Bacteria in Market Milk. 213 



this amount. This test is also useful in the selection of 

 milk or cream that is to be used for special purposes, 

 such as pasteurization. 



3. Alcohol test. A test giving similar information is 

 made by adding two parts of 70 per cent alcohol to one 

 part of milk, and noting whether curdling occurs. 



4. Curd test. The curd test described on p. 100 gives 

 no indication of the number of bacteria present, only 

 concerning the types present. It has been proposed to 

 combine the fermentation test with the reduction test 

 referred to below and thus gain some idea of, not only 

 the number, but the kinds of bacteria present. 



5. Reduction test. The reduction test is made by 

 adding to twenty cubic centimeters of milk, one-half 

 cubic centimeter of a solution of methylene blue, a coal 

 tar dye. A saturated solution of the dye is made in 

 alcohol, and 2.5 per cent of this solution added to water. 

 The time required for the reduction of the dye or the 

 change of the color from blue to white when the samples 

 are placed in tubes and kept at 98 to 100 F., is depend- 

 ent upon the number of bacteria present. By allowing 

 the tubes to stand until curdling occurs, and noting the 

 nature of the curd, whether the solid curd of the de- 

 sirable acid-forming bacteria or the gassy curd of the 

 harmful types is produced, knowledge is gained of the 

 kinds of bacteria present. 



According to Barthel, milks that reduce the methylene 

 blue within fifteen minutes contain hundreds of thou- 

 sands of bacteria per cubic centimeter. Those that re- 

 quire from fifteen minutes to one hour for the disappear- 

 ance of the color are also high in bacteria, and are to 

 be classed as a poor grade of market milk. If one to 

 three hours is required, the milk is comparatively low in 

 bacteria, and is to be classed as a good grade of market 



