RELATION TO AIR 113 



to begin on the surface of things, extending towards the 

 interior. This is not true, however, of all bacteria. Some 

 species can grow perfectly well without air, and others, 

 indeed, cannot grow at all if they are in contact with 

 air. The latter bacteria, which live without oxygen, are 

 known as anaerobic ; the former, which demand oxygen, 

 are called aerobic. The aerobic bacteria are by far the most 

 important in the affairs of the household, but the anae- 

 robic bacteria, on the other hand, produce certain types of 

 putrefaction which are sometimes more serious, inasmuch 

 as the products of putrefaction which take place without 

 air are likely to be more poisonous than those products 

 of decay taking place in contact with the air. We must 

 remember, then, that whereas most bacteria grow best in 

 the air, we cannot protect any material from the growth 

 of microorganisms simply by keeping air away from it, 

 inasmuch as some species grow perfectly well, and even 

 better, out of contact with the air. Hence, in canning 

 food, it is not the exclusion of air that makes preserva- 

 tion of food possible, but the exclusion of bacteria. 



Moisture. Like yeast and molds, bacteria require 

 water. Dry food is protected from their action because 

 they cannot obtain water sufficient for their life processes. 

 Bacteria, in general, require more water than molds. Vari- 

 ous materials, if simply damp, will mold or mildew, but 

 they will not support bacteria life unless the amount of 

 water is considerable, 25 % to 30% of water being necessary 

 for any growth, and a larger amount still for vigorous 

 growth. Hence they may be expected to grow in all 

 kinds of food which are thoroughly wet, but they will not 

 grow in any of the dried forms of food which we keep 



