BACTERIA IN OUR DAIRY PRODUCTS. 765 



Fortunately, this immense number of bacteria in milk need not 

 especially alarm us, for they are not disease-germs and are harm- 

 less to the healthy person. Nevertheless, they are undoubtedly a 

 nuisance in the milk. They can not grow there without producing 

 some effect upon it. Commonly the first change noticeable is the 

 appearance of the well-known odor and taste of sour milk, followed 

 shortly by its curdling. This souring is undoubtedly the result 

 of bacteria growth, and it was at first supposed that there was a 

 single definite species which alone had this power of producing 

 lactic acid. So thought Pasteur and Lister at first, and such a 

 species they described. The species of bacterium studied by them 

 certainly had this power, and it was named Bacterium lactis by 

 Lister. In later years the name Bacillus acidi lactici has been 

 given to it. By the work of the last six years we have learned 

 that more than one species has the power of souring milk by the 

 formation of lactic acid. Lactic-acid formation is the character- 

 istic of a class of bacteria comprising many species, and even in 

 the ordinary souring of milk under normal conditions it is not 

 always the same species of bacteria which produces the mis- 

 chief. 



While it is true that any one of a number of species of bacteria 

 may produce lactic acid by their growth in milk and thus cause its 

 souring, in other respects these different species do not have the 

 same effect. The formation of lactic acid is not the only change 

 that occurs in the souring of milk. Sour milk has a well-known 

 odor, but this is not due to the lactic acid, since lactic acid is odor- 

 less. The formation of such an odor tells us, therefore, that there 

 are other changes going on in the souring of milk. The fact is, 

 that a decomposition of the albuminoids and other substances in 

 the milk is going on, and it is these decomposition products that 

 give the odor. Now, the different species of bacteria do not all pro- 

 duce the same sort of decomposition products. All who are fa- 

 miliar with milk will recall that the character of sour milk is by 

 no means uniform. It differs in the hardness of the curd, in the 

 amount of the whey, in odor, and even in taste. "When different 

 specimens of milk are examined just before or just after souring, 

 it is found that the species of bacteria are by no means the same 

 in the different specimens. Each will contain some of the acid- 

 forming class, but the particular species which happen to be 

 present in the different specimens will vary with the different 

 conditions. Different localities and different methods of handling 

 the milk will affect the variety of bacteria that it contains. It will 

 sour in all cases, since all have some of the members of the acid- 

 forming class ; but the other accompanjdng phenomena may be 

 different. Thus we have learned to attribute all the differences in 

 the different specimens of sour milk to the fact that the souring 



