Physical Methods of Control

Physical Methods of Control - three hours may be required...

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Physical Methods of Control Physical methods for controlling the growth of microorganisms can be divided into heat  methods and nonheat methods. The lowest temperature at which all microorganisms  are killed in 10 minutes is the  thermal death point , while the minimum amount of time  required to kill microorganisms at a given temperature is known as the  thermal death  time.  The time for destruction of 90 percent of the microbial population is the  decimal  reduction time.   Dry heat. Dry heat  kills microorganisms by reacting with and oxidizing their proteins.  Dry heat can be used in incineration devices, such as the  Bunsen burner  or the  hot-air  oven.  In the hot-air oven, a temperature of about 170°C for two hours will bring about  sterilization.  Moist heat. Moist heat  is used to kill microorganisms in such things as  boiling water.  Most vegetating microorganisms are killed within two or three minutes, but over two or 
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Unformatted text preview: three hours may be required for destruction of bacterial spores. In moist heat, the microbial proteins undergo denaturation , a process in which the three-dimensional form of the protein reverts to a two-dimensional form, and the protein breaks down. Moist heat is used in the autoclave , a high-pressure device in which steam is superheated (Figure 1 ). Steam at 100C is placed under a pressure of 15 pounds per square inch, increasing the temperature to 121C. At this temperature, the time required to achieve sterilization is about 15 minutes. The autoclave is the standard instrument for preparing microbial media and for sterilizing instruments such as syringes, hospital garb, blankets, intravenous solutions, and myriad other items....
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