84 General Bacteriology. 
EXERCISE LXX. QUANTITATIVE ANALYSIS OF MILK. 
. Obtain a sample of milk in a sterile vessel. 
. Dilute milk 1000 times with sterile water. 
. Make plates as under soil (LX VIII). 
. Count colonies and estimate number of bacteria per cc. 
a we 
a 
EXERCISE LXXI. EFFICIENCY OF PASTEURIZATION, 
a. Place same milk as used in previous experiment in the bottles of a pasteurizing 
apparatus, such as Freeman’s, and pasteurize as per printed directions, or place the milk 
in ordinary milk bottles or fruit jars, filling 
to a uniform level; these are then to be 
placed in a flat bottomed pail which is to be 
filled with water and heated to71° C.(160° F.), 
remove source of heat, cover and allow to 
stand 30 minutes. Remove bottles and cool 
as quickly as possible without danger to glass. 
b. Determine bacterial content of pas- 
teurized product by making plates. A dilu- 
tion of 100 will probably be sufficient. Ex- 
press results so as to indicate per cent. of or- 
ganisms destroyed by the process. Compare 
the keeping qualities of the pasteurized pro- 
duct with that of the raw milk by keeping 
samples of both under similar conditions, . 
e. g. in locker or ice chest, making frequent Fic. 17. A home-made pasteurizer (Russell.) 
observations. 
Pasteurized milk should not have a permanently cooked taste. 
REFERENCES. Bull. Wis. Exp. Station No. 44. Russell, Outlines of Dairy Bacte- 
riology, 95 (4th Edit.). 
EXERCISE LXXH. TESTING ANTISEPTIC ACTION OF CHEMICALS. 
GENERAL DIRECTIONS. 
a. Fill a number of test-tubes with a measured quantity of agar (5 cc.). 
b. Add to the agar varying but measured amounts of the substance to be tested. If 
the antiseptic is not volatile, or affected by heat, sterilize. 
ce. Inoculate the tubes thus prepared, together with a control, with B. colt and make 
rolls. : 
d. Keep these cultures under observation in the 28° C. incubator. 
e. If no growth appears within 96 hours repeat the experiment, using smaller amounts 
of the antiseptic. In this way determine the amount of chemical (in %) which just pre- 
vents growth. 
SpeciaL Directions. Test in this way carbolic acid (5 %), aleohol (95 %). 
REFERENCES. A. 566; H. 411; N. 527; 8, 156. 
