282 FERMENTATIONS IN MILK 



casein and the cream becomes blood-red in colour and slightly 

 alkaline. The colonies liquefy gelatine and produce a red 

 coloration upon it, and upon agar when grown in the dark. In 

 the light the colonies are yellowish. 



(b) Micrococcus roseus, L. & N., is a very common bacterium 

 found in the air. It grows readily in milk, forming a red sedi- 

 ment in the latter without otherwise effecting much change. 

 Nearly allied to this is Sarcina rosea, Schroter, which has been 

 found to give a red coloration to milk, with slow precipitation 

 and solution of casein. 



(c) Bacillus prodigiosus, Fliigge (L. & N.) (Micrococcus pro- 

 digiosuS) Cohn), is also a common air and water organism which 

 appears as a large coccus i ^ in diameter on solid media and as 

 rods in liquid media. It is sometimes motile, and when grown 

 on potato and agar produces rose-red to reddish-purple colonies. 

 It liquefies gelatine and the colonies are often greyish-white 

 instead of reddish in colour. On the surface of milk or cream 

 it may produce its characteristic blood-red pigment, but in the 

 liquid it precipitates and dissolves the casein with the production 

 of a faint yellow colour only. 



(ii) Yellow Milk. A number of bacteria have been 

 found to give rise to a yellow or orange coloration in 

 milk, especially if the latter is kept for a long time. The 

 species, which has been most carefully investigated, is 



Bacterium synxanthum, Ehr, a short, thin rod-like organism 

 obtained from boiled milk, in which it gives rise to a bright 

 yellow colour; the casein is dissolved, the solution becoming 

 alkaline. In acid milk the colour is not developed. 



(iii) Blue Milk. The development of a sky-blue colour 

 in milk has occurred from time to time in certain dairies, 

 especially on the Continent, and so far back as 1838 

 investigations were made to determine its cause. That 

 it was due to the work of micro-organisms was demon- 



