Moist heat sterilization using autoclave is commonly used for the sterilization of biohazards trash, heat and moisture resistant materials such as aqueous preparation (culture media). Moist heat in the form of pressurized steam is regarded as the most dependable method for the destruction of all forms of life, including bacterial spores. Over 100 years ago, French and German microbiologist developed the autoclave as an essential component of their laboratories.

Principle of Autoclaving:
Autoclaving is the most common methods of sterilization. In this method sterilization is done by steam under pressure. Steaming at temperature higher than 100°C is used. The boiling temperature depends upon the surrounding atmospheric pressure. Higher temperature of steaming is obtained by employing higher pressure. When the autoclave is closed, the water starts boiling and the inside pressures increases and now the water boils above 100°C. At 15 lb per sq. inch pressure, 121°C temperatures are obtained. This is kept for 15 minutes for sterilization to kill spores. It works like a pressure cooker. In this way steam is a gas, increasing its pressure in a closed system increases its temperature. As the water molecules in steam become more energized, their penetration increases substantially. This principle is used to reduce cooking time in the home pressure cooker and to reduce sterilizing time in the autoclave. It is important to note that the sterilizing agent is the moist heat, not the pressure.

Preparation of items for Autoclaving:
In preparing items for autoclaving, containers should be unsealed and articles should be wrapped in materials that allow steam penetration. Large packages of dressings and large flasks of media require extra time for heat to penetrate them. Likewise, packing many articles close together in an autoclave lengthens the processing time to as much as 60 minutes to ensure sterility. It is more efficient and safer to run two separate, uncrowned loads than one crowded one. Wrapping objects in aluminum foil is not recommended because it may interfere with steam penetration. Steam circulates through an autoclave from a steam outlet to an air evacuation port (figure).
Moist heat sterilization describes sterilization techniques that utilize saturated steam. Moist heat sterilization using autoclave is commonly used for the sterilization of biohazards trash, heat and moisture resistant materials such as aqueous preparation (culture media).

The Basic Autoclave Parts

1. Water intake –Water intake provides the water required to produce the steam that is used in sterilization process. Smaller autoclaves have a reservoir that is filled with water when required whereas large autoclaves have their water intake connected to a continuous water supply

2. The chamber –it is a space that accommodates the load to be sterilized. This chamber contains wire racks that hold items to be sterilized so that steam can penetrate them.

3. The control panel –The control panel allows the user to controls temperature, pressure and time for the autoclave process. Some items require higher temperatures other require more time inside the autoclave so as to achieve their maximum sterility. So using the control panel these parameters can be controlled.

4. Air pump system – The Air pump system pumps out air from the chamber so as to create a vacuum which will then be filled with steam. It is important to remove the air so as to avoid any chemical reactions between the steam and the items to be sterilized.

The chamber in which all the sterilization is done should be an air-free and air-tight zone. The air in the chamber should be removed either through vacuum pumping, steam pulsing and pumping, or upward displacement of air by steam.Once the zone has been made a vacuum, air should be prevented from entering again during the cleansing process. Hence, the door is the most important of all the autoclave parts.The door usually contains an insulation pad, which can wear and lose its air-tight properties with time. This pad gets thinner, folds in and loses its insulating properties. This then calls for a quick response to its replacement so as to keep the autoclave fully operational without faults

Working of Autoclave:
Most autoclaves contain a sterilizing chamber into which articles are placed and a steam jacket where steam is maintained. As steam flows from the steam jacket into the sterilizing chamber, cool air is forced out and a special valve increases the pressure to 15 pounds/square inch above normal atmospheric pressure. The temperature rises to 121.5oC, and the superheated water molecules rapidly conduct heat into microorganisms. The time for destruction of the most resistant bacterial spore is now reduced to about 15 minutes. For denser objects, up to 30 minutes of exposure may be required. The conditions must be carefully controlled or serious problems may occur.

Uses of Autoclave:
1) Autoclaving is used to sterilize culture media, instruments, dressings, intravenous equipment, applicators, solutions, syringes, transfusion equipment, and numerous other items that can withstand high temperatures and pressures.
2) The laboratory technician uses it to sterilize bacteriological media and destroy pathogenic cultures.
3) Autoclaves are also used on large industrial scale. Large industrial autoclaves are called retorts

Limitations and Disadvantages of Autoclave:
1) Some plastic ware melts in the high heat, and sharp instruments often become dull.
2) Many chemicals breakdown during the sterilization process
3) Oily substances cannot be treated because they do not mix with water.
4) Heat requires extra time to reach the center of solid materials, such as canned meats, because such materials do not develop the efficient heat-distributing convection currents that occur in liquids.
5) Heating large containers also requires extra time.