128 BACTERIA, YEASTS, AND MOLDS 



degrees of temperature may have upon the rate of growth 

 of this bacterium. The figure teaches a very practical 

 lesson in regard to the influence of cold in delaying the 

 growth of bacteria and thus protecting food from spoiling. 



If the temperature is raised too high, it has an injurious 

 action upon the growth of bacteria. Each species of bac- 

 teria grows best at a certain temperature, growing less 

 rapidly if warmed above this point or cooled below it. 

 Most, though not all, of the bacteria against which the 

 housewife has to contend grow best at temperatures 

 between 70" and 95". If the temperature is raised above 

 95°, many cease to grow so rapidly, and at still higher tem- 

 peratures — between 125° and 140° — a large majority are 

 quite incapable of growing at all. At the higher temper- 

 atures food would hardly decay. There are, however, a 

 few species which grow only at very high temperatures, 

 not developing at all unless it is above 125°. 



It is perfectly evident that all problems connected with 

 the protection of food from the action of microorganisms 

 will be dependent upon the temperature at which the bac- 

 teria grow most rapidly. Food which is kept in an ice 

 chest, although it may be protected from the action of 

 those bacteria which grow only at room temperatures, will 

 be exposed to other species that grow best at lower tem- 

 peratures. When we remember that some kinds of bac- 

 teria grow at temperatures close to freezing, we can readily 

 see that no method of cooling food short of actually 

 freezing it will totally protect it from decay. 



Death Temperatures. All bacteria are killed by excess- 

 ive heat, but the temperature which kills them is some- 

 what variable. Bacteria exist, as we have seen, in two 



