3^4 



How Living Things Affect One Another unit vi\ 



Fig. 333 '■'■Wheels" of Swiss 

 cheese in a curing room. 

 The distinctive holes and 

 the distinctive flavor are 

 due to bacteria. (kraft 



FOODS COiMPANy) 



the tenderizing is done by bacteria 

 which also give the meat a pleasant fla- 

 vor. If the hanging is allowed to continue 

 too long, we say the meat goes bad or 

 decays. Some cheeses are the product of 

 bacterial action on milk. Other cheeses 

 are produced by the action of molds. 

 Whether cheeses are made from cream, 

 whole milk, or skim milk, particular 

 microorganisms do the work. 



Bacteria are also used in changing al- 

 cohol to vinegar, and in numerous other 

 industries. Flax, jute, hemp, and other 

 fibrous plants are "retted" (rotted) in 

 water through bacterial action to separate 

 fibers from the rest of the plant. 



How do microorganisms harm us? You 

 have already studied something of our 

 relationship as hosts to the parasites 

 called disease germs. Other microorgan- 

 isms harm us in less spectacular but quite 

 important ways. Curiously some of 

 them are the same organisms that at 

 times are helpful. 



The bacteria and molds that make 

 meat tender and that flavor cheese make 

 both the meat and the milk unfit for 

 food by decaying it, if their growth is 

 not stopped soon enough. The decay 

 microorganisms, which are helpful in 

 changing dead plants into humus, are 

 harmful when they decay wharf pilings, 

 building timbers, and telephone poles. 

 Yeasts which are very useful to us are 

 also harmful when they spoil fruit. 



We have long worked on the problem 

 of keeping foods from decay. The In- 

 dians smoked and dried meats and fish. 

 For hundreds of years meat which was 

 to be used on lontj ocean voyafjes \vas 

 packed in barrels of brine (strong salt 

 water). And early in the seventeenth 

 century Francis Bacon, long before bac- 

 teria w^ere known, proved that cold pre- 

 serves food. For a long time, too, vine- 

 gar, spices, and sugar have been used. If 

 sufficiently concentrated, all act as mild 

 antiseptics and preser\'e food. Sometimes 



