274 PHYSICAL INFLUENCES 



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. 



FREEZING. The discussion of the relation of temperature to 

 microorganisms 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 below their minimum temperature they do not die immedi- 

 ately. 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. An example is given in the follow- 

 ing table relevant to the number of bacteria in frozen milk (after 

 Bischoff). The decrease in numbers is not very uniform, since there are 

 many different bacteria in milk, but the general tendency is the same 

 as in the dried bacteria. 



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 72,300 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 40,500 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 

 freezing, 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 water above all. If food substances change in cold storage 

 (and some food products do deteriorate), this must either be due to 

 changes other than microbial or the material was not completely 

 frozen as is probably the case with salted butter. 



