114 BACTERIA AND FOOD 



stroyed by heat (Fig. 48). Uncooked water contains numerous 

 active bacteria, but well boiled water contains no living bac-i 

 teria. 



If you wish to prove that strong heat destroys bacteria, fill 

 two sterilized test tubes half full of milk from the same can. 

 Put one of the test tubes in a bath of boiling water and 

 allow it to remain there for five minutes or longer; then re- 

 move it and plug it with cotton. Plug the test tube of unheated 

 milk also, and set both tubes in a warm place side by side. 

 Within a short time, sometimes less than an hour, the unheated 

 milk will sour, but the heated milk will remain in good condition 

 for hours or even a few days. The unheated milk contains 

 bacteria which at the warm temperature multiply rapidly 

 and sour the milk ; the heated milk contains few or no living 

 bacteria and hence remains sweet even in a warm temperature. 

 But the heated milk does not remain good permanently; be-* 

 cause while the heat was strong enough to kill all of the bac- 

 teria, it was not continued long enough to kill the spores and 

 to protect the milk against the bacteria which develop from 

 them when the heating is over. Spores are always harder to; 

 kill than bacteria, and unless the heating is strong and prolonged 

 they escape death and grow and multiply when the heated food , 

 cools. 



It is important to kill the spores as well as the bacteria in 

 fruits and vegetables which are to be canned. The bacteria 

 which grow on corn have spores which are very difficult to kill, 

 and unless corn is boiled an hour or more these spores escape 

 death and later develop colonies of bacteria which spoil the 

 corn. The bacteria which grow in apples, peaches, plums, and 

 most fruits have spores which are killed by a few minutes of 

 vigorous boiling, and hence the canning of these fruits seldom 

 troubles the housewife. 



Strong prolonged heat kills the bacteria which are in the 



