Sterilization methods

Heat is the most widely used means of sterilization, which can be employed for both liquid medium and heatable solid objects. It can be applied as dry or moist heat (steam). The moist heat is more effective than the dry heat, because the intrinsic heat resistance of vegetative bacterial cells is greatly increased in a completely dry state. As a result the death rate is much lower for the dry cells than for moist ones. The heat conduction in dry air is also less rapid than in steam. Therefore, dry heat is used only for the sterilization of glassware or heatable solid materials. By pressurizing a vessel, the steam temperature can be increased significantly above the boiling point of water. Laboratory autoclaves are commonly operated at a steam pressure of about 30 psia, which corresponds to 121°C. Even bacterial spores are rapidly killed at 121 °C. 

Chemical agents can be used to kill microorganisms as the result of their oxidizing or alkylating abilities. However, they cannot be used for the sterilization of medium because the residual chemical can inhibit thefermentation organisms. Chemical agents are frequently 

Many cellular materials absorb ultraviolet light, leading to DNA damage and consequently to cell death. Wavelengths around 265 nm have the highest bactericidal efficiency. However, ultraviolet rays have very little ability to penetrate matter. Therefore, their use is limited to the reduction of microbial population in a room where sterility needs to be maintained, such as hospital operating rooms or clean chambers in a laboratory. X-rays are lethal to microorganisms and have penetration ability. However, they are impractical as sterilization tools due to their expense and safety concerns. 

Sonic or ultrasonic waves of sufficient intensity can disrupt and kill cells. This technique is usually employed in the disruption of cells for the purpose of extracting mtracellular constituents rather than as a sterilization technique. 

Filtration is most