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THE MICRO-ORGANISMS OF MOST IMPORTANCE IN
THE FERMENTATION INDUSTRY.1
The micro-organisms to be described now are partly useful, partly disadvantageous to the alcohol fermentation industry; their importance is therefore of widely different character. They all belong to that branch of the plant kingdom called fungi. The fungi are divided into two large groups : true fungi (Eumycetes) and fission fungi (Schizomycetes). The first of these two groups is divided into that of the algæ fungi (Phycomycetes) and that of the higher fungi (Mycomycetes). Of the numerous fungi belonging to the phycomycetes only a single group comes to be considered here, namely, that of the Zygomycetes, and in this only the family of Mucoracea.
Among the mycomycetes we shall refer partly to the sac fungi (Ascomycetes), taking representatives of the four orders, the gymnoasceæ with the family of saccharomycetes, the perisporaceæ with the family of aspergilleæ, the sphæriaceæ with the family of sphærieæ, and the discomycetes with the family of pezizaceæ-partly to a large group of fungi, the imperfect fungi (Fungi imperfecti), of no less importance, but which cannot yet be classified ; they are in all probability only stages of development of other forms of fungi.
A bibliography is given at the end of the book.
2 We refer those wishing a more detailed description of the fungus system and the general morphology and physiology of fungi to Zopf's Handbuch der Pilze.
A review of the systematic connection of the microorganisms to be described here may be made by aid of the classification given on page 173.
I.—TRUE FUNGI (EUMYCETES) is the name given to those fungi of which the vegetative organ is a mycelium. This consists of long threads which possess growing points and exhibit true branching. The first division is distinguished by this means from the second, which contains the fission fungi.
1.–Alge fungi (Phycomycetes). The whole mycelium consists as a rule only of one single, very much branched cell. Septa only appear under special conditions, and first appear normally when fructification begins. Endogenous spores are formed in sporangia. The only group belonging to this division which we will consider here is that of the
Zygomycetes. Multiplication takes place in these fungi partly by means of spores in sporangia, partly by means of the socalled zygospores, and in some forms by budding, by gemmae (chlamydospores) and by conidia.
Mucoracee. The spores develop from the mass of plasma in the interior of the sporangium, a part of the plasma being left which swells by taking up water. This happens as soon as the spores are ripe, and the wall of the sporangium bursts, setting free the spores. These fungi can propagate themselves not only by means of endospores, but also by zygospores (Fig. 55, V. and VI.). The latter are produced in the following manner: Two club-shaped swellings develop on two neighbouring mycelial threads; these grow towards one another until their ends touch, which then become flat. The two flattened end membranes then coalesce. A septum is then formed in these club-shaped growths, so that an end cell—copulation cell (Fig. 55, V., c)--and a suspensor (Fig. 55, V. and VI., b) appear. Finally the two end cells melt into one and a zygospore is thus formed (Fig. 55, VI., a). Some species form zygospores easily; with others it seems to be accidental; in short, the conditions of their formation are not yet known. The communications made by Bainier and others on this subject do not hold as regards those species with which the author has experimented. that the temperature maximum for development of sporangia and zygospores is lower than that for development of mycelium, yeast cells and gemma formation, and that the temperature limits for sporangium and zygospore formation change with the species; thus, in Mucor alpinus, sporangium formation has a higher temperature maximum than zygospore formation, whilst for Mucor neglectus the reverse is the case.
A third means of propagation is possessed by some species ; this consists of the so-called gemmæ or chlamydospores (Fig. 58, a). When the mycelium is immersed in a culture liquid containing sugar, numerous dividing walls make their appearance; short members are thus formed, which swell to a barrel shape and become highly refractive, after which their cell walls thicken. The separate members may grow into mycelium, or develop sporangium carriers at once (Fig. 58, 6), or they may separate from one another and increase by budding like yeast cells; the so-called “spherical yeast” is thus formed (Fig. 59). The spores may also behave in this manner.
Hansen has established the following general law for fungi, that the temperature maximum for the development of the organs of propagation lies lower than the maximum for the development of the vegetative organs. This, of course, applies also to the Mucoracea. Not long ago he described two new species, Mucor alpinus and Mucor neglectus, which can both develop zygospores. He showed
(1) Genus : Pin Moulds, Mucor, Micheli. Characteristic of this order is the ball-shaped sporangium (Fig. 54, III. and IV.) which occurs at the point of the sporangium carrier which is undivided in most kinds, a “columella” separating sporangium and carrier (Fig. 54, III., b). This columella is formed by the more or less vesicular end of the sporangium carrier projecting into the sporangium. A crust of calcium oxalate (Fig. 54, III., c) is often present on the outer wall of the sporangium.
The species belonging to this order live either as saprophytes, i.e., on dead animal or plant matter, or as parasites, i.e., on living organisms. They may be frequently seen as a white, gray or brown felt on dung, bread, fruit, corn, malt, etc., and they also thrive in beer wort. Some species can cause dextrin to ferment, others again contain diastase.
Hansen investigated several of these species with regard to their action on the four sugars: saccharose, maltose, lactose and dextrose. It appeared that lactose is fermented by no species, and saccharose only by one single species after previous inversion, while on the other hand all the species investigated ferment dextrose and maltose. The fermentation of maltose proceeds very slowly and only forms higher percentages of alcohol after a relatively long time. Thus, for example, Mucor Mucedo in wort gave after fifteen days at 23° C. Only 0:4 vol. per cent, alcohol; after two and