160 Microbes and You 



those organisms capable of withstanding high temperatures. They 

 usually do not multiply at high temperatures. 



The probable temperature limits between which bacterial life 

 is possible are absolute zero ( — 273° C.) and 160° C, according 

 to some authorities in the field, but the time factor must be borne in 

 mind. Life activities, however, are confined to a narrower range- 

 about 0° C. to 90° C, and in general, a growth temperature cooler 

 than optimum will increase the life span of microbes, while tem- 

 peratures higher than optimum tend to "burn up" the cells more 

 quickly. This same observation has been reported for fruit flies. 

 When kept at 10° C. they survived much longer than flies kept at 

 30° C. where they lived at a killing pace. 



THERMAL DEATH POINT 

 Because heat is relatively cheap and most readily controlled, it 

 is quite commonly employed for killing microorganisms. Most 

 non spore-formers are found to have a thermal death point between 

 55 and 60° C, while spore-formers may withstand boiling for hours. 

 The thermal death point (abbreviated T.D.P. ) is determined by 

 exposing a 24-hour nutrient broth culture to increasing degrees of 

 heat for a constant time period of 10 minutes. The lowest tem- 

 perature which sterilizes the culture during the stated time period 

 is called the T.D.P. 



THERMAL DEATH TIME 



Thermal death time is an important consideration in food in- 

 dustries where undesirable flavors and colors may be imparted to 

 the food by excessive exposure to high temperatures. It may be 

 more desirable to heat at a lower temperature for a longer time 

 period to accomplish the same effect as higher temperatures for a 

 shorter time period. Each product has its own optimum conditions 

 based upon bulk, the nature of the product, etc. About seven 

 billion #2 cans of food are processed annually, and the importance 

 of heat in destroying undesirable organisms is only too apparent. 



All cells of a given species of bacteria do not perish at once 

 when exposed to heat. Some of the more resistant strains in a 



