Sterilization is defined as the process where all the living microorganisms, including bacterial spores are killed. This can be achieved by physical, chemical and physiochemical methods.

Heat is one of physical method of sterilization. Heat acts by oxidative effects as well as denaturation and coagulation of Proteins. Those materials/articles that cannot withstand high temperatures can still be sterilized at lower temperature by prolonging the duration of exposure.

When heat is used as a sterilizing agent, the vibratory motion of every molecule of microorganism is increased to level that induce the cleavage of intramolecular hydrogen bonds between proteins. Death is therefore caused by an accumulation of irreversible damage to all metabolic functions of the organism.

Factors affecting heat sterilization are:

• Nature of heat; moist heat is more effective than dry heat.
• Temperature and time; temperature and time are inversely proportional. As temperature increases the time taken decreases.
• Number of microorganism; more the number of microorganisms, higher the temperature or longer the duration required.
• Nature of microorganism; depends on species and strain of microorganism, sensitivity to heat may vary, spores are highly resistant to heat.
• Types of material; articles/materials that are heavily contaminated require higher temperature or prolonged exposure. Certain heat materials/articles must be sterilized at lower temperature.
• Presence of organic material; organic material such as protein, sugars, oils and fats increase the time required.

Moist heat acts by coagulation and denaturation of proteins.

It has more penetration power than dry air, It moistens the spores (moisture is essential for coagulation of proteins), condensation of steam on cooler surface releases latent heat, condensation of steam draws in fresh steam.

Moist heat sterilization technology is the most widely used given its versatility and effectiveness for heat resistant products. Depending on the type of product to be sterilized, different techniques are used:

• Steam sterilization
• Steam-air mixture sterilization
• Steam-water mixture sterilization

Steam sterilization is basically used to sterilize different types of solid materials: clothing, stainless steel equipment, filters, component parts of other equipment, etc., and also for liquids in sealed or ventilated containers, etc.

Steam-air mixture and steam-water mixture sterilization Autoclaves have been specially designed for the terminal sterilization of liquid solutions in sealed glass or plastic containers and the process controls the differential pressure existing between the inside of the container and the chamber.

Moist heat sterilization at above 100 degree C

Sterilization can be effectively achieved at a temperature above 100 degree C using an Autoclave. In an Autoclave the water is boiled in a closed chamber. As the pressure rises, the boiling point of water also raises. At a pressure of 15lbs inside the Autoclave, the temperature is said to be 121 degree C exposure of materials/article to this temperature for 15 minutes sterilize them. To destroy infectious agents higher temperature or longer times are used; 135 degree C or 121 degree C for at least one hour.

Different types of Autoclave; A simple autoclave has vertical or horizontal cylindrical body with a heating element, a perforated tray to keep the material/article, a lid that can be fastened by screw clamps, a pressure gauge, a safety valve and discharge tap.

• Simple laboratory Autoclave; pressure cooker type
• Steam jacketed downward displacement laboratory Autoclave
• High pressure pre-vacuum Autoclave

Heat can be used either alone or mixed with steam. In the Autoclave Chamber, terminal sterilization is based on highly efficient heat transfer from the saturated steam to autoclave load. Heat transfer occurs by the release of the latent heat from saturated steam under pressure as it condenses. Heat transfer from saturated steam to the chamber environment is much more effective and timely for the coagulation and denaturation of nucleic acid and proteins than from dry heat or superheated heat. Moist heat in the form of saturated stem under pressure is therefore the most reliable sterilizing agent and is the method of choice whenever it can be used, especially for aqueous preparations.

The physical parameters governing the efficiency of the sterilization process are exposure time, temperature and pressure. Generally a moist heat sterilization cycle is comprises of heating, sterilizing and cooling phases.

Autoclaves are useful in some industrial application where parts and materials need to be thoroughly sterilized during the production process; this is relatively common in industries working with high performance composite materials, particularly in the aerospace sector.

Autoclaves are also used in

• Veterinary medicine
• Dentistry
• Research and development for Pharmaceutical
• Food production facilities

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