59 
Exercise 29. Milk Fermentation 
Acid Fermentation Caused by Lactobacillus Butgaricus Group 
MATERIALS: 
Raw skimmed milk 
1 125 cc. bottle, cork, and string 
Outfit for determination of acidity in milk 
There are present in ordinary milk acid-forming bacteria which develop 
only under certain conditions. If milk is kept tightly stoppered to prevent 
mold growth, and held at a comparatively high temperature, it will become 
very acid due to the growth of certain bacteria usually termed the Lacto¬ 
bacillus Bulgaricus group. 
1. Fill a 125 cc. bottle to the neck with raw skimmed milk. Cork tightly 
and secure the cork with a string as shown by the instructor. Label the 
bottle. Place in the incubator at 37° C. 
2. At the end of 2, 7, and 14 days, open the bottle and examine the milk as 
to odor, acidity (by titration), and type of organism present. Microscopic 
preparations should be made after thorough shaking of the bottle. Some of 
the curd should be spread as thinly as possible on the slide. Methylene blue 
is a good stain to use. 
3. Tabulate the observations on the regular note paper, showing the time, 
acidity, and morphology of the organisms present. See Table X. 
TABLE X.—The Development of High-Acid Organisms in Milk 
Incubation period 
Drawing of 
organisms present 
Acidity of milk 
2 days 
7 days 
♦ 
14 days 
4 . 
(a) How does the final acidity of the milk compare with that 
produced by Streptococcus lactis and Aerobacter aerogenes. 
( b ) What type of organism finally predominates? 
(c) Is the flavor of the milk desirable or undesirable? 
(d) What is the significance of the organism in fermented milk 
drinks? 
