264 Principles of Legislative Milk Control. 



be used- for plugs. It is well to use care in selecting cotton for this purpose 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 e. c. pipettes are more easily handled than those with 

 bulbs; they may be made from ordinary three-sixteenths inch glass tubing and should 

 be about 10 inches in length. 



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

 water bath between 40 C. and 45 C. for at least 15 minutes 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 dilutior water and rinse the pipette by drawing dilution water to the mark and ex- 

 pelling; 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 minutes 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 e. c. of 

 agar shall be used for each plate. 



67. Determination of taste and odor of milk. After the 

 plates have been prepared and placed in the incubator, the taste 

 and odor of the milk shall be determined after warming the milk 

 to 100 F. 



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 bac- 

 terial tests shall be kept on file by the secretary of each commis- 

 sion, 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 chemi- 

 cal 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 examination 

 those procedures which, whilst they might be helpful and interest- 

 ing, 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. 



70. The chemical analyses shall be made by a competent 

 chemist designated by the medical milk commission. 



71. Method of obtaining samples. The samples to be exam- 

 ined by the chemist shall have been examined previously by the 



