Bacteria: Chapter IV - BACTERIA AND FERMENTATION

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CHAPTER IV. BACTERIA AND FERMENTATION

It was Pasteur who in 1857 first propounded the true cause and process of fermentation. The breaking down of sugar into alcohol and carbonic acid gas had been known, of course, for a long period. Since the time of Spallanzani (1776) the putrefactive changes in liquids and organic matter had been prevented by boiling and subsequently sealing the flask or vessel containing the fluid.

Moreover, this successful preventive practice had been in some measure correctly interpreted as due to the exclusion of the atmosphere, but wrongly credited to the exclusion of the oxygen of the air. It was not until the beginning of the present century that authorities modified their view and declared in favour of yeast cells as the agents in the production of fermentation. That this process was due to oxygen per se was disproved by Schwann, who showed that so long as the oxygen admitted to the flask of fermentative fluid was sterilised no fermentation occurred. It was thus obvious that it was not the atmosphere or the oxygen of the atmosphere, but some fermenting agent borne into the flask by the admission of unsterilised air.

It was but a step to further establish this hypothesis by adding unsterilised air plus some antiseptic substance which would destroy the fermenting agent. Arsenic was found by Schwann to have this germicidal faculty. Hence Schwann supported Latour's theory that fermentation was due to something borne in by the air, and (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.).

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.

There are four chief conditions common to all these five kinds of organised fermentation. They are as follows:—1. The presence of the special living agent or organism of the particular fermentation under consideration. This, as Pasteur pointed out, differs in each case.2. A sufficiency of pabulum (nutriment) and moisture to favour the growth of the micro-organism.3. A temperature at or about blood-heat (35–38° C., 98.5° F.).4. The absence from the solution or substance of any obnoxious or inimical substances which would destroy or retard the action of the living organism and agent. Many of the products of fermentation are themselves antiseptics, as in the case of alcohol; hence alcoholic fermentation always arrests itself at a certain point.

We are now in a position to consider particular fermentations and their causal micro-organisms. These latter are of various kinds, belonging, according to botanical classification, to various different subdivisions of the non-flowering portion of the vegetable kingdom. A large part of fermentation is based upon the growth of a class of microscopic plants termed yeasts.

These differ from the bacteria in but few particulars, mainly in their method of reproduction by budding (instead of dividing or sporulating, like the bacteria). Their chemical action is closely allied to that of the bacteria. Secondly, there are special fermentations and modifications of yeast fermentation due to bacteria. Thirdly, a group of somewhat more highly specialised vegetable cells, known as moulds, make a perceptible contribution in this direction. According to Hansen, these latter, so far as they are really alcoholic ferments, induce fermentation, not only in solutions of dextrose and invert sugar, but also in solutions of maltose. Mucor racemosus is the only member that is capable of inverting a cane-sugar solution; M. erectus is the most active fermenter, yielding eight per cent. by and secondly the change of the sugar into alcohol and carbonic acid gas by the process of fermentation, an organic change brought about by the living yeast cells.

In all these three forms of alcoholic fermentation the principal features are the same, viz., the sugar disappears; the carbonic acid gas escapes into the air; the alcohol remains behind. Though it is true that the sugar disappears, it would be truer still to say that it reappears as alcohol. Sugar and alcohol are built up of precisely the same elements: carbon, hydrogen, and oxygen. They differ from each other in the proportion of these elements. It is obvious, therefore, that fermentation is really only a change of position, a breaking down of one compound into two simpler compounds. This redistribution of the molecules of the compound results in the production of some heat. Thus we must add heat to the results of the work of the yeasts.

When alcohol is pure and contains no water it is termed absolute alcohol. If, however, it is mixed with 16 per cent. of water, it is called rectified spirit, and when mixed with more than half its volume of water (56.8 per cent.) it is known as proof spirit.

We shall have to consider elsewhere a remarkable faculty which some bacteria possess of producing products inimical to their own growth. In some degree this is true of the yeasts, for when they have set up fermentation in a saccharine fluid there comes a time when the presence of the resulting alcohol is injurious to further action on their part. It has become indeed a poison, and, as we have already mentioned, a necessary condition for the action of a ferment is the absence of poisonous substances. This limit of fermentation is reached when the fermenting fluid contains 13 or 14 per cent. of alcohol.

Having discussed shortly the "medium" and the results, we may now turn to the bacteriology of the matter, and enumerate some of the chief forms of the yeast plant. Professor Crookshank gives more than a score of different members of this family of Saccharomycetes. Before dwelling upon some of the chief of these, it will be desirable to consider a number of properties common to the genus.

The yeast cell is a round or oval body of the nature of a fungus, composed of granular protoplasm surrounded by a definite envelope, or capsule. It reproduces itself by budding, or, as it is sometimes termed, gemmation. At one end of the cell a slight swelling or protuberance appears, which slowly enlarges. Ultimately there is a constriction, and the bud becomes partly and at last completely separated from the parent cell. In many cases the capsules of the daughter cell and the parent cell adhere, thus forming a chain of budding cells.

The character of the cell and its method of reproduction do not depend merely upon the particular species alone, but are also dependent upon external circumstances. There are differences in the behaviour of species towards different media at various temperatures, towards the carbohydrates (especially maltose), and in the chemical changes which they bring about in nutrient liquids. In connection with this Professor Hansen has pointed out that, whilst some species can be made use of in fermentation industries, others cannot, and some even produce diseases in beer.Ascospore Formation

One of the most remarkable evidences of the adaptability of the yeasts to their surroundings and a specific characteristic occurs in what is sometimes called ascospore formation. If a yeast cell finds itself lacking nourishment or in an unfavourable medium, it reproduces itself not by budding, but by forming spores out of its own intrinsic substance, and within its own capsule. To obtain this kind of spore formation Hansen used some gypsum blocks as medium on which 

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