l6 PRACTICAL DAIRY BACTERIOLOGY 



others at 130. But the majority of them grow most rapidly 

 at temperatures somewhere between 80 and 100, a fact that 

 explains the rapidity of the souring of milk in hot weather, and 

 the quickness with which any organic material decays during 

 the summer. Above 100 most bacteria are injured, and if the 

 temperature is raised, they soon stop growing. There are, how- 

 ever, a few species which grow at unexpectedly high tempera- 

 tures; some types are found that grow most rapidly at a tem- 

 perature as high as 140 and some even higher. These peculiar 

 bacteria are called thermophiles. Such bacteria may be found 

 in milk by proper means, but it is needless to say that they have 

 no important relations in dairy problems, for -dairy products 

 are never kept at a temperature as high as these. 



Death Temperature. Most active bacteria are killed by a 

 temperature of 150 kept up for half an hour. This tempera- 

 ture may, therefore, be satisfactorily adopted for pasteurizing. 

 (See page 151.) At this temperature, however, they die slow- 

 ly, while a temperature of 160 destroys them much more 

 quickly, and 170 to 180 more rapidly still. Hence, the higher 

 temperatures are usually adopted for pasteurization. A total 

 destruction of bacteria, including their spores, can be brought 

 about only by temperatures above that of boiling water, and this 

 is usually accomplished in the case of liquids in a closed 

 chamber, in which the steam can be generated under consider- 

 able pressure. If the steam is allowed to collect in such a 

 chamber at a pressure of 115 Ibs., the temperature, then, will 

 be about 240. This temperature kept up for half or three 

 quarters of an hour destroys even the most resisting spores 

 and may be relied upon to produce the perfect sterilizing of any 

 liquid. Laboratories usually have a small apparatus designed 

 for this purpose, called an autoclav (Fig. 76), and this is used 

 constantly for sterilizing of liquids. 



Relation to Cold. While heat will destroy all bacteria, cold 

 will not do so. It is practically impossible to destroy the life 



