GENERAL FOOD RELATIONSHIPS— METABOLISM 73 



'^ spoiling" of food material, especially liquids. The reac- 

 tions brought about by bacteria on substances which are 

 soluble and diffusible are essentially "surface reactions." 

 The material diffuses into the cell over its entire surface 

 with little hindrance. The bacteria are usually distributed 

 throughout the medium, so that there is very intimate con- 

 tact in all parts of the mass which favors rapid chemical 

 action. The following calculation illustrates this: 



The volume of a coccus 1 ^i in diameter is 0.5236 x lO-^^ cc. 

 The surface of a coccus 1 /x in diameter is ir x 10-* sq. cm. 



It is not uncommon to find in milk on the point of sour- 

 ing 1,000,000,000 bacteria per cc. 



Assuming these to be cocci of l^x diameter the volume of 

 these bacteria in a liter is only 0.05 cc, or in the liter there 

 w^ould be 19999 parts of milk and only 1 part bacteria. The 

 surface area of these bacteria is 3141.6 sq. cm. With this 

 large surface exposed, it is not strange that the change from 

 ''on the point of souring" to ''sour" occurs within an hour 

 or less. 



It is an old behef, and undoubtedly a true one, that milk 

 sours more rapidly during thunder-storms. It is not true 

 that the electricity in the air has anything to do with it. 

 Thunder-storms usually occur during periods of warmer 

 weather. It is the high temperature which warms the 

 milk and this favors more rapid growth of the bacteria. 

 If the milk is kept cool during thunder-storms it will sour 

 no more rapidly than at other times. 



Although large numbers of bacteria can and do cause 

 great chemical changes the amount of material actually 

 utilized for maintenance of the cell is very slight, infinitesi- 

 mal almost, and yet is fairly comparable to that required 

 for man, as is illustrated by the following computations : 



E. Kohn has shown that certain water bacteria grew well 

 in water to which there was added per liter 0.000002 mg. 

 dextrose, 0.00000007 mg. (NH4)2 SO4 and 0.0000000007 mg. 

 (NH4)2 HPO4. The bacteria numbered about 1000 per cc. 

 Taking the specific gravity at 1 (a little too low) the mass 



