PRODUCED BY MICROORGANISMS 129 



there is a very marked increase within a few feet of the 

 top of the silo where air may have access to the material. 

 The heat of oxidation is also made use of in certain methods 

 of curing hay in which it is allowed to heat in piles, and is 

 then spread and dried rapidly. It is possible by this means 

 to secure a palatable product in countries where the cli- 

 mate is too moist to dry the hay in the ordinary manner. 



CHANGES IN VISCOSITY 



Some organisms may markedly increase, others decrease 

 the viscosity of solutions in which they are growing, as will 

 be discussed in greater detail in a later chapter. Many 

 species of bacteria are known to produce gums. The accu- 

 mulations of these gummy or mucilaginous materials in 

 the solution in which the organism is growing will increase 

 materially its viscosity. For example, certain bacteria 

 growing in milk may cause it to become ropy or stringy. 

 It is possible that in some cases the increase in viscosity is 

 due to partial digestion or hydrolysis of protein material. 

 Under certain conditions bread may become viscous or ropy 

 as the result of the growth of certain putrefactive bacteria. 

 Apparently the most marked change in this case is the 

 transformation of the gluten. 



Conversely some bacteria are markedly capable of reduc- 

 ing the viscosity of the medium in which they are grown. 

 Certain bacteria digest gelatin and thus decrease its vis- 

 cosity. Other organisms are able to soften starch paste, or 

 to liquefy coagulated blood serum; a few species can even 

 digest agar-agar. 



EFFECT ON OSMOTIC PRESSURE 



The osmotic pressure of any solution varies directly with 

 the total number of ions and molecules dissolved. Conse- 

 quently when complex molecules are broken down into 

 simple molecules there is a resultant increase in the osmotic 



