METHODS OF FOOD PRESERVATION 401 



I. Keep one Petri dish closed tliroughout the experiment. This is the con- 

 trol and will serve as a comparison with the exposed dishes. 



II. Expose one dish of the agar media in a classroom at head level ; ex- 

 pose one in the hallway tlirough which many pupils pass ; one in the lunch 

 room, one in the gymnasium, one in the street, and one in a theater. Cover 

 all dishes immediately after exposure. 



A. Incubate the dishes between 30° to 40° C. until colonies appear on 

 the sLU"faces. 



B. Compare the dishes with the control. 



C. Which dishes show the greater number of colonies ? 



D. Which dishes show the greater number of different kinds of colonies ? 



E. What seems to be the relation between dust and bacteria? 



Problem. What is the bacterial content of different kinds of milkf * 



I Put a couple of drops of certified milk in a Petri dish containing 

 sterile agar. Cover the dish immediately and permit the milk to completely 

 cover the agar. 



II. Repeat the experiment for grade A and grade B milk. 



III. Incubate the dishes for twenty-four hours at between 30° and 40° C- 



IV. Which milk shows the greater number of colonies? 



V. Which milk shows the greater number of kinds of colonies? 



VI. What is the danger of having a great number of kinds of colonies ? 



VII. In selecting milk, state anything else, besides bacterial content, that 

 must be considered. 



VIII. State one possible reason for using pasteurized milk and one reason 

 for using certified milk. 



Problem. What is the effect of various antisej)tics on the growth of 

 bacteria f 



Expose each of five dishes of sterile agar media to dusty air in order to 

 inoculate them or introduce germs into them. 



I. Cover one and keep it closed througliout the experiment. 



II. Pour a little boric acid into dish number 2. Cover it and tip the dish 

 repeatedly until the boric acid completely covers the surface of the agar. Be 

 sure not to use any more than just enough to cover the agar. 



III. Repeat the experiment, using iodine, salt solution, and mercurochrome 

 in each of the other three dishes. 



* All cities do not grade milk as A, B, and certified. If different forms of grading 

 milk are used in your community, use those in performing this experiment. 



