94 MILK [vi 



and bacteria have been studied by Hueppe and the 

 above-mentioned investigators, which cause lactic 

 fermentation. Lactic bacteria do not, as a rule, 

 develop spores, and consequently have not very 

 great resistant powers ; a temperature of 70 C. 

 (158 Fahr.) usually killing them. The chief diffi- 

 culty of keeping milk is due to the presence of this 

 class of bacteria ; and hence it is that heating milk 

 even to a comparatively low temperature has such 

 an effect in preserving it. 



Lactic ferments play a very important role in the 

 dairy in the case of making butter from slightly 

 soured cream. This is generally done by adding 

 small quantities of sour milk to the cream, and then 

 permitting it to stand for some time before churning. 

 But here there is a danger, since it often occurs 

 that the milk thus added to the cream contains 

 bacteria which give rise to disagreeable odours and 

 a bad taste. From a bacteriological point of view, 

 therefore, it may be said that butter should only be 

 made from sweet cream. Weigmann has attempted 

 to prepare pure culture of cream -ripening ferment. 

 In this attempt he has been successful, and he has 

 succeeded in isolating a micrococcus which sets up 

 normal lactic fermentation. 



Certain of the lactic bacteria exert, as we have 

 just indicated, a harmful action. This is seen in the 

 manufacture of cheese. They split up the milk- 



