FERMENTATION (ante), a chemical term originally applied to natural processes in which bubbles of air seem to be generated, producing what is called effervescence. Effervescence is, however, only a phenomenon which accompanies one of the most familiar instances of fermentation, and does not exist in all its forms. The commonest examples of fermentation are: the change of the juices of fruits to wine, cider, etc. ; the souring of milk; and the putrefaction of animal or vegetable matter. As these changes occur without any notably exciting cause, they have been thought to be spon taneous; but no such thing as simple spontaneity exists in the case. On the contrary, experiment shows that no fermentable chemical species will ferment except it is in the presence of water, and is kept by that water in contact with some specific substance which by its presence excites and maintains the chemical activity of the kind in ques tion. The substance which is thus the occasion of the chemical action is called a fer ment. Even the simple fact of presence is not deemed to be enough. The ferment must itself change, but the cycle of change may cause a continuous reproduction of the agency, and thus result in the of the fermentation.
Vinous fermentation may be selected for illustration, as one which is familiar in some of its many illustrations, such as the making of wine from grapes and currants, cider from apples, beer from grain, etc. The juice of grapes is an intensely sweet yel lowish liquid, which may be made perfectly limpid and transparent by filtration through bibulous paper. If thus clarified, it will remain unchanged indefinitely; but if to it be added even a small quantity of the unclarified juice, fermentation will ultimately begin, and the liquid will become turbid. A finely divided substance is formed in the liquor, which rises to the surface as a scum, and is called yeast. The production of yeast is accompanied by the evolution of carbonic acid, which also comes to the surface, and is retained in bubbles by the viscous nature of the scum. The chemical change once begun becomes accumulatively more active in the presence of the increasing volume of yeast, until it reaches a climax, and then it dies away because the whole substance has been acted upon. . The yeast settles to the bottom; a clear liquid remains, whose sweet ness has given place to a vinous taste; from which it appears that the sugar has van ished, and instead, a new, volatile, inflammable substance called alcohol is present. The temperature of vaporization being lower for alcohol than for water, it may be driven off by processes of distillation, each repetition furnishing a greater proportion of alcohol, and may finally be obtained in a pure, or "absolute," form, when treated with some chemical which takes away the remnant of the water. It appears then that the
vinous fermentation has occasioned a change in which sugar has given place to alcohol. The analytical statement of this change is expressed by Gay Lussac's formula, substan tially, but not critically. He assigned to grape sugar the simple formula C1120, and for the reaction gives the equation— Sugar. Alcohol. Carbonic Acid.
6(01120) = 06111202 = + or, 45 units of sugar give 23 units of akohol and 22 of carbonic acid. Cane sugar has the formula, C 0 12 22 - 11 = 2(06111206)—H20, or, two equivalents of grape sugar with one It appears, on further investigation, that certain other compounds arc formed even this does not account for the formation of a small quantity of fusel-oil, and some ethers.
Vinous fermentation is induced by saccharomyces, a genus of fungi, consisting of minute cells, sometimes isolated, sometimes grouped, but never forming a continuous tissue. Of the several species, S. cerevithe, the fungus of common yeast, used in making beer, is most important. Its cells have a diameter of about millimeter. Of the genesis of the yeast plant little is known. Its germs abound in harvest time about the vines and stalks of the grape, and in breweries and wine-cellars, but they are by no means generally diffused through the air.
The change in lactic fermentation is expressed by the equation— Hydrated milk sugar. Lactic acid.
= 4C2H808 Ordinary glucose dissolved in milk ferments to lactic acid, with the milk sugar, up to a certain maximum of acidity, when the change stops. Chalk or carbonate of soda neu tralizes a part of the acid, and revives the fermentation. The agent of this fermentation is a microscopic fungus, consisting of single cells, much smaller than those of the brewers' yeast. Lactic ferment sometimes annoys brewers as an impurity in their yeast. The lactic ferment is not chargeable as the agent which ordinarily sours milk; this result is caused by a motionless bacterium which Lister calls B. &teas. Yet this bacterium, if made to pass through a certain round of changes, also produces lactic fer mentation. The germs of this bacterium are thought to abound in the air of dairies and cow-stables, but are not generally diffused through the air.
Butyric fermentation is a change which occurs in milk or cheese, in which the lactic acid is broken up, as shown by the equation— Lactic acid. Butyric. Carb. acid. Hydrogen.