144 Dairy Bacteriology. 



dually extended, until it became almost universal in the 

 larger butter-producing districts. 



In 1890 a more refined and scientific process was in- 

 troduced by the Danish bacteriologist, Storch. Recog- 

 nizing the fact that butter flavor was attributable to the 

 development of the bacteria present in the ripening 

 cream, he conceived the idea of isolating the various 

 types of organisms found in milk and testing them as 

 to their effect on the quality of flavor. Selection was 

 then made of the most favorable flavor-producing types, 

 and these were propagated in suitable culture media, 

 such as skim milk, which was rendered more or less per- 

 fectly sterile by pasteurization or sterilization. Under 

 such conditions the addition of a selected ferment could 

 be made to the fresh cream, and so control the type of 

 fermentation which occurred therein. An essential re- 

 quisite in any organism used for this purpose must be 

 the ability to produce relatively large amounts of acid 

 rapidly at ordinary ripening temperatures,' and also to 

 form sufficient-quantities of the proper flavor-produc- 

 ing substances to impart a suitable flavor to the butter 

 fat. Such starters are known as pure culture or com- 

 mercial starters, and are prepared in both liquid and 

 dry form. At present they are used to a greater or 

 less extent in all of the leading dairy districts. 



Liquid starters consist of a mass of sterile nutrient 

 medium, milk or beef broth, inoculated with the pure 

 culture. The dry starters are made by adding liquid 

 cultures, containing the growing bacteria, to some ab- 

 sorbing material, such as milk sugar, milk powder, or 

 starch, the whole mass being dried at low temperatures, 

 so as not to injure the bacteria. Under such conditions 

 the bacteria exist in a dormant state, and are protected 



