2 5 8 



CLEAN MILK 



it is well to use a dilution of 1/10,000. To make this dilution, use 

 Fig. 64. two bottles of sterile water each containing 99 c.c. 



Shake the milk sample twenty-five times, then with a 

 sterile pipette remove 1 c.c, put into the first dilution water 

 and rinse the pipette by drawing dilution water to the mark 

 and expelling; this gives a dilution of 1 to 100. 



Shake the first dilution twenty-five times, then with a 

 fresh sterile pipette remove 1 c.c, put into the second dilu- 

 tion water, rinsing the pipette to the mark as before; this 

 gives a dilution of 1 to 10,000. Shake the second dilution, 

 twenty-five times, then with a sterile pipette remove 1 c.c. 

 and put it into the petri dish, using care to raise the cover 

 only so far as necessary to insert the end of the pipette. 



Taking a tube of agar from the water bath, wipe the 

 water from outside the tube with a piece of cloth, remove 

 the plug, pass the mouth of the tube through the flame, and 

 pour the agar into the plate, using the same care as before 

 to avoid exposure of the plate contents to the air. 



Carefully and thoroughly mix the agar and diluted 

 milk in the petri dish by a rotary motion, avoiding the form- 

 ation of air bubbles or slopping the agar, and after allow- 

 ing the agar to harden for at least fifteen minutes at room 

 temperature place the dish bottom down in the incubator. 

 The practice of mixing the diluted milk with the agar in the 

 tube leaving a certain portion of the bacteria unplated, is 

 not recommended by the Committee. 



Controls. — Plating should always be checked by con- 

 trols. A blank plate should be made with each set of milk 

 . c.pip- P^tes for control of the water, petri dishes, pipettes, etc. 



For control on technique of plating it is recommended 



\J 



I c 



ettes in- 

 closed in 



steriliz- tnat ^ or wor ^ on " market milk," duplicates be made each 



m S- day on several plates. 



" Certified milk " should always be plated in duplicate, and 



