278 PRACTICAL DAIRY BACTERIOLOGY 



steam sterilizer 20 minutes. Allow to stand 24 hours in room 

 temperature, and steam again for 20 minutes. 24 hours later 

 steam a third time. It is now sterile and may be reserved for 

 future use. 



*No. 8. Litmus Gelatin. For the study of milk bacteria it is 

 very desirable in most cases to detect the production of acid. 

 For this purpose a litmus culture medium is used, the prepara- 

 tion of which is as follows: 



a. To 500 c.c. bouillon add 15% gelatin and i% lactose 

 (milk sugar). The rest of the process up to the point of filling 

 the test tubes is exactly as above described in 5, a to c. In filling 

 test tubes, exactly 8 c.c. of the medium is to be placed in each 

 of the test tubes, and the test tubes are to be sterilized on three 

 successive days. 



b. Prepare a litmus solution by weighing out 50 grams of 

 solid litmus (the litmus cubes preferred). Soak for 24 hours 

 in 5 times its weight of water; filter through filter paper, and 

 a deep blue solution will be obtained. This is too alkaline, and 

 it is necessary to bring its reaction to the same point as the 

 reaction of the gelatin namely, 1.5%, as follows: 



c. Measure out 5 c.c. of the litmus solution in an evaporating 

 dish; dilute with 10 times its bulk of water, and heat. Add to 

 it from the burette i/io normal HC1, drop by drop, until the 

 blue turns from a deep blue to a faint red. This gives the 

 neutral point of litmus. 



d. By the reading on the burette before and after neutraliza- 

 tion determine the number of c.c. required to neutralize 5 c.c. 

 of the litmus. If we divide by 5 to give the amount required 

 to neutralize one c.c. and multiply by 495, we shall have the 

 number of cubic centimeters necessary to neutralize the whole 

 litmus solution. If this amount of acid is added it will bring 

 the litmus solution to the litmus neutral point, which is different 

 from the phenolthalein neutral point used in 4, b. The litmus 

 neutral point is 2.5% more acid than the phenolthalein neutral 



