THE EXAMINATION OF MILK 417 



as a yellowish layer, and the casein is precipitated, 

 though the reaction remains alkaline and the clear serum 

 is pink. 



B. lactis erythrogenes. — This organism was isolated by 

 Hueppe and Grotenfeldt from red milk. It occurs as short 

 rods, the growth of which liquefies gelatine. The colonies 

 are of a yellow colour when first seen on the plate, but 

 after liquefaction they become rose-red. A yellowish 

 deposit occurs on agar, which soon changes to yellowish- 

 red. The cultures give rise to an unpleasant, sweet smell. 

 Other organisms which give rise to red milk are the 

 following: B. prodigiosus, Sarcina rosea, Saccharomyces 

 ruber (red yeast) of Demme (Festschrift, Hirschwald, 

 Berlin,1890). The latter is liable to cause infantile diarrhoea. 

 A red colour in milk may be due to the presence of blood, 

 as a result of disease of the udders. 



Yellow Milk. — According to Freudenreich many organisms, 

 especially those of putrefaction, can produce a yellow colour 

 in milk, but this is rare in practice, as the milk is very 

 seldom kept long enough for this change to take place. 

 The best-known organism which gives rise to a yellow 

 colour in milk is the Bacillus synxanthus. This organism 

 was first found in a sample of boiled milk which had 

 assumed a yellow colour. It is a motile rod, which curdles 

 milk by means of a rennet-like ferment, which afterwards 

 re-dissolves the curd and produces a yellow pigment. 



Bitter Milk. — This fault may be produced in milk by the 

 cows eating certain plants, but there are a number of 

 bacteria which give rise to bitterness in milk. 



Bitter milk is recognisable by the taste, which is very 

 often also mouldy and accompanied by coagulation, though 

 the latter phenomenon may not occur until the milk is 

 warmed. Several organisms have been isolated which 

 have the capacity of producing bitterness in milk, notably 



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