no BACTERIA AND THE CARBON DIOXIDE CYCLE 



In the same way, invertasc, an enzyme secreted by yeast cells, converts 

 one molecule of cane sugar into one molecule of glucose and one of fructose : 



C 12 H 22 U +H 2 = C 6 H 12 6 + C 6 H 12 6 . 



The peptic enzyme (pepsin) of the stomach changes insoluble proteids 

 into soluble peptones and albumoses. All these reactions are complete in 

 themselves, and there are no by-products such as CO 2 or other gases. The 

 action of organized ferments is quite different. Byjthern the highly complex 

 molecules of organic compounds are split up, and numerous substances of a 

 totally different character arise, generally with the evolution of gases and 

 other by-products. For this reason it is impossible to represent the reaction 

 by a simple chemical equation. Ordinary alcoholic fermentation, for in- 

 stance, cannot be written 



C 6 H ]2 O 6 = 2C 2 

 nor butyric fermentation, 



C 6 H J2 O 6 = C 4 H 8 O 



for, as we shall see presently, numerous other compounds are formed. 



Carbonic acid being evolved in almost all instances, it is customary to 

 designate each process by the name of some other conspicuous product, as 

 the preceding examples show. A by-product of one kind of fermentation 

 may be the chief product of another kind. Acetic acid, for instance, is a 

 by-product of alcoholic fermentation, but the principal substance formed in 

 vinegar fermentation. 



The organized ferments are often regarded as comprising three 

 classes of micro-organisms, yeasts, bacteria, and moulds. The last- 

 named, however, are of rare occurrence, e. g. the mucor yeast that causes 

 the decomposition of wine. The so-called ' aspergillus yeast,' used in China 

 and Japan in the preparation of soy and sake, works only by means of an 

 enzyme that it secretes, the alcohol being produced, not by the aspergillus, 

 but by a true yeast. Similarly in ragi, the ' yeast ' of the arrack industry, 

 there are present both a true yeast and a mucorine fungus (Rhizopns Oryzae). 

 The function of this latter is to ' invert ' the rice starch and make it accessible 

 to the yeast (80). 



Mould fungi are sometimes employed on an industrial scale in the 

 preparation of citric acid (81). 



The discrimination of the various species of fermentative organisms is 

 of very recent date. When Pasteur first showed that fermentation was the 

 work of living cells, we had not the technical means which we now possess 

 of distinguishing closely related species from each other. It was sufficient 

 to assume that every fermentation had its own peculiar fungus, a Bacterium 

 aceticnm in vinegar fermentation, a B. btityricum in butyric fermentation, 

 and so forth. In the same way it was not until the publication of Hansen's 



