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High Temperatures
Autoclaves operate on the principle that pathogens, like all organic matter, can be killed by prolonged exposure to high temperatures. This was first seized upon by Louis Pasteur, who developed a way to prevent wine from spoiling by briefly heating it almost to its boiling point. The high temperatures cause proteins and other building blocks of life to disintegrate or reconstitute, thus killing microbial organisms that might spread disease.
Limitations to Pasteurization
Pasteurization, which is most commonly associated with milk, is limited to sterilizing milk at low temperatures (around 70 degrees Celsius) because if it is done with too much heat, the milk itself will be destroyed. Because autoclaves are used to sterilize abiotic materials such as stainless steel and glass, they can be raised to much higher temperatures than pasteurized foods. Higher temperatures guarantee a more efficient sterilization process.
High Pressure
Autoclaves are often compared to pressure cookers because they often operate at very high internal pressures. This is because when water becomes steam it follows the Ideal Gas Law, which dictates that the pressure and volume of a gas are directly proportional to its mass and temperature. When water is heated to above 100 degrees Celsius in a confined volume, the pressure within the autoclave quickly increases. The increased pressure also forces more thorough contact between the heated steam and the object being sanitized.
Specific Timing
The advantage of having extremely hot steam within an autoclave is that the high temperature and high pressure drastically increase the rate of exchange of thermal energy. As a result, items sterilized in an autoclave will have foreign microbial matter die much sooner than by other methods. Because different organisms are more resilient than others, microbes are referenced as having a thermal death time, or TDT, that should be observed to guarantee sterilization in an autoclave.