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...