240 MICRO-ORGANISMS AND FERMENTATION. 



must be a food-stuff for yeast. In general, he proved that no 

 decay of yeast takes place during fermentation, and that the 

 presence of assimilable albuminoids in the liquor is unnecessary, 

 for by sowing a minute portion of yeast in a liquid which only 

 contains sugar, ammonium tartrate and a few salts, fermenta- 

 tion can be brought about with development of young cells 

 capable of propagation. 



At the same time, he showed that the reason why many of 

 the early experiments, which should have refuted the older 

 theories, did not succeed was due to the fact that it was im- 

 possible at the time to secure absolute sterilisation of the 

 liquids. 



He then produced further proofs that the acetic acid 

 fermentation, already recognised by Kiitzing as due to physio- 

 logical activity, must certainly be regarded as having this 

 character. 



One further observation must be mentioned on account 

 of its wide-reaching importance. He proved that calcium 

 lactate can undergo fermentation resulting in the formation 

 of butyric acid, and that the active organism can exist without 

 access of air. He gradually extended his observations in this 

 entirely new field (anaerobiosis = life without air), and de- 

 finitely distinguished between aerobic and anaerobic life. It 

 was this remarkable discovery, which at a later stage included 

 the alcoholic yeasts, that led the distinguished scientist to a, 

 solution of the problem under what conditions yeast cells can 

 decompose sugar. 



In 1876, in his Ktudea sur la Here, he formulated his cele- 

 brated theory of fermentation, based upon a series of actual 

 .experiments, details of which cannot be given here, a theory 

 which has served both as the basis and the starting point by 

 which progress has been made throughout a long series of years, 

 and one which will always retain its importance. It starts 

 essentially from the thought that living yeast cells under 

 certain conditions are obliged to live apart from air, and that 

 they then react as exciters of fermentation. Fermentation 

 is, therefore, bound up with the life of yeast cells ; it is life 

 without air. As yeast under these conditions is obliged to 

 obtain its necessary demands of oxygen from sugar in order 



