56 
Exercise 28. Milk Fermentation 
Acid Fermentation Caused by Aerobacter aerogenes group 
MATERIALS: 
a. 1 tube of plain broth 
b. 1 Durham tube of lactose broth 
C. 1 Durham tube of saccharose broth 
d. 1 Durham tube of glycerin broth 
e. 1 fermentation tube of dextrose broth 
f. 1 lactose agar slope 
g. 3 tubes lactose agar for plates 
h. 1 tube gelatin 
i. 1 small tube of plain milk 
j. 1 small tube of litmus milk 
k. 1 large tube of plain milk 
l. 4 sterile Petri dishes 
m. Ice water for gelatin 
n. Culture of Aerobacter aerogenes 
0. Outfit for determination of acidity 
A number of acid-forming bacteria also produce gas and undesirable flavors 
and odors in milk. On account of the second characteristic, their presence is 
very undesirable in milk, especially in that which is to be used for the manu¬ 
facture of butter and cheese. 
One of the organsims occuring in the intestines of cattle produces acid and 
gas from most sugars. This organism is A erobacter aerogenes. Because manure 
so frequently gets into milk, most milk under favorable temperature condi¬ 
tions will show gas formation. 
1. Label each of the culture tubes and the Petri dishes with the number of 
the exercise and the letter indicating the kind of culture medium. 
2. Prepare loop dilution plates for colony formation. Label 3 sterile Petri 
dishes 1, 2, and 3. Melt three tubes of lactose agar and cool them not lower 
than 45° C. Label 1 tube number 1, one number 2, and the remaining one 
number 3. Transfer one loopful of the culture to the agar tube number 1. 
Mix the organisms and agar well. Transfer 3 loopfuls of the agar and organ¬ 
isms from tube number 1 to tube number 2. Mix well. Transfer 4 loopfuls 
from tube number 2 to tube number 3. Pour the agar in tube number 1 into 
Petri dish number 1, and the agar in tube number 2 into Petri dish number 2, 
and that in tube number 3 into Petri dish number 3. This gives a dilution of 
the organisms and enough scattered colonies on one of the plates so that the 
colonies may be studied in detail. 
4. Make a stab culture in the gelatin by thrusting the inoculated straight 
wire into the center of the solid medium nearly to the bottom of the test tube. 
Be sure the wire is perfectly straight. 
5. Inoculate all of the remaining tubes of media by means of the wire loop. 
6. Incubate the gelatin culture in the ice box. Incubate all of the other 
cultures 48 hours in the incubator. 
7. Record the morphological, cultural, and biochemical properties of the 
organism according to the outline in Appendix A. The description should be 
recorded on the regular note paper following the order given below: 
