The different varieties of fermentation to which glucose is liable will here be treated of separately.
Alcoholic Fermentation.—Five agents, each acting in a different direction, are necessary to produce this; in the absence of auy one of them, fermentation cannot proceed. They are (1) Sugar, (2) Water, (3) A ferment, (4) Heat, and (5) Air. The part played by each of these five indispensable agents will now be examined.
Sugar.—Sugar when dissolved and brought into contact with a ferment is decomposed, yielding alcohol and carbonic anhydride. Before fermentation, the sugar has to be converted into glucose, by combination with two equivalents of water. This hydration is very easily effected; simple beating of a saccharine solution is sometimes sufficient ; the presence of ferments themselves produces it, and a thousand other causes will bring it about when water is present. It is this ready conversion of sugar into glucose that renders saccharine matters so useful in the production of alcohol. The best proportion of sugar in an unfermented liquor or " must " is about 12 per cent. More than this hinders the fermentation.
Water.—The proportion of water employed in dissolving the glucose exercises considerable influence upon the products of the fermentation, as well as upon the time occupied by the process. The operation may be hurried or kept back by adding or subtracting water ; the latter is effected by evaporation. The relative amount of water present is ascertained by means of an instrument called a " saccharometer." The water employed should contain no organic matter, and only a small proportion of mineral salts; it should always be clear and bright.
The Ferment.—A ferment is a substance undergoing decomposition, the ultimate particles of which are in a state of continual motion. When brought into contact with sugar, this atomic motion is communicated to the atoms of carbon, hydrogen, and oxygen of which the sugar is made up, the carbon dividing itself between the hydrogen and oxygen in such a manner that in place of the sugar, two more stable compounds are formed, viz., carbonic anhydride ,and alcolaol. The elements of the ferment take no part in the formation of these products, but only act as the stimulant which provokes the change without participating therein chemically.
As stated above, brewers' yeast is the ferment chiefly employed by distillers. It is a frothy
substa,nce formed during the fermentation of the worts of beer. It collects on the surface, and is bkimmed off and rendered clry and solid by the action of a press. That obtained from a strong beer is much to be preferred, as it is more certain in its action and less liable to engender acetous fermentation. It is best when newly prepared : old yeast should never be used when fresh can be obtained.
The best yeast for fermenting grain spirit is the London porter yeast, which is bought up by the grain distillers for this purpose. The proportions of yeast and sugar for quick fermentation are 5 parts of sugar to 1 part of yeast, although the same quantity of yeast will ferment a much larger quantity of sugar. Any nitrogenous substance, such as albumen, fibrin, gluten, &e., possesses the power of converting sugar into alcohol, when in a state of incipient decomposition, though in a less degree than yeast.
When required for storing, the yeast is subjected to processes of washing and pressing in order to get rid of the water and other impurities which it contains. It is pressed through linen or through a hair sieve, and the filtered liquid is then allowed to stand until the yeast has settled to the bottom. The clear liquid is then decanted off, and the yeast is washed several times with cold water, and well stirred up, until the wash water exhibits no acid reaction. It is finally mixed with per cent. of starch, filled into bags, and pressed.
Heat.—Ileat is as necessary to fermentation as water, and, like water, may be the cause of hastening or checking the process. The lowest temperature at which the action is sustained is about ', and it becalms more energetic and perfect as the temperature is increased up to 28° or 30°. A higher temperature than this should be avoided, as likely to excite acid fermentation. As a means of cooling the vat rapidly, in case of necessity, a coil of pipe in which cold water circulates is sometimes laid in the bottom of the vats. Since heat is retained longer in large masses than in snail], and tho heat generated by the rapidity of the chemical action is in proportion to the bulk of liquor, it follows that the temperature should be raised in inverse Proportion to the bulk of the liquor undergoing fermentation.