FERMENTATION 3 3 7 



3. Fermentation by Simple Decomposition. — As is well 

 known, milk on standing, especially in warm weather, 

 becomes acid and coagulates. The change which takes 

 place is one of simple decomposition, one molecule of milk- 

 sugar (lactose) becoming converted into four molecules of 

 lactic acid, which latter coagulates the milk by precipitating 

 the albuminoids. This change is shown by the following 



equation : 



Ci,H,Ai+H20 = 4C3He03 

 Lactose. Lactic acid. 



The fermentation ceases after a certain amount of lactic 

 acid has been formed, but it will recommence, however, if 

 the liquid be neutralised with carbonate of lime. 



The Bacillus acidi lactici consists of short, somewhat 

 thick non-motile bacilli 1 to 1-7 fi long by about 0'3 to 0'4 yu. 

 broad, generally occurring in pairs and in strings of 

 4 elements. The bacilli form spores, each situated at one 

 extremity of the bacillus. On the surface of gelatine, in 

 ' streak ' cultures, a thin delicate growth is formed along 

 the whole track of the needle. The gelatine is not liquefied. 



The Bacillus acidi lactici sets up the lactic fermentation 

 in solutions of milk-sugar, cane-sugar, dextrose and 

 mannite. In the case of the first two sugars, the ferment 

 appears to first exert an ' inverting ' action, whereby one 

 molecule of these two sugars is respectively converted into 

 two molecules of dextrose or glucose, which in turn is 

 broken up by the action of the ferment into two molecules 

 of lactic acid. These changes are shown by the two 

 following equations : 



(a) Ci2H2Ai+H,0 = 2C,Hi206 



Milk-sugar. Dextrose. 



(b) CeH,A = 2C3He03 

 Dextrose. Lactic acid. 



The most favourable temperature for the fermentation 



appears to be from 35° to 42° C, while at 45° C. it ceases. 



22 



