80 EXPERIMENTAL DAIRY BACTERIOLOGY 



Norway (B. lactis longi Troili-Petersson), belong to the 

 acid-producing group. 



In the study of this type of fermentation especial atten- 

 tion should be devoted to the cause of the ropy condition. It 

 may be due to the presence of a greatly thickened cell wall 

 (capsule), which may be detected by appropriate staining 

 (p. 48), or to the presence of a mucus-like substance in the 

 milk (galactan). 



Exercise. Each student will make a detailed study of the organ- 

 isms furnished, B. lactis lonf/i. 



Bitter fermentation. A bitter taste in milk may be pro- 

 duced by a large variety of bacteria. Many of the digesting 

 bacteria produce a bitter taste in milk, due to the formation 

 of peptones from the albumen and casein. Nonliquefying 

 acid-forming bacteria may produce a specific bitter principle 

 in milk, and the same may be formed by certain types of 

 liquefying acid-producing bacteria. 



Exercise. The various milk cultures studied in the digesting and 

 sweet-curdling fermentations should be tested for bitterness. 



Cycle of fermentations in milk. Under natural conditions 

 the decomposition of organic matter is accomplished not by 

 a single form of life, but by a greater or less variety of organ- 

 isms growing together (symbiosis) ; or one form prepares the 

 way for another, through the establishment of a favorable 

 environment, as in the preparation of a suitable food or a 

 favorable reaction in the medium (metabiosis). Milk is no 

 exception to the rule. During the act of withdrawal from 

 the animal it becomes contaminated with certain classes of 

 bacteria and molds, which make themselves manifest in a 

 constant and regular sequence in ordinary milks. On ac- 

 count of their small numbers it may be impossible to find in 



