As in all the conclusions arrived at by Pasteur, so in those relating to fermentation, there were a number of different experiments which were performed by him to elucidate the same point. We will choose one of many in relation to fermentation. If a sugary solution of carbonate of lime is left to itself, after a time it begins to effervesce, carbonic acid is evolved, and lactic acid is formed; and this latter decomposes the carbonate of lime to form lactate of lime. This lactic acid is formed, so to speak, at the expense of the sugar, which little by little disappears. Pasteur demonstrated the cause of this transformation of sugar into lactic acid to be a thin layer of organic matter consisting of extremely small moving organisms. If these be withheld or destroyed in the fermenting fluid, fermentation will cease. If a trace of this grey material be introduced into sterile milk or sterile solution of sugar, the same process is set up, and lactic acid fermentation occurs.
Pasteur examined the elements of this organic layer by aid of the microscope, and found it to consist of small short rods of protoplasm quite distinct from the yeast cells which previous investigators had detected in alcoholic fermentation. One series of experiments was accomplished with yeast cells and these bacteria, a second series with living yeast cells only, a third series with bacteria only, and the conclusions which Pasteur arrived at as the result of these labours were as follows:
"As for the interpretation of the group of new facts which I have met with in the course of these researches, I am confident that whoever shall judge them with impartiality will recognise that the alcoholic fermentation is an act correlated to the life and to the organisation of these corpuscles, and not to their death or their putrefaction, any more than it will appear as a case of contact action in which the transformation of the sugar is accomplished in the presence of the ferment without the latter giving or taking anything from it."
Pasteur occupied six years (1857–1863) with further elucidation of his wonderful discovery of the potency of these hitherto unrecognised agents, and the establishment of the fact that "organic liquids do not alter until a living germ is introduced into them, and living germs exist everywhere."
It must not be supposed that to Pasteur is due the whole credit of the knowledge acquired respecting the cause of fermentation. He did not first discover these living organisms; he did not first study them and describe them; he was not even the first to suggest that they were the cause of the processes of fermentation or disease. But, nevertheless, it was Pasteur who "first placed the subject upon a firm foundation by proving with rigid experiment some of the suggestions made by others." Thus it has ever been in the times of new learning and discovery: many contributors have added their quota to the mass of knowledge, even though one man appearing at the right moment has drawn the conclusions and proved the theory to be fact.
In order that no confusion may arise in the mind of the reader, we may here say that, although fermentation is always due to a living agent, as proved by Pasteur, the process is conveniently divided into two kinds.[30] (1) When the action is direct, and the chemical changes involved in the process occur only in the presence of the cell, the latter is spoken of as an organised ferment; (2) when the action is indirect, and the changes are the result of the presence of a soluble material secreted by the cell, acting apart from the cell, this soluble substance is termed an unorganised soluble ferment, or enzyme. The organised ferments are bacteria or vegetable cells allied to the bacteria; the unorganised ferments, or enzymes, are ferments found in the secretions of specialised cells of the higher plants and animals. With the former this book deals in an elementary fashion; with the latter we have little concern. It will be sufficient to illustrate the enzymes by a few of the more familiar examples. They form, for example, the digestive agents in human assimilation. This function is performed, in some cases, by the enzyme combining with the substance on which it is acting and then by decomposition yielding the new "digested" substance and regenerating the enzyme; in other cases, the enzyme, by its molecular movement, sets up molecular movement in the substance it is digesting, and thus changes its condition. These digestive enzymes are as follows: in the saliva, ptyalin, which changes starch into sugar; in the gastric juice of the stomach, pepsin, which digests the proteids of the food and changes them into absorptive peptones; the pancreatic ferments, amylopsin, trypsin, and steapsin, capable of attacking all three classes of food stuffs; and the intestinal ferments, which have not yet been separated in purer condition. In addition to these, there are ferments in bitter almonds, mustard, etc. Concerning these unorganised ferments we have nothing further to say. Perhaps the commonest of them all is diastase, which occurs in malt, and to which some reference will be made later.
Its function is to convert the starch which occurs in barley into sugar. These unorganised ferments act most rapidly at about 75° C. (167° F.).[31]
We may now return to the work of Pasteur and the question of organised ferments. Let us preface further remark with an axiom with which Professor Frankland sums up the vitalistic theory of fermentation, which was supported by the researches of Pasteur: "No fermentation without organisms, in every fermentation a particular organism." From these words we gather that there is no one particular organism or vegetable cell to be designated the micro-organism of fermentation, but that there are a number of fermentations each started by some specific form of agent. It is true that the chemical changes induced by organised ferments depend on the life processes of micro-organisms which feed upon the sugar or other substance in solution, and excrete the product of the fermentation. Fermentation nearly always consists of a process of breaking down of complex bodies, like sugar, into simpler ones, like alcohol and carbonic acid. Of such fermentation we may mention at least five: the alcoholic, by which alcohol is produced; the acetous, by which wine absorbs oxygen from the air and becomes vinegar; the lactic, which sours milk; the butyric, which out of various sugars and organic acids produces butyric acid; and ammoniacal, which is the putrefactive breaking down of compounds of nitrogen into ammonia. We have already referred at some length to this process when considering denitrifying organisms in the soil.