MILK FKKMKXTATIOXS 77 



Nearly all of the acid-forming bacteria which are of prac- 

 tical importance may be placed in two great groups: (1) the 

 group including the bacteria which are desirable, especially 

 from the standpoint of the butter and cheese maker, B. lactis 

 acidi Leichrnann (Streptococcus lacticus Kruse) being the most 

 important representative of this group ; (2) the group includ- 

 ing the bacteria which are undesirable in milk, B. coli corn- 

 munis and the related form B. lactis aerogenes, both appearing 

 in a number of varieties, being the important representatives. 

 A detailed study should be made of pure cultures of these 

 organisms. In this study attention should be directed to the 

 changes produced in milk, especially to .the nature of the curd 

 formed, the odor and taste of the milk, and the rate and 

 quantity of acid produced at 20 C. and at 37 C. 



The student should also collect samples of milk, allow them 

 to sour, and judge by the nature of the curd produced the type 

 of organisms concerned in the souring. From the samples of 

 milk showing different types of curd, plates should be prepared 

 with lactose agar in order to isolate the organisms. Kepre- 

 sentatives of the various groups should be studied in detail. 



Determination of the group to which the organism be- 

 longs. The grouping may best be made by the inoculation 

 of litmus-milk tubes from the individual colonies. The re- 

 sults thus obtained should show whether the organism is 

 to be included in either of the groups mentioned above, or 

 belongs to some other. 



Exercise. Each student will make a detailed study of the cultures 

 furnished in accordance with the outline given on p. 66. Collect 

 samples of milk, and from them isolate representatives of the two 

 groups of lactic bacteria and study in detail. 



Digesting and sweet-curdling fermentation. These changes 

 are produced by various kinds of bacteria. Many of the coccus 



