Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/107

FERMENTATION 97 B. Fermentations which are known only as Physiological Processes. L Vinous Fermentation. This case having already been con sidered, we confine ourselves here to a few additions and qualifica tions. Vinous fermentation, as we see it going on in the brewers vats and in the wine-producers casks, is a function of Saccha- romyccs, a genus of fungi, consisting of minute cells, which sometimes are isolated from one another, sometimes grouped to gether in a variety of forms, but never united into an organized tissue. There is a variety of species, of which S. cercrisicc (the main constituent of ordinary yeast, as produced in the high fermen tation of beer) is the most important. It consists of cells of about -j^ millimetre diameter. According to Pasteur, saccharomyccs thrives best when immersed in grape juice or wort, or similar liquids. It multiplies only by budding, never by sporification. In pure sugar-water it lives, so to say, at its own expense, and gradually becomes exhausted ; but on addition of phosphates (yeast-ash works best) and ammonia salt to the sugar, the plant thrives as well almost as in native sugar juices. "When saccharomyces is not fully immersed in the liquor, and otherwise constrained to live under abnormal conditions, it passes into "aerobiotic" forms which are similar to inucors andmucedos (mould plants), and which, like these, live on atmospheric oxygen. But these abnormal forms, when re- immersed in wort, &c., always relapse into the non-aerobiotic form of saccharomyces. Keal mucedos, &c., for instance Mycoilcrma vini and ccrcvisicc, which by nature are aerobiotic, when immersed in wort or grape juice, and thus placed in what to them is an abnormal condition, assume non-aerobiotic forms, and produce vinous fermenta tion, but (contrary to what was formerly assumed by Pasteur himself) they are never converted into saccharomyces, and their fermentative power soon conies to an end, unless they are occasionally revived by re-exposure to the atmosphere. The power of inducing vinous fermentation, however, is by no means confined to microscopic organisms. It has long been known, from the experiments of Dobereincr and others, that sweet fruit, when kept within an inert atmosphere devoid of free oxygen, evolves carbonic acid with formation of alcohol, and it has been proved by Pasteur that this fermentation, which may extend to a considerable portion of the sugar present, is not accompanied by the development of any microscopic species. Closely related to this fact is the well-established experience that large quantities of sugar may be made to ferment by means of yeast without the latter multiplying to any noteworthy extent. Oil the other hand, large growths of yeast may bo obtained (and as a matter of fact arc obtained every day by the makers of German barm) without pro ducing much alcohol. Oskar Brefeld, by means of a peculiar arti fice, succeeded in growing saccharomyces in brewers wort, without producing a trace of alcohol. From these experiences we must conclude that vinous fermentation, far from being the characteristic life-function of healthy saccharomyces, is dependent on a certain pathologic condition of " non-photobiotic " plant-cells (i.e., cells which Habitually live in darkness) generally, which is brought about by immersing them in saccharifcrous fluids and shutting them out from the oxygen-gas which they need for their healthy development. Hence, even in the ordinary cases of fermentation, the normal life of the yeast-plant on the one hand, and the dis sociation of the sugar on the other, are not only not necessarily re lated, but, in the individual cell, positively exclude each other. In any given mass of yeast, healthy cells and diseased cells are in general mixed up together, and thus, in practice, the two pheno mena come to be accidental concomitants. But this brings us back almost precisely to the later views of Liebig, as set forth in his last memoir on the subject. In regard to the genesis of the yeast plant little is known. According to Pasteur s experiments and observations the yeast which forms spontaneously in grape juice is derived chiefly from certain germs which abound about harvest time on the grapes, and still more on the grape-stalks. These germs are largely diffused also through the atmosphere of breweries, wine cellars, and labora tories where fermentation experiments are carried on, but they are not by any means widely diffused through the atmosphere generally. 2. Lactic Fermentation. Milk when left to itself in warm weather, as everybody knows, soon turns sour, the main feature in the chemical process being the transmutation of the milk sugar into lactic acid, as expressed by the equation C JS H ss O n .H,0 - 4C 3 H 6 3 Hydrated Milk sugar. Luctic acid. The milk sugar, before assuming the form of lactic acid, probably passes through the condition of glucose. At any rate, ordinary glucose, when dissolved in milk, ferments into lactic acid along with the milk sugar originally present. But in this case, if the total percentage of sugar goes beyond a certain limit, the reaction comes to a stop as soon as the acidity of the liquid has attained a certain limit-value. Addition of chalk or carbonate of soda, i. c. , conversion of the lactic acid into a neutral lactate, then revives the process. A solution of "in vert-sugar" (as produced by boiling cane sugar water With a little vitriol), when mixed with excess of chalk and some