92 MILK [vi 



which takes place on keeping milk for any length 

 of time, and which results sooner or later in 

 its so-called spontaneous coagulation. This sour- 

 ing is due to the formation of lactic acid, derived 

 from the decomposition of the milk-sugar in the 

 manner explained in a former Chapter. The pro- 

 duction of lactic acid from milk-sugar may be 

 effected by quite a large class of bacteria ; and it has 

 been found that while one ferment may be the 

 common exciting cause in one district, another 

 ferment may be the exciting cause in another 

 district. In the case of some of these bacteria the 

 lactic acid is merely incidentally formed along with 

 other products. Such bacteria are those producing 

 different colours in the milk, which we have already 

 referred to. Another interesting fact with regard to 

 this class of fermentation is that it is highly doubtful 

 whether any two of the organisms effecting it act on 

 the milk in exactly the same manner. An interesting 

 observation made by Grotenfelt has shown that the 

 lactic bacilli lose their power of producing lactic 

 acid if they be cultivated for a time in a medium 

 free from sugar. Again, the amount of lactic acid 

 produced depends on the original condition of the 

 milk, as well as the temperature. One of these 

 ferments splits up the molecule of milk-sugar com- 

 paratively easily into four molecules of lactic acid, 

 producing at the same time an extremely slight 



