DETERMINATION OF BACTERIA IN MILK 569 



To make the milk-powder solution "a" use a good grade of 

 skimmed-milk powder made by the spray process, and prepare 

 the following solutions: 



(1) 5 grams milk powder 

 20 cc. distilled water 



(2) 1 gram sodium dibasic phosphate (Sorensen's phosphate) 

 5 cc. distilled water 



(When making more than 1 liter, the same proportions of milk powder 

 to water and phosphate to water must be used; therefore, to make 5 liters 

 multiply each amount by 5.) 



Weigh out 5 grams of skimmed-milk powder and pour on to 

 20 cc. of cold distilled water in a small beaker. Stir until thor- 

 oughly dissolved. In another beaker, dissolve 1 gram of sodium 

 dibasic phosphate (Sorensen's phosphate) in 5 cc. of distilled 

 water. Warm to dissolve phosphate quickly. Sorensen's phos- 

 phate, Na 2 HP0 4 + 2H 2 0, must be used. 



Add the phosphate solution (2) to the milk powder (1). Place 

 the beaker, containing (1) and (2) mixed, in a water bath with 

 water at about 30°C. and heat the milk powder phosphate solu- 

 tion to about 60° C. This should take about 10 minutes. At 

 temperatures between 50 and 60°C. a flocculent grayish pre- 

 cipitate will appear. Continue the heating until the precipitate 

 appears, then steam in an Arnold or other steamer for five minutes, 

 or until the precipitate appears white. Then dilute the milk- 

 powder solution about one-third with distilled water and steam 

 five minutes longer. Too long steaming will cause the solution 

 to turn dark and should be avoided. The whole heating period 

 should not be more than twenty or twenty-five minutes. 



Decant the solution while hot on to a filter paper, taking care 

 to keep the precipitate in the beaker until most of the liquid 

 is through. Then pour the precipitate on the filter and wash 

 with a little distilled water. If the milk-powder solution has 

 been properly heated it will filter readily, provided the filter 

 paper is not too hard. "J. Green" Grade 588 and "Ilmenau" 

 filter paper have given good results. 



