Media sterilization is carried out with the autoclave, basically a huge steam cooker. Steam enters into a jacket surrounding the chamber. When the pressure from the steam is at a certain point in the jacket, a valve allows the steam to enter the chamber. The pressure will go up over 15 pounds per square inch (psi): at this point the timer begins to count down--- usually for 15 minutes, depending on the type of media. The high pressure in a closed container allows the temperature to go above the highest temperature one could get by just boiling, around 121 degrees C. Therefore, the parameters for sterilization with an autoclave are 121 C at >15 psi for 15 minutes. Fifteen minutes is the thermal death time for most organisms (except some really
hardy sporeformers).