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Sterilization – Introduction, Classification, Methods, Applications

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  • Last Updated : 17 Aug, 2022
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Sterilization is an effective method or a process used to kill all the vegetative spores and disease-causing microorganisms. Sterilization deactivates microorganisms including bacteria, spores, fungi, and unicellular and other biological agents which are present in food products or on any certain surface of the object. Sterilization can be achieved by various methods and sources including heat, chemicals, radiation, gases, and filtration. Sterilization is making a substance liberated from all microorganisms both in vegetative and sporing states. Spore is a regenerative structure that is adjusted for dispersal and surviving for expanded timeframes in horrible circumstances. Spore’s structure is part of the lifecycles of numerous microscopic organisms, plants, green growth, parasites, and some protozoa. For the greatest ideal, the Steam process is utilized where the temperature is high and strain can be observed giving for check of sterilization. Elevated degrees of sterilization can be accomplished by utilizing the Steam Sterilization methodology, and the steam sterilizer or autoclave is the most widely recognized piece of instrument utilized in labs and medical clinics.




Sterilization is defined as the process of destruction or elimination of all pathogenic microorganisms and organisms capable of giving rise to infection such as all spores, bacteria, fungi, etc., all disease-causing. It can be achieved by both physical, chemical, and other effective methods that inhibit growth or are free from food products, fluids, objects, solid materials, food packaging materials, raw materials, and other different products.

Importance of Sterilization

  • Sterilization is used to prevent the transmission of certain pathogenic disease-causing microorganisms into the body.
  • It helps in sterile products to prevent contamination.
  • Sterilization is an important process in research development laboratories.
  • It prevents the contamination of instruments and areas in the pharmaceutical industry.
  • It is used in the preparation process of cultures and other microbiology experiments.
  • The sterilization process is used in the food industries such as canning, and high-pressure methods.
  • Sterilization is used for the preparation of sterile dosage forms and sterility testing.

Methods of Sterilization

There are many different methods that are effective to achieve the sterilization process. They are

Physical method

By using heat

The most effective and reliable method of the sterilization process. There are two methods by using heat-dry heat and moist heat.

Dry heat 

  • Flaming In this method heating instruments over the fire until they become hot in red. Instruments that are used such as point of forceps, Spatulas, inoculating loops, and Wires.
  • Incineration It is a process that involves the combustion of organic substances contained in waste materials. Items such as contaminated cloth, animal carcasses, and pathological material. PVC and polythene can be dealt with.
  • Hot air oven Hot air ovens are the electrical devices used for sterilization. The oven uses dry heat to sterilize items. Generally, they can be set at minimum to maximum temperature from 50˚C to 300˚C. The thermostat is present to control the temperature. This is the most widely used method of sterilization by dry heat. 

Moist heat

Moist heat can be categorized into 3 groups: 

  • Temperature below 100 C Pasteurization of milk: This method is done by holding period at 63˚C for 30 minutes or 72˚C for 15-20 minutes followed by cooling. This method is not suitable for killing all spores.
  • Temperature at 100 C Steam at atmospheric pressure is used to sterilize culture media. This is an inexpensive method. The principle of this method is first exposure kills vegetative bacteria and next exposure will kill vegetative bacteria that mature from the spore. It is intermittent sterilization by holding at a temperature of 100˚C for 20 minutes on three consecutive days. 
  • Temperature above 100 C An autoclave or steam sterilizer is an instrument that uses steam to sterilize equipment and other objects. This implies that all microscopic organisms, infections, parasites, and spores are inactivated. In any case, prions may not be annihilated via autoclaving at the regular 134˚C for 3 minutes or 121˚C for 15 minutes. It is suitable for the Items such as dressings, instruments, laboratory ware, media, and medical products.


Filtration assists with eliminating microorganisms from heat-labile fluids such as sera and solutions of antibiotics. Its working principle as viruses go through the normal filter channels, filtration can be used to obtain bacteria-free filtrates of clinical samples for virus isolation. 

Types of filters

  • Candle filters These filters are used for the purification of water for industrial and drinking purposes. These are made under various grades of porosity. 
  • Asbestos filters are disposable and single usage. They tend to alkalinize filtered fluids. Their usage is less because of their carcinogenic property. 
  • Sintered glass filters These filters have low absorptive properties. They are brittle and costly. 
  • Membrane filters These filters are made of cellulose esters or other polymers. They are usually used for water purification and analysis, sterility testing, and preparation of solutions.


There are 2 types of radiation: Ionizing radiation & non-ionizing radiation

  • Non-ionizing radiation In the non-ionizing method infrared is used for rapid mass sterilization of prepacked items such as syringes, and catheters. UV is used for disinfecting enclosed areas such as halls, operation theatres, and labs. 
  • Ionizing radiation In ionizing radiation, gamma rays and x-rays are used for sterilizing plastics, syringes, swabs, catheters, animal feeds, cardboard, oils, and metal foils.

