The must - that is, the juice expressed from the grape, but in which (juice) fermentation has not yet taken place - is a fluid of very complex composition. It is made up of a variety of ingredients, with which it is necessary to become familiar in order to follow, during the process of fermentation, its change into wine. We find, therefore, that a large part of the must consists of water; this serves to dissolve the other constituents, and to dilute them to the required extent. For instance, the sugar in the must needs to be considerably diluted for the purposes of fermentation. In too concentrated a form it actually prevents it, as we see when fruits are preserved in syrup.
Next to water, sugar is the material which exists in the largest proportions in the must; it is, however, that peculiar kind of sugar termed "glucose," which may be described as uncrystallisable sugar, and as consisting of half grape sugar and half fruit sugar. It possesses the property of being able to ferment, which cane or crystallisable sugar cannot do, unless, indeed, it first be changed into glucose. Now, it is a curious fact that although cane sugar can be transformed into glucose, yet the latter form of sugar has never, so far, been changed into cane or crystallisable sugar. As Mr. J. A. Despeissis points out, the invention of a process that would achieve this would be worth more than all the mines of New South Wales put together.
In the process of fermentation the glucose is broken up into a number of substances, which differ entirely from it; and as these different bodies are very important they deserve much attention. Under the influence of fermentation glucose undergoes a great change, of which the principal products are alcohol and carbonic acid gas. The alcohol is, of course, the one predominant feature in wine; and according to the amount of alcohol which wine contains, so it varies in strength.
In addition to these two main products of glucose by fermentation, namely, alcohol and carbonic acid gas, there are glycerine and succinic acid, as well as a lesser proportion of other derivatives, very much akin to alcohol. Of all these glycerine is by no means unimportant, as it confers a blandness or mellowness upon the wine. The succinic acid, also, is distinctive for this reason, that it is the source of that characteristic flavour in wine known as "vinosity."
Besides the water and the glucose, the must likewise contains quite an appreciable amount of those important bodies, the various acids. These consist of tartaric acid, so frequently met with all through the vegetable world; of malic acid, which is the acid almost distinctive of apples; of tannic acid or "tannin," and of other acids. These different acids play an important part in the production of wine; without them, in truth, it would be a mere admixture of spirits and water - a colourless, flavourless, and insipid product. By their assistance, however, wine is endowed with the brilliancy it possesses. And more than this, the action of the alcohol on these acids develops those exquisitely delicate ethers - the oenanthic and other ethers - which constitute, in fact, the bouquet of the wine. At the same time, it has also to be remembered that while these many acids constitute the life and soul, so to speak, of the wine, their very presence is absolutely necessary for the process of vinous fermentation. That is to say, the active agents of vinous fermentation are only enabled to work perfectly in a liquid which is somewhat acid.
There is an astringent principle, named tannin, which calls for attention in any reference to wine-making. It is almost the same body - not quite - as the tannin obtained from galls, and so largely employed in tanning. This vine-tannin, if it may be so termed, does not exist in the juice of the grape, but in the stalk and the skin. The white wines, in which the juice is almost always freed from the skins and stalks, contain but little tannin; while, on the contrary, most red wines, in which juice, skins, and stalks are all included together in the fermenting-vat, contain a good deal. Some white wines derive their tannin from the oaken casks which hold the wine; and their colour, in consequence, subsequently deepens. Other red wines, strange to say, gradually lose their dark colour from a certain action of the tannin. So that tannin is the cause of some white wines deepening in colour, while it renders other red wines of a lighter colour. Now, tannin has the effect of preserving albuminous substances, and in this way it may be beneficial in rendering red wines more durable. But although this may be advisable in wines which are liable to turn, it is certain that excess of tannin is most undesirable. In fact, the practice of placing the stalks in the fermenting-vat is in many cases, as I have previously stated, an unnecessary proceeding.
The mineral kingdom is not unrepresented in must, and certain saline substances are found in it. Of these, the salts of potash are uniformly present, and the most important is, without doubt, the acid tartrate of potash. This is the salt so well known in commerce under the name of cream of tartar. The lees of wine contain it in considerable quantity, and it is also found as a crystalline deposit in the inside of the casks. As the alcohol begins to develop in the must this salt is precipitated, and the more so the lower the temperature. Thus it is that a light wine of low alcoholic strength, if it be markedly acid, will lose the acidity in a cool, underground cellar. And, as a matter of fact, the proper maturation of a wine is impossible without a due amount of tartar; besides this, it develops in the wine a well-defined vigour and tonicity, which improves its taste, while it also increases its alimentary qualities.
There are a few other ingredients in must, namely, the colouring matters and essential oils, and the albuminoids, or nitrogenous substances. The colouring matters and oils appear to be contained in the cells of the inner side of the skin. Of these, the purpose of the colouring matter is obvious; while the essential oils are believed to contribute to the "aroma" of the wine. The albuminoids or nitrogenous substances are of the nature of white of egg; and, when in small proportion, are necessary for the due performance of the fermentative process. But, in excess, they are a source of considerable anxiety to the vigneron, in that they are the cause of much of the wine going wrong.