Physical methods of sterilization- Heat, Filtration, Radiation

Sterilization is the complete removal of microorganisms from an object or surface. Microorganisms are tiny living organisms, such as bacteria, viruses, fungi, and parasites, that can cause infections or diseases in humans, animals, and plants. Sterilization is important for preventing the transmission of infectious pathogens and ensuring the safety and quality of various products and services.

Sterilization can be achieved by applying different methods that destroy or eliminate all forms of microbial life. These methods can be classified into physical, chemical, and physiochemical methods. Physical methods use heat, radiation, filtration, or mechanical forces to kill or remove microorganisms. Chemical methods use liquid or gaseous chemicals to disrupt the structure or function of microorganisms. Physiochemical methods combine physical and chemical factors to enhance the sterilization effect.

The choice of sterilization method depends on several factors, such as the type and number of microorganisms present, the nature and composition of the object or surface to be sterilized, the desired level of sterility assurance, the availability and cost of the method, and the environmental and safety implications of the method. Some methods are more suitable for certain materials or products than others. For example, heat sterilization is effective for thermostable items but may damage heat-sensitive items. Similarly, chemical sterilization may be convenient for liquids or gases but may leave toxic residues on solid items.

In this article, we will focus on some of the most common physical methods of sterilization: heat, filtration, and radiation. We will discuss how these methods work, what are their advantages and disadvantages, and what are their applications in various fields. We will also briefly mention some other physical methods of sterilization that are less commonly used but may have potential benefits in certain situations.

Heat sterilization is the most effective and widely used method of sterilization, where the bactericidal activity results from the destruction of enzymes and other essential cell constituents . The effects of heat sterilization occur more rapidly in a fully hydrated state, as it requires a lower heat input, with low temperature and less time, under high humidity conditions where the denaturation and hydrolysis reactions are predominant, rather than in the dry state where oxidative changes take place. Under circumstances where thermal degradation of a product is possible, it can usually be minimized by adopting a higher temperature range, as the shorter exposure times generally result in a lower partial degradation. This method of sterilization is applicable to thermostable products. Still, it can be applied to both moisture-sensitive and moisture-resistant products, for which dry (160–180°C) and moist (121–134°C) heat sterilization procedures are respectively used.

The principle of heat sterilization is based on the concept of sterility assurance level (SAL), which is the probability of a single viable microorganism occurring on a product after sterilization. SAL is normally expressed as 10^-n^. For example, if the probability of a spore surviving were one in one million, the SAL would be 10^-6^. In short, a SAL is an estimate of lethality of the entire sterilization process and is a conservative calculation. Medical devices that have contact with sterile body tissues or fluids are considered critical items. These items should be sterile when used because any microbial contamination could result in disease transmission. Such items include surgical instruments, biopsy forceps, and implanted medical devices. If these items are heat resistant, the recommended sterilization process is steam sterilization, because it has the largest margin of safety due to its reliability, consistency, and lethality. However, reprocessing heat- and moisture-sensitive items requires use of a low-temperature sterilization technology (e.g., ethylene oxide, hydrogen peroxide gas plasma, peracetic acid).

Heat sterilization can be classified into two types: moist heat sterilization and dry heat sterilization. The following table summarizes the main differences between them:

Some examples of moist heat sterilization methods are autoclaving, pasteurization, boiling, and tyndallization. Some examples of dry heat sterilization methods are hot air oven, incineration, flaming, infrared radiation, and red heat . Each method has its own advantages and disadvantages depending on the type of material to be sterilized and the level of sterility required. The specific times, temperatures, and other process parameters must be determined for each type of material being sterilized and the amount and configuration of the load in the autoclave chamber.

Heat sterilization is a time- and temperature-dependent variable that can effectively eliminate microorganisms from various objects and surfaces. However, it also has some limitations such as thermal degradation of some materials, poor penetration power of dry heat, risk of burns or fire hazards, etc. Therefore, it is important to select the appropriate method of heat sterilization based on the nature and compatibility of the material to be sterilized .

