Viscous Fermentation.—This is usually the result of allowing the vats to stand too long before fermentation begins. It is characterized by the formation of viscous or mucilaginous matters, which render the liquor turbid, and by the evolution of carbonic anhydride and hydrogen gases, the latter acting as in the case of lactic fermentation, and converting the glucose into marmite. Viscous fermentation may generally be attributed to the too feeble action of the ferment. It occurs prin cipally in the fermentation of white wines, beer, and beet-juice, or of other liquors containing much nitrogenous matter. It may be avoided by the same precautions as are indicated for the prevention of lactic fermentation.
It remains now to describe briefly the vessels or vats employed in the processes of fermentation. They are made of oak or pine, firmly bound together with iron bands, and they should be somewhat deeper than wide and slightly conical, so as to present as small a surface as possible to the action of the air. Their dimensions vary, of course, with the nature and quantity of the liquor to be fermented. Circular vats are preferable to square ones as being better adapted to retain the heat of their con tents. The lid should close securely, and a portion of it should be made to open without uncovering the whole. For the purpose of heating or cooling the contents when necessary, it is of great advan tage to have a copper coil at the bottom of the vat, connected with two pipes, one supplying steam and the other cold water. The diameter of the coil varies according to the size of the vat.
The room in which the vats are placed should be made as free from draughts as possible by dispensing with superfluous doors and windows ; It should not be too high and should he enclosed by thick walls in order to keep in the heat. As uniformity of temperature is highly desirable, a thermometer should be kept in the room, and there should be stoves for supplying heat in case it be required. Every precaution must be taken to ensure the most absolute cleanliness; the floors should be swept or washed with water daily, and the vats, as pointed out above, must be cleaned out as soon as the contents are removed. For washing the vats, lime-water should be used when the fermentation has been too energetic or has shown a tendency to become acid ; water acidulated with sulphuric acid is used when the action has been feeble and the fermented liquor contains a small quantity of undecomposed sugar. Care must be taken to get rid of carbonic anhydride formed during the operation. Buckets of lime-water are sometimes placed about the room for the purpose of absorbiog this gas ; but the best way of getting rid of it is to have a number of holes, 3 or 4 in. square, in the floor, through which the gas escapes by reason of its weight. The dangerous action of this gas and its effects upon animal life when unmixed with air are too well known to necessitate any further enforcement of these precautions.
D/STILLATION.—The fermented liquors obtained in the manner described above, are composed essentially of volatile substances, such as water, alcohol, essential oils and a little acetic acid, and of non-volatile substances, such as cellulose, dextrine, unaltered sugar and starch, mineral matters, lactic acid, &e, The volatile constituents of the liquor possess widely different degrees of volatility ; the alcohol has the lowest boiling point, water the next, then acetic acid, and last the essential oils. It will thus be seen that the separation of the volatile and non-volatile constituents by evaporation and condensation of the vapours given off is very easily effected, and that also by the same process, which is termed distillation, the volatile substances may be separated from one another. As the acetic acid
and essential oils are present only in very small quantities, they will not require much consideration. The aim of the process is to separate as completely as possible the alcohol from the water which dilutes it. At the beginning of this article, we have given a table showing the amount of alcohol contained in the vapours given off from alcoholic liquids of different strengths, and also their boiling points. A glance at this table will show to what an extent an alcoholic liquor may be strengthened by distillation, and how the quantity of spirit in the distillate increases in proportion as that contained in the original liquor diminishes. It will also be seen that successive distillations of spirituous liquors will ultimately yield a spirit of very high strength. As an example, suppose that a liquid containing 5 per cent. of alcohol is to be distilled. Its vapour condensed gives a distillate containing 42 per cent. of alcohol, which, if re-distilled, affords another containing 82 per cent. This, subjected again to distillation, yields alcohol of over 90 per cent, in strength. Thus three siiceessive distillations have strengthened the liquor from 5 per cent. to 90 per cent. This, of course, is Bp, liking theoretically ; in practise it ia possible to obtain results so absolutely perfect, only by leaving behind a considerable quantity of spirit in the dietilling itpparatus after each It will thus be clear that the richness in alcohol of tho vapours given off from boiling alcoholic liquids is not a constant quantity, but that it necessarily dimiuishes as the ebullition is contiuued. For example, a liquor containing 7 per cent of alcohol yields, on boiling, a vapour containing 50 per cent. (see table, p. 192). The first portion of the distillate wilt, therefore, be of this atrength. But, as the vapour is proportionally richer in alcohol, the boiling liquer must become gradually weaker, and, in consequence, must yield weaker vapours. Thus, when the proportiou of alcohol in the boiling liquid has sunk t,o 5 per cent., the vapours condensed at that time will contain only 40 per cent. ; at 2 per cent. of alcohol in the liquor, the vapours yield only 28 per cent., aod at 1 per cent., they will be found when condensed to contain only 13 per cent. From this it will be understood that if the distillation be stopped at any given point before the complete volatilization of all the alcohol, the distillate obtained will be considerably stronger than if the process had been carried on to tho end. Moreover, azother advantage derived from checking the process before the end, and keeping the last portions of the distillate separate from the rest, besides that of obtaioiog etronger spirit, is that a much purer one is obtained also. The volatile, essential oils, mentioned above, are soluble only in strong alcohol, and insoluble in ite aqueous solutions. They distil also at a much higher temperature than alcohol, and se are found only among the last products of tho distillation, which result from raisiog the temperature of the boiling liquid. This system of checking the distillation and removing the products at different points is frequently employed in the practice of rectification.