156 BACTERIA IN MILK. 



over an extensive territory, the cause was found to be due to a yeast 

 that infected the factory and the utensils used in cheese-making 

 (Fig. 37, b). The remedy in such cases lies in a thorough disinfec- 

 tion of the cans, vats, etc. 3. Microorganisms in boiled milk. It 

 may frequently happen that boiled milk will become bitter. The 

 boiling destroys the acid-forming bacteria, but leaves alive some of 

 the spore-producing organisms. These may subsequently develop 

 and produce bitter products. This type of bitter milk is of little 

 significance, however, since the practice of keeping milk after it is 

 boiled has almost disappeaied. 



Fermentations Changing the Color of Milk. The first 

 bacterial fermentation of milk clearly described was that of blue 

 milk, noticed and studied over sixty years ago. This 

 trouble has, therefore, an historic interest because of 

 * ts connect i n w ith tne early development of bacteri- 

 ology. It has little practical interest, however, since 

 FIG. 38. jj- j s very rare occurrence. It is caused by a well- 



The organism J 



producing known bacterium named B. cyanogenes (Fig. 38) that 

 cyanogenes nas Deen found in this country as well as in Europe. 

 When the organism is inoculated into milk it produces 

 no visible effect until the milk is two or three days old. Then blue 

 patches appear in it that extend as the milk sours, until the whole 

 becomes of a sky-blue color. As a dairy infection it is very unusual, 

 and no well-marked case of such blue milk infecting a dairy has 

 apparently been reported from this country. 



Other fermentations producing pigments are also reported by 

 bacteriologists. Red milk has occasionally been mentioned. Milk 

 may sometimes be red when it is drawn from the cow because of the 

 presence of blood, due to some trouble in the udder. But there are 

 also types of red milk developing slowly, and due to the growth of 

 bacteria. B. prodigiosus, B. erythrogenes, and B. lacto rubifaciens 

 are three species that have been described as having this power. 

 None of them is of practical importance in the dairy. Red spots 

 in cheeses do sometimes result from the growth of bacteria, but red 

 milk is the rarest of occurrences. In addition to these we some- 

 times hear of yellow milk, orange milk, green milk, amber- colored 



