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PLANTS AND ANIMALS 



a warm place. The fourth 

 tube should have a plug 

 of cotton put into the top 

 and then it should be 

 placed in the can of cold 

 water which should 

 gradually be heated to 

 176F. and kept at about 

 that temperature for at 

 least five minutes. This 

 heating is called pasteur- 

 ization. Then set it in 

 a warm place. Examine the milk once a day. Which 

 tube sours first? Which keeps sweet the longest? 



b. Add a little boiling soda to some of the sour 

 milk. Notice the bubbles. They are carbon dioxide 

 How does the milk taste? 



The illustrationshows the kind of bacteria which caused 

 the milk to sour. They are very much magnified. 



Bacteria can be killed by heat and by poisons. The 

 poisons are called preservatives, and they should never 

 be used. The experiment was to show that poisons may 

 preserve food but they are harmful and unnecessary. 

 The growth of bacteria may be made very slow by keep- 

 ing the food cold. Since bacteria require moisture in 

 order to live, if the food is dried it may be protected from 

 them. Smoking of meat and fish, as well as drying, will 

 preserve them for a long time. Food may be kept moist 

 and yet free from bacteria by the use of salt, sugar, or 

 spices and vinegar. 



After the bacteria, which are in or on the food, are 

 killed bv heat it is necessary to exclude others. This is 



Cut supplied through United States Department of Agriculture. 



