FERMENTATION AND GERM DISEASES 



Bacteria 



FlG. 34. A DROP OF MILK UNDER 



THE MICROSCOPE 

 Showing fat drops and bacteria. 



decaying bodies. Nearly all kinds of decay are thus produced 

 by bacteria or closely allied organisms. The putrefaction of 

 meats and eggs, the souring of bread and milk, and hosts of 

 other processes by which 

 fooa is spoiled are instances 

 of bacterial action; Fig. 34. 



Since the spoiling of food is 

 produced by bacteria, it fol- 

 lows that the preservation of 

 food for an indefinite length 

 of time is possible if bacteria 

 can be kept from it. This 

 is not an easy matter, how- 

 ever, because of the wide 

 distribution of these plants. 

 They are sure to get into the foods in spite of all 

 ordinary caution. But many foods may be protected 

 from them by the process of canning. In canning, foods 

 are first subjected to a heat (commonly boiling) sufficient 

 to kill any bacteria in them, and then are sealed up in jars 

 or cans so tightly that no air or bacteria can reach them. 

 If this is done thoroughly and carefully, the food will keep 

 indefinitely. At any time afterward the cans may be opened 

 with the certainty that the food will be found in good condi- 

 tion. The discovery of the methods of canning has been of 

 extreme value, for it made it possible to preserve for winter 

 use vast quantities of food grown in the summer, which would 

 otherwise spoil before they could be consumed. 



When such materials are decomposed by bacteria the prod- 

 ucts that come from them pass into the soil and air in a form 

 which can be used by subsequent generations of plants. In 

 this way soil is kept fertile, and we can depend upon getting, 

 year after year, abundant harvests. Were it not for the 

 action of bacteria the soil would soon cease to yield crops, 

 and we should eventually starve. 



