88 EXPERIMENTAL DAIRY BACTERIOLOGY 



exposure is made. Where the vegetative, growing organism is 

 subjected to heat, it is killed at approximately a scalding 

 temperature, varying from 60 to 80 C., depending on the 

 period of exposure. Endospores cannot be destroyed unless 

 the temperature is raised to the boiling point or above, and 

 the exposure continued for a considerable time. No process 

 under commercial conditions completely destroys all traces 

 of germ life, so that subsequent chilling of the heated milk 

 is necessary in order to prevent rapid growth of the remain- 

 ing cells. 



In condensed milk the main factor accounting for its keep- 

 ing qualities is the concentrated nature of the liquid. By the 

 application of heat in a partial vacuum a portion of the water 

 is evaporated. The concentration of the milk is further in- 

 creased by the addition of cane sugar. The keeping quality 

 is thus dependent upon the same condition that is found in 

 the case of sirups, which " work," i.e. ferment, unless suffi- 

 ciently concentrated. By heating during the evaporation many 

 of the bacteria are killed. Those remaining are unable to mul- 

 tiply until the milk is diluted with water. 



Exercise. EFFECT OF CONCENTRATION ON BACTERIAL GROWTH. 

 Place about 2 cc. of sweetened condensed milk in each of two sterile 

 test tubes. To one add 10 cc. of sterile water. Inoculate both tubes 

 with the same amount of a culture of a digesting organism, and note 

 changes that occur. 



Effect of temperature on bacterial growth. The three 

 temperatures which are usually employed for storage of 

 milk under practical conditions are 10 C. (the temperature 

 of an ordinary ice box), 20 C. (room temperature), and 30- 

 37 C. (temperatures approximating blood heat, as when a 

 large can of milk fresh from the cow is allowed to stand 

 without cooling). 



