EXPERIMENT STATION BULLETINS. 199 



if they simply developed, and consumed a portion of the milk, produced 

 no diseased conditions nor altered the composition of the milk. But when 

 we consider that all of the usual changes in milk are due to the actions of 

 these bacteria, their rdle at once begins to assume an important part in 

 dairy economy. 



Our acquaintance with the usual fermentations about us, such as the 

 changing of cider into vinegar, grape juice into wine and the like, helps 

 us to conceive of milk fermentations. In order that we may arrive at a 

 systematic understanding of this question, it will be necessary to divide 

 the subject into several heads: 



Although some of these functions cannot with propriety be assigned to 

 fermentation, still these bacterial functions can with profit be considered 

 coordinately. 



PIGMENT FORMATION. 



There is reason why this is not more commonly observed. Inasmuch as 

 many bacteria which produce pigment will not grow and give rise to pig- 

 ment in an acid medium, any pigment-bearing bacteria are likely to be 

 passed over unnoticed; still anyone familiar with the bacteria of milk 

 will tell you that a failure to isolate pigment from any ordinary sample 

 of milk is a rare occurrence. They are common in milk and either do 

 not succeed in developing along with the many other species present or 

 the acidity destroys the pigment. Although this is usually the case, there 

 are many plagues of colored milk occurring in dairies. 



A common color in milk is red. This may be due to a variety of bac- 

 teria. A yeast plant has also been found by the author in many samples 

 of milk originating from the same source. The color produced by these 

 pigment producing micro-organisms may be simply a scum over the sur- 

 face or may be uniformly distributed throughout the milk. 



There are many yellow pigment bacteria found in milk. All shades 

 will be noticed, from an orange yellow to a lemon yellow. These bac- 

 teria may find their way into the milk from the air, dairy or cow, since 

 they are widely distributed. Most of the yellow pigmented bacteria pro- 

 duce a peptonizing ferment which is not advantageous to the quality of 

 cream or milk. 



A greenish color is also quite frequently present. It usually originates 

 from flourescing bacteria which have fallen in generally from the cow. 

 In their action upon milk they produce a very unpleasant odor which 

 seems to be characteristic of this class. 



Perhaps the most common pigmented disease of milk is what the Ger- 

 mans call " blau milch." It is uniquely common in the northern part of 