Moist heat sterilization is one of the most effective methods of sterilization where the steam under pressure acts as a bactericidal agent. Moist heat sterilization usually involves the use of steam at temperatures in the range 121–134°C . High pressure increases the boiling point of water and thus helps achieve a higher temperature for sterilization. High pressure also facilitates the rapid penetration of heat into deeper parts of material and moisture present in the steam causes the coagulation of proteins causing an irreversible loss of function and activity of microbes. The high temperature-short time cycles not only often result in lower fractional degradation, but they also provide the advantage of achieving higher levels of sterility assurance due to more significant inactivation factors. The most commonly used standard temperature-time cycles for clinical porous specimens (e.g. surgical dressings) and bottled fluids are 134°C for 3 minutes and 121°C for 15 minutes, respectively .

An autoclave is a device that works on the principle of moist heat sterilization through the generation of steam under pressure . In this method, the microorganisms are killed by coagulating their proteins, and this method is much more effective than dry heat sterilization where microbes are killed through oxidation. In the pharmaceutical and medical sectors, it is used in the sterilization of dressings, sheets, surgical and diagnostic equipment, containers, and aqueous injections, ophthalmic preparations, and irrigation fluids, in addition to the processing of soiled and contaminated items .

Moist heat can be used in sterilization at different temperatures:

• At temperatures below 100°C: The sterilization technique employed at a temperature below 100°C involves pasteurization. In this process, all non-spore forming microbes are killed in milk by subjecting the milk to a temperature of 63°C for 30 minutes (the holder method) or 73°C for 20 seconds (the flash method). In pasteurization, however, not all the pathogenic organisms are killed. The principle of pasteurization is the logarithmic reduction in the number of viable microbes so that they can no longer cause diseases. All mesophilic non-sporing bacteria can be killed by exposure to a moist heat at 60C for half an hour with the exception of some organisms which require different temperature-time cycles. The milk is not heated above its boiling point as the milk might curdle, and its nutritional value might be destroyed. Besides milk, other fluids and equipment like vaccines of non-sporing bacteria are also pasteurized at 60°C for 1 hour in special water baths. Similarly, serum and body fluids with congealable proteins are also sterilized at 56°C for 1 hour in water baths.
• At a temperature of 100°C: Boiling at 100°C is a moist heat sterilization technique that doesn’t ensure complete sterility, but is enough for the removal of pathogenic vegetative microbes and some spores. In this case, the items to be sterilized are immersed in boiling distilled water for 30-40 minutes. Distilled water is preferred because hard water might result in the formation of a film of calcium salts on the instruments. Tyndallization is a method that is used for sterilization of media with sugar and gelatin at 100°C for 30 minutes on three successive days so as to preserve sugar which might be decomposed at a higher temperature. Moist heat at 100°C is applicable for contaminated dishes, beddings, pipettes, and other instruments that are not soiled or contaminated as well as for objects that are temperature sensitive.
• At temperatures above 100°C: Moist heat sterilization above 100°C involves sterilization by steam under pressure. Water usually boils at 100°C under normal atmospheric pressure (760 mm of Hg); however, the boiling point of water increases if the pressure is to be increased. This principle is employed in an autoclave where the water boils at 121°C at the pressure of 15 psi or 775 mm of Hg. As a result, the steam under pressure has a higher penetrating power. When this steam comes in contact on the surface, it kills the microbes by giving off latent heat. The condensed liquid ensures the moist killing of the microbes. Autoclaves are used for the sterilization of contaminated instruments along with different culture media as it ensures complete sterility.

Dry heat sterilization is the process of removing microorganisms by applying moisture-free heat which is appropriate for moisture-sensitive substances. The dry heat sterilization process is based on the principle of conduction; that is the heat is absorbed by the outer surface of an item and then passed onward to the next layer. The entire item will eventually reach the proper temperature needed to achieve sterilization.

