Thermal death is the death of a population of microorganisms due to exposure to an elevated temperature.
The nature of the thermal death varies depending on the source of the heat. The heat of an open flame incinerates the microorganisms. The dry heat of an oven causes the complete removal of water, which is lethal for biological structures. In contrast, the moist heat delivered by a sterilizer such as an autoclave causes the proteins in the sample to coagulate in a way that is analogous to the coagulation of the proteins of an egg to form the familiar cooked egg white.
The coagulation of proteins by heat is a drastic alteration in the three-dimensional shape of these protein molecules. Typically, the alteration is irreversible and renders a protein incapable of proper function.
Thermal death also involves the destruction of the membranes surrounding microorganisms such as bacteria . The high temperatures can cause the phospholipid constituents of the membrane to dissolve and thus destroy the membrane structure. Finally, the high heat will also cause the destruction of the nucleic acid of the target microorganism. In the case of double-stranded DNA , the heat will result in the disassociation of the two DNA strands.
Thermal death can be related to time. A term known as the thermal death time is defined as the time required to kill a population of the target microorganism in a water-based solution at a given temperature. The thermal death time of microorganisms can vary, depending on the thermal tolerance of the microbes. For example, thermophilic bacteria such as Thermophilus aquaticus that can tolerate high temperatures will have a thermal death time that is longer than the more heat-sensitive bacterium Escherichia coli .
Another aspect or measure of thermal death is termed the thermal death point. This is defined as the lowest temperature that will completely kill a population of a target microorganism within 10 minutes. This aspect of thermal death is useful in purifying water via boiling. Whereas Escherichia coli populations will be readily killed within 10 minutes at 212°F (100°C), spores of bacteria such as Bacillus subtilus and Clostridium perfringens will have a higher thermal death point, because a higher temperature is required to kill spores within 10 minutes.
Exact temperatures and times are usually used in calculating thermal death variables because terms such as "boiling" are not precise. For example, the boiling point of water (i.e., the temperature of boiling water) depends upon pressure. As altitude above sea level increases, the boiling temperature of water (H2O) lowers.
See also Laboratory techniques in microbiology; Sterilization