382 



STATE BOARD OF AGRICULTURE. 



make the color of the broth solution compare with tube pH 7.0. Broth 

 without indicator niu.st be used as a compensating blank, in front of the 

 standard tube. The broth is noAV ready to use and is transferred in 10 cc. 

 portions to a 2.8 x 16 cm. test tubes using a sterile 10 cc. pipette. 



Preparation of samples. It is desired to add 0.1 cc. of milk to each 

 tube of brom-'thymol blue broth. To reduce the error due to measuring 

 direct as small an amount of milk as 0.1 cc, sterile 9 cc. water blanks in 

 test tubes are prepared and 1 cc. of the milk to be tested added to these. 

 After thoroughly mixing 1 cc. of each diluted sample is transferred to a 

 tube of the broth. 



Incubati-on. After the 0.1 cc. portions of milk have been added to the 

 tubes containing b^om-th3^nol blue broth they are placed in a water 

 bath or incubator at a temperature of 100°F. Hourly observations are 

 made to detect the first change in color of the indicator due to the devel- 

 opment of acidity resulting from the activity ol the bacteria and enzymes 

 l^resent. As soon as pronounced changes are noticed, hourly readings 

 are made by placing the tubes containing milk broth indicator in the com- 

 parator and comparing with the standard tubes as mentioned below. The 

 samples must be placed in a water bath and cooled to 20°C. before read- 

 ings are made. Reading's should be made at a constant temperature as 

 otherwise they varj- somewhat. 



Standards. The standards are the buffer solutions described by Clairk 

 and Lubs (5). Ten cc. portions are placed in the 2.8 x 16 cm. test tubes 

 with the same amount of indicator, that was placed in each 10 cc. of the 

 broth solution and sealed. • In this way the standards will keep for a 

 considerable period. The standards with the following readings are used 

 in this work, pH 7.1, 7.2, 7.0, 6.8, 6.6, 6.4, 6.2, 6.0, and 5.8. These stand- 

 ards may be prepared in any chemical laboratoiy, or, if one has not the 

 facilities for making, may be obtained from Hynson, Wescott and Dun- 

 ning of Baltimore at a reasonable price. A suitable comparator accom- 

 panies the standards furnished by the above company. If a large number 

 of samples are to be examined the special comparator designel by Cool- 

 edge (6) and made by the Central Scientific Company is desirable. 



For convenience, samples of milk aire given a sicore depending upon the 

 pH reading as follows. 



