ROUTINE TESTS FOR THE IDENTIFICATION OF BACTERIA 



165 



ACID PRODUCTION IN MILK 



Acid production in milk maj^ be determined verj^ simply, but the 

 opacity of the milk must be taken into account if accurate determinations 

 are desired. The milk must be considerably diluted before adding indi- 

 cator for comparison with a buffer standard. 



Indicator milk is often useful. Litmus has been used most frequently, 

 as it indicates reduction as well as pH changes (although roughly) . Neu- 

 tral litmus milk (about pH 6.8) has a lavender color, which becomes red 

 with acid production or blue with production of alkalinity. Reduction is 

 indicated by a partial or complete fading of the color. The use of litmus 

 milk has been seriously criticized because of the inaccurate nature of 

 litmus as a pH indicator; nevertheless the differences it brings out have 

 enough practical value so that it has not yet been superseded by any 

 other indicator in milk. 



The use of bromcresol purple, as was recommended by Clark and Lubs 

 (1917), does not show changes in 0/R potential. 



It is possible to recognize the five degrees of acidity listed in Table 16 

 by the use of bromcresol purple (either in the milk before inoculation or 

 added after incubation), the subsequent addition of bromphenol blue, 

 and observation as to the presence of curdling. This is only a rough 

 method of measurement, but in the routine study of milk cultures it will 

 often be found valuable. 



H. C. Brown (1922) proposed condensed milk diluted with 4 parts of water contain- 

 ing phenol red. The reaction is adjusted by addition of alkali until first appearance 

 of a brick red. Subsequent changes of reaction in either direction can be observed. 



RENNET PRODUCTION 



The production of the enzyme rennet (lab) can sometimes be recognized 

 in litmus milk by noticing the occurrence of coagulation without the 



