ACID FERMENTATION 149 



The true explanation of this phenomenon was afforded by 

 the researches of Pasteur, though others, e.g., Person in 1822, 

 had noticed the growth of organisms as a fine film on tlie 

 surface of such a Hquid and had given the name Mycoderma 

 aceti to the growth. 



Pasteur showed that certain rod-like bacteria were the true 

 causes of the formation of acetic acid, while other organisms 

 which might be present, such as yeasts, etc., carried the oxida- 

 tion further to COo and HgO. Hansen was the first to obtain 

 pure cultivations of Mycoderma aceti, and discovered also 

 further species capable of bringing about the same change. 

 As a matter of fact, as already indicated, quite a large number 

 of organisms can effect the formation of acetic acid, not only 

 from ethyl alcohol, but from other alcohols and carbohydrates 

 which contain the characteristic group CHgCH^OH. 



It should be pointed out that the formation of acetic acid 

 by bacterial action can only take place within certain limits 

 of concentration, and in presence of the essential ingredients of 

 bacterial food, that is, nitrogen must be present in some form, 

 e.g., as peptone or albumin, and phosphorus as phosphate. 



The Lactic Acid Fermentation.— As already mentioned, 

 the souring of milk is due to the formation of lactic acid by 

 decomposition of milk sugar. The simple chemical equation 

 in this case is as foUows : The milk sugar is first inverted, 

 forming two hexose molecules — 



By a simple molecular decomposition one hexose molecule 

 yields two molecules of lactic acid, thus : — 



The production of lactic acid from milk can be brought 

 about by the addition of a small quantity of previously soured 

 milk. The reaction quickly reaches a limit if the solution is 

 allowed to become too acid, and therefore chalk is generally 



