THE KINDS OF MICRO-ORGANISMS IN MILK 377 



the presence of pigment when the skimmed milk below has a nor- 

 mal appearance. 



Colored milk has never, as far as our knowledge goes, been 

 produced by bacteria that are injurious to health. However, the 

 appearance of colored milk naturally arouses the suspicion of the 

 consumer, and is, therefore, frequently the source of considerable 

 annoyance to the producer and dealer. Colored milk is an an- 

 noyance for the additional reason that the milk usually shows no 

 evidence of color when it leaves the producer's or even the dealer's 

 hands. The pigment appears after the milk has been standing 

 for some time, so that pigment-forming bacteria have had oppor- 

 tunity to multiply. 



As indicated, the pigment frequently affects only limited por- 

 tions of the milk. The surface may be uniformly colored; or 

 colored spots appear which gradually expand and coalesce, and 

 finally penetrate to some depth; or the whole volume of the milk 

 is affected; or, finally, the pigment may in rare instances appear 

 in lower portions of the milk only. 



There are cases occasionally where milk is colored by agen- 

 cies other than micro-organisms. The presence of blood is not 

 an infrequent cause of a red tint in milk, and, according to Olson, 

 milk which has been allowed to stand in iron dishes for several 

 hours may assume a bluish-gray color due to solution of some 

 iron. Milk from diseased udders may have a decided yellowish 

 tint. It has furthermore been held that some pasture plants 

 which are eaten by cows influence the milk so as to pro- 

 duce coloration, but observations of this sort are not entirely 

 trustworthy. 



Blue Milk 



Blue color in milk has probably been more frequently observed 

 than any other "color fermentation" of milk. Sometimes the 

 blue pigment appears only after the acidity of the milk is pro- 

 nounced, and observers agree that the pigment becomes more 

 intense as acidity increases. Probably this phenomenon is not 

 due to a chemical influence as much as to the fact that the milk 

 becomes more suitable for growth of the organism when acidity 

 develops. It is an example of an associative effect of lactic acid 

 bacteria and the pigment-producing types. Hammer has ob- 

 served that the pigment develops more rapidly in raw milk than 

 in pasteurized milk, and in pasteurized milk more rapidly than 

 in sterilized milk. A blue color develops quite rapidly in pas- 

 teurized milk when it curdles promptly. The influence of lactic 

 acid bacteria is quite obvious, since raw milk sours more rapidly 

 than pasteurized milk, and pasteurized milk more rapidly than 



