134 



GENERAL BACTERIOLOGY 



ties or fruit jars, filling to a uniform level; these are then to be 

 placed in a flat bottomed pail 

 (Fig. 29) which is to be filled 

 with water and heated to 71° 



C. (160° F.). Remove source 

 of heat, cover and allow tc 

 stand 30 minutes. Remove 

 bottles and cool as quickly as 

 possible without danger to 

 glass. 



Fig. 23. Home-Made Pasteurizers. 



6. Determine bacterial content of pasteurized product by mak- 

 ing plates. A dilution of 100 will probably be sufficient. Express 

 results so as to indicate per cent, of organisms 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 observations. 



Pasteurized milk should not have 

 a permanently cooked taste. 



References. H. 485 ; Wis. Exp. 

 Station Bull. No. 44 and 18th. An. 

 Rept. 185. Russell, Outlines of Dairy 



The Freeman Pasteurizer. Bacteriology, (5th Edit.) 113. 



Fi(, iO 



EXERCISE 72. TESTING ANTISEPTIC ACTION OF CHEMICALS. 



General Directions. 



a. Fill a number of test-tubes with a measured quantity of agar 

 (5cc). 



b. Add to the agar varying but measured amounts of the sub- 

 stance to be tested. If the antiseptic be not volatile, or affected by 

 heat, sterilize. 



c. Inoculate the tubes thus prepared, together with a control, 

 with B. coJi or M. pyogenes and make rolls. 



d. Keep these cultures under observation in the 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 prevents growth. 



Special Directions. Test in this way carbolic acid (5%), 

 alcohol (95 %). 



References. A. 619 ; H. 506 ; M. & R. 140 ; McP. 248. 



