METHODS OF STUDYING PHYSIOLOGIC CHARACTERS 97 



the various organic acids have not in most instances been 

 developed. Something of their nature, however, may be 

 determined by separating them into volatile and nonvola- 

 tile. If a solution containing an acid is acidified with 

 sulphuric acid and the materials distilled, certain acids, 

 particularly acetic, propionic and butyric will pass over, 

 while other acids, particularly lactic, will not. This makes 

 it possible by titration of the distillate to determine the 

 relative proportion of volatile and nonvolatile acid. 



Determination of Alcohol Production. Yeasts produce 

 considerable quantities of ethyl alcohol, and smaller amounts 

 are formed as a result of the growth of a few bacteria and 

 molds. Occasionally other alcohols may be developed such 

 as amyl, butyl and propyl. With yeasts alcoholic fermen- 

 tation is accompanied by the production of carbon dioxide. 

 The presence of ethyl alcohol may be detected by placing a 

 few cubic centimeters of the material to be tested in a 

 suitable container such as a test tube, and adding a small 

 crystal of iodine and several cubic centimeters of a strong 

 solution of sodium hydrate. If alcohol is present heat- 

 ing this material over a Bunsen flame will give a distinct 

 odor of iodoform. The amount of alcohol developed may 

 be determined by distillation and test of the specific grav- 

 ity of the distillate. 



Determination of Aldehyde. Certain microorganisms, 

 particularly when growing in carbohydrate solutions, pro- 

 duce aldehyde in sufficient quantities to give a distinctive 

 reaction. The detection of the presence of aldehyde is 

 best accomplished by the use of a fuchsin indicator. If a 

 solution of basic fuchsin is decolorized by the addition of 

 sodium sulphite (or better suphurous acid) until the color 

 has just disappeared or until the material is of a very 

 light pink color, and added to a solution in which aldehyde 

 has developed, the color of the fuchsin is restored. This 

 is the principle made use of in the so-called Endo medium. 



