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Encyclopedia Britannica - Main :: CHA-CHR |
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CHEMISTRY OF WINE Maturation of the Grape.The processes which take place in the grape during its growth and maturation are of considerable interest
The following figures obtained by Mach afford an interesting illustration
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
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 Constitution of Grape Juice at Various Periods of Maturation. (E. Mach.) Date of Analysis of Juice. 6th July. 12th Aug. 9th Sept. 12th Oct. Specific gravity . 'salts I.029 1.083 I.093 Sugar Per cent. Per cent. Per cent. Per cent. 0.86 2.02 18.52 23.17 Total acid (as 2.66 3.46 0.87 0.71 tartaric acid) . Tartar 0.67 0.55 0'54 0.55 Malic acid 1.16 2.47 0.55 0.42 Tannin oto6 ooI2 .. 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
mineral
special
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
body
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 evaporationparticularly in the case of light wineshas 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, &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, how-ever, 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 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 Cu 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 720 another. The great epidemic in the French vineyards in the years 1882 to 1885 led to a reduction of the yield of about 50 %. Many remedies for this disease have been suggested, including total submersion of the vineyards, the use of carbon bisulphide for spraying, and of copper salts, but there appears to be little doubt that a really serious epidemic can only be dealt with by systematic destruction of the vines, followed by replanting with resistant varieties. This, of course, naturally leads to the production of a wine somewhat different in character to that produced before the epidemic, but this difficulty may be overcome to some extent, as it was in the Bordeaux vineyards, by grafting ancient stock on the roots of new and resistant vines. Oidrum or mildew is only second in importance to the phylloxera. It is caused by a species of mould which lives on the green part of the plant. The leaves shrivel, the plant ceases to grow, and the grapes that are formed also shrivel and die. The most effective cure, short of destruction and replantation, appears to be spraying with finely divided sulphur. Another evil, which is caused by unseasonable weather during and shortly after the flowering, is known as coulure. This causes the flowers
Diseases of Wine.These are numerous, and may be derived either directly from the vine, from an abnormal constitution of the grape juice, or to subsequent infection. Thus the disease known as tourne or casse is generally caused by the wine having been made or partly made from grapes affected by mildew. The micro-organism giving rise to this disease generally appears in the form of small jointed rods and tangled masses under the microscope. Wine which is affected by this disease loses its colour and flavour. The colour in the case of red wines is first altered from red to brown, and in bad cases disappears altogether, leaving an almost colourless solution. This disease is also caused by the wine lacking alcohol, acid and tannin, and to the presence of an excess of albuminous matters. The most common disease to which wine is subject by infection is that caused by a micro-organism termed mycoderma-vini (French fleurs de vin). This micro-organism, which resembles ordinary yeast cells in appearance, forms a pellicle on the surface of wine, particularly when the latter is exposed to the air more than it should be, and its development is favoured by lack of alcohol. The micro-organism splits up the alcohol of the wine and some of the other constituents, forming carbonic acid and water. This process indicates a very intensive form of oxidation inasmuch as no intermediary acid is formed. One of the most common diseases, namely that producing acetous fermentation, differs from the disease caused by M. vini in that the alcohol is transformed into acetic acid. It is caused by a micro-organism termed Mycoderma aceti, which occurs in wine in small groups and chaplets of round cells. It is principally due to a lack of alcohol in the wine or to lack of acidity in the must. The micro-organism which causes the disease of bitterness (amer) forms longish branched filaments in the wine. Hand in hand with the development of a disagreeable bitter taste there is a precipitation of colouring matter and the formation of certain disagreeable secondary constituents. This disease is generally caused by infection and is favoured by a lack of alcohol, acid and tannin. Another disease which generally occurs only in white wines is that which converts the wine into a thick stringy liquid. It is the viscous or graisse disease. As a rule this disease is due to a lack of tannin (hence its more frequent occurrence in white wines). The mannitic disease, which is due to high temperatures during fermentation and lack of acid in the must, is rarely of serious consequence in temperate countries. The micro-organism splits up the laevulose in the must, forming mannitol and different acids, particularly volatile acid. The wine becomes turbid and acquires a peculiarly bitter sweet taste, and if the disease goes further becomes quite