July 10, 1917 A Substitute for Litmus for U^e in Milk Cultures 107 



The range of Pg within which bromcresol purple exhibits its color 

 changes is well suited to the Pg values of milk cultures, which is not 

 entirely true of litmus and azolitmin. There are found in the literature 

 various directions for "adjusting the reaction" of milk in order to bring 

 out a favorable color with litmus or azolitmin. 



Litmus is seldom pure, and even azolitmin has been reported to be of 

 uncertain composition. Bromcresol purple can be obtained in crystalline 

 form. When one considers the relatively high concentrations in which 

 litmus must be used and the very high dilutions in which bromcresol 

 purple is serviceable, it is evident that, if the introduction of impurities 

 is to be avoided, the advantage of bromcresol purple is great. 



The impurity in a litmus preparation often changes the Pq of the milk 

 to which it is added. Because of considerable variation in the quantity 

 of impurity, it is difficult to obtain the same color in different batches of 

 litmus milk even when the milks, the actual concentration of dye, and 

 the time and temperature of sterilization are constant. Since fresh 

 milks are fairly constant in their initial Pq, while litmus preparations 

 have variable neutralizing power. Dr. P. Rupp, of the Dairy Division, 

 has found it advisable to adjust the neutralizing power of the litmus 

 solution rather than to attempt any adjustment of the Pg of the 

 milk either before or after the addition of the litmus solution. By this 

 procedure he has been able to increase very materially the reproduci- 

 bility of the color in litmus milk. 



Bromcresol purple is obtainable now in very pure form, and the 

 ability of a low concentration of its sodium salt or of the acid itself 

 to change the Pg of milk is practically nil; consequently, when it is 

 added to milk in the concentration recommended, no adjustment of the 

 dye solution, of the milk, or of the mixture is necessary. 



Particular attention should be called to the fact that milk is not suited 

 to accurate estimations of its hydrogen-ion concentrations by the colori- 

 metric method. This is chiefly because the turbidity of milk is so intense 

 that it can not be compensated for in making the comparisons with the 

 clear, colorless standards. There is also a probable ' ' protein ' ' error. None 

 of the methods which we have successfully applied to other colored and 

 turbid-culture media has proved to be very successful when applied to 

 milk. These considerations reduce but do not wholly destroy the value 

 of comparative measurements. Even though a definite Pg value can 

 not be assigned to the reaction in any particular culture, the direction 

 of the fermentation and, roughly, its intensity can still be determined. 



A much more serious aspect of the subject is the considerable change 

 in Ph which occurs when milk is sterilized and the consequent difficulty 

 in reproducing a particular initial color in different batches. Quite 

 aside from the temporary reduction which occurs when litmus milk is 

 sterilized, there is a permanent color change which must be ascribed to 

 the change in the hydrogen-ion concentration of the milk. A similar 

 change in color will be observed when milks containing bromcresol 



