Physical and Chemical Method of Sterilization

What is Sterilization?

Sterilization is a process of destruction of all forms of living microorganisms from a substance.

The Methods of Sterilization

·         Physical methods

·         Chemical methods

Physical Methods of Sterilization:

·         Heat method of sterilization

·         Radiation

·         Filtration

Heat Method of Sterilization

This is the most common method of sterilization. The heat used kills the microbes in the substance. The temperature of the heat and duration of heating are the factors that affect the extent of sterilization.

In heat sterilization process, the longer the exposure to heat the better is the sterilization at a given temperature. As the temperature of heat raises the timespan required for sterilization decreases.

Further, the sterilization time increases with a decrease in temperature and vice-versa. But one needs to maintain minimum sterilization time or minimum contact time for the heat to be in touch with microbes or bacteria and thereby kill them.

The heat method of sterilization is again of two types based on the type of heat used.

1.      Moist heat methods

2.      Dry heat methods

Moist heat method of sterilization:

Here heat is applied in the form of steam or just boiling.

This method includes techniques like:

·         Boiling

·         Pasteurization

·         By use of steam (Autoclave)

·         Boiling is preferred for metallic devices like surgical scissors, scalpels, needles, etc. Here substances are boiled to sterilize them.

It is preferred for metallic devices like surgical scissors, scalpels, needles, etc. Here substances are boiled to sterilize them.

·         Pasteurization is the process of heating the milk at a temperature of 6o degrees or 72 degrees 3 to four times. Here alternative heating and cooling kills all the microbes and molds without boiling the milk.

·         Using Steam (autoclaving) Here the substances are subjected to sterilization in an autoclave a steam sterilization equipment. The process is carried out at a temperature of 115 degrees for 60 min or 121 degrees for 20 min at 15psi pressure.

This method is used for sterilization of;

·         Flammable substances

·         Culture media

·         Solutions

·         Equipment

·         Glass wares

Bacterial spores are the forms of bacteria which are inert. They form a rigid cover over the cell wall during harsh climate. This cover prevents any damage to cell and drying of the cell. By steam sterilization, these forms of bacteria are also killed as steam destroys the cell wall.

Dry heat methods:

Following methods are subjected for dry heat sterilization:

·         Flaming

·         Incineration

·         Hot air oven.

·         Radiation sterilization

Flaming is the process of exposing metallic device like the needle, scalpels, and scissors to flame for few minutes. The fire burns the microbes and other dust on the instrument directly.

Incineration is done especially for inoculating loops used in microbe cultures. The metallic end of the loop is heated to red hot on the flame. This exposure kills all the germs.

Hot air oven is suitable for het sterilization of dry material like powders, metal devices, glassware, and other such laboratory stocks. Dry heat destroys microorganisms by dehydration and oxidation or even incineration.


This method involves exposing the packed materials to radiation for sterilization. There are two types of radiations available for sterilization i.e.

·         non-ionic and

·         ionic radiation.

a) Non-ionic radiations are safe to the operator of sterilization, and they are like Ultra Violet radiations, they can be used even at the door entrances to prevent entry of live microbes through the air.

b) Ionizing radiation sterilization. They are powerful radiation and very useful for sterilization. The operator needs to protect himself from exposure from these radiations by use of special clothing.  Ex: X-rays, γ-rays, etc.


In this method, liquids are filtered through bacterial filters to remove any microbes present. This method is very effective for sterilization of heat sensitive liquids. The chances of clogging and long time duration for the process to happen are drawbacks.

For sterilization three types of filters are used:

·         A) Membrane filters: These are thin filters which are made of cellulose. They can be employed for online sterilization during injection by placing the membrane between the syringe and needle. Used for sterilization of solvents, gasses.

·         B) Seitz filters: These are made of asbestos or other material. They are pad like and thicker than membrane filters. They do not rupture during filtration. But the solution might get absorbed by the filter pad itself.

·         Sintered glass filters: These are made of glass and hence do not absorb liquids during filtration. The disadvantage is that they are very brittle and break easily.

·         c) Candle filters: These are made of clay like diatomous mud. This special mud has minute pores made by algae. The filters have many minute lengthy pores. The microbes get stuck during their travel through the pore in the candle.

Chemical Methods of Sterilization:

The articles are subjected to sterilization by using toxic gasses. The gas penetrates quickly into the material like steam so, the sterilization is effective. But the chances of explosion and cost factors are to be considered.

Gasses used for sterilization are very poisonous. The commonly used gas is ethylene oxide with a combination of carbon-dioxide. Carbon dioxide is added to minimize the chances of an explosion.

Hydrogen Peroxide Sterilization, also known as hydrogen peroxide gas sterilization, is a low temperature sterilization process commonly used to sterilize heat-sensitive devices. A hydrogen peroxide sterilization process involves H2O2 vapor filling the sterilizer chamber, contacting and sterilizing exposed device surfaces. Once the sterilization cycle has completed, the vapor is vacuumed from the chamber and converted to water and oxygen.

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