Exercise 30. Milk Preservation 
61 
MATERIALS: 
6 sterile test tubes 
Sample of raw skimmed milk 
It is very desirable to keep milk in as nearly the natural condition as possible 
until it is consumed. Bacteria are responsible for practically all of the changes 
that render milk undesirable for food purposes. With even the best methods 
of production, the contamination of milk is so great and the growth of bacteria 
so rapid that the milk becomes undesirable for food purposes in a short time. 
On this account methods for the preservation of milk are necessary. 
1. Fill each of 3 sterile tubes one-third full of sweet milk. 
2. Label the tubes and store one in the ice box, one in the desk and one in 
the incubator. Examine them at each laboratory period until the milk is 
curdled. Tabulate the time of curdling. 
3. Fill 3 sterile test tubes one-third full of sweet milk. 
4. Heat the tubes to 90° C. in water in the cup for 30 minutes. 
5. Place one of the tubes in the ice box. Place one tube in the incubator. 
Permit the other tube to remain in the desk. 
6. Examine the tubes after 24 and 48 hours. Tabulate the time of curdling 
on the regular note paper. See Table XI. 
TABLE XI.—Effect of Temperature and Pasteurization on the 
Quality of Milk 
Sample 
Treatment 
Time to curdle 
7 . 
(a) In what order did the unheated tubes of milk curdle? 
( b) What can you say of the temperature necessary to keep milk 
sweet for as long a time as possible? 
(c) Why does it remain sweet under these conditions? 
( d ) Which tube of milk in the heated set showed signs of curdling 
or spoiling first? 
