CONTROL OF THE PUBLIC MILK SUPPLY 415 



to avoid short-fiber or dusty cotton, which give a cloud of lint-like particles on shaking. 

 Bottles * * * should be filled a little over the 99 c.c. * * * to allow for loss during 

 sterilization. 



Pipettes. Straight sides 1-c.c. pipettes are more easily handled than those with 

 bulbs; they may be made from ordinary %6-in. glass tubing and should be about 10 

 in. in length. 



Plating Technique. The agar after melting should be kept in the water-jacketed 

 water bath between 40C. and 45C. for at least 15 min. before using to make 

 sure that the agar itself has reached the temperature of the surrounding water. If 

 used too warm, the heat may destroy some of the bacteria or retard their growth. 



Shake the milk sample 25 times, then with a sterile pipette transfer 1 c.c. to the 

 first dilution water and rinse the pipette by drawing dilution water to the mark and 

 expelling; this gives a dilution 1 to 100. 



* * * Then with a sterile pipette transfer 1 c.c. to the Petri dish, using care to 

 raise the cover only as far as necessary to insert the end of the pipette. 



Take the 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 a 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 formation of air bubbles or slopping the agar, and after 

 allowing the agar to harden for at least 15 min. at room temperature, place the dish 

 bottom down in the incubator. 



Plating should always be done in a place free from dust or currents of air. 



In order that colonies may have sufficient food for proper development 10 c.c. 

 of agar shall be used for each plate. 



67. Determination of Taste and Odor of Milk. After the plates have been pre- 

 pared and placed in the incubator, the taste and odor of the milk shall be deter- 

 mined after warming the milk to 100F. 1 



68. Counts. The total number of colonies on each plate should be counted, 

 and the results expressed in multiples of the dilution factor. Colonies too small 

 to be seen with the naked eye or with slight magnification shall not be considered 

 in the count. 



69. Records of Bacteriologic Tests. The results of all bacterial tests shall be 

 kept on file by the secretary of each commission, copies of which should be made 

 available annually for the use of the American Association of Medical Milk 

 Commissions. 



CHEMICAL STANDARDS AND METHODS 



The methods that must be followed in carrying out the chemical investigations 

 essential to the protection of certified milk are so complicated that in order to keep the 

 fees of the chemist at a reasonable figure, there must be eliminated from the exami- 

 nation those procedures which, whilst they might be helpful and interesting, are in no 

 sense necessary. 



For this reason the determination of the water, the total solids and the milk sugar is 

 not required as a part of the routine examination. 



1 Should it be deemed desirable and necessary to conduct tests for sediment, the 

 presence of special bacteria, or the number of leucocytes the methods adopted by the 

 committee of the American Public Health Association should be followed. 



