yso bacteria: their work 



attempt to do so is frequently made. The products of fer- 

 mentation are variable in amount and kind, according to the 

 substances fermented, the species of bacteria carrying on the 

 work, and external conditions such as temperature and absence 

 or presence of oxygen. In most instances, however, there is 

 in each case a characteristic production of one or two com- 

 pounds from which the fermentations are ordinarily named; 

 thus it is customary to speak of lactic fermentations, but)Tic 

 fermentations, and marsh-gas fermentations, in which cases lactic 

 acid, butyric acid, and marsh-gas are the chief products of the 

 respective processes. 



The organisms to which the fermentations are due are known 

 as organised ferments to distinguish them from enzymes or un- 

 organised ferments, the latter, as indicated in chapter xviii., being 

 soluble organic but lifeless substances secreted by plants and 

 animals, and capable of inducing hydrolysis and other changes in 

 various chemical compounds. Enzymes are manufactured by 

 many species of bacteria, and to these bodies are due some of 

 the special powers of fermentation possessed by the organisms 

 in question ; nevertheless, it is not at present possible to explain 

 all the phenomena of bacterial fermentations by the action of 

 enzymes. 



2. In order that advantage may be taken of the processes 

 which are useful to mankind, it is important to study the nature 

 of the species of bacteria to which they are due, and the con- 

 ditions under which the particular organisms carry on their 

 work most satisfactorily. 



About many of these processes little is yet accurately known ; 

 the most important of those which are best understood are 

 described in the present chapter. 



3. Lactic Fermentations. — When milk is left exposed to the 

 air in a warm room for a few hours it develops a sour taste. 

 Investigations show that the latter is caused by the presence of 

 lactic acid which has been produced from a portion of the milk- 



