288 GENERAL MICROBIOLOGY 



EXERCISE 9. A COMPARISON OF THE BACTERIAL 

 CONTENT OF SWEETENED AND UNSWEETENED 

 CONDENSED MILKS 



Apparatus. Six sterile Petri dishes; six tubes of litmus 

 lactose agar; 99 c.c. dilution flasks; two 95 c.c. dilution 

 flasks; tubes of sterile litmus milk; two sterile 5 c.c. pipettes 

 with large aperture for delivery; can-opener. 



Culture. Unopened can of sweetened condensed milk; 

 unopened can of unsweetened condensed milk (contents 

 guaranteed to be sterile) . 



Method. 1. Sterilize the can-opener in the flame. 



2. Thoroughly cleanse the outside of the unopened cans 

 of condensed milk and then submerge in boiling water for 

 five or ten minutes. 



3. Remove the cans from the water, being careful in 

 handling them not to contaminate the upper surface of the 

 cans. 



4. With the sterile can-opener make an opening in the 

 can only large enough to admit the introduction of a 5 c.c. 

 pipette. 



Note. Only one can is to be opened at a time to avoid contam- 

 ination. ' 



5. With a sterile pipette obtain a 5 c.c. sample from the 

 can just opened and transfer to a 95 c.c. dilution flask. 



Note. As the condensed milk is very viscous and adheres to tho 

 sides of the pipette, after delivering the 5 c.c. into the dilution flask 

 blow out the remainder into the sink or other suitable place, then 

 replace in the dilution flask and wash out the adhering fluid by draw- 

 ing the diluting fluid up into the pipette several times. The use of a 

 5 c.c. volumetric pipette having a large aperture for delivery would 

 lessen the possibilities of contamination. 



6. This resulting dilution is a 1 : 20 dilution of the con- 

 densed milk, or a 1 : 40 dilution of the original milk (if 

 the directions on the can give a dilution of 1 : 1 for pro- 

 ducing a milk of original composition). 



