ROUTINE PROCEDURE IN MILK EXAMINATION 75 



Routine Procedure in the Systematic Examination 



OF Milk 



Physical examination (temperature, reaction, colour, cream, 

 deposit, specific gravity, etc) of the milk should be made if necessary. 

 The microscopical examination of the milk before and after centri- 

 fugalisation or sedimentation will likewise often yield useful 

 results. 



1. Plate cultivation. — Dilute as required {see p. 49) and make 

 plate cultivations in Petri dishes or flat-bottomed flasks. Six or 

 more gelatine plates should be made and incubated at room 

 temperature. Plates should also be made with nutrient agar for 

 incubation at 37^ C. Other media may also be selected. 



The plates should be counted on the second, third, and fourth 

 days, and the necessary subcultures made. Agar plates incubated 

 wholly at 18' or 22^ C, will in the long run show more colonies 

 than when incubated at 37° C. and then at 22° C, or at 37"^ C. 

 throughout. 



2. Anaerobic cultivation. — At the same time that the primary 

 aerobic plate cultivations are made, similar plates should be made 

 on lactose-gelatine and lactose-agar for anaerobic culture (see 

 p. 83 et seq.). 



3. Primary tube cultivation. — Take ten tubes of 10 c.c. of the 

 milk under examination and place three of them in the incubator 

 at room temperature and three of them at 37"^ C. Place four of 

 them in a water bath heated to 80° C. for fifteen minutes, and then 

 enclose each of the four tubes in a Buchner's tube or vacuum desic- 

 cator (p. 89). These primary cultures may be tested in forty-eight 

 hours for B. coli, the presence of indol, and B. enteritidis sporogenes. 



4. Secondary or subcultures. — From the primary cultivations 

 make subcultures on selected media for the isolation of organisms 

 making their appearance on the plates, or what is often preferable 

 make a separate set of plates for qualitative examination only, 



5. Examination for special micro-organisms. — The milk must be 

 centrifugalised or the particulate matter allowed to gravitate by 

 sedimentation. It is, as a rule, useless to attempt examination 

 microscopically or otherwise without first using the centrifuge or 

 sedimentation flask. The deposit is then to be stained for the 

 particular organism for which search is being made. 



For centrifugalisation take two or three samples of the milk 

 under examination to the amount of about 40 c.c each and place 



