BACTERIA IN MILK 179 



the plate method in certainty, and the latter should always 

 be employed where it is at all possible. 



EXAMINATION OF MILK AND ITS PEODUCTS 



Methods. Milk may be contaminated either during 

 milking or in the subsequent manipulation, and may exhale 

 an odour owing to the substances so acquired, or become 

 slimy or stringy, or take on a bitter or an acid taste, and 

 its colour may turn blue or red. Moreover, various patho- 

 genic micro-organisms may be imparted to milk from the 

 animal furnishing it. To examine, Arens recommends that 

 a loopful of milk be diluted with another of distilled water 

 upon a cover-glass, dried, and fixed by heat, which should 

 not be too strong. The cover-glass so treated is now 

 brought into chloroform methyl blue, prepared by mixing 

 twelve to fifteen drops of saturated alcoholic solution of 

 methyl blue with three or four cubic centimeters of chloro- 

 form. It is then waved to and fro for from four to six 

 minutes, so as to allow the chloroform to evaporate, and 

 the adherent methyl blue is rinsed away. In fresh milk and 

 in cream the bacteria alone are dark blue ; in curdled milk 

 the flakes of caseine are also stained, but only a pale blue. 



Another method of examining milk bacteriologically 

 consists in treating a drop placed upon a cover-glass with 

 two or three drops of a one per cent, solution of sodium 

 carbonate, the fluids being mixed as thoroughly as possible 

 with the platinum needle, after which the cover-glass is 

 carefully heated over a small flame until evaporation is 

 complete. The result of this process is saponification of the 

 fat, so that the cover-glass appears overlaid with a thin film 

 of soap, and the preparation so treated can be submitted to 

 the ordinary staining processes. 



Bacillus lacticus (Bacillus acidi lactici). Pasteur pointed 



