TYPES OF BACTERIA FOUND IN MILK. 157 



milk, and black milk. "By carefully selecting the varieties of bacteria, 

 and inoculating them into tubes of sterile milk there may be produced 

 samples of milk, each showing different colors, all the colors of the 

 rainbow being thus obtained. All of these phenomena do certainly 

 occur in the bacteriological laboratory, and all are produced by the 

 growth of different species of microorganisms. But they are usually 

 procured by inoculating sterile milk with particular kinds of bacteria 

 and allowing them to act on the milk for many days. They are not 

 ordinarily dairy phenomena, and will hardly ever be likely to ap- 

 pear as dairy infections. They are of scientific rather than of 

 practical interest. 



Miscellaneous Faults. There is a considerable list of troubles 

 appearing occasionally in milk that are due to the growth of unusual 

 bacteria. Some of these are the following: Premature curdling, the 

 milk curdling too quickly and without souring; failure to curdle at 

 all, even after several days; bad tastes such as turnip taste, rancid 

 taste, putrid taste; difficulty in churning; bad tasting sour milk; yeasty 

 smell; soapy consistency. These faults are all unusual and in all cases 

 the growth of unusual bacteria is the cause. The remedy is al- 

 ways the same, more care in cleanliness and more thorough steriliza- 

 tion of the milk-vessels. Any sample of milk in which lactic acid 

 bacteria fail to develop normally will be sure to show some trouble 

 due to the growth of bacteria that happen to be present and whose 

 rapid growth is not prevented by the acid-foiming bacteria. Some- 

 times such troubles may be remedied by the addition to the milk of 

 a culture of ordinary lactic acid bacteria. 



Alcoholic Fermentation of Milk. Most sugar solutions will 

 readily undergo an alcoholic fermentation, but milk sugar does not 

 easily make this change. It may be converted into lactic acid, 

 but not readily into carbon dioxid and alcohol. Hence an alcoholic 

 fermentation of milk is not a normal phenomenon, although it may 

 be pioduced by the addition of a little cane-sugar to the milk. The 

 possibility of making milk undergo an alcoholic fermentation by the 

 addition of yeasts is made use of in the manufacture of kummys. 

 This beverage was originally prepared by the Arabs from mare's 

 milk, which will normally undergo an alcoholic fermentation; but 



