juiyi6, I9I7 A Substitute for Litmus for Use in Milk Cultures 1 1 1 



as a good indicator should. This alone is a sufficient reason for recom- 

 mending that it be substituted for litmus in milk cultures and in similar 

 instances in which it is considered ad\'isable to have an indicator present 

 during a fermentation. 



SUMMARY 



The color changes which occur in litmus-milk cultures may be due to 

 changes in the hydrogen-ion concentration of the medium or to reduction 

 or even destruction of the dye. If it is the degree of acid or alkali fer- 

 mentation which is sought, it is advisable to use an indicator which will 

 not be affected except by a change in the hydrogen-ion concentration. 

 Dibromoorthocresolsulfonphthalein, for which the short name bromcresol 

 purple is suggested, fulfills this condition. 



Litmus undergoes a temporary reduction during steriUzation in the 

 presence of milk. Bromcresol purple does not. 



The coloring power of litmus is relatively weak; bromcresol purple in 

 very high dilution is useful. 



Ivitmus and azolitmin are indicators of uncertain composition; brom- 

 cresol purple is a definite individual compound obtainable in crystalline 

 form and therefore reproducible. Its cost is not excessive. 



The impurities of litmus preparations vary in their effect upon the Ph 

 of milk and often necessitate elaborate adjustment either of the litmus 

 solution, of the milk, or of the mixture if reproducible color is to be 

 obtained. Bromcresol purple, on the other hand, may be used with the 

 assurance that, if other conditions are constant, it will always produce 

 the same coloration. 



Some of the difficulty experienced in reproducing a particular initial 

 color with either indicator is shown to be due to the changes in Ph which 

 occur when milk is sterilized by heat. 



The comparative value of litmus and bromcresol purple in milk cul- 

 tures was tested with a variety of organisms. It was found that no 

 change in reaction could be observed with litmus which could not be 

 followed equally well with bromcresol purple. In many instances litmus 

 was rendered useless by reduction or destruction while bromcresol purple 

 continued to act as a true indicator of the hydrogen-ion concentration. 



