XII 



CHEMICAL TESTS OF MILK 



207 



M 



r hydrogen peroxide, and from O'l to 1 c.c. of the peroxidase 



reagent (i.e. phenol-phthalein, guaiacum, or paraphenylenedia- 

 mine). They obtained much more reliable results with the 

 addition of the sensitising agent. 



Eaw milk gives a positive reaction with these tests at 

 once ; milk heated to boiling gives no reaction. The exact 

 temperature at which the ferments are destroyed so that no 

 reaction is obtained is usually given at about 80 C., but 

 slightly different results are recorded by different workers. 

 The killing of the enzymes depends not only upon the tem- 

 perature, but also upon the duration of heating, although the 

 temperature reached is the most important factor. 



The following table shows results obtained by the writer 

 as the mean of a number of experiments with different samples 

 of milk, using the above four tests : 



=no reaction. 



+ = a positive reaction. 



+ a = positive reaction very slightly delayed. 



+ b = after about 30 seconds. 



p = a, partial and indefinite reaction: no reaction at first, but after 

 standing a few minutes some colour alteration occurs, but not the 

 true tint. 



Experiments were also carried out to ascertain what pro- 

 portion of raw milk must be present, added to the cooked milk, 

 to cause the mix tare to give the reactions of raw milk. A 

 quantity of milk was heated to 80 C. for five minutes, and 



