Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/104

FERMENTATION (4.) Fermentation is impeded and may be entirely stopped by addition of alcohol. Hence the wines produced from the rich juices of southern grapes always contain unfermented sugar. (5.) Fermentation may be stopped more or less completely by addition to the liquid of even small quantities of certain reagents called antiseptics. Of these corrosive sublimate (and many other heavy metallic salts), sulphuric acid, sulphurous acid, bisulphide of carbon, and carbolic acid may be mentioned as examples. (6.) Perfectly pure grape juice does not ferment, unless the pro cess has been started by at least temporary contact with ordinary air. This cardinal fact was discovered by Gay-Lussac in a now classical series of experiments. He caused clean grapes to ascend through the mercury of a large barometer into the Toricellian vacuum, where he crushed them by means of the mercurial column. The juice thus produced and preserved remained unchanged ; but the addition to it of ever so small an air-bell (as a rule) induced fermentation, which, when once started, was always found to take care of itself. (7.) Ordinary vinous fermentation always involves the formation of yeast. This is the most important of positive facts made out. (8.) The rate at which a fermentation progresses is (in a limited sense) determined by the quantity of yeast present within the liquid. (9.) Spontaneous fermentation of grape juice is always slow in beginning ; addition of yeast from without starts it immediately. From these facts it is legitimate to conclude that it is the yeast or some constituent of the yeast which somehow or other causes the sugar to break up into alcohol, carbonic acid, glycerine, and succinic acid. But what is the rationale of the action 1 Chemically speaking, it would ap pear to be vain to attempt an answer without first knowing what yeast is made of in the chemical sense. Unfortu nately the present state of our knowledge on this point is very unsatisfactory. All we know is that yeast is a highly complex mixture of chemical substances which may be arranged under the four heads of (1) fats (forming about 2 per cent, of the whole); (2) cellulose (about 18 per cent,) ; (3) nitrogenous substances more or less closely allied to white of egg, some of them soluble, some insoluble in water (about 60 per cent.) ; (4) incombustible salts, which, when the yeast is burned, remain as "ash" (about 7 per cent.). According to Mitscherlich s analysis, yeast-ash consists mainly of phosphoric acid (54 to 59 per cent.), united with potash (28 to 40 per cent.), magnesia (6 to 8 per cent.), and lime (1 to 4 per cent.). That such a complex mixture should act, chemically, as a whole cannot reasonably be assumed ; chemists, accordingly, have always been unani mous in thinking that it is some one constituent or set of constituents of the yeast which constitutes the characteristic reagent in vinous fermentation; but none of them has suc ceeded in isolating that reagent. The only clue in this respect which we have is an important discovery of Mitscherlich s, who showed that an aqueous extract of yeast, although capable of converting cane-sugar into glucose, does not induce fermentation in the glucose formed, whence it at once follows that the ferment must be sought for amongst the insoluble portion of the yeast. Their non-success in isolating the vinous ferment did not prevent chemists from speculating on its mode of action. Berzelius gave it as his opinion (which was adopted by Mitscherlich and others of the leading chemists) that the action was a purely "catalytic" one. What he meant by this is best explained by an example. Peroxide of hydro gen (a compound of the elements of water and oxygen) is perfectly stable at ordinary temperatures. Add to it a mere speck of platinum black (a peculiar form of finely divided platinum), and it at once breaks up into water and oxygen, the platinum which caused the decomposition remaining unchanged. In an exactly similar manner Berzelius thought the yeast acted upon the sugar, and caused it to break up into alcohol and carbonic acid. The merit of the idea was that it apparently reduced the explanation of a seemingly complex to that of an undoubtedly simpler phenomenon. But unfortunately neither Berzelius nor any of his followers succeeded in proving the objective existence of the analogy by experimental evidence. Hence Berzelius s theory really amounted to no more than showing that vinous fermentation and the "catalytic" reactions of inorganic chemistry were both unexplained phenomena. Something far more worthy of the name of a theory had been offered 200 years before by Stahl. The originator of the phlogiston theory justly divined that vinous fermenta tion and putrefaction are phenomena of the same order, and, starting from the well-known infectious nature of the latter, explained both as disturbances in the "molecules" of the fermenting body, brought about by a pre-existing mole cular motion. " Ein Kb rper der in der Faulung begriffen 1st bringet in einem anderen von der Faulung annoch befreiten sehr leichtlich die Verderbung zu Wege, ja es kann ein solcher, bereits in innerer Bewegung begriffener Ko rper einen anderen annoch ruhigen, jedoch zu sothaner Bewe gung geneigten sehr leicht in cine solche innere Bewegung hineinreissen." These ideas of Stahl s, at the time of Berzelius s catalytic theory, had long been forgotten, and they remained lost to science until they were revived and brought into a more definite form by Justus Liebig, who, in a powerful and com prehensive memoir on fermentative changes, which he pub lished in 1848, used them as the basis of a new theory of these phenomena, which justly attracted universal attention, as it or rather the wonderfully lucid memoir which embodied it exhibited the subject in a clearer light than anything else that had been said or written on it before. With Liebig as with Stahl, all " fermentations " and " putrefactions " are analogous phenomena. Putrefactions are owing mainly, to the inherent instability of the albuminoid constituents of the respective substances in presence of water. So unstable are these albuminoids that even an incipient oxidation (see Gay-Lussac s experiment) may suffice to disturb their chemical equilibrium to such an extent as to cause the whole of the atoms of the mass to gradually rearrange themselves into new products of lesser complexity and consequently higher stability. The decom position when once started, readily propagates itself through the whole mass, aided as it is by the inherent tendency of the molecule to pass into more highly stable forms, just as a stone which rolls down a hill and strikes other stones on its way causes them to roll down likewise. This is so clear and plausible as almost to command assent. It is less easy to agree with Liebig when he tries to explain fermentation, when he says, for instance, that the sugar in grape juice, although not naturally gravitating towards a rearrangement as alcohol plus carbonic acid, is nevertheless caused to undergo this change by its immediate contact with the albuminoids of the juice or yeast, which are in a state of atomic commotion; and it is still less easy to see how such an atomic revolution could progress from sugar to sugar, as he says it may. That the nitrogenous matters of the juice, in all ordinary cases of vinous fermentation, assume the form of yeast, is, according to Liebig, a purely accidental phenomenon, and, if yeast is so characteristically powerful as a ferment, it is so only through its consisting largely of exceptionally unstable albuminoid substances. Liebig s ideas, more perhaps through the brilliancy of his mode of exposition than the force of his arguments, took firm hold of the scientific mind of the time ; amongst chemists at least the general impression was, and it prevailed for a considerable time, that Liebig s theory in a satisfac tory manner summed up the whole of the empirical know ledge of the subject although it totally ignored at least one most important feature in the phenomena which had been brought out and firmly established by Schwann and Cagniard-Latour. In 1680 a Dutch philosopher, Leuwenhoek, fell upon examining yeast under the microscope, and found it to con-