FERMENTATION. 391 



less apt will they be to ferment. Exposure to heat favors 

 fermentation, and hence preserves should be kept in a cool 

 place. A high degree of heat will, however, destroy the 

 power of the ferment, and hence preserves are scalded when 

 there is a suspicion that fermentation is commencing in 

 them. For the same reason vinegar is boiled to arrest the 

 formation of the mother in it. 



553. Sour Bread. When bread is sour it is because the 

 vinous fermentation has been followed by the acetous. 

 This may arise from two causes. Either the fermentation 

 has been allowed to go on too long before the bread was 

 baked, or the ferment used has been kept so long as to 

 enter into that state which makes it capable of producing 

 the acetous fermentation. If a flour paste stand in a ves- 

 sel covered with a board for six or eight days, it acquires 

 a pleasant smell in the change which has taken place in it, 

 and is now fit to act as an alcoholic ferment. Bread raised 

 by it will be sweet. But if this paste or dough be left to 

 stand a little longer, it acquires an acid taste, and will now, 

 indeed, excite an alcoholic fermentation in sugared water 

 or in bread, but this will at once pass on to the acetous fer- 

 mentation. 



554. Explanation of Acetous Fermentation. The change 

 which alcohol undergoes on conversion into acetic acid is 

 not strictly a result of fermentation, because this conversion 

 may be effected in various ways which exclude the idea 

 of any vegetable or animal growth. It is rather a case of 

 oxidation, for alcohol contains more oxygen and less hydro- 

 gen than acetic acid, as shown in the following equation : 



Alcohol. Oxygen. Acetic acid. Water. 

 C a H 6 + O a = C a H 4 O a + H0 a 



Since, however, pure alcohol may be exposed to the air> 

 either alone or mixed with water, for any period without 

 suffering oxidation, and the change is induced by the pres* 



