142 MODERN DAIRY PRACTICE. 



both parties concerned. It is fortunate that the milk in 

 cities is consumed so soon, for otherwise the injurious re- 

 sults of this careless treatment would oftener show them- 

 selves. 



Milk kept in the dark in covered and unventilated 

 cupboards often does not sour, but assumes an insipid, sick- 

 ening, or bitter taste, and shows distinct signs of putrefac- 

 tive fermentations on standing for a longer time. This 

 led me to investigate why especially putrefactive bacteria 

 gain the upper hand in such milk; and I found that the 

 conditions of light are first of all the causes of it. 



Although bacteria, generally speaking, develop best in 

 the dark, we observe in some a greater dislike to light than 

 in others. Bacteria of milk may be separated in two dis- 

 tinct groups, according to their behavior in this respect. 



1. Those that can stand daylight without injury. 



2. Those that develop only with difficulty in daylight. 

 The lactic-acid bacteria belong to the former group, and 

 the great majority of the putrefactive bacteria and the 

 butyric-acid bacteria to the latter. 



If the milk-room is airy and well ventilated as well as 

 light and clean, we can conclude with a great deal of cer- 

 tainty, provided the milk has been properly handled at the 

 farm, first, that a comparatively small number of bacteria 

 is found in it, and second, that putrefactive bacteria are in 

 the minority. The lactic- acid bacteria, which are practi- 

 cally everywhere, will then thrive best, and keep the putre- 

 factive bacteria in check at least as long as the nutritive 

 conditions are favorable. 



Sometimes a slight acidity in the milk can also be dis- 

 covered by chemical reactions: this has arisen partly 

 through the lactic-acid bacteria, which owing to their 