putrid cheese, and kept at 30-35 C., soon ferments, with formation of large quantities of lactate of lime (Beusch). Lactic fermentation, according to Pasteur, is caused by the development in the mass of a microscopic fungus, consisting "of cylindrical cells which are far smaller than those of saccharomyces. We are not aware that this "lactic ferment" has ever been seen in ordinary sour milk; in Bensch s process it is produced largely as a greyish deposit on the chalk, from which pure growths of the fungus may be obtained by Pasteur s method (see above). The lactic ferment, to the annoyance of brewers, frequently occurs in ordinary yeast as an impurity. There is no doubt that that fungus which Pasteur calls (fie lactic ferment is capable of inducing lactic fermentation ; but it does not by any means follow that it is the thing which actually causes the souring of milk under ordinary circumstances. On the con trary, from a remarkable set of experiments made by Lister in 1873, this appears not to be the case. According to him, milk can be completely purged of germs by exposing it (within a germ- less flask) to the temperature of boiling water for some hours, and, when protected against atmospheric germs by a slightly carbolized stopper of cotton wool, keeps sweet for an indefinite time. Speci mens of such gcrmless milk, when exposed to the atmosphere of his study, were found by Lister to undergo a variety of fermentative changes, accompanied sometimes by the development of an acid reaction, but none of them set into sour milk. A specimen of ordinary unboiled dairy milk when kept in the same room did get sour as usual, and, when examined under the miscroscope, was found to contain, not Pasteur s fungus, but a kind of motionless bacterium which Lister calls B. lactic, because the introduction of it (or rather of a trace of the sour milk containing it) into the germless milk determined normal lactic fermentation, the Bacterium lactis multiplying at the same time. The same bacterium, when made to pass successively through germless urine and other germless organic liquids, underwent a series of metamorphoses, but, when ultimately put back into milk, caused normal lactic fermentation. The germs of this bacterium must be assumed to abound in the atmosphere of cows stables and dairies, although they do not seem to be abundantly diffused through the atmosphere generally. 3. Viscous Fermentation is a peculiar change which has long been known to occasionally accompany vinous fermentation, and which manifests itself in this that the wine becomes thick and viscous, so that, when poured from one vessel into another, it draws into long threads. This property is caused by the presence of a kind of gum (of the composition C 12 H ao 10 ) which is invariably accompanied by mannite, a sweet crystalline substance of the composition 6 H ]4 6 (i.e. containing the elements of glucose, C 6 H 12 6 + those of hydrogen, H 2 ). The exact nature of the reaction is not established ; in fact, we do not know whether it is one reaction or a set of reactions going on simultaneously. According to the usually adopted equation, 100 parts of cane sugar should yield 51 of mannite, 45 5 of gum, and 6 of carbonic acid. According to Peligot (supported by Pasteur) the "viscous ferment" is a fungus consisting of very minute spherical cells (of O OOl to 0014 millimetre diameter). 4. Butyric Fermentation. In the lactic fermentation of glucose, as induced by milk or cheese in the presence of chalk, the lactate of lime is no sooner formed than it undergoes itself a further change, Avhich, chemically, is represented approximately by the equation 2C 3 H 6 3 = C 4 H 8 2 + 2CO S + 211, Lactic acid. Butyric acid. Carbonic acid. Hydrogen gas. The temperature most favourable to the change lies near 40 C. A number of similar changes (of other organic acids than lactic) are known, but they are passed over here, being of a more purely scientific interest. According to Pasteur, butyric fermentation is caused by the development in the mass of a special kind of vibrio, a worm-shaped animalcule, consisting of a number of longitudinal cells, each about 002 millimetre thick, and from 002 to 02 mm. long. Butyric fermentation, strictly speaking, is only one of a large genus of changes customarily summed up under the generic name of putrefaction. 5. Putrefaction. The scientific meaning of this term coincides pretty much with its popular acceptation, except that it must be understood to be exclusiveof all cases of oxidation. In olden times it was assumed that organized matter (the tissues of plants and animals, blood corpuscles, &c.) could hold together even chemically only as long as supported by the vital force. But this is a long exploded notion. In absolute absence of water, or at very low temperatures, dead organized matter remains chemically (and even structurally) unchanged. In support of this assertion we need only refer to that well known case of the mammoth of the Siberian cave, which was found sweet and fresh thousands of years after the extinction of life. And since the time of Appert (who discovered the now so ex tensively used process of preserving meats in sealed-up tins) we know that prolonged exposure to boiling heat and subsequent absolute exclusion of air prevent putrefaction, even in presence of liquid water and at the ordinary temperature, as long as the air remains excluded. Chemically speaking, ordinary putrefaction is a most complex phenomenon, always involving the simultaneous on-going IX - T