A. P. H. A. MILK ANALYSIS 



provided special attention be given to 

 the H-ion concentration of the medium. 

 The agar must be of the best quality. 

 If oven-dried at 105° C. just before 

 using, take 1.2%; if used just as 

 obtained in the market without oven- 

 drying, use 1.5%. Remove salts and 

 any dirt present by soaking, washing 

 and draining. Distilled water is to 

 be used for dissolving the ingredients. 



Reaction. 



A medium consisting of the above 

 ingredients, including a suitable pep- 

 tone, ordinarily has a reaction between 

 pH = 6.2 and 7.0. If within these 

 limits, the reaction requires no adjust- 

 ment for milk analysis. The most 

 desirable reaction is about pH = 6.5 

 to 6.6; but any reaction between 

 pH = 6.2 and 7.0 is allowable. No 

 change in reaction should be made 

 without carefully determining the 

 H-ion concentration of the finished 

 medium by the method described below. 



Inasmuch as the range of H-ion 

 concentration recommended for water 

 analysis* is pH = 6.8 to 8.4, it is 

 permissable, if desired, to use a single 

 agar for both purposes with a reaction 

 of pH = 6.8 to 7.0. If Witte's pep- 

 tone is used in the above formula, this 

 will ordinarily be the reaction without 

 adjustment. 



Each batch of finished medium 

 should be tested before use as to its 

 final reaction after sterilization. This 

 test is to be made as follows: 



Put 4 cc. of distilled water at 30 to 

 40° C. {not warmer) in a test tube. 

 Add 1 cc. of the agar to be tested and 

 then 10 drops of the indicator, brom 

 thymol blue* (0.04 per cent solution 

 in 95 per cent alcohol). The resulting 

 color should be either a yellowish green 

 or vary to a deeper shade of grass green. 



♦Prepared by Hynson, Wescott & Dunning, Baltimore.Md. 



To one whose eye is trained this shade 

 of color is sufi&cient. 



These shades may be accurately 

 determined by means of the buffered 

 solutions^ of Sorensen or of Clark and 

 Lubs. 



However, they may be approximate- 

 ly determined by comparing the tube 

 of agar containing the indicator with a 

 set of color tubes after the method of 

 Barnett and Chapman. '" 



Select 12 test tubes of even caliber 

 and place in two rows of 6 each. In 

 each tube of one row put 5 cc. of a 

 dilute alkali (as, for example, twentieth 

 normal sodium hydroxid). In each 

 tube of the other row put 5 cc. of very 

 dilute acid (one drop of concentrated 

 sulphuric or of hydrochloric to 100 cc. 

 of distilled water is sufficient) < Avoid 

 stronger acid. 



Add indicator to the tubes as follows: 



The tubes are to be viewed in pairs 

 of acid and alkali, each pair containing 

 the sum of ten drops of indicator. 



If preferred, double these quantities 

 may be used throughout and the 

 indicator measured in fractions of a 

 cubic centimeter instead of drops. 

 That is, two cc. of agar should be 

 taken for testing. This should be 

 added to 8 cc. of distilled water. One 

 cc. of indicator should be used. In 

 comparing with the Barnett and Chap- 

 man tubes, use 10 cc. of dilute acid or 

 alkali in each tube, and add the indi- 

 cator in tenths of a cubic centimeter 

 instead of in drops. 



All of the test tubes used in this 



