CHAPTER IX 



THE ACID FERMENTATION OF ALCOHOLS AND 

 CARBOHYDRATES 



It is probable that the earKest fermentation known to man 

 was the souring of milk ; this we now know to be due to the 

 fermentation of milk sugar, and it is one of the more important 

 of a class of fermentation changes, all of which essentially 

 consist in the oxidation of the characteristic alcohol group 

 CHgOH to the group characteristic of acids, viz. CO2H or 

 carboxyl, either by addition of oxygen or by intra-molecular 

 change. 



The simpler carbohydrates or sugars are, as we have learnt, 

 ketone or aldehyde alcohols, and therefore lend themselves to 

 this change. 



The oxidation of the alcohol group can of course be brought 

 about by purely chemical reactions. The chemical method 

 which is of most interest in the present connection is the 

 oxidation of alcohols by means of platinum black ; the latter 

 is obtained as a black precipitate when solutions of platinum 

 salts are treated with certain reducing agents. This finely 

 divided platinum, to which reference has already been made 

 on p. 3, has the power of causing oxidisable vapours to 

 combine with oxygen when the two are led over it together ; 

 thus, e.g., ordinary formaUn or formaldehyde is prepared by 

 bubbUng air through methyl alcohol, and leading the mixture 

 of air and methyl alcohol vapour over platinum black. In 

 this case indeed it is sufficient to heat a spiral of platinum wire, 

 and plunge it into the mixture of methyl alcohol vapour and 



