The method of moist heat sterilization is based on the steam under pressure. An autoclave is used for this purpose. Moist heat sterilization is the method preferred for the pharmaceutical products that can withstand heat. Microorganisms are coagulated and destroyed at a lower temperature in the presence of water. The mechanism of microbial destruction in moist heat is by denaturation and coagulation of the organism’s essential protein, which occurs by the hot moisture in the microbial cell that permits destruction at relatively low temperature. Increase in pressure helps in killing the microbes in combination with the temperature. Under atmospheric conditions it is not possible to increase the temperature of steam above 100°C. Hence, pressure is employed to achieve a higher temperature. An autoclave is fitted with temperature, pressure and time gauges which help the operator to fix the sterilization conditions required for different sterilizing substances. The usual steam pressures, the temperatures obtainable under these pressures, and the approximate length of time required for sterilization after the system reaches the indicated temperatures are provided in Table 8.4.

Table 8.4 Sterilization Conditions by Autoclaving

Moist heat sterilization is applicable to preparations that can withstand heat under pressure; examples are ampoules containing solutions, sealed empty vials containing small quantity of water, bulk solutions, glassware, surgical dressings and instruments. However, it is not applicable to exposed powders that may be damaged by the condensed moisture and oils, fats, oleaginous preparations and other preparations that are not penetrated by the moisture.