LABORATORY WORK 1N T DAIRY BACTERIOLOGY 2/9 



point, and it is desired to make the reaction 1.5%. To do this 

 several methods are possible, the simplest of which is as fol- 

 lows: To the litmus solution add the amount of HC1 necessary 

 to bring it to the phenolthalein neutral point, and then add 

 10 c.c. per liter of normal NaOH to bring it back to 1.5% . 



An actual example will make this clear. Suppose we start 

 with 500 c.c. and find that it requires 1.5 c.c., i/io normal HC1 

 to neutralize 5 c.c. 



1.5 -:- 5 = .3 x 495 = 148.5 c.c. i/io normal required for 

 495 c.c. of solution. 



148.5 _:_ 10 14.8 c.c. normal HC1. Add 14.8 c.c. to the lit- 

 mus solution. 



495 x Io 4-9- Hence, by adding 4.9 c.c. normal NaOH, 

 the litmus solution will be brought to the desired reaction, 1.5% 

 acid. Since this solution and the gelatin made in a are of the 

 same degree of acidity, they may be mixed in any proportion 

 without changing the reaction. 



e. Sterilize the litmus solution in the same way as the gelatin. 

 The method of combining the gelatin and litmus will be men- 

 tioned below. 



No. 9. Litmus Agar. This is prepared by adding to ordinary 

 a ar (5) l % m lk sugar and enough litmus solution to give it 

 a blue color. It has some advantages over plain agar, since it 

 differentiates acid colonies. It is not so useful as litmus gelatin. 



*No. 10. Gelatin Plates. Gelatin culture media offer a num- 

 ber of advantages over agar, since they enable us to tell more 

 satisfactorily the kinds of bacteria which may be present in the 

 milk. It is more difficult to use, however, and results more 

 frequently in failure, for reasons to be noted presently. 



In making gelatin plates from milk, it is desirable to dilute 

 the milk to such an extent that the number of bacteria in each 

 plate shall be not more than 500 or 600, preferably not more 

 than 200. If the numbers are higher than these, there is likely 

 to be trouble and inaccurate determinations. For this reason, 



