Heat can be applied either in dry form, as in flaming and baking or moist, as in steaming, boiling or autoclaving.
The thermal death point
is the lowest temperature at which bacteria die in a broth incubated for
10 minutes at a pH of 7.
The decimal reduction time (D) represents the time it takes for 90% of a population of microbes to be killed by a particular method of microbial control, such as autoclaving.
Water heats up quickly but cools slowly. In liquid form it can retain and deliver heat energy much more efficiently that dry heat alone since molecules in a liquid are much closer together than those in a gas.
Boiling (raising water temperature to 100 oC) kills most vegetative cells within 10 minutes at sea level. Since water boils at a lower temperature at higher altitudes, it is necessary to boil for a longer period of time to achieve the same level of disinfection.
Some viruses, the cysts of protozoa and bacterial endospores are very resistant to boiling. For example, the spores of Clostridium botulinum can be boiled for 5-20 hours and still remain viable. Boiling should not be considered an agent of sterilization.
Though the heat energy in steam can dissipate quickly in the atmosphere, water under pressure has a higher boiling point, increasing the amount heat delivered to a microbial population.
An autoclave is a chamber that uses steam under pressure to sterilize media, glassware, surgical implements and other materials that are not sensitive to heat. At standard operating temperature (121 oC) and pressure (15 lb/in2), it takes about 10 minutes (though standard operating time is generally 15 minutes) to kill vegetative cells, endospores, cysts of protists and inactivate viruses. When operating correctly, an autoclave delivers twice as much heat energy to the surface of an object than steaming alone.
Sterility indicators, such as autoclave strips and spore ampules are used to test how well the device functions. Bacillus stearothermophilus spores are used in spore ampules and strips, since the endospores of this bacterium are very thermoduric and will only be killed if the autoclave operates properly.
Autoclaves cannot be used to sterilize powders, oils or any material or medium (examples, egg media, whole blood and many carbohydrate broths) that is sensitive to heat. Also, since some bacterial exotoxins can withstand autoclaving, bandages sterilized in this manner must be thoroughly rinsed in sterile water prior to use.
This process reduces microbial populations in liquids such as milk and juices, but does not sterilize. Pasteurization is used primarily to kill and inhibit the growth of pathogens found in dairy products such as E. coli, Brucella melitensis, Mycobacterium bovis and Listeria monocytogenes.
Standard (batch method) pasteurization - raising the temperature of the fluid in large containers to 63 oC for 30 minutes.
High temperature-short time (flash) pasteurization - passing the fluid through tubes raised to 72 oC for 15 seconds.
Ultra-high temperature pasteurization - raising the temperature of milk to 134 oC for 1 second.
All three of these techniques produces approximately the same effect in terms of the reduction of microbial populations, thus are considered to be equivalent treatments.
Powders, oils and other substances that cannot be sterilized using moist heat can be sterilized with dry heat. However, it takes greater temperatures and longer application times to achieve the same effect.
Baking, flaming and incineration are all methods for the application of dry heat.