Dry heat does most of the damage by oxidizing molecules. The essential cell constituents are destroyed and the organism dies. The temperature is maintained for almost an hour to kill the most difficult of the resistant spores. The most common time-temperature relationships for sterilization with dry heat sterilizers are 170°C (340°F) for 30 minutes, 160°C (320°F) for 60 minutes, and 150°C (300°F) for 150 minutes or longer depending on the volume. Bacillus atrophaeus spores should be used to monitor the sterilization process for dry heat because they are more resistant to dry heat than the spores of Geobacillus stearothermophilus.

There are two types of dry-heat sterilizers: the static-air type and the forced-air type. The static-air type is referred to as the oven-type sterilizer as heating coils in the bottom of the unit cause the hot air to rise inside the chamber via gravity convection. This type of dry-heat sterilizer is much slower in heating, requires a longer time to reach sterilizing temperature, and is less uniform in temperature control throughout the chamber than is the forced-air type. The forced-air or mechanical convection sterilizer is equipped with a motor-driven blower that circulates heated air throughout the chamber at a high velocity, permitting a more rapid transfer of energy from the air to the instruments.

Dry heat sterilization has some advantages and disadvantages compared to moist heat sterilization. Some of the advantages are:

• A dry heat cabinet is easy to install and has relatively low operating costs;
• It penetrates materials;
• It is nontoxic and does not harm the environment;
• And it is noncorrosive for metal and sharp instruments.

Some of the disadvantages are:

• Time-consuming method because of a slow rate of heat penetration and microbial killing;
• It may damage some materials such as rubber, plastics, and paper;
• And it requires higher temperatures than moist heat sterilization.

Dry heat sterilization is used for the sterilization of materials which are difficult to sterilize by moist heat sterilization for several reasons. Substances like oil, powder, and related products cannot be sterilized by moist heat because moisture cannot penetrate into deeper parts of oily materials, and powders are destroyed by moisture. Similarly, laboratory equipment like Petri dishes and pipettes are challenging to sterilize by moist heat due to the penetration problem. Dry heat ovens are used to sterilize items that might be damaged by moist heat or that are impenetrable to moist heat (e.g., powders, petroleum products, sharp instruments). Dry heat sterilization also destroys bacterial endotoxins (which are products of Gram-negative bacteria that cause fever when injected into the body) which are difficult to eliminate through other sterilization techniques.

Some examples of dry heat sterilization methods are:

• Red Heat: Rest heat sterilization is the process of instant sterilization by holding the instruments in a Bunsen flame till they become red hot. This method is commonly used for sterilization of instruments like incubation loops, wires, and points of forceps.
• Flaming: Flaming is a type of dry sterilization that involves exposure of metallic objects to flame for some time where the flame burns microbes and other dust presents in the instrument. In this case, the instrument is dipped in alcohol or spirit before burning it in a gas flame. This process doesn’t ensure sterility and is not as effective as red hot sterilization.
• Incineration: Incineration is the process of sterilization along with a significant reduction in the volume of the wastes. It is usually conducted during the final disposal of hospital or other residues. The scraps are heated till they become ash which is then disposed of later. This process is conducted in a device called incinerator.
• Infrared radiation: Infrared radiation (IR) is a method of thermal sterilization in which the radiation is absorbed and then converted into heat energy. For this purpose, a tunnel containing an IR source is used. The instruments and glassware to be sterilized are kept in a tray are then passed through the tunnel on a conveyer belt, moving at a controlled speed. During this movement, the instruments will be exposed to the radiation, which will result in a temperature of about 180°C for about 17 minutes.
• Hot air oven: Hot air oven is a method of dry heat sterilization which allows the sterilization of objects that cannot be sterilized by moist heat. It uses the principle of conduction in which the heat is first absorbed by the outer surface and is then passed into the inner layer. A hot air oven consists of an insulated chamber that contains a fan, thermocouples, temperature sensor, shelves and door locking controls. The commonly-used temperatures and time that hot air ovens need to sterilize materials are 170°C for 30 minutes, 160°C for 60 minutes, and 150°C for 150 minutes or longer depending on the volume . These ovens have applications in the sterilization of glassware, Petri plates, and even powder samples.

