204 GENERAL MICROBIOLOGY 



water) and heat slowly up to 60 (thermometer in each flask). 

 Keep at 60-65 for twenty minutes. 



7. Remove the flasks from water bath and cool* quickly. 



8. Make dilution plates, using the same dilutions as 

 before, and place the flask and plates at room temperature, 

 marking each carefully. 



9. .Watch daily for signs of growth in each medium. 



10. Make plates from each flask after six days, deter- 

 mining the range of dilutions by consulting your former 

 plates. Will the organisms have increased or decreased in 

 this time? W^hy? 



11. Compare the types of organisms on the plates before 

 and just after pasteurizing and six days after pasteurizing. 

 Examine each type microscopically. Of what does the 

 predominant flora of each nutrient fluid consist before 

 pasteurization? After pasteurization? 



Note. The fruit juice may be saved for the experiment on meta- 

 biosis. 



12. Count each set of plates and record the average 

 number of microorganisms per c.c. 



13. Plot the curve to show the destruction of micro- 

 organisms by pasteurization. 



Compare the milk data with milk data and also the cider 

 with cider. 



14. Keep the original flasks for one or two weeks. If 

 any marked changes occur, plate qualitatively and ascer- 

 tain the type of organism causing the change. 



15. How does the physical nature of the two nutrient 

 substances influence their response to pasteurization? 

 Give reasons for explanations offered. 



What changes are brought about in milk by pasteuriza- 

 tion? In cider or other fermenting fruit juice? 



16. Give all data and results in full and draw any conclu- 

 sions. Point out any practical applications that may be 



made. 



* See note on page 203. 



