158 PHYSICAL INFLUENCES. 



fermentation in the beginning will be higher at the higher temperature 

 but the end-point is lower. The end-point of the lactic cultures A, B, 

 and C at 6 is probably not final, because thirty-four days is a short time 

 of growth at so low a temperature. Above the optimum the rate of 

 decomposition will decrease rapidly with the rising temperature and the 

 end-point will also be lower. As already mentioned above, pathogenic 

 bacteria lose to some extent their virulence, that is, the power of producing 

 disease, if grown above their optimum. 



FREEZING. The discussion of the relation of temperature to micro- 

 organisms has so far considered only the temperatures within the limits 

 of growth. However, the temperatures below the minimum and above 

 the maximum are also of greatest importance. If bacteria are cooled be- 

 low their minimum temperature they do not die immediately (except 

 a few thermophile bacteria) . They remain alive in a dormant condition 

 ready to multiply as soon as the temperature rises. Even the freezing 

 of a liquid will not kill them immediately. Of course, they cannot 

 multiply in ice, because they have no water, consequently no food, and 

 they cannot thaw the ice to get their water and food for lack of body 

 temperature of their own. As long as liquids are frozen solid the bacteria 

 in them will remain dormant much like dried organisms, and like them 

 their number will decrease very slowly. If ice has been kept frozen 

 many months, it may be sterile, while the fresh ice has nearly as many 

 bacteria as the water it came from. An example is given in the following 

 table relevant to the number of bacteria in frozen milk (after Bischoff). 



Milk kept at 3 to 7. 



Freshly frozen 200,000 bacteria per c.c. 



After i day 105,500 bacteria per c.c. 



After 2 days 7 2 ,3o bacteria per c.c. 



After 3 days 62,000 bacteria per c.c. 



After 4 days 46,400 bacteria per c.c. 



After 7 days 44,000 bacteria per c.c. 



After 14 days 40,50 bacteria per c.c. 



After 21 days 30,300 bacteria per c.c. 



After 35 days 22,500 bacteria per c.c. 



After 49 days 14,200 bacteria per c.c. 



The table shows plainly that it is impossible to sterilize milk by freez- 

 ing, but as long as it is frozen it will keep; there is no possibility of any 

 microorganisms decomposing a frozen liquid, for the organisms need 



