16 
Exercise 7. Sterilization 
MATERIALS: 
Beef broth 
Skimmed milk 
Eight empty test tubes 
Two sterile test tubes 
Cotton batting 
Note. —Where the test tubes contain cultures and experimental material, 
they are handled in tumblers, and to avoid breaking the tubes a cushion of 
cotton is placed in the bottom of each tumbler. 
1. Prepare five tubes of milk and five tubes of beef broth according to the 
following outline, making plugs as shown by the instructor. 
No. 
Material 
Kind of tube 
Treatment 
la 
beef broth 
sterile tube 
not 
lb 
milk 
plugged 
heated 
2a 
beef broth 
unsterilized 
not plugged 
2b 
milk 
tubes 
not heated 
3a 
beef broth 
unsterilized 
heat to 100° C. 
3b 
milk 
tubes, plugged 
for ten minutes 
4a 
beef broth 
unsterilized 
heated to 120° C. 
4b 
milk 
tubes, plugged 
for 30 minutes 
5a 
beef broth 
unsterilized 
heat to 120° C. 
5b 
milk 
tubes, plugged 
and inoculate 
2. Treat tubes 3a and 3b in a cup of boiling water 10 minutes over the 
Bunsen flame. 
3. Prepare a labeled piece of paper giving the experiment and desk number. 
Fasten this paper around the tubes 4a and 4b, and 5a and 5b with a rubber 
band. Place them in the autoclav to be heated for 30 minutes at 116°-120°C. 
FIG ll-CORRECT METHOD OF HOLDING TUBES WHEN TRANSFERRING CULTURES 
Note how the plugs are held between the fingers of the right hand. The plugs ..re never laid down- 
The tubes are held as nearly horizontal as possible. 
