126 DAIRY TECHNOLOGY 



Reduction-Fermentation Test. This test was origi- 

 nated by Barthel of the Swedish Experiment Station and 

 is used as follows: To 20 .cubic centimeters of milk add 

 0.5 cubic centimeter methylene blue and 2.5 per cent satu- 

 rated alcoholic solution in water. Keeping the samples at 

 about one hundred degrees F., the rapidity of the reduc- 

 tion of the color, the change from blue to white, varies 

 directly as the number of bacteria in the milk. This re- 

 duction test is combined with the fermentation test by 

 keeping the samples until curdling takes place and noting 

 the time required to curdle and the presence of gas or bad 

 flavors in the curd. 



According to Barthel, if the milk contains several hundred 

 thousand bacteria per cubic centimeter, reduction will take 

 place within fifteen minutes. If the blue color disappears 

 within an hour, the milk is not first class. One to three 

 hours are required for the reduction in high-grade market 

 milk, but even a greater length of time is required for very 

 fresh, clean milk. 



Of course, this test is merely an indication of the numoer 

 of bacteria present, but it may be found very useful to the 

 dealer trying to buy a high-grade milk, or to the butter 

 or cheese maker, who wishes to guard against the evils of 

 over-ripe milk. 



