FACTORS INFLUENCING BACTERIA 65 



freezing. A very interesting condition is often found in natural 

 ice filled with air bubbles. On examining the ice surrounding 

 the air bubble it will be found to contain many more bacteria than 

 does the rest of the block. And curious enough, these organisms 

 are the oxygen-loving ones which have been attracted to this 

 region by the air and then caught on the congealing of the water. 



The bacteriologist recognizes what he calls a thermal death- 

 point. This is that temperature which under given conditions 

 will kill an organism in a given time. It is evident to all that a 

 living organism may withstand a comparatively high temperature 

 for a short time, whereas a much lower temperature will kill in 

 a longer time. The time, therefore, is usually taken as ten 

 minutes. 



One can place his hand in an oven at a temperature of 130° 

 C. for a short time, whereas if placed for this same length of 

 time in steam, the temperature of which is 100^ C, a painful 

 burn may be the result. This is due to a number of factors: (i) 

 Moist heat is more penetrating than is dry heat. (2) The steam 

 on changing to water liberates large quantities of heat. (3) 

 Death is probably due to the coagulation of the proteins which 

 compose the bacterial body, and the temperature at which albu- 

 men coagulates varies with the quantity of water present, as may 

 be seen from the following: 



Egg albumen + 50 per cent of water coagulates at ^6° C. 



Egg albumen + '^S P^^ c^"* °^ water coagulates at 74- 80° C. 

 Egg albumen + 18 per cent of water coagulates at 80- 90° C. 

 Egg albumen + 6 per cent of water coagulates at 145° C. 



Egg albumen + o per cent of water coagulates at 160-170° C. 



That is, we may heat dry egg albumen to the ignition poipt 

 without causing it to coagulate. It is evident that the thermal 

 death-point for microorganisms in water would be much lower 

 than for the same microorganism in dry air. 



The housewife cans strawberries, raspberries, plums, and other 

 acid fruits with considerable ease, but corn and peas give con- 

 siderable trouble. One is acid and the other neutral, and mi- 