Filtration is a method of sterilization that removes microorganisms from liquids and gases by passing them through a filter with pores too small for the microbes to pass through. It is suitable for heat-sensitive substances that cannot be sterilized by moist or dry heat. Filtration does not destroy the microorganisms, but rather separates them from the fluid or gas.

Filtration works by physically trapping particles larger than the pore size and by retaining somewhat smaller particles via electrostatic attraction of the particles to the filters. The pore size of the filter depends on the size range of the contaminants to be excluded. The most commonly used filter is composed of nitrocellulose and has a pore size of 0.22 μm, which can retain all bacteria and spores but not all viruses.

Filtration can be performed by applying positive pressure above the fluid or gas, or by applying negative pressure below the filter. The filter material can be made of different materials, such as asbestos, diatomaceous earth, porcelain, sintered glass, cellulose, or borosilicate glass fiber. Some filters are disposable, while others can be reused after cleaning or sterilizing.

Filtration has several applications in sterilization, such as:

• Sterilization of heat-sensitive liquids, such as antibiotic solutions, vaccines, ophthalmic preparations, irrigation fluids, and biological products.
• Sterilization of medical gases, such as oxygen, nitrogen, and carbon dioxide.
• Sterilization of air in isolation rooms, operating rooms, biological safety cabinets, and laminar air flow cabinets.
• Sterilization of venting or displacement air in tissue and microbiological culture.
• Sterility testing of solutions by trapping and concentrating contaminating organisms on the filter and then incubating them in a nutrient medium.

Some advantages of filtration sterilization are:

• It is nontoxic and does not harm the environment.
• It does not alter the chemical or physical properties of the fluid or gas.
• It does not require high temperature or pressure.
• It is fast and effective for small volumes.

Some limitations of filtration sterilization are:

• It cannot be used for solids or viscous liquids.
• It may clog or damage the filter if the fluid or gas contains large particles or debris.
• It may not remove all viruses or prions.
• It may require prefiltration or postfiltration steps to ensure sterility.
  
  
  

  Irradiation

Irradiation is the process of exposing surfaces and objects to different kinds of radiation for sterilization. Mainly electromagnetic radiation is used for sterilization. The major target for these radiations is considered to be microbial DNA, where damage occurs as a result of ionization and free radical production (gamma-rays and electrons) or excitation (UV light).

There are two types of irradiation sterilization: ultraviolet (non-ionizing) radiation and ionizing radiation.

Ultraviolet (non-ionizing) radiation

Ultraviolet radiation includes light rays from 150-3900 Å, of which 2600 Å has the highest bactericidal effect. Non-ionizing waves have a very little penetration power, so microorganisms only on the surface are killed. Upon exposure, these waves are absorbed by many materials, particularly nucleic acids. The waves, as a result, cause the formation of pyrimidine dimers which bring error in DNA replication and cause the death of microbes by mutation.

UV radiation owing to its poor penetrability of conventional packaging materials is unsuitable for sterilization of pharmaceutical dosage forms. It is, however, applied in the sterilization of air, for the surface sterilization of aseptic work areas, and the treatment of manufacturing-grade water.

Ionizing Radiation

X-ray and gamma rays are the commonly used ionizing radiation for sterilization. These are high energy radiation which causes ionization of various substances along with water. The ionization results in the formation of a large number of toxic O2 metabolites like hydroxyl radical, superoxide ion, and H2O2 through ionization of water. These metabolites are highly oxidizing agents and kill microorganisms by oxidizing various cellular components.

With ionizing radiation, microbial resistance decreases with the presence of moisture or dissolved oxygen (as a result of increased free radical production) and also with elevated temperatures. Radiation sterilization is generally exposed to items in the dried state which include surgical instruments, sutures, prostheses, unit-dose ointments, plastic syringes, and dry pharmaceutical products.

The most common source of gamma rays for sterilization is Cobalt 60, which emits high-energy photons that can penetrate deeply into materials and disrupt the DNA of microorganisms. Gamma irradiation is a physical/chemical means of sterilization, because it kills bacteria by breaking down bacterial DNA, inhibiting bacterial division. Gamma irradiation can also destroy bacterial endotoxins (pyrogens) which are difficult to eliminate through other sterilization techniques.

