Air.—Air, though indispensable at the beginning of the process, becomes useless, and indeed injurious, during its continuation. It is essentially the initial force, but when once the impulse has been given, it is no longer necessary. Therefore air should be excluded as carefully as possible, by keeping the vat covered and allowing no movement to displace the layer of carbonic anhydride, resting on the surface of the liquor, because contact with the air is certain to produce an rien1 fermon• tattoo in place of the alcoholic; this is especially liable to occur towards the end of the operation.
The whole process of alcoholic fermentation may be briefly described as follows:— The liquor in the vat having been heated to the right temperature, the ferment, previously mixed with a small quantity of the saccharine liquor and then left to stand until fermentation begins, is thrown in, and the whole is well stirred together. In about three hours' time, the com mencement of the fermentation is announced`by small bubbles of gas which appear on the surface of the vat, and collect round the edges. As these increase in number, the whole contents are gradually thrown into a state of motion, resembling violent ebullition, by the tumultuous disengage ment of carbonic anhydride. The liquor rises in temperature and becomes covered with froth. At this point, the vat must be covered tightly, the excess of gas finding an exit through holes in the lid ; care must now be taken to prevent the temperature from rising too high, and also to prevent the action from becoming too energetic, thereby causing the contents of the vat to overflow. In about twenty-four hours, the aotion begins to subside, and the temperature falls to that of the surrounding atmosphere. An hour or two later, the process is complete ; the bubbles disappear, and the liquor, which now possesses the characteristic odour and taste of alcohol, settles out perfectly clear. The whole operation, as here described, usually occupies about forty-eight hours, more or less. The duration of the process is influenced, of course, by many circumstances, chiefly by the bulk of the liquor, its richness in sugar, the quality of the ferment, and the temperature.
Acctous Ils.mcntatin.—This perplexing occurrence cannot be too carefully guarded against. It results, as mentioned above, when the fermenting liquor is exposed to the air. When this is the ease, the liquor absorbs a portion of the oxygen, which unites with the alcohol, thus converting it into acetic acid as rapidly as it is formed. When acetone fermentation begins, the liquor becomes
turbid, and a long stringy substance appears which after a time settles down to the bottom of the vat. It is then found that all the alcohol has been decomposed, and that an equivalent quantity of acetic acid remains instead. It has been discovered that the presence of a ferment and a tempera ture of 20° to 35° are indispensable to acetous fermentation, as well as contact with the atmosphere. Hence, in order to prevent its occurrence, it is necessary not only to exclude the air, but also to guard against too high a temperature and the use of too much ferment. The latter invariably tends to excite neetous fermentation, It should also be remarked that it is well to cleanse the vats and utensils carefully with lime water before using, in order to neutralize any acid which they may contain ; for the least trace of acid in the vat has a tendency to accelerate the conversion of alcohol into vinegar. A variety of other circumstances are favourable to acetification, such as the use of a stagnant or impure water, and the foul odours which arise from the vats; stormy weather or thunder will also engender it.
Lactic Fcrincatation.—Uuder the influence of lactic fermentation, sugar and starch are converted into lactic acid. When it has once begun, it dovelopes rapidly, and soon decomposes a large quantity of glucose ; but as it can proceed only in a neutral liquor, the presence of the acid itself speedily checks its own formation. Then, however, another ferment is liable to act upon the lactic acid already formed, converting it into butyric acid, which is easily recognized by its odour of rank butter. Carbonic anhydride and hydrogen are evolved by this reaction. The latter gas acts powerfully upon glucose, converting it into a species of gum called nzannite, so that lactic fermen tation—in itself an intolerable nuisance—becomes the source of a new and equally objectionable waste of sugar. It can be avoided only by keeping the vats thoroughly clean ; they should be washed with water acidulated with 5 per cent. of sulphuric acid. An altered ferment, or the use of too small a quantity, will tend to bring it about. The best preventives are thorough cleanliness, and the use of good fresh yeast in the correct proportion.