92 MILK 



Procedures to determine the Manurial and General 



Bacterial Contamination of Milk. 



The above considerations show that the sources whereby 

 bacteria gain access to milk are very numerous and diverse 

 while they are operative very unequally, according as cleanliness 

 conditions are or are not practised. 



To gauge this general bacterial contamination, the following 

 estimations have to be considered. 



A. Estimation of the Number of Bacteria. 



The estimation of the total number of bacteria in milk is 

 impossible, and it is necessary to select an arbitrary basis for 

 enumeration. There are no known nutrient media, and no 

 known conditions of growth which will allow all the bacteria in 

 milk to develop. All we can say is that some media and some 

 conditions are more favourable to the growth of a larger number 

 of the bacteria in milk than are others, and by their employment 

 a higher count is obtained than by the use of less favourable 

 media and conditions. The bacterial count being relative, not 

 absolute, any enumeration method for the bacteria in milk should 

 aim not so much at obtaining the largest count, but one which 

 shows best the evidences of manurial pollution. Some investi- 

 gators use nutrient gelatine, others nutrient agar, others whey 

 agar, etc. In the writer's opinion nutrient agar (of + 1 reaction), 

 with the plates incubated for forty-two to forty-six hours at 37 C, 

 is the most convenient and satisfactory, but gelatine plates grown 

 for three days at 20 to 22 C. are used by many. 



With unknown samples an extended series of dilutions will 

 have to be employed for plating in order not to have overcrowded 

 plates. A rough idea of the number of bacteria in a given milk 

 sample, and so a guide obtained as to the dilutions to plate, 

 can be ascertained from an examination of the stained centri- 

 fugalized deposit. 



In milk examinations involving bacterial estimations, the 

 presence or absence of preservatives should be ascertained, as if 

 present they will have an inhibitory or retarding action on the 

 number of bacteria, and a stale milk so treated may show a low 

 bacterial figure. 



