What is Sterilization ? 9 Types and Methods in Microbiology

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

Articles having a direct application on humans and animals are subjected to sterilization.

These materials include drugs, nutraceuticals, surgical equipment, food, etc.

This sterilization helps to preserve the substance for a long time without decay.

It also eliminates the possibility of transmitting infectious microbes from a substance when consumed or administered to living beings.

So sterilization becomes an essential process.

The microbes are invisible to the naked eye, and even those like bacteria have a protective sheath on their surface, making them resistant to sterilization.

For this, effective sterilization techniques are designed that can be studied in microbiology.

Types of sterilization techniques in microbiology

There are basically three methods of sterilization used in industry like

  1. Heat methods.
  2. Chemical sterilization.
  3. Filtration method

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 the heat sterilization process, the longer the exposure to heat, the better is the sterilization at a given temperature. As the temperature of heat rises, the time span 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.

  • Moist heat methods.
  • 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.
  • Autoclave (By use of steam).


  • It is preferred for metallic devices like surgical scissors, scalpels, needles, etc.
  • Here substances are boiled to sterilize them at 100 degree centigrade temperature.


  • It 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 kill all the microbes and molds without boiling the milk.

By autoclave:

  • Here the substances are subjected to sterilization in autoclave steam sterilization equipment.
  • The process is carried out at a temperature of 115 degrees for 60 minutes or 121 degrees for 20 minutes at 15psi pressure.
Modern laboratory autoclave sterilizer
  • The saturated steam is formed at a boiling temperature of the water, i.e.,100 degrees.
  • This steam condenses on the material and relieves the latent heat repeatedly to convert back into the water.
  • Further, the saturated steam under pressure penetrates all the narrow spaces leaving no microbes alive, thereby making the sterilization very efficient.
  • It is the most common method used for drugs as it is powerful enough even to kill bacterial spores.
  • Bacterial spores are the forms of bacteria that are inert.
  • They form a rigid cover over the cell wall during the harsh climate. This cover prevents any damage to the cell and the drying of the cell.

By steam sterilization, these forms of bacteria are also killed as steam destroys the cell wall.

Dry heat methods

Here the substances are subjected to dry heat like

  • Flaming
  • Incineration
  • Hot air oven.
  • Radiation sterilization


As the name indicates, the metallic objects to be sterilized are exposed to direct flame. The metallic devices like the needle, scalpels, scissors are kept in the flame for a few minutes. The fire burns the microbes and other dust on the instrument directly.


It is done primarily 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

This is suitable for dry materials like powders, metal devices, glassware, etc.

hot air oven
Hot Air Oven by barcode-labels.com

Here thermostable materials are on the racks inside the hot air oven.

Then in the closed oven, hot air is circulated at a particular temperature and time.

The radiation

This method involves exposing the packed materials to radiation for sterilization. There are two types of radiation available for sterilization, i.e., non-ionic and ionic radiation.

  • Non-ionic radiations are safe to the operator of sterilization, and they are like UltraViolet radiation; they can be used even at the door entrances to prevent the entry of live microbes through the air.
  • Ionizing radiation sterilization. They are powerful radiation and very useful for sterilization. The operator needs to protect himself from exposure from these radiations by the use of unique clothing.  Ex: X-rays, γ-rays, etc.
UV sterilization
Ultraviolet sterilizer with medical instruments in it.

Chemical method of sterilization

Here the articles are subjected to sterilization by using toxic gasses.

The gas penetrates quickly into the material like steam so, sterilization is effective.

But the chances of explosion and cost factors are to be considered.

The 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.


As the name indicates, the liquids are passed through bacterial filters to remove any microbes present in them.

This method is beneficial for the sterilization of heat-sensitive liquids. The chances of clogging and the long time duration of the process are significant drawbacks.

The sterilization by filtration can be done by three types of filters like

  • Membrane filters
  • Seitz filters
  • Candle filters

A) Membrane filters

These are thin filters that are made of cellulose. They can be employed for online sterilization during injection by placing the membrane between the syringe and needle. They are highly efficient in sterilizing liquid, solvents, gasses.

Membrane filter structure-Sterilization Methods
The minute pores in the membrane

The disadvantage is there are chances of rupture of membrane leading to improper sterilization.

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.

An alternative type of filter is 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 of algae. The filters have many tiny lengthy pores.

The microbes get stuck during their travel through the pore in the candle.

Also, ultrasound waves are being tested for sterilization.

Though it is not as effective as other methods, it was found to be helpful in tissue cultures.

Here the aim is to sterilize or even prevent the growth of bacteria during culturing of tissue.

Application of the sterilization methods

1. Methods of sterilization of surgical instruments are Boiling, Incineration, Autoclave.

2. Methods of sterilization of glassware are autoclave, boiling, and also the hot-air oven.

3.  Methods of sterilization of water we use filtration and other moist liquid material autoclave.

4.  For powders and other dry forms, it is a hot air oven if thermostable or gaseous methods and radiation.

5. Methods of Sterilization employed in hospitals for metallic surgical instruments are boiling, autoclave, incineration.

6. To prevent microbial contamination due to air and mobility, UV radiation lamps are arranged at the doors.


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  1. Question: What is the commercial sterility method used for evaporated Milk?

    Respond: We put the cans into the thermostat (37 ° C for 3 days) after sterilization – acceptable result – the total number of microorganisms (TPC) after this thermostat test is max. 100 TPC.

    I still don’t know what method is this… any one please can help me?.. or at least tell me what is the most common sterility method for evaporated milk?

  2. Hello, can i know the boiling method for sterilization such as temperature and time required for boiling.. In my laboratory not have autoclave.


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