PUKE MILK COMPARED WITH MARKET MILK 279 



Apparatus. Three sterile 1 liter Erlenmeyer flasks; 

 one sterile 2 liter Erlenmeyer flask; twenty-four sterile 

 Petri dishes; sterile 10 c.c. pipettes; sterile 1 c.c. pipette 

 graduated to 0.1 c.c.; twenty-four tubes of litmus lactose 

 agar; 90 c.c. and 99 c.c. dilution flasks; ice and salt for pre- 

 paring freezing mixture; fresh milk and bottled milk. 



Method. 1. In a sterile 2 liter Erlenmeyer flask place 

 about 1 J liters of milk from a can of milk immediately after 

 it has been filled by the milkers. 



Note. This exercise is to be repeated, for purposes of comparison, 

 using three pint bottles of milk all obtained at one time from the 

 same milkman. In this case the first plating is to be made from 

 each separate bottle. 



2. Record the temperature. Plate from the sample 

 immediately in litmus lactose agar, making dilutions of 

 1 : 100 and 1 : 500. Determine the acidity of the sample, 

 using a sterile 5 c.c. pipette to obtain the sample. 



Note. Portions for acidity determination and plating should be 

 removed with sterile pipettes in all instances. 



3. Transfer the sample " aseptically " into the three 

 1 liter flasks, placing an equal portion as nearly as possible 

 in each flask. Label the flasks A, B, C. 



4. Cool flask A in a freezing mixture to 10 C., and set 

 away in refrigerator to maintain the low temperature. 



5. Cool flask B in a freezing mixture to 10 C., then place 

 it at a constant temperature of 21 C. 



6. Place flask C at a constant temperature of 21 C. 



7. At the end of twenty-four hours determine and record 

 the acidity of each of the three portions of the original 

 sample. 



8. Plate in litmus lactose agar, using the following 

 dilutions : 



Flask A, 1 : 100 and 1 : 1,000. 



Flasks B and C, 1 ; 10,000 and 1 ; 1,000,000. 



