258 BACTERIOLOGICAL ANALYSIS OF MILK, 



Several of the small flasks marked to hold 99 c.c. are 

 rilled to this mark with water and placed in an autoclave for 

 sterilization. If it is desired to dilute the milk more than one 

 hundred times there should, at the same time, be placed in the 

 autoclave a number of the smaller vials filled with water to 

 the 19-c.c. mark and others to the 5-c.c. mark (Fig. 39). 

 All of these vessels of water are to be sterilized for an hour 

 at a temperature of 120 (a steam pressure of ten pounds 

 will do), after which they are to be removed'. If an auto- 

 clave is not at hand the water may be sterilized by steaming 

 for two hours. 



There is now taken from the milk to be tested a 

 single cubic centimeter in one of the sterilized pipettes. 

 Taking of the sample of milk is the most important point in 

 the analysis and most liable to introduce errors. The num- 

 ber of bacteria found in different parts of a can of milk is 

 by no means uniform, the surface layers containing different 

 numbers from the deep layers of the milk. To avoid this 

 irregularity it is necessary to give the milk a very thorough 

 stirring or shaking immediately before the sample is taken, 

 so as to distribute the bacteria as uniformly as possible. . 



A cubic centimeter of milk is transferred by a sterilized 

 pipette into one of the loo-c.c. flasks of sterilized water. 

 The mixture is then to be very thoroughly shaken, so as to 

 distribute the milk uniformly through the water. This 

 thorough shaking is extremely important to break up the 

 clumps of bacteria shown in Fig. 9, page 34. 



Meantime six test-tubes of the agar culture medium have 

 been melted by placing them in water over a gas flame. 

 The tube should be kept in warm water at a temperature just 

 sufficient to keep its contents from solidifying. A single cubic 

 centimeter of the mixture of milk and water is removed 

 with a second sterilized pipette and placed in each of the 

 test-tubes of melted culture medium. The test-tubes are then 

 to be gently but thoroughly shaken, so as to distribute the 



