The organized ferments, or micro-organisms concerned in fermentation, all belong to the group of crytogamic plants known as Fungi, and their near allies the Bacteria. The most highly organized members of this group con cerned in fermentation are some of the molds, but their part in fermentation is of little im portance, and we need not further consider them. We may therefore discuss the micro organisms of fermentation under the two heads: (1) Saccharomycetes, or Yeast-Fungi, and (2) Schizomycetes, or Bacteria. The Sac charomycetes are minute unicellular plants of an oval shape, surrounded by a cell-wall and containing granular protoplasm in which several vacuoles are •conspicuous. They re produce themselves by gemination or budding, and by this means chains of cells closely re sembling the hyphz of a mycelium are formed; but very few species develop any true mycelium, such as we find in the molds and other fungi. The Bacteria, Schizomycetes, or Fission-Fungi, are a very important group of microscopic cryptogams usually classed with the fungi. Their importance arises not only from the part they play in fermentations, but also from their widespread activity in the order of nature, and especially from their action in producing numerous zymotic diseases in man and other animals. See 13AcrEam.
The yeast-fungi cannot assimilate free nitro gen, but they can extract it from salts of am monia and from various organic bodies. They can take up carbon from sugars, glycerin, tar taric acid, citric acid, acetic acid, ethyl alcohol, benzoic acid, phenol and other organic bodies; but they can obtain their nutriment only in the presence of such substances as potash, lime and phosphoric acid. Unlike nearly all the bacteria, the yeasts can live in acid media if the acid be not present in excessive quantity. The presence of free oxygen is very favorable to growth of yeast-fungi, but under these conditions the transformation of sugar into alcohol greatly diminishes in vigor. If, however, free oxygen be excluded, fermentation proceeds vigorously. The optimum temperature for the growth of yeast-fungi is from 77° to 86° F., and they seem to be quite indifferent to light and electric ity. Bacteria assimilate nitrogen most readily from peptone, but many other substances can also yield. Carbon is most readily taken up from sugars, glycerin, tartrates, citrates, lac tates, acetates, etc. Most bacteria are unfavor ably influenced by light, and in respect to temperature there is a great diversity among the different species, some flourishing at 60° to 70° C. and others at freezing point, but for several of the best-known species the optimum temperature is 25° to 35° C. The obligate aerobic forms cannot live in the absence of free oxygen, while the obligate anaerobic kinds are destroyed by the smallest trace of free oxygen. Between these two extremes we have facultative aerobic species, which are generally anaerobic, but can tolerate some free oxygen, and facultative anaerobic species, generally aerobic but able to live when the free oxygen is much diminished. Phosphoric acid, potash,
lime and similar bodies are necessary to their growth.
The most important of all fermentations due to Saccharomycetes is that by which alcohol is obtained from sugar (the sugar itself being largely that derived from the starch of barley or other cereal), and it is in connection with this process that the subject has been most thoroughly studied. The species of fungus or yeast used in this process is known as Saccharo myces cerevisice, but of this two kinds are dis tinguished, namely, top yeast and bottom yeast. The former is used in top fermentation, which is earned on at a rather high temperature (60° to 85° F.) and is rather vigorous, carbon dioxide being freely evolved and carrying the yeast to the surface. Bottom yeast is used in the slower bottom fermentation at about 40° to 50° F., in which carbon dioxide is more gently evolved and the yeast therefore remains at the bottom. Bottom fermentation is chiefly used in the manufacture of lager beer. The sugars directly fermentable by this process are those with the formula G.H.O., namely, dextrose, levulose and galactose. Cane-sugar (Ca-L*0u) is first changed by an enzyme invertase (see above) contained in the yeast into dextrose and levulose, which are then fermented. The chief products of the fermentation are alcohol and car bon dioxide, but other bodies, such as glycerin, succinic acid, propyl, amyl and other higher alcohols, are also produced. The process of fermentation continues until the alcohol forms about 14 per cent of the solution, when the yeast-plant cannot continue its action any longer.
Many theories of alcoholic fermentation have been advanced. Lavoisier, treating sugar as an oxide, thought that it was chemically re solved by fermentation into two other oxides, carbon dioxide and alcohol. When the organic nature of yeast was proved beyond doubt, the physiological theory was advanced, to which the sugar is the food of the plant and the products of fermentation are its excretions. The celebrated chemist Liebig, however, con tended that fermentation was a purely chemical process, and his later opinion has been in part reverted to by more recent observers. Pas teur championed. the Schulze theory that fer mentation was produced directly by living or ganisms. Traube regarded the cells as centres for the production of enzymes, thus reducing fermentation by organized ferments to that produced by unorganized ferments. Traube's theory has gained considerable ground lately on account of the successful extraction from yeast by Buchner of an enzyme, zymase, which can produce all the phenomena of alcoholic fer mentation. In consequence, Buchner held that the fermentative process is a result of the activ ity of the enzymes produced by the living micro-organisms, and that the process could be initiated and carried on independently of the presence of the living organisms.