208 MODERN DAIRY PRACTICE. 



tion is in the former kind usually caused by bacteria found 

 in large numbers on the walls and bottom of the pans, or 

 is started by means of buttermilk, sour milk, and skim- 

 milk. No pure cultures are here applied; but the bacteria 

 happening to be found in the pans or the milk are given 

 full liberty to produce their characteristic fermentations, 

 in the hope that the lactic-acid bacteria will be victorious. 

 This hope in some cases miscarries other fermentations 

 occurring along with the lactic-acid fermentation, while 

 in other cases this planless ripening yields surprisingly 

 good results. As before stated, I have thus at several 

 farms met with buttermilk obtained after ripening of such 

 spontaneously soured cream which proved to contain almost 

 exclusively lactic-acid bacteria. 



The reason why these samples of buttermilk contained 

 practically pure cultures of lactic-acid bacteria doubtless 

 lay in the great cleanliness and strict care of which all the 

 dairy work at these farms bore witness, as well as in the 

 skill with which the creaming and ripening process was 

 conducted. I have still oftener met with ice or sep- 

 arator creameries where practically pure cultures were 

 found in the buttermilk. 



The method used in bacteriological investigations for 

 obtaining pure cultures originated with Pasteur, who pub- 

 lished his first experiments in this line in 1857. Succes- 

 sive inoculations are made of mixtures of bacteria in differ- 

 ent samples of the same substratum in order to learn 

 which of the bacteria thrive best in the medium at hand. 

 If different kinds of bacteria are inoculated in a certain 

 medium, the organisms for whom this medium is most 

 favorable easily gain the upper hand over the others. 



If a mixture of bacteria, e.g., those found in ripened 



