Exercise 26. Milk Contamination 
50 
MATERIALS: 
10 tubes of lactose agar 
10 sterile Petri dishes 
1 tube of sterile milk 
3 9cc. water blanks 
4 99cc. water blanks 
Case of pipettes 
Sample of aseptic milk 
Cow hairs 
A 1 to 1000 suspension of manure 
Small pail containing skimmed milk 
PART I. 
Contamination from the Interior of the Udder, Aseptic Milk, 
The milk upon the desk was procured as follows: The cow’s udders were 
carefully cleaned. The milker’s hands were carefully washed in a disinfectant 
and the milk drawn in as nearly a dust free atmosphere as possible. The first 
streams of milk were throwm away and then the milk was drawn as carefully 
and quickly as possible into a sterile flask. In this manner the danger from 
outside contamination was reduced to the minimum. 
1. Melt two tubes of lactose agar and cool to 45° C. Dilute 1 cc. of the milk 
in a 9 cc. water blank and prepare one lactose agar plate, using 1 cc. of the 
dilution. Dilute 1 cc. of the milk in a 99 cc. water blank. Prepare a lactose 
agar plate, using 1 cc. of this dilution. Be sure that each one of the plates is 
labeled with the dilution, name of the milk, and the desk number. Incubate 
the plates, inverted, in the incubator one week. 
2. Set the sample of milk away in the locker and allows it to incubate for 
the same length of time as the plates. 
3. Observe the number of colonies that have developed on the plates. 
Tabulate the results on the regular note paper as in Table VII. 
Table VII.—Bacterial Contents of Aseptic Milk 
Dilution 
Colonies 
on plate 
Bacteria 
per cc. 
Number and 
types of kinds 
PART 2. 
Contamination from Cow Hairs 
1. Melt a tube of lactose agar. Pour the agar into a Petri dish and allow 
it to solidify. 
2. When the agar is solid, with sterile forceps carefully remove two or three 
cow hairs from the dish upon the desk and place them upon the surface of the 
agar. Make sure that the hairs are in contact with the surface of the culture 
medium for their entire length. Press them dowm with the sterile loop. Be 
careful not to break the surface of the medium. 
