XII CHEMICAL TESTS OF MILK 205 



Smidt suggested that this estimation might be used to 

 judge the degree of bacterial contamination of milk samples. 

 The test has been investigated, as regards its ability for 

 this purpose, by several workers with somewhat variable 

 results. The writer's own results ^ show that very broadly 

 there is a relationship between reduction time and the fresh- 

 ness of milk. With but one exception, fresh milk samples 

 always took 7 minutes or longer to reduce the blue at 50° C, 

 while milk samples incubated for 24 hours at 15° C. or 

 20° C, with but few exceptions, always took less than 

 7 minutes to reduce. 



This information, however, is not very valuable, and 

 as more reliable indications can be obtained by other tests, 

 is not worth obtaining as a gauge of present bacterial 

 pollution. 



To estimate, place 1 c.c. of each of the milk samples in clean 

 test tubes. To each add 0"5 c.c. of a solution consisting of 

 5 c.c. saturated alcohohc methylene blue solution, 5 c.c. form- 

 aldehyde and distilled water to 200 c.c. The milk and 

 methylene blue solution is well mixed, and the tubes at once 

 placed in the inner receptacle of a double water bath kept 

 accurately at 50° C. The tubes are watched, and the time 

 taken to discharge the blue colour carefully recorded. Difficulty 

 is sometimes experienced owing to the end point not being 

 always sharply defined. 



The Detection oe Heated Milk 



While boiled milk has a definite and peculiar taste, the 

 heating of milk to temperatures somewhat short of boiling 

 does not alter either its appearance or taste. Ordinary 

 pasteurisation cannot therefore be detected by inspection or 

 tasting. There is no legal obligation upon milk sellers to 

 label or otherwise declare if their milk has been pasteurised. 

 It is of value to ascertain if milk has been pasteurised, and 

 if bacteriological standards are made use of in sampling milk, 

 it becomes of great importance to be able to detect whether 

 heat has been applied to the milk. 



A number of tests have been introduced to detect cooked 



^ Loc. cil. 



