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 number, 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- 



w 



2 



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 forms 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 pipet (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 



