50 THE STORY OF GERM LIFE. 



in the fermentation is the presence of contami- 

 nating bacteria among the yeasts. These bac- 

 teria have been more or less carefully studied by 

 bacteriologists, and their effect upon the beer or 

 wine determined. Some of them produce acid 

 and render the products sour; others make them 

 bitter ; others, again, produce a slimy material 

 which makes the wine or beer "ropy." Some- 

 thing like a score of bacteria species have been 

 found liable to occur in the fermenting mate- 

 rial and destroy the value of the product of both 

 the wine maker and the beer brewer. The spe- 

 cies of bacteria which infect and injure wine are 

 different from those which infect and injure beer. 

 They are ever present as possibilities in the great 

 alcoholic fermentations. They are dangers which 

 must be guarded against. In former years the 

 troubles from these sources were much greater 

 than they are at present. Since it has been dem- 

 onstrated that the different imperfections in the 

 fermentative process are due to bacterial impuri- 

 ties, commonly in the yeasts which are used to 

 produce the fermentation, methods of avoiding 

 them are readily devised. To-day the vintner 

 has ready command of processes for avoiding 

 the troubles which arise from bacteria, and the 

 brewer is always provided with a microscope to 

 show him the presence or absence of the con- 

 taminating bacteria. While, then, the alcoholic 

 fermentations are not dependent upon bacteria, 

 the proper management of these fermentations 

 requires a knowledge of their habits and char- 

 acters. 



There are certain other fermentative processes 

 of more or less importance in their commercial as- 

 pects, which are directly dependent upon bacte- 



