about the alcoholic fermentation, and that the means which are adopted by the brewer and the distiller for checking the action of these undesirable organisms cannot be employed by the wine-maker, it is no doubt remarkable that the natural wine yeast so seldom fails to assert a preponderating action, particularly as the number of yeast cells at the beginning of fermentation is relatively small. The fact is that the constitution of average grape juice and the temperatures of fermentation which generally prevail are particularly well suited to the life action of wine yeast, and are inimical to the development of the other organisms. When these conditions fail, as is, for instance, the case when the must is lacking in acidity, or when the weather during the fermentation period is very hot and means are not at hand to cool the must, bacterial side fermentations may, and do, often take place. The most suitable temperature for fermentation varies according to the type of wine. In the case of Rhine wines it is between 20 and 25° C. If the temperatures rise above this, the fermentation is liable to be too rapid, too much alcohol is formed at a relatively early stage, and the result is that the fermentation ceases before the whole of the sugar has been transformed. Wines which have received a check of this description during the main fermentation are very liable to bacterial troubles and frets. In the case of wines made in more southerly latitudes temperatures between 25 and 30° are not excessive, but temperatures appreciably over 30° frequently lead to mischief. The young wine immediately after the cessation of the main fermentation is very differently constituted from the must from which it was derived. The sugar, as we have seen, has disappeared, and alcohol, glycerin and other substances have been formed. At the same time the acidity is markedly reduced. This reduction of acidity is partly due to the deposition of various salts of tartaric acid, which are less soluble in a dilute alcoholic medium than in water, and partly to the action of micro-organisms. Young wines differ very widely in their composition according to class and vintage. The alcohol in naturally fermented wines may vary between 7 and 16%, although these are not the outside limits. The acidity may vary between 0.3 and 1% according to circumstances. The normal proportion of glycerin varies between 7 and 14 parts for every 100 parts of alcohol in the wine, but even these limits are frequently not reached or exceeded. The total solid matter or “extract,” as it is called, will vary between 1.5 and 3.5% for dry wines, and the mineral matter or ash generally amounts to about one-tenth of the “extract.” The tannin in young red wines may amount to as much as 0.4 or 0.5%, but in white wines it is much less. The amount of volatile acid should be very small, and, except in special cases, a percentage of volatile acid exceeding 0.1 to 0.15%, according to the class of wine, will indicate that an abnormal or undesirable fermentation has taken place. As the wine matures the most noticeable feature in the first instance is the reduction in the acidity, which is mainly due to a deposition of tartar, and the disappearance of tannin and colouring matter, due to fining and the action of oxygen.

Constitution of Grape Juice at Various Periods of Maturation (E. Mach.)

The taste and bouquet of wines in the earlier stages of their development, or within the first four or five years of the vintage, are almost entirely dependent upon constituents derived from the must, either directly or as a result of the main fermentation. In the case of dry wines, the quality which is known as “body” (palate-fulness) is mainly dependent on the solid, i.e. non-volatile, constituents. These comprise gummy and albuminous matters, acid, salts, glycerin and other matters of which we have so far little knowledge. The apparent “body” of the wine, however, is not merely dependent upon the absolute quantity of solid—non-volatile—matters it contains, but is influenced also by the relative proportions in which the various constituents exist. For instance, a wine which under favourable conditions would seem full and round may appear harsh or rough, merely owing to the fact that it contains a small quantity of suspended tartar, the latter causing temporary hyperacidity and apparent “greenness.” It has been found by experience also that wines which are normally constituted as regards the relative proportions of their various constituents, provided that the quantities of these do not fall below certain limits, are likely to develop well, whereas wines which, although perfectly sound, show an abnormal constitution, will rarely turn out successful. The bouquet of young wines is due principally to the compound esters which exist in the juice or are formed by the primary fermentation. It was at one time thought that the quality of the bouquet was dependent upon the absolute quantity of these compound esters present, but the author and others have plainly shown that this is not the case. Among the characteristic esters present in wine is the well-known “oenanthic ether,” which consists principally of ethylic pelargonate. It does not follow that a wine which shows a pretty bouquet in the primary stages will turn out well. On the contrary, it is frequently the case that the most successful wines in after years are those which at first show very little bouquet. The maturation of wine, whether it be in bottle or in cask, is an exceedingly interesting operation. The wines which remain for a long period in cask gradually lose alcohol and water by evaporation, and therefore become in time extremely concentrated as regards the solid and relatively non-volatile matters contained in them. As a rule, wines which are kept for many years in cask become very dry, and the loss of alcohol by evaporation—particularly in the case of light wines—has as a result the production of acidity by oxidation. Although these old wines may contain absolutely a very large quantity of acid, they may not appear acid to the palate inasmuch as the other constituents, particularly the glycerin and gummy matters, will have likewise increased in relative quantity to such an extent as to hide the acid flavour. In the case of maturation in bottle the most prominent features are the mellowing of the somewhat hard taste associated with new wine and the development of the secondary bouquet. The softening effect of age is due to the deposition of a part of the tartar together with a part of the tannin and some of the colouring matter. The mechanism of the development of the secondary bouquet appears to be dependent firstly on purely chemical processes, principally that of oxidation, and secondly on the life activity of certain micro-organisms. L. Pasteur filled glass tubes entirely with new wine and then sealed them up. It was found that wine so treated remained unchanged in taste and flavour for years. On the other hand, he filled some other tubes partly with wine, the remaining space being occupied by air. In this case the wine gradually matured and acquired the properties which were associated with age. Wortmann examined a number of old wines and found that in all cases in which the wine was still in good condition or of fine character a small number of living organisms (yeast cells, &amp;c.) were still present. He also found that in the case of old wines which had frankly deteriorated, the presence of micro-organisms could not be detected. It is, however, not absolutely clear whether the improvement observed on maturation is actually due to the action of these micro-organisms. It may be that the conditions which are favourable to the improvement of the wine are also favourable to the continued existence of the micro-organisms, and that their disappearance is coincident with, and not the cause of, a wine’s deterioration. It is frequently assumed that a wine is necessarily good because it is old, and that the quality of a wine increases indefinitely with age. This is, however, a very mistaken idea. There is a period in the life history of every wine at which it attains its maximum of quality. This period as a rule is short, and it then commences “to go back” or deteriorate. The age at which a wine is at its best is by no means so great as is popularly supposed. This age naturally depends upon the character of the wine and on the vintage. Highly alcoholic wines, such as port and sherry, will improve and remain good for a much longer period than relatively light wines, such as claret, champagne or Moselle. As regards the latter, indeed, it is nowadays held that it is at its best within a very short period of the vintage, and that when the characteristic slight “prickling” taste due to carbonic acid derived from the secondary fermentation has disappeared, the wine has lost its attraction for the modern palate. In the same way champagne rarely, if ever, improves after twelve to fourteen years. With regard to claret it may be said that as a general rule the wine will not improve after twenty-five to thirty years, and that after this time it will commence to deteriorate. At the same time there are exceptional cases in which claret may be found in very fine condition after a lapse of as much as forty years, but even in such cases it will be found that for every bottle that is good there may be one which is distinctly inferior.

Diseases of the Vine.—The vine is subject to a number of diseases some of which are due to micro-organisms (moulds, bacteria), others to insect life. The most destructive of all these diseases is that of the phylloxera. The Phylloxera vastatrix is an insect belonging to the green fly tribe, which destroys the roots and leaves of the growing plant by forming galls and nodosities. Practically every wine-growing country has been afflicted with this disease at one time or