Chemical methods of Sterilization 

Using chemicals

  • Chemical agents: The action of chemical agents are protein coagulation and Disruption of cell membrane resulting in exposure, damage, and loss of contents. 
  • Chemical Alcohols: Commonly used are Ethyl alcohol, and Isopropyl alcohol and must be utilized at concentrations of 60-90%. Isopropyl alcohol is used in the sanitization of the clinical thermometer. Methyl alcohol is viable against contagious spores, treating cabinets, and incubators. It is toxic and inflammable. 
  • Aldehyde Formaldehyde: It contains bactericidal and sporicidal and it has a great effect on viruses. It is used to preserve anatomical specimens and destroy anthrax spores on hair and wool
  • Phenols: These are acquired from the distillation of coal tar between 170˚ to 270˚C. The deadly impacts include- it can cause cell membrane damage, releasing cell contents, and causing lysis. 


Types of gases used for sterilization are:

  • Ethylene oxide works because of its alkylating of the amino, carboxyl, hydroxyl, and sulfhydryl groups in protein molecules and furthermore on DNA and RNA. It very well may be utilized on instruments such as heart-lung machines, respirators, dental equipment, books, and clothing. 
  • Formaldehyde gas This is generally utilized for fumigation of operational theatres and other rooms in clinics. Formaldehyde is formed by the addition of 150g of KMnO4 to 280ml of formalin for each 1000cu.ft of room volume, in the wake of shutting the windows and different outlets. After fumigation, the doors ought to be sealed and left unopened for 48 hours. 
  • Beta propiolactone (BPL) is a result of ketene and formaldehyde with a boiling point of 163˚C. It has fat bactericidal activity yet is cancer-causing. It is equipped for killing all microorganisms and it is exceptionally dynamic against infections.

Applications of Sterilization

In the medical industry and surgeries

Normally all the surgical instruments and medications that are used for the treatments undergo contact with the aseptic part of the body. Therefore, they must be sterile. Examples of such instruments are scalpels, hypodermic needles, and artificial pacemakers. 

Preparation of injectable medications and intravenous solutions for fluid replacement therapy requires sterility as well as all-around planned compartments to forestall sections of unusual agents after initial product sterilization.

Most clinical and surgical devices used in medical facilities are made of materials that can go under steam sterilization. Ethylene oxide gas has been used since the 1950s for heat- and moisture-sensitive medical devices. In the last few years, a number of various new, low-temperature sterilization systems have been developed and are being utilized to sterilize medical instruments.

Research and laboratories

Many research facilities, laboratories, and their instruments are mainly dependent on the effective sterilization process.

Food Industry

The father of food science – ‘Nicolas Appert’ introduced modern methods using the sterilization process. He found that the application of heat over a long period slowed down the deterioration of foods and various fluids by preserving them for safe consumption over an overtime period. Canning of foods works on a similar principle and it helped to reduce food poisoning and disease-causing microorganisms. Food irradiation and high-pressure methods using steam are also used in the food industries.

Vaccine and biological productions

The preparations of vaccines and other biological products are dependent on the sterilization process. To sterilize certain heat-liable enzyme preparations, antibiotics, and other drugs.


A huge number of instruments utilized on shuttle can’t endure exceptionally high temperatures, so strategies not needing extreme temperatures are utilized as endured, including warming no less than 120°C, chemical sterilization, oxidization, ultraviolet, and irradiation.

Frequently Asked Questions

Question 1: What are the factors that influence sterilization by heat?


The factors influencing sterilization by heat are:  

  1. Nature of heat. (dry or moist) 
  2. Temperature and time.
  3. Number of microorganisms present 
  4. Characteristics of organisms (species, strain, sporing capacity).
  5. Type of material from which organisms have to be eliminated. 
  6. Effect due to protein denaturation, oxidation, coagulation, and toxic effect.

Question 2: What is TDT?


The term TDT indicates – ‘Thermal Death Time’. Thermal death time is defined as the minimum time required to kill a suspension of organisms at a predetermined temperature in a specified environment. TDT is inversely proportional to temperature. Thermal death time is increased in presence of organic substances, proteins, nucleic acid, starch, gelatin, sugar, fats, and oils.

Question 3: What are the factors that determine the type of sterilization?


The factors that determine the type of sterilization or disinfecting process are: 

  • Time and temperature.
  • Stage of growth of the organism. 
  • Nature of the medium, in which the organism is suspended (air, gas, liquid).  
  • The number of organisms present. 

Question 4: How do determine the potential of Ideal antiseptics or disinfectants?


They should be effective against all microorganisms. The concentration of the solution is active in presence of organic matter substance. The time of action of the disinfectant and how effective acid and alkaline media are, the pH of the medium. It should have faster action, temperature, and nature of the organisms during usage. It should have a high penetrating power. It should be compatible with other antiseptics and disinfectants. It should not corrode metals and not cause local irritation, non-toxic and inexpensive, easily available.

Question 5: What are the different types of steam sterilizers?


There are several types of steam sterilizers that are presently used. They are:

  1. Hospital dressing sterilizers
  2. Bowl sterilizers
  3. Laboratory autoclaves
  4. Rapid cooling sterilizers
  5. Instrumental sterilizers
  6. The domestic pressure cooker can also act as a sterilizer.

Question 6: What are spores?


A spore is a regenerative structure that is adjusted for dispersal and surviving for expanded time periods in horrible circumstances. Spore’s structure is part of the lifecycles of numerous microscopic organisms, plants, green growth, parasites, and some protozoa. They are not easily killable. Pasteurization and other heat treatments cannot affect the state of spores. it can only be easily achieved by the process of sterilization. 

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