422 



MILK 



to counting bacteria in milk. This method consists of collecting 

 the sediment of a definite quantity of milk by centrifugation and 

 smearing the sediment over a surface of about 4 sq. cm. with 

 the aid of a drop or two of sterile water. The sediment is stained 

 with methylene-blue after having been dried with gentle heat. 

 Bacteria can be counted under the microscope, and the method 

 is sufficiently accurate to eliminate sam- 

 ples which are below the standard. 

 When, however, the milk contains more 

 than the permissible mmiber, as deter- 

 mined by the microscopic examination, 

 plate counts should be made to confirm 

 the direct count. 



Prescott and Breed worked out a 

 method for enumerating cells in milk 

 by direct microscopic observation, and 

 Breed applied a similar method for 

 enumerating bacteria. This is de- 

 scribed by Brew and is carried out as 

 follows: 0.01 c.c. of milk is taken di- 



Fig. 188. — Nivellating apparatus 

 for setting agar on little plates in a 

 film of uniform thickness. (Frost, 

 in Jour. Amer. Med. Assoc., vol. 66, 

 1916.) 



Fig. 189. — Convenient foitns of 

 capillary pipets. (Breed and Brew, 

 Tech. Bull. 49, New York Agric. Exp. 

 Sta., 1916.) 



rectly from a well-shaken sample by means of a specially gradu- 

 ated pi^et (Fig. 189). The drop of milk is deposited on a clean 

 glass slide and spread over an area of 1 sq. cm. with a stiff, straight 

 needle. Duplicate smears of each sample are made on the same 

 slide. The milk is then dried by gentle heat which is obtained 

 over a level wooden surface placed on a steam radiator. The 



