EFFICIENCY OF INTERMITTENT HEATING 139 



EXERCISE 46. TO DEMONSTRATE THE EFFICIENCY 

 OF INTERMITTENT HEATING AS A METHOD OF 

 STERILIZING MEDIA. ALSO TO COMPARE THE 

 EFFICIENCY OF CONTINUOUS AND INTERMITTENT 

 HEATING 



Apparatus. 400 c.c. fresh skim milk; forty sterile 

 test tubes; 2% azolitmin solution. 



Method. 1. Prepare litmus milk according to direc- 

 tions on p. 25. 



2. Fill the tubes, using approximately 8 c.c. per tube. 



3. Set five away without heating. 



Heat five for fifteen minutes on the first day. 

 Heat five for one hour on the first day. 

 Heat five for fifteen minutes on two successive days. 

 Heat five for fifteen minutes on three successive days. 

 Heat five for fifteen minutes on four successive days. 

 Heat ten for fifteen minutes on five successive days. 



4. Keep all tubes at room temperature. Examine 

 every two or three days and describe the macroscopical 

 changes of each set, as described under the discussion on 

 litmus milk (pp. 23-25.) 



Why do not all the tubes of a set change alike? Why 

 do not all sets present the same appearance? 



Save all tubes that do not show macroscopical changes. 

 These are probably sterile. 



6. Tabulate your results after ten days to two weeks, 

 recording the number and per cent of each lot that shows 

 macroscopical changes. 



6. Is milk difficult to sterilize? Why? What other media 

 present the same problem of sterilization as milk? Why? 



Would any other method for the sterilization of milk be 

 preferable to the ones you used? Give reasons for your 

 answer. 



7. State your results in detail and point out any con- 

 clusions that may be drawn and any practical applications 

 that may be made. 



