4 BACTERIA, YEASTS, AND MOLDS 



the only serious foe with which she has to contend in her 

 attempt to keep food in proper condition for use. 



These invisible plants are constantly on the alert to con- 

 sume for themselves the foods which the housewife designs 

 for the table. If they have a chance to get at the food, 

 she soon notices that it undergoes a series of changes, 

 characterized by what we call putrefaction, decay, souring, or 

 perhaps some other change not properly classed under 

 any of these terms. The general rotting of fruit, the 

 decay of meat, the souring of milk, and a host of other 

 similar phenomena which occur in every pantry if the food 

 is not carefully protected, represent some of the effects 

 produced in foods when microorganisms begin to feed 

 upon them. Thus it is evident that these microscopic 

 plants play a very great part in domestic economy. 

 This fact, however, has not been thoroughly appreciated 

 until recent years, and indeed it is only just beginning 

 to be recognized to-day that the housewife's knowledge 

 should comprise an understanding of the nature and hab- 

 itat of these microscopic foes, their methods of distribu- 

 tion from place to place, the conditions under which they 

 grow and fail to grow, together with the various devices 

 which may be adopted for checking their active growth 

 where they are not wanted. Although the facts have 

 only recently been appreciated, it is known to-day that a 

 very considerable part of the duties in every household is 

 concerned with these microscopic organisms, known and 

 unknown. 



The chief desire of the housewife is to prevent the 

 growth of these microorganisms in places where they 

 are not wanted. For this purpose have been invented 



