METHODS OF EXAMINING MILK 263 



Restore the weight lost by evaporation. It is imperative 

 that the agar be of the best quality and clean, or it is 

 impossible to obtain a clear medium. (When an auto- 

 clav is at hand the following modification is convenient. 

 Add the peptone and beef extract to about 300 c.c. of 

 water, and the agar to 700 c.c. Heat both in an autoclav 

 under 15 Ibs. pressure one-quarter hour. Filter the broth 

 while hot through filtered paper, and then mix the broth 

 with the melted agar and filter through absorbent 

 cotton. ) 



" Determine the acidity of the medium by titrating 

 5 c.c., diluted with 45 c.c. water, with 1 / 2 o normal NaOH, 

 phenolphthalein being used as an indicator. Any at- 

 tempt to adjust the acidity is likely to make undesirable 

 changes in the medium. Inasmuch as variations between 

 +.5 and +1. acid make no appreciable difference in the 

 results, the reaction of the medium is not to be changed 

 if it falls between + .5 and + .1 acid. It will usually be 

 between these limits. If it should be above + 1. acid, add 

 enough normal NaOH to bring it to + 1., and if it is 

 below + .5 acid add enough normal HC1 to bring it to 

 + .5, always adding the smallest amount of the reagent 

 possible to bring the reaction within these limits. 



" Cool to 45 C. and then heat to boiling ( 15 minutes) 

 and filter through filter paper or absorbent cotton. It 

 is necessary that the filtering be continued till the medium 

 is clear. If the procedure here given is followed and a 

 high grade of agar and other materials is used there will 

 be no difficulty in getting the agar clear. Egg should 

 not be used as a clarifier. 



" After filtering, the agar may be either tubed, 10 

 c.c. in each tube, or placed in flasks containing about 150 

 c.c. each. The latter method requires less glassware and 



