Fermentation 67 



treatment of sewage by the "septic tank " method, the 

 organic matter contained in the water is consumed through 

 the agency of anaerobic and aerobic bacteria, until its con- 

 sumption leaves the water once more clear and pure, the no 

 longer useful bacteria dying out as the nutrition becomes ex- 

 hausted. 



The promptness with which bacteria attack organic 

 matter is seen in the changes brought about in foods, some 

 of which are ruined in flavor or quality, though others 

 are thought to be improved. Thus, the flavor of butter, 

 sausage, and cheese, the aroma of wines, and many other 

 important gustatory characteristics of our foods depend 

 solely upon the activity of bacteria or other micro-organisms. 



Many of these activities are harmless, and, indeed, ad- 

 vantageous, though the fact that they are not infrequently 

 accompanied by chemic changes, some of which are poison- 

 ous, makes it necessary to watch and time their operations 

 lest acridity, acidity, insipidity, or toxicity of the food re- 

 place the desired effect. 



Briefly considered, the best-known phenomena resulting 

 from micro-organismal energy are as follows: 



Fermentation. Fermentation is a chemic transforma- 

 tion of carbohydrates resulting from the activity of micro- 

 organismal enzymes. The alcoholic fermentation, which is 

 a familiar phenomenon to the layman as well as to the brewer 

 and chemist, depends upon the activity of an yeast-plant, one 

 of the saccharomyces fungi by which the sugar is broken up 

 into alcohol and carbon dioxid, with some glycerin and other 

 by-products. The following equation shows the chief changes 

 produced : 



C 6 H 12 6 2C 2 H 5 OH -f 2C0 2 



Sugar. Alcohol. Carbon dioxid. 



There are also several bacteria which produce the acetic 

 fermentation, though it is generally attributed to Bacillus 

 aceticus. There are two equations to express this fermen- 

 tation : 



I. CH 2 CH 2 OH + O = CH 3 CHO + H 2 O 

 Alcohol. Oxygen. Aldehyd. Water. 



II. CH,CHO + O = CH 3 COOH 



Aldehyd. Oxygen. Acetic acid. 



A number of different bacilli seem capable of converting 

 milk-sugar into lactic acid, though Bacillus acidi lactici is 



