Physical Methods of Sterilisation — Steam Sterilisation

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Steam Sterilisation by Autoclave — Above 100ºC

Steam sterilisation is effective sterilisation that has more significant lethal action. An autoclave instrument is the most preferred instrument used for the sterilisation of surgical equipment and culture plates. It is a must for all laboratories and hospitals and was invented by Charles Chamberland, an associate of Louis Pasteur.

The principle is based on moist heat, similar to a pressure cooker- steam under pressure. Water boils at 100ºC, and the steam accumulates in a closed container, increasing pressure.

Heat above 100ºC is an efficient way of sterilisation than dry heat. It has greater penetrating power and more significant lethal action. As pressure increases above atmospheric pressure, temperature also increases. The temperature of the steam increases the pressure, which is indicated in the pressure gauge.

The Autoclave usually operated at 15 lbs pressure for 15 minutes which corresponds to a temperature of 121ºC. This temperature is enough to kill all the spores and cells of microorganisms.

The Autoclave is usually of a pressure cooker type made up of gunmetal sheets supported in an iron case. It is closed by a swing door which is fastened by radical bolts tightly. The Autoclave is a Cylindrical Vessel having Double Walls around all parts except the upper side. The Autoclave has a Discharge Tap for Air and Steam on its lid, a Pressure Gauge, and a Safety Valve.

A sufficient amount of water is placed inside the Autoclave. The material is placed inside the Autoclave for sterilisation. The Autoclave lid should be tightened with the help of screws, then switch on the plug.

The steam outlet is kept open until we feel that the air from outside the Autoclave has been evacuated and close the steam outlet. The pressure is allowed to remain at 15 lbs for 15 to 30 minutes., which is done by controlling the steam in the valve. After the sterilisation cycle is complete, the Autoclave is switched off, allowed to cool till the pressure gauge is equal to atmospheric pressure. The discharge tap is opened slowly.

Applications

• The autoclave is a must for every hospital and microbiological lab environment.
• The autoclave is used to sterilise usual non -carbohydrate media, broths, agar media, contaminated media, aprons, rubber tubes, rubber gloves.
• This type of sterilisation is also used in the commercial canning of fruits and vegetables and sterilised milk.
• Autoclaves are also used for decontamination before disposal for all medical and biological waste.

Precautions

• Air present in the autoclave must be driven out, and the level of water should be checked before operating.
• Place the articles to be sterilised on the shelf to give adequate space for the steam to escape in between, and air in between is replaced with steam. The air should be completely evacuated, and the steam must have access to the materials to be sterilised.
• Heat sensitive substances should not be sterilised by autoclaving.
• The sterilisation cycle and decontamination cycle should be independent.