Sterilization is the process of eliminating all forms of microbial life from an object or surface. This is typically achieved through the use of heat, chemicals, irradiation, high pressure or filtration. For heat sterilization, the required temperature and duration depends on what is being sterilized and the type of microorganisms that need to be destroyed. There are a few common heat-based methods used to achieve sterility:
Autoclaving
Autoclaving is a common heat-based sterilization technique that uses steam heat under pressure. An autoclave is a pressure chamber that allows hot steam under high pressure to penetrate objects and kill microbes. The typical temperature range for sterilization with an autoclave is 121-134°C (250-273°F), with an exposure time of 15-20 minutes at 15 psi of steam pressure. This temperature and pressure combination is sufficient to kill endospores, the most heat resistant form of bacteria. Autoclaving is commonly used to sterilize medical equipment, laboratory glassware, media bottles, and other heat stable items.
Dry Heat Sterilization
Dry heat sterilization involves the use of hot air at temperatures of 160-180°C (320-356°F). The required exposure time is longer, from 2-6 hours, to effectively penetrate objects and kill microbial contaminants without moisture. Dry heat tends to penetrate objects better than moist heat. It is commonly used for sterilizing glassware, powders, oils and anhydrous liquids. The high temperatures dehydrate bacterial endospores and oxidize cell constituents to destroy microorganisms.
Steam Sterilization
Steam sterilization or moist heat sterilization uses saturated steam under pressure. Typical conditions are 121°C (250°F) for 15-20 minutes or 134°C (273°F) for 3-4 minutes. Steam sterilization penetrates objects better than dry heat and kills microbes quickly due to the moist environment. It is used for sterilizing medical devices, reusable labware, culture media, and liquids. Care must be taken with moisture sensitive items.
Pasteurization
Pasteurization applies milder heat of 60-100°C (140-212°F) to destroy pathogenic microbes. It is not true sterilization but extends the shelf life of foods by significantly reducing microbial loads. The time and temperature parameters vary based on the type of food being treated. For example, milk may be pasteurized by heating it to 72°C (161°F) for 15 seconds. Pasteurization allows foods to retain more nutrients and flavor than full sterilization.
Flaming
Flaming is a quick and simple heat-based sterilization method that involves passing an object through the blue cone portion of a Bunsen burner flame. The flame burns off microbial contaminants. It is suitable for sterilizing small metal instruments like inoculating loops. Proper technique is required to ensure no microbes survive.
Incineration
Incineration is burning in a very hot oven or furnace at temperatures from 200-1000°C. It completely destroys microorganisms and is used to sterilize medical waste before disposal. However, toxic emissions have limited the use of incinerators. A minimum temperature of 800°C for at least 1 second is typically used for medical waste.
Chemical Sterilization
Chemical sterilizing agents can also be used to denature proteins and enzymes in microbes and achieve sterility. Common chemical sterilants and disinfectants include:
- Ethylene oxide – a highly effective gas sterilant for heat sensitive items. It alkylates nucleic acids and proteins.
- Formaldehyde – an effective surface and instrument disinfectant that denatures proteins and DNA.
- Glutaraldehyde – a liquid chemical sterilant and high level disinfectant.
- Sodium hypochlorite (bleach) – a broad spectrum surface disinfectant effective against bacteria and viruses.
- Alcohols – commonly used to disinfect skin and surfaces; inhibit protein and DNA synthesis.
- Phenolics – surface disinfectants that disrupt cell membranes.
Chemical sterilization has the advantages of penetrating objects, low temperature use, and compatibility with heat sensitive items. However, chemicals must be rinsed from objects that will contact patients or food products.
Radiation Sterilization
Ionizing radiation can penetrate deeply into materials to kill microbes and achieve sterility. Gamma irradiation and electron beam radiation break microbial DNA chains and disrupt cell proteins. Typical radiation doses are in the range of 25-40 kGy. Radiation works well for sterilizing medical products, pharmaceuticals, and heat-sensitive disposables.
Filtration Sterilization
Filters with very small pore sizes can remove bacteria and other microorganisms from gases and liquids. Membrane filtration uses filters with pore sizes of 0.2 μm or less to achieve sterility by mechanically trapping microbes larger than the pore size. Common sterilizing grade filters are made of cellulose acetate, polyvinylidene fluoride (PVDF), nylon, and PTFE. Filtration is useful for sterilizing heat sensitive solutions such as nutrients broths.
Conclusion
In summary, sterile conditions are typically achieved by removal or destruction of all microbial life through the use of heat, radiation, chemicals, or filtration. Autoclaving at 121-134°C is a common heat-based sterilization method used in medical settings. However, lower temperatures such 60-100°C achieved through pasteurization can reduce pathogenic microbes and extend food shelf life. Chemical disinfectants and antiseptics also play an important role in preventing transmission of pathogens. Understanding the different sterilization methods and required parameters allows proper selection based on the items to be sterilized and desired degree of microbial inactivation.