QUALITATIVE ANALYSIS OF MILK. 27! 



METHOD OF QUALITATIVE ANALYSIS OF MILK. 



Dilution. In the use of these media the first thing to be 

 done is to determine upon the proper dilution of the milk. 

 The milk must always be diluted with sterilized water, but 

 the determination of the proper amount of dilution is the 

 most difficult point of the analysis, from the fact that different 

 samples will require different dilutions. The dilution should 

 be such that the plates which are to be made should contain, 

 if possible, from 100 to 300 colonies. The extent of dilution 

 varies with the age of the milk, with the temperature and 

 with the season. In cool winter weather with common milk 

 a dilution of 200 or 600 is commonly satisfactory. In warm 

 weather it must be higher, and it is always best to prepare 

 plates of more than one dilution. 



At the beginning of experiments there should be sterilized 

 a series of flasks marked to hold 99 c.c. of water, and some 

 small vials marked at 5 c.c. and at 19 c.c. There should also 

 be one or more large flasks of sterilized water at hand. The 

 first step is to dilute the milk with sterilized water. It is 

 always wise to make two different dilutions in order that the 

 chances of a proper dilution may be greater than if only one 

 should be made. In the study of market milk in ordinary 

 winter weather it is best to choose the two dilutions of 200 

 and 600. If the milk is old or the weather is warm the dilu- 

 tion must be 2,000-4,000, or even greater. Having selected 

 the dilution to be used, a single cubic centimeter of the thor- 

 oughly mixed milk is removed with a sterilized pipette and 

 placed in a flask containing 99 c.c. of sterilized water. This, 

 after thorough shaking, is to be further diluted according to 

 the amount of dilution required. The following table .will 

 show the method of obtaining different dilutions. 



A. First dilution, I c.c. of original milk in 100 c.c. of ster- 

 ilized water = a dilution of 100. 



1 c.c. of A placed in 5 c.c. of water gives a dilution of 600. 



2 c,c, of A in 4 c.c. of water gives a dilution of 300, 