Gamma irradiation has several advantages over other sterilization methods:

• It can be applied after packaging, thus eliminating the risk of recontamination.
• It does not leave any toxic residues or harmful by-products.
• It does not affect the physical or chemical properties of most materials.
• It can be easily validated and monitored with dosimeters.
• It can achieve high levels of sterility assurance.

However, gamma irradiation also has some limitations and challenges:

• It requires a radioactive source that needs proper handling and disposal.
• It may cause some changes in certain polymers (plastic or resin), such as discoloration, embrittlement, or cross-linking.
• It may affect some biological products, such as enzymes or hormones.
• It may be affected by environmental factors, such as temperature or humidity.
• It may be expensive and not widely available.

Ionizing radiation can also be generated by X-ray machines or electron beam accelerators. These sources have similar effects as gamma rays but differ in their penetration power and dose rate. X-rays have higher penetration power than gamma rays but lower than electron beams. Electron beams have lower penetration power but higher dose rate than gamma rays or X-rays. Therefore, X-rays and electron beams are more suitable for thin or low-density materials.

X-rays and electron beams have some advantages over gamma rays:

• They do not require a radioactive source and can be switched on and off as needed.
• They have shorter exposure times and higher throughput rates.
• They have less environmental impact and regulatory issues.

However, X-rays and electron beams also have some disadvantages compared to gamma rays:

• They have lower penetration power and may require multiple passes or orientations to achieve uniform dose distribution.
• They may cause more damage to heat-sensitive materials due to higher dose rate.

• They may require more shielding and safety measures due to higher energy output.

  Sound Waves Vibration

Sound waves vibration is a method of sterilization that uses high-frequency sound waves to kill microorganisms by disrupting their cell membranes and causing them to lyse. This method is also known as ultrasonic cleaning or sonication.

Sound waves vibration is based on the principle of cavitation, which is the formation and collapse of microscopic bubbles in a liquid due to the alternating compressive and tensile forces of the sound waves. The collapse of these bubbles generates shock waves and high temperatures that damage the microorganisms and other debris present in the liquid.

Sound waves vibration can be applied to both liquids and solids, as long as they are immersed in a suitable solvent that can transmit the sound waves. The solvent can be water or a detergent solution, depending on the type and amount of soil to be removed. The frequency of the sound waves can range from 20 to 40 kHz, depending on the size and shape of the objects to be sterilized.

Sound waves vibration has several advantages over other methods of sterilization, such as:

• It can reach inaccessible areas of complex instruments, such as lumens, joints, and serrations.
• It can remove organic and inorganic matter, such as blood, tissue, salts, and rust.
• It can be gentle on delicate instruments, such as microsurgical and ophthalmic devices.
• It can be fast and efficient, requiring only a few minutes of exposure.

However, sound waves vibration also has some limitations, such as:

• It cannot ensure complete sterility, as some spores and viruses may resist the sound waves.
• It cannot penetrate conventional packaging materials, such as plastic or paper.
• It may cause damage to some materials, such as rubber or glass.
• It may generate aerosols that can contaminate the environment or pose a health risk.

Therefore, sound waves vibration is usually used as a pre-cleaning step before terminal sterilization by other methods, such as moist heat or radiation. Sound waves vibration is commonly used in healthcare facilities to clean surgical and dental instruments, as well as in industrial applications to clean filters, valves, and pipes.

Pressure (Pascalization)

Pressure sterilization, also known as pascalization, bridgmanization, or high-pressure processing (HPP), is a method of preserving and sterilizing food by applying very high pressure (typically 40,000 to 80,000 psi) to the food products inside pressure chambers.

The high pressure causes the inactivation of certain microorganisms and enzymes in the food without affecting the covalent bonds of the food molecules. This preserves the sensory and nutritional qualities of the food while extending its shelf life and ensuring its safety.

