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 disappeared. 
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 
its connection with the early development of bacteri- 
ology. It has little practical interest, however, since 
FIG. 38. j t j s o f verv rare occurrence. It is caused by a well- 
The organism ' J 
producing known bacterium named B. cyanogenes (Fig. 38) that 
blue milk. 
cyanogenes. 
' ' has been 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 
