Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/106

9G up into alcohol and carbonic acid, under certain other sets of conditions ferments into lactic acid, or through lactic into butyric acid, or into gum plus mannite. This has long been known. What Pasteur showed is that each of these changes is the exclusive function of a certain species (or at least genus) of organism. What the yeast plant is for vinous a certain other organism is for lactic, a third for mannitic, a fourth for butyric fermentation. No two of these species, even if they belong to the same genus, will ever pass into each other. Pasteur arrived at this great generalization by means of his invention of an ingenious method for cultivating pure growths of the several species, so that each of them could be examined separately for its chemi cal functions. To explain the method, let us suppose we wanted an unmixed growth of the species of yeast-plant called Saccharomyces cerevisice. The first step of course is to procure a specimen of yeast which, when examined under the microscope, proves a fair approximation to what is wanted. This being done, we place a quantity of brewers wort in a flask provided with two necks, one long and very narrow and bent like a gas-evolution tube, the other short and wide like the tubulus of a retort. We boil the wort in the flask to kill the germs, and during the boiling close first the tubulus by means of a (germless) glass stopper, then the narrow tube by means of a previously ignited plug of asbestos. We now allow the flask to cool down slowly, and by exposing it for several days to the proper temperature make sure that there is no potential life in our medium. We then, by means of a thin platinum wire, introduce a speck of the yeast through the tubulus, which, of course, is stopped up again without delay. We then place the flask in a chamber kept at the particular tempera ture which is most favourable to the development of " saccharomyces." The saccharomyces-cells, being in the majority and enjoying a position of advantage, will multiply at a greater rate than the foreign cells, of which many in fact will go to the wall, so that what we ultimately obtain is a closer approximation to pure saccharomyces than the speck of yeast which we used as seed, and which, in general, was already a little better (in all probability) than the bulk of the yeast it came from. Of the relatively pure yeast we now again take a speck and sow it in a fresh supply of germless wort, and so on until the foreign cells can be assumed to be " Darwinized " out of existence. This method of Pasteur s, apart from its scientific value, is of the greatest practical importance ; supposing it to be carried out on a large scale which, in fact, has been done with some measure of success it will enable the brewer to grow and use pure saccharomyces, and thus to avoid all the many " diseases " which beer, as brewed with ordinary yeast, is liable to. After Pasteur s researches it became impossible not to admit that those fermentative changes which he investi gates are, at least in a practical sense, physiological and not purely chemical phenomena ; but there appears to be no necessity for assuming, as many do, that it is the life of those minute organisms, qualite, which directly causes the phenomena. It seems far more rational and philosophic to adopt the view taken by Berthelot and Hoppe-Seyler, who go no further than to admit that those living organisms are the only known sources for the ferments proper, which in themselves are chemical substances pure and simple. Proceeding now to give a short account of the different fermentative changes, we begin with those that are proved to be functions of purely chemical reagents. A. Fermentations proved to he purely chemical reactions. These are conveniently arranged according to the respective cata lytic agents. 1. Acvl<s. Many organic substances, when boiled with water and a small quantity of sulphuric, muriatic, or other strong mineral acid, undergo hydra! ion and decomposition, or other chemical trans mutation, the acid remaining ultimately in its original condition. Thus, under the circumstances named, (1.) Cane sugar is "inverted," i.e., converted into dextrose and levulose, thus : C u H M O n + 11,0 C 6 H 12 6 + C 6 II 1S 6 . (2.) The same equation applies to the case of milk sugar, one of the products C 6 rI 12 6 in this case being a peculiar substance called galactose. " (3.) Starch passes into dextrine (a kind of gum) and dextrose, thus : 3H,0 Water. CH 10 O a Dextrine. 3C 8 H JS 6 Dextrose. (4.) Salicine (a bitter principle in willow bark) breaks up into glucose and "saligenine," thus : C 13 H 18 7 + H 2 C 6 H 12 6 + C 7 H 8 0., Salicine. Glucose. Saligenine. Many other "glucosides" (native substances containing potential glucose) behave in a similar manner. The exact mechanism of these reactions is scarcely understood ; pending exact investigations, they may be explained, according to Lyon Flayfair, by a tendency of the acid to combine with the elements of one of the products, which tendency, although sufficient to sever these elements from the rest, is defeated ultimately by the stability of the compound formed by their union with one another. 2. Diastase is a peculiar substance which is formed in the ger mination of grain (in malting), and which has the power of convert ing many times its weight of starch into dextrine and dextrose when made to act on it in the presence of water at about 66 C. Diastase has not yet been isolated in the pure state. In the process referred to it is changed ; but it is not known into what. 3. Emulsine is a constituent of almonds (both of bitter and sweet), which is known chiefly for its power of decomposing amygdaline (a crystalline substance contained in bitter almonds, and extractable therefrom by alcohol), with formation of bitter- almond oil and glucose, thus : C 20 H 27 NO n + 2H 2 = 2C B H 1S 8 + C 7 H 6 O.NCH Amygdaline. Glucose. Volatile oil. The oil, as the formula shows, is a (loose) compound of pmssic acid, KCH, and benzaldehyde, C 7 H 6 0. The common idea that bitter almonds contain prussic acid is erroneous ; that acid, like the benzaldehyde, is present only potentially, viz., as amygdaline, which, when the almond meal is treated with water, undergoes the above fermentative change. Many other glucosides are decomposed by emulsine, as they are by dilute acids. It may be said, in pass ing, that the acrid volatile oil contained in table-mustard is not found ready formed in the mustard seed, but is produced from a constituent of the seed by a fermentative action closely analogous to that we have just been explaining. 4. Soluble Yeast Ferment. It has already been stated that an aqueous extract of yeast, though devoid of the power of inducing vinous fermentation, converts cane sugar into dextrose and levulose. The ferment, as Berthelot showed, can be precipitated from the liquid, in an impure state, by addition of alcohol. 5. Pcpsinc. Stomach digestion (in man and animals nearly re lated to man) consists mainly in this that the gastric juice dis solves the albuminoids of the food, as hydrochlorates of peptones, the only form, it seems, in which they can be assimilated by the system. The juice owes this power to the presence in it of small percentages of two things, namely, of free hydrochloric acid and of " pepsinc," botli of which are continuously produced by the mucous membrane. Real pepsine has never been seen ; but an impure substance, possessing the specific properties of the ferment, can be extracted from the mucous membrane of the stomach by a laborious process which we have no space to describe. Highly dilute hydrochloric acid alone dissolves certain albuminoids, but it does not convert them into peptones ; it acquires this property by the addition to it of a small quantity of the preparation named. It is as well to state in passing, that the so-called pepsine of the pharmaceutist is only a very poor apology for even the pepsine of the physiological chemist. C. Fancrcatinc. What pepsiuc is to the gastric juice pancreatine is to the secretion of the pancreas gland, whose function it is to digest the starchy and fatty portions of the food. This "pancrea tine" seems to include three ferments, namely, a kind of diastase (see above), a ferment similar in its functions to pepsine, and a fer ment which has the power of converting fats into fatty acids and glycerine. None of these has been isolated. 7. Erythrozyme. This is a peculiar ferment Avhich Ed waul Schunck, in 1854, extracted from madder-root, and which was found by him to possess the power of inducing vinous fermentation in solutions of sugar, a most important discovery, which ought to be further investigated. Note. All these ferments, the acids of course exccpted, lose their efficacy at temperatures near 100 C. in presence of water. In the dry state they may survive boiling heat.