The principle of pressure sterilization was named after Blaise Pascal, a 17th century French scientist who studied the effects of pressure on fluids. The technique was first demonstrated to kill microorganisms by B. H. Hite in 1899.

Pressure sterilization is suitable for heat-sensitive foods that can withstand high pressure without being crushed or deformed. Some examples of foods that can be treated by pressure sterilization are juices, fruits, yogurts, salad dressings, rice cakes, guacamole, fish, shellfish, and ready-to-eat meats.

Pressure sterilization can kill most vegetative microorganisms, such as bacteria, yeasts, and molds, but it is less effective against spores. Therefore, pressure sterilization is often combined with other treatments, such as heat or acidification, to achieve complete sterility or pasteurization.

Pressure sterilization has some advantages over other methods of sterilization, such as:

• It does not require high temperatures or chemical additives, which can alter the flavor, texture, color, or nutrient content of the food.
• It can be applied to both solid and liquid foods in their final packaging, reducing the risk of recontamination and saving energy and time.
• It can destroy bacterial endotoxins (pyrogens), which are heat-stable toxins produced by some gram-negative bacteria that can cause fever when injected into the body.
• It can reduce the allergenicity of some foods by modifying their protein structure.

Some challenges and limitations of pressure sterilization are:

• It requires specialized equipment and facilities that are costly and complex to operate and maintain.
• It may not be effective against some viruses or prions that are resistant to high pressure.
• It may cause some changes in the physical properties of some foods, such as texture, water activity, or color.
• It may not be able to penetrate into air pockets or dry solids in some foods.

Pressure sterilization is a promising technique for food preservation and sterilization that offers many benefits for consumers and producers. However, it also requires further research and development to optimize its performance and applicability for different types of foods and microorganisms.

Sunlight (Solar Disinfection)

Sunlight is a natural source of energy that can be used for disinfection of water. Solar disinfection, or SODIS, is a method of using sunlight to inactivate microbes in biologically contaminated water. The contaminated water is placed in transparent containers and is then exposed to strong sunlight for at least 6–8 hours .

The principle of solar disinfection is based on the germicidal effect of UV light and its synergistic effect with rise in water temperature. The UV-A rays (wavelength 320–400 nm) in sunlight can damage the DNA and RNA of microorganisms, preventing them from reproducing or infecting . The heat from the sun can also increase the water temperature, which enhances the disinfection process. Water heated to 50°C or more can kill most pathogens within an hour.

Solar disinfection is a simple, low-cost and environmentally friendly method that can be applied at the household level. It is especially suitable for rural areas where access to safe drinking water is limited and electricity is scarce. It can also reduce the use of chemical disinfectants such as chlorine, which may have adverse effects on human health and the environment.

However, solar disinfection also has some limitations and challenges. It requires clear PET plastic bottles or glass containers, which may not be readily available or durable in some regions. It also depends on the weather conditions, such as cloud cover, humidity and air pollution, which can affect the intensity and duration of sunlight exposure. Moreover, it cannot remove non-biological contaminants such as toxic chemicals or heavy metals, which may require additional treatment steps .

Therefore, solar disinfection should be used with caution and proper monitoring. The water quality should be tested before and after the treatment to ensure its safety and effectiveness. The turbidity of the water should be less than 30 NTU, otherwise it should be filtered or settled before exposure to sunlight. The containers should be clean and transparent, without any scratches or labels that may block the sunlight. The containers should be placed on a flat surface with maximum exposure to direct sunlight, preferably on a reflective surface such as metal sheets or aluminum foil. The water should be consumed within 24 hours after the treatment, or stored in a dark and cool place to prevent recontamination.

Solar disinfection is a promising method for providing safe drinking water to millions of people who lack access to other sources of water treatment. However, it requires further research and development to optimize its performance and overcome its limitations. It also needs more awareness and education among the potential users to ensure its proper application and acceptance.

Pressure sterilization, also known as pascalization, bridgmanization, or high-pressure processing (HPP), is a method of preserving and sterilizing food by applying very high pressure (typically 40,000 to 80,000 psi) to the food products inside pressure chambers.

