INFLUENCE OF TEMPERATURE. 159 



water above all. If food substances change in cold storage (and 

 some food products do deteriorate), this must be either due to changes 

 other than microbial or the material was not completely frozen as is 

 probably the case with salted butter. 



After bacteria are once frozen, they do not seem to be affected by any 

 lower temperature. Macfadyen and Rowland found that they tolerate 

 very low temperatures remarkably well. Many bacteria were not killed 

 by a twenty-hours' exposure to the temperature of liquid hydrogen 

 (-252). Yeasts are not quite so resistant and the mycelium of most 

 molds is easily destroyed by freezing, while the spores are hardier. 



THERMAL DEATH-POINT. Heating above the maximum temperature 

 is decidedly harmful to microorganisms; if the exposure to such unsuit- 

 able conditions is prolonged for a certain length of time, it results in the 

 death of the organism. The length of time required to kill a certain 

 organism will depend mainly upon how high the temperature is above 

 the maximum. A few degrees above this point, it will take several or 

 even many hours to kill the organism; the higher the temperature the 

 shorter the time. An exposure to 10 to 20 above the maximum takes 

 generally but a few minutes. It can be stated generally that the lower 

 the maximum temperature the more easily is an organism destroyed 

 by heat. It is customary to speak of the thermal death-point of an 

 organism as the lowest temperature which will kill it within ten minutes. 

 (Instead of this time adopted by the Society of American Bacteriologists, 

 some investigators prefer one or five minutes as the standard time.) 



The thermal death-point does not depend upon the species and the 

 temperature only. It varies with the age of the culture since older cells 

 are less resistant than younger ones especially if heated in their own pro- 

 ducts. The medium in which the organisms are heated is also of great 

 significance. The fact that acid liquids, as fruit juices, are more easily 

 sterilized than neutral meat or vegetables is largely due to a chemical 

 (poisonous) action of the acids upon bacteria. But the greater resistance 

 of tubercle bacteria in sputum compared with those suspended in salt 

 solution cannot be so readily accounted for. 



A necessary factor for the prompt destruction of organisms by heat 

 is the presence of moisture. The resistance of dry organisms is remark- 

 ably higher than that of the same organisms in a liquid culture. The 

 following table shows the death-point of yeast cells and spores in a dry 

 and moist state. 



