MICROORGANISMS AS USEFUL AGENTS 25 



refrigerators and all devices for cold storage and for cool- 

 ing and keeping cold any food products ; to this end, too, 

 are designed the various methods of preserving food and 

 fruits. The immense industry of canning, either on a 

 large scale as is done in factories, or on a small scale as 

 is done in the household, is dependent upon the relation 

 of microorganisms to food. The sterilizing or pasteuriz- 

 ing of milk, as well as other foods, is also a bacteriological 

 problem, and, indeed, many other phases of household 

 life are really bacteriological phenomena. Whether or 

 not she possesses a scientific knowledge of bacteria and 

 their allies, the housewife must have a certain practical 

 knowledge of their nature and of their powers, for this 

 practical knowledge is absolutely necessary to enable her 

 to preserve her food successfully from the microorganisms 

 which are so liable to spoil it. , 



2. Microorganisms as Useful Agents 



It must not be understood, however, that micro- 

 organisms are always our foes. It is true that in the 

 household they are commonly a source of trouble, but 

 it is also true that some of them are distinctly friends. 

 To appreciate that they are sometimes useful, even in our 

 foods, one needs only to remember that under this head 

 are included the great group of yeasts that play such an 

 important part in the household in the raising of bread 

 and in all types of fermentation. Yeasts, as well as bac- 

 teria, are microscopic plants, of which the microscopist 

 recognizes many kinds. Some of these are troublesome, 

 but, so far as concerns their relations in the household, 