The high pressure causes the inactivation of certain microorganisms and enzymes in the food without affecting the covalent bonds of the food molecules. This preserves the sensory and nutritional qualities of the food while extending its shelf life and ensuring its safety.

The principle of pressure sterilization was named after Blaise Pascal, a 17th century French scientist who studied the effects of pressure on fluids. The technique was first demonstrated to kill microorganisms by B. H. Hite in 1899.

Pressure sterilization is suitable for heat-sensitive foods that can withstand high pressure without being crushed or deformed. Some examples of foods that can be treated by pressure sterilization are juices, fruits, yogurts, salad dressings, rice cakes, guacamole, fish, shellfish, and ready-to-eat meats.

Pressure sterilization can kill most vegetative microorganisms, such as bacteria, yeasts, and molds, but it is less effective against spores. Therefore, pressure sterilization is often combined with other treatments, such as heat or acidification, to achieve complete sterility or pasteurization.

Pressure sterilization has some advantages over other methods of sterilization, such as:

• It does not require high temperatures or chemical additives, which can alter the flavor, texture, color, or nutrient content of the food.
• It can be applied to both solid and liquid foods in their final packaging, reducing the risk of recontamination and saving energy and time.
• It can destroy bacterial endotoxins (pyrogens), which are heat-stable toxins produced by some gram-negative bacteria that can cause fever when injected into the body.
• It can reduce the allergenicity of some foods by modifying their protein structure.

Some challenges and limitations of pressure sterilization are:

• It requires specialized equipment and facilities that are costly and complex to operate and maintain.
• It may not be effective against some viruses or prions that are resistant to high pressure.
• It may cause some changes in the physical properties of some foods, such as texture, water activity, or color.
• It may not be able to penetrate into air pockets or dry solids in some foods.

Pressure sterilization is a promising technique for food preservation and sterilization that offers many benefits for consumers and producers. However, it also requires further research and development to optimize its performance and applicability for different types of foods and microorganisms.

Sunlight is a natural source of energy that can be used for disinfection of water. Solar disinfection, or SODIS, is a method of using sunlight to inactivate microbes in biologically contaminated water. The contaminated water is placed in transparent containers and is then exposed to strong sunlight for at least 6–8 hours .

The principle of solar disinfection is based on the germicidal effect of UV light and its synergistic effect with rise in water temperature. The UV-A rays (wavelength 320–400 nm) in sunlight can damage the DNA and RNA of microorganisms, preventing them from reproducing or infecting . The heat from the sun can also increase the water temperature, which enhances the disinfection process. Water heated to 50°C or more can kill most pathogens within an hour.

Solar disinfection is a simple, low-cost and environmentally friendly method that can be applied at the household level. It is especially suitable for rural areas where access to safe drinking water is limited and electricity is scarce. It can also reduce the use of chemical disinfectants such as chlorine, which may have adverse effects on human health and the environment.

However, solar disinfection also has some limitations and challenges. It requires clear PET plastic bottles or glass containers, which may not be readily available or durable in some regions. It also depends on the weather conditions, such as cloud cover, humidity and air pollution, which can affect the intensity and duration of sunlight exposure. Moreover, it cannot remove non-biological contaminants such as toxic chemicals or heavy metals, which may require additional treatment steps .

Therefore, solar disinfection should be used with caution and proper monitoring. The water quality should be tested before and after the treatment to ensure its safety and effectiveness. The turbidity of the water should be less than 30 NTU, otherwise it should be filtered or settled before exposure to sunlight. The containers should be clean and transparent, without any scratches or labels that may block the sunlight. The containers should be placed on a flat surface with maximum exposure to direct sunlight, preferably on a reflective surface such as metal sheets or aluminum foil. The water should be consumed within 24 hours after the treatment, or stored in a dark and cool place to prevent recontamination.

Solar disinfection is a promising method for providing safe drinking water to millions of people who lack access to other sources of water treatment. However, it requires further research and development to optimize its performance and overcome its limitations. It also needs more awareness and education among the potential users to ensure its proper application and acceptance.

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