22 



HUMAN BIOLOGY 



19. Relation of bacteria to the flavors of food. Again, 

 many of the flavors of food are due to the action of bacteria. 

 The flesh of animals, for instance, that have just been killed, 

 is often tough and tasteless. If allowed to stand, however, 

 these meats become tender and acquire their distinctive fla- 

 vors by the decomposing action of bacteria. A similar action 

 takes place when butter or cheese ripens, and the dairy in- 

 dustry has been perfected to such a degree that bacteria of 



certain kinds have been 

 proved to give rise to defi- 

 nite flavors, and these 

 bacteria may be produced 

 in pure cultures for the 

 dairymen. 



20. Bacteria in the in- 

 dustries. Without the 

 help of bacteria the prep- 

 aration of linen, jute, 

 and hemp would be im- 

 possible. All these valu- 



^^ able products are plant 



fibers which are connected with woody materials so closely 

 that they cannot be separated without first subjecting 

 the stems of flax, hemp, and jute to a process of decay 

 in large tanks of water. Moisture and warmth induce 

 the rapid growth of germs, and the resulting decay loosens 

 the tough fibers so that they may be separated from the 

 useless parts of the plant. The change of alcohol into 

 vinegar is also caused by bacteria. Formerly in the 

 preparation of indigo other forms of bacteria were all- 

 important, but at the present time indigo is largely made 

 artificially. 



FIG. 10. Bacteria from root tubercles. 