undrinkable. It would appear from the researches of the author and others that the mannitol ferment is more generally present in wines than is supposed to be the case. Thus the author found in some very old and fine wines very appreciable quantities of mannitol. In these cases the mannitic fermentation had obviously not developed to any extent, and small quantities of mannitol appear to exercise no prejudicial effect on flavour. Treatment of Diseases.It was found by Pasteur that by heating wine out of contact with air to about 66 C. the various germs causing wine maladies could be checked in their action or destroyed. The one disadvantage of this method is that unless very carefully applied the normal development of the wine may be seriously retarded. In the case of cheap wines or of wines which are already more or less mature, this is not a matter of any great importance, but in the case of the finer wines it may be a serious consideration. Pasteurizing alone, however, will only avail in cases where the disease has not gone beyond the initial stages, inasmuch as it cannot restore colour, taste or flavour where those have already been affected. In such cases, and also in others where pasteurizing is not applicable, some direct treatment with a view to eliminating or adding constituents which are in excess or lacking is indicated. In this regard it is somewhat difficult to draw the line between that which is a rational and scientific method for preventing waste of good material and sophistication pure and simple. It appears to the author, how-ever, that where such methods are employed merely with a view to overcoming a specific malady and there is no intention of increasing[WINES OF FRANCE the quantity of the wine for purposes of gain, or of giving it a fictitious appearance of quality, these operations are perfectly justifiable and may be compared to the modifications of procedure which are forced upon the brewer or distiller who has to deal with somewhat abnormal raw material. It has been found, for instance, that in the case of the mannitic disease the action of the micro-organism may be checked, or prevented altogether, by bringing the acidity of the must up to a certain level by the addition of a small quantity of tartaric acid. Again, it is well known that in the case of the viscous disease the difficulty may be overcome by the addition of a small quantity of tannin. In the same way the disease caused by the mildew organism may be counteracted by a slight addition of alcohol and tannin. One method of assisting nature in wine-making, which is, in the opinion of the author, not justifiable if the resulting product is sold as wine or in such a manner as to indicate that it is natural wine, is the process termed " gallisizing," so called from its inventor H. L. L. Gall, which has been largely practised, particularly on the Rhine. The process of Gall consists in adding sugar and water in sufficient quantity to establish the percentages of free acid and sugar which are characteristic of the best years in the must obtained in inferior years. Although there is no objection to this product from a purely hygienic point of view, it is not natural wine, and the products present in the must other than sugar and acid are by this process seriously affected. Another method of dealing with inferior must, due to J. A. C. Chaptal, consists in neutralizing excessive acid by means of powdered marble, and bringing up the sugar to normal proportions by adding appropriate. amounts of this substance in a solid form. There is less objection to this process than to the former, inasmuch as it does not result in a dilution of the wine. It is scarcely necessary to say that the indiscriminate addition of alcohol and water, or of either to must or to wine, must be regarded as a reprehensible operation.Plastering.In some countries, particularly in Italy, Spain and Portugal, it has been and still is a common practice to add a small quantity of gypsum to the fermenting must or to dust it over the grapes prior to pressing. It is said that wines treated in this manner mature more quickly, and that they are more stable and of better colour. It certainly appears to be the case that musts which arc plastered rarely suffer from abnormal fermentation, and that the wines which result very rarely turn acid. The main result of plastering is that the soluble tartrates in the wine are decomposed, forming insoluble tartrate of lime and soluble sulphate of potash. It is held that an excess of the latter is undesirable in wine, but unless the quantity appreciably exceeds two grams per litre, no reasonable objection can be raised. Basis Wines.Wines which are made not from fresh grape juice but from raisins or concentrated must, or similar material, are generally termed basis wines. They are prepared by adding water to the ' concentrated saccharine matter and subsequently pitching with wine yeast at an appropriate temperature. Frequently alcohol, tannin, glycerin, and similar wine constituents are also added. If carefully prepared there is no objection to these basis wines from a hygienic point of view, although they have not the delicate qualities and stimulating effects of natural wines; unfortunately, however, these wines have in the past been vended on a large scale in a manner calculated to .deceive the consumer as to their real nature, but energetic measures, which have of late
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