THERMAL DEATH POINT 



99 



That moisture is essential for coagulation of proteins is illustrated 

 by the following table from the work of Hiss and Zinsser: 



Egg albumen in dilute aqueous solution coagulates at 56 C. 



" with 25 per cent, water coagulates at 74-80 C. 



" " 18 " 80-90C. 



" 6 145 C. 



Absolute anhydrous albumin, according to Haas, may be heated 

 to 170 C. without coagulation. 



Moreover, moist heat is much more penetrating than is dry heat. 

 This is illustrated by an experiment carried out by Koch and his 

 associates. Small packages of garden soil were wrapped with vary- 

 ing thicknesses of linen with thermometers so placed that the tem- 

 perature under a definite number of layers could be determined. 

 These were exposed to hot air and steam for four and three hours, 

 respectively, with the following results: 



The comparatively low specific density of the steam enables it to 

 displace the air from the interior of materials. Furthermore, when 

 the steam comes in contact with the substance to be sterilized it 

 condenses with a liberation of heat. This in the case of water vapor 

 amounts to 536.6 calories. 



Although the spores of certain bacteria of the soil can withstand 

 live steam for several hours, they may be destroyed in a few minutes 

 or even instantaneously in compressed steam ranging in temperature 

 from 120 to 140 C. * 



The germicidal action of the great majority of disinfectants is 

 due to a chemical reaction taking place between the protoplasm of 

 the bacterial cell and the germicide. This reaction follows the 

 temperature law of Van't Hoff and Arrhenius. Hence, the mere 

 raising of the temperature a few degrees of a sugar, salt, acid or 

 alkali solution makes of it a disinfectant. 



Thermal Death Point. The thermal death point of an organism 

 is the lowest temperature that will certainly destroy it under definite 

 conditions. These conditions are time (which is generally taken as 

 ten minutes), amount of moisture present, the reaction and com- 

 position of the medium in which the organism is heated, and the 



