But yeast-fungi and mould-fungi, like bacteria or fission-fungi, are micro-organisms, belonging to two specific orders, the Saccharomycetes and the Hyphomycetes, which are intimately related to each other, but quite distinct from bacteria. Their germs occur widely distributed in air, soil, and water. Many species are of hygienic, while others are of pathological interest and importance in being either accidentally associated with, or the cause of, disease processes, while others are fermentations of very essential service in various industrial processes. The making of beers, wines, and spirits, as we understand them, constitutes but a small part of the province of fermentation. The life activities of ferments open out a study of vast importance to mankind, and while they have only been regarded in their worst aspect—that of a bane—they are, nevertheless, a boon to mankind. The first clear view we obtained of this was that of Reess, who in 1870 showed there were several species or forms of the yeast-fungus. Hansen followed up this discovery in 1883, and, taking advantage of the strict methods of culture introduced by bacteriologists, found that by cultivating yeast on a solid media from a single spore it was quite possible to obtain constant types of pure yeasts, each possessing its own peculiar properties. One consequence of Hansen’s labours was that it now became possible for every brewer to work with a yeast of uniform type instead of with haphazard mixtures, in which serious disease forms might predominate and injure the beer. Among other things made clear was that a true yeast may have a mycelial stage of development. Furthermore, there is the influence exercised by the nucleus of the yeast cell. Many other points of interest arose out of these investigations; one was, that many higher fungi can assume a yeast-like stage of development if submerged in fluids, as, for instance, various species of Mucor, Ustilago, Exoascus, and numerous others. Ascomycetes, and Basidiomycetes as well, are known to form budding cells, and it was thought that the yeasts of alcoholic fermentation are merely reduced forms of these higher fungi, which have become habituated to the budding condition—a conclusion supported by Hansen’s discovery that a true Saccharomyces can develop a feeble, but a true, mycelium.
Fig. 279.—Saccharomyces and Moulds.
1. Section from a tomato, showing spores growing from cuticle; 2. Portion detached to show budding-out process; 3. Lateral view of spore sac with oospores issuing forth; 4. Apiculated ferment spores; 6 and 7. Mycoderma cerivisiæ in different stages of growth, as seen on wine bottles; 8 and 9. Torulæ diabeticæ, torulæ and fission spores.
“This view has been entirely confirmed by an inquiry into the mode of brewing saké by the Japanese, by the aid of the Aspergillus fungus. Further researches established the fact that other forms of fungi, e.g., those on the surface of fruits, developed endogenous spores, which cause alcoholic fermentation. More recently, and by further experimental inquiry, partly by pure cultures of separate forms, and partly by well-devised cultures on ripening fruits still attached to the plant but imprisoned in sterilised glass vessels, it has been found that yeast and moulds are separate forms, not genetically connected, but merely associated in nature, as are so many other forms of yeasts, bacteria, and moulds. Further, Hansen has discovered that several yeasts furnish quite distinct races or varieties in different breweries in various parts of the world, so that we cannot avoid the conclusion that their race characteristics have been impressed on the cells by the continued action of the conditions of culture to which they have so long been exposed—they are, in fact, domesticated races.”
The environments of yeasts are peculiar. Sauer found that a given variety of yeast, whose activity is normally inhibited when the alcohol attains a certain degree of concentration in the liquid, can be induced to go on fermenting until a higher degree is attained by the addition of a certain lactic acid bacterium. The latter, indeed, appears to prepare the way for the yeast. It has been shown, also, that damage may be done to beers and wines by allowing plant germs to gain access with the yeast; there are, too, several forms of yeast that are inimical to the action of the required fermentation. Other researches show that associated yeasts may ferment better than any single yeast, and such symbiotic action of two yeasts of high fermenting power has given better results than either alone. English ginger-beer furnishes a curious symbiotic association of two organisms—a true yeast and a true bacterium—so closely united that the yeast cells become imprisoned in the gelatinous meshes of the bacterium; and it is a curious fact that this symbiotic union of yeast and bacterium ferments is far more energetic than either when used alone, and the product is different, large quantities of lactic and carbonic acids being formed, and little or no alcohol.
Many years ago I gave an account of similar curious symbiotic results obtained by introducing into a wort-infusion a small proportion of German yeast, an artificial product composed of honey, malt, and a certain proportion of spontaneously-fermented wheat flour. This, to my astonishment, produced ten per cent. more alcohol than any of its congeners, and did not so soon exhaust itself as brewer’s yeast.[54]
In the hephir used in Europe for fermenting milk, another symbiotic association of yeast and a bacterium, it is seen that in this process no less than four distinct organisms are concerned. I have already instanced the fermentation of rice to produce saké, which is first acted upon by an Aspergillus that converts the starch into sugar and an associated yeast, and this is also shown to be a distinct fungus, symbiotically associated in the conversion. “Starting, then, from the fact that the constitution of the medium profoundly affects the physiological action of the fungus, there can be nothing surprising in the discovery that the fungus is more active in a medium which has been favourably altered by an associated organism, whether the latter aids the fungus by directly altering the medium, or by ridding it of products of excretion, or by adding gaseous or other body. It is not difficult to see, then, that natural selection will aid in the perpetuation of the symbiosis, and in cases like that of the ginger-beer plant it is extremely difficult to get the two organisms apart, a difficulty similar to that in the case of the soredia of lichens.”
Buchner discovered that by means of extreme pressure a something can be extracted from yeast which at once decomposes sugar into alcohol and carbon-dioxide. This something is regarded as a kind of incomplete protoplasm—a body, as we have already seen, composed of proteid—and in a structural condition somewhere between that of true soluble enzymes like invertin and a complete living protoplasm. This reminds me of an older experiment of mine, the immediate conversion of cane-sugar into grape-sugar. If we take two parts of white sugar and rub it up in a mortar with one part of a perfectly dry solid, the German yeast before spoken of, it is immediately transformed as if by magic into a flowing liquid mass—a syrup. This process of forming “invert sugar” can be watched under the microscope; the liberation of carbonic acid gas in large bubbles is seen to go on simultaneously with the assimilation of the dextrose, and the breaking up of the crystals of sugar; the cell at the same time increasing in size as well as in refractive power; a curious state of activity appears to be going on in the small mass, which is very interesting to watch throughout.
However, the enzymes of Buchner are probably bits off the protoplasm, as it were, and so the essentials of the theory of fermentation remain, the immediate agent being not that of protoplasm itself, but of something made by or broken off from it. Enzymes, or similar bodies, are known to be very common in plants, and the suspicion that fungi do much work with their aid is abundantly confirmed. It seems, indeed, that there are a whole series of these bodies which have the power of carrying over oxygen to other bodies, and so bringing about oxidations of a peculiar character. These curious enzymes were first observed owing to studies on the changes which wine and plant juice undergo when exposed to the action of the oxygen of the air.