PREPARATION OF CULTURE MEDIA. 253 



outset, so as to have them on hand. Some of the cubic centi- 

 meter pipettes should be graduated to one tenth of a c.c., and 

 it is best to place them in a larger glass tube whose ends are 

 plugged with cotton, as shown in Fig. 38, in which the 

 pipettes can be kept after being sterilized. After this appa- 

 ratus has been sterilized it should be set aside where it can 

 be kept free from dust until needed. 



Preparation of Culture Media. Any of the methods of 

 preparation of culture media used in bacteriological labora- 

 tories are satisfactory for the purpose of milk analysis. The 

 method of making a satisfactory medium for quantitative 

 analysis is as follows : 



Peptone- A gar Culture Medium. 



A. In a large porcelain cooking dish place the following 

 ingredients : 



Water 500 liters. 



Peptone, dry 10 grams. 



Common salt 5 " 



Liebig's Extract of Beef 5 " 



Milk sugar 30 " 



These materials are to be mixed together and heated to a 

 temperature not above 150 F., which will bring the materials 

 into solution. There should then be added to the mixture a 

 sufficient amount of water to make up for that which is 

 evaporated in the heating. This may best be determined by 

 weight rather than by measure, weighing the dish with its 

 contents before heating, and then weighing again after the 

 solution is complete, adding water to make the weight equal 

 to the original weight. 



B. Cut into fine pieces 15 grams of agar and dissolve it in 

 500 c.c. of water. This will require considerable heat. After 

 the agar is dissolved cool to 150 C., add water to restore 

 evaporation, and mix with solution A. The mixture must 

 then be neutralized carefully. This neutralization is the most 



