DETERMINATION OF BACTERIA IN MILK 571 



ready at any time for use. The agar should be put up in flasks 

 in amounts suitable for the amount of medium to be made at 

 any one time. It is not desirable to use part of a flask of agar, 

 then resterilize and hold for future use. Repeated heating 

 lowers its jelly strength. The flasks should be stoppered to 

 prevent evaporation. To prepare a liter of 3 per cent washed 

 agar, weigh out 30 grams of agar and place in a flask with 2000 

 cc. of distilled water. This proportion should always be used. 

 Allow it to stand for twenty-four hours, at room temperature, 

 with occasional shaking. Then pom' off as much water as pos- 

 sible, using a piece of cheesecloth over the top of the flask, and 

 add distilled water enough to make up again the original volume. 

 Allow the agar to stand another twenty-four hours, then pour 

 off the agar on to a cotton-flannel cloth in a funnel and wash 

 once with a liter of distilled water. Let the agar drain and then 

 press out as much water as possible by squeezing the filter cloth 

 with the hands. A container large enough to hold the agar is 

 counterpoised on the laboratory scales, and the agar placed in it. 

 In the opposite pan is placed 30 grams for the agar and 1000 

 grams for the weight of the water in which the agar is to be 

 dissolved. Then water enough is added to make up this weight. 

 This will make a liter of 3 per cent agar. Dissolve the agar 

 by heating in the Arnold sterilizer, then filter through cotton 

 flannel or absorbent cotton until clear. 



REASONS FOR USING WASHED AGAR 



There are two reasons for using washed agar; first, because 

 it makes possible the preparation of the previously described 

 medium without the formation of a precipitate upon sterili- 

 zation; and second, because washed agar gives higher counts 

 than ordinary agar, at least with some samples of milk. 



When ordinary or so-called purified agars are used in a medium 

 with milk powder, a heavy precipitate forms upon sterilization, 

 probably due to the precipitation of calcium and magnesium 

 phosphate with some casein. This will not occur if enough 

 sodium bicarbonate is added to the medium to bring the hydro- 



