This section is from the book "The Manufacture Of Liquors, Wines, And Cordials, Without The Aid Of Distillation", by Pierre Lacour. Also available from Amazon: Manufacture of Liquors, Wines, and Cordials, Without the Aid of Distillation.
In the chemical sense, is a liquid generated for the most part in vegetable juices and infusions by a peculiar fermentation called the vinous or alcoholic, The liquids which have undergone it, are called vinous liquors, and are of various kinds. Thus, the fermented juice of the grape is called wine; of the apple, cider; and the fermented infusion of malt, beer. With regard to the nature of the liquids sus-ceptible of the vinous fermentation, one general character prevails, however various they may be in other respects; that, namely, of containing sugar in some form or other. It is found further, that after they have undergone the vinous fermentation, the sugar They contain has either wholly or in part disappeared, and that the only new products are alcohol, which remains in the liquid, and carbonic acid which escapes during the process, and these when taken together, are found to be equal in weight to the sugar lost; it is hence inferred that sugar is the subject matter of the changes that occur during the vinous fermentation, and that it is resolved into alcohol and carbonic acid. Sugar will not undergo the vinous fermentation of itself, but requires to be dissolved in water, subjected to the influence of a ferment, and kept at a certain temperature.
Accordingly, sugar, water, and the presence of a ferment and the maintenance of an adequate temperature, may be deemed the pre-requisites of the vinous fermentation. The water acts by giving fluidity, and the ferment and temperature operate by commencing and maintaining the chemical changes. The precise manner in which the ferment operates in commencing the reaction is not known, but the fermentative change seems to be intimately connected with the multiplication of a microscopic vegetable, in the form of diaphanous globules contained in the ferment, and called "torula cervisice." The ferment is generally considered to contain a peculiar nitrogenous principle having a close analogy to albumen and casein.
Certain vegetable infusions, as those of potatoes and rice, though consisting almost entirely of starch, are nevertheless capable of undergoing the vinous fermentation, and form seeming exceptions to the rule that sugar is the only substance susceptible of this fermentation. The apparent exception is explained by the circumstance that starch is susceptible of a spontaneous change which converts it into sugar. How this change takes place is not well known, but it is designated by some authors as the saccharine fermentation. It has been proved that if a mixture of gluten from flour, and starch from potatoes, be put into hot water, the starch will be converted into sugar. When, therefore, starch is apparently converted into alcohol by fermentation, it is supposed that during the change it passes through the intermediate state of sugar. Alcohol being the product of the vinous fermentation, necessarily exists in all vinous liquors, and may be obtained from them by distillation. Formerly it was supposed that these liquors did not contain alcohol, but were merely capable of furnishing it in consequence of a new arrangement of their ultimate constituents - the result of the heat applied. This idea has been disproved by showing that alcohol may be obtained from all vinous liquors without the application of heat, and, therefore, must pre-exist in them. The method consists in precipitating the acid and coloring matter from each vinous liquor, by subacetate of lead, and separating the water by carbonate of potassa.
In vinous liquors, the alcohol is largely diluted with water, and associated with coloring matter, volatile oil, extractive, and various acids and salts. In purifying it, we take advantage of volatility, which enables us to separate it by distillation, combined with some of the principles of the vinous liquor employed, and more or less water. The distilled product of vinous liquors forms the different ardent spirits of commerce. When obtained from wine, it is called brandy; from fermented molasses, rum; from cider or peaches, it is called apple or peach brandy; from malted barley, rye, or corn, it is known as whiskey; from malted barley and rye meal, with hops, and rectified from juniper berries, it is known as Holland gin; from malted barley, rye, or potatoes, and rectified from turpentine, it is called common gin; and from fermented rice, arrack. The spirits are of different strengths, that is, contain different proportions of alcohol, and have various peculiarities by which they are distinguished by the taste. Their strength is accurately judged of by the specific gravity, which is always less in proportion as their concentration is greater. When they have the sp. gr. 0.920, they are designated in commerce as proof spirit; if lighter than this, they are said to be above proof; if heavier, below proof; and the percentage of water or of spirit of 0.825 necessary to be added to any sample of spirit to bring it to the standard of proof spirit, indicates the number of degrees the given sample is above or below proof: thus, if 100 volumes of spirit require 10 volumes of water to reduce it to proof, it is said to be "10 over proof." On the other hand, if 100 volumes of spirit require 10 volumes of a spirit of 0.825 to raise it to proof, the sample is said to be 10 under proof.
Thus, for instance, these marks will be observed on the heads of rectified whiskey barrels, the initials "A. B. P.," signifying above proof, and "B. P.," below proof. This whiskey should contain about 40 per cent, of alcohol, of the strength of 92 per cent.; thus it will be seen that a barrel of forty gallons of whiskey is composed, as far a? the fluid measure extends, of sixteen gallons of alcohol and twenty-four gallons of water; this is called "rectified proof spirit," or "proof spirit."Should the spirit contain above forty per cent, of alcohol, it will be denoted on the head of the barrel by the initials, "A. B. P.' with the figures denoting the per centage. And if the spirit contains less than forty per cent of alcohol, it will be known by the initials "B. P.," or below proof, with the less per centage indicated by figures.
Proof spirit is far from being pure, as it contains a considerable quantity of grain oil and other foreign matters; it may be further purified and strengthened by distillation, or the impurities may be driven off by filtration through charcoal. Alcohol thus purified, is known in commerce as neutral spirits, and is used in the manufacture of the imitation of foreign liquors, cordials, syrups, aromatic waters, essences, perfumes, etc, etc.