OHROMOGENIC ORGAXIS^WS. 281 



on the surface of milk, having the appearance of a drop of blue- 

 black ink which has fallen in, is noticed. This is due to an 

 organism called B. syncyanus or B. cymiogenus ; when cultivated 

 alone a grey colour is produced, which turns an intense blue on 

 the addition of acids ; the blue colour is only noticed if the mUk 

 be sour with lactic acid. 



Hed Milk. — This is occasionally due to the action of micro- 

 organisms ; it is usual to ascribe the formation of red milk to 

 Micrococcus prodigiosus, which forms an intense blood-red sub- 

 stance, but it is doubtful whether this organism is always the 

 cause. The colour is often of a pinkish tinge, and is due to the 

 pink yeast, Micrococcus rosaceus, Bacillus laclis erythroqenes, 

 or Sarcina rosea. The organism causing a red colour varies 

 according to the district, and only one organism is usually found 

 in any district. 



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

 in the food eaten by the cattle, but far more frequently arises 

 from the presence of blood ; this is produced by a diseased state 

 of the udder, but far more frequently by some slight local damage, 

 through a kick or a blow resulting in the breaking of a small 

 blood-vessel in the udder. 



Yellow Milk. — An organism which curdles milk and redissolves 

 the curd to form a yellow liquid has been described as Bacillus 

 synxanthus ; there are probably several organisms which produce 

 a yellow colour ; all seem to have proteolytic functions. Yellow 

 milk is very rare, though it is very common to see dirty vessels 

 which have contained milk become quite jellow. 



Green milk, violet milk, and biltrr milk have been found to be 

 produced by micro-organisms. Bitter principles may be derived 

 from the food of the cattle, and some, though not all, of the 

 butyric ferments give rise to a bitter taste. Peptones produced 

 from casein may also be the cause of bitterness. 



Ropy Milk. — Milk occasionally, instead of remaining liquid, 

 becomes a thick slimy mass ; if a glass rod is dipped into milk 

 which has become ropy and withdrawn, a portion of the milk 

 adheres and can be drawn out in long threads. Sometimes this 

 action is confined to the cream on the surface, but with other 

 organisms the whole milk becomes ropy. 



The organisms which produce ropy milk do not grow well 

 at a low temperature, and it frequently happens that milk at 

 a dairy goes ropy in the summer, is free from this trouble in the 

 winter, and becomes ropy again in the spring. 



^\'hen milk is found to become coloured, or to be ropy, the dairy, 

 and all vessels used for nulk, should be submitted to a thorough 

 disinfection, which will remove the cause. The Bacillus of Mas-iol 

 tends to produce a sour, ropy milk. 



