264 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



less time, and is found to be equally satisfactory for 

 plating, as explained below. 



" Sterilize in an autoclav at 15 Ibs. pressure (never 

 above) for one-half hour after dry steam comes from the 

 autoclav. As an alternative the medium may be steri- 

 lized on three successive days in streaming steam, in 

 which it must remain 20 minutes each day after the agar 

 has completely melted. 



" If the flasks are to be kept for more than a couple 

 of days, or are to be shipped, before using, cover the 

 mouths of the flasks before sterilizing with paper fastened 

 by a rubber band or by tinfoil to retard evaporation." 



Dilutions. "For miscellaneous milk samples the 

 character of which is not known three dilutions shall be 

 made: 1-100, 1-1000, 1-10,000. Where the character 

 of the milk is known the number of dilutions may be 

 reduced. If the milk is pasteurized, certified, or 

 known to be fresh and of high grade, the 10,000 

 and 1000 dilutions may be omitted ; if the milk is known 

 to be old and of high bacterial content the 100 dilution 

 may be omitted. In no case shall less than two plates 

 be made of each sample. 



" Any convenient method of making dilutions may 

 be used, always using pipettes and sterile water blanks. 

 The water used for dilutions may be placed in dilution 

 bottles (99 c.c. and 9 c.c. are convenient sizes) and 

 sterilized for one hour in an autoclav at 15 Ibs. pressure. 

 These should be marked so that it can be determined that 

 they have neither gained nor lost water during or subse- 

 quent to sterilization. Or the water may be sterilized 

 in bulk, if kept in a properly guarded container, and 

 subsequently measured directly into dilution bottles with 

 sterilized pipettes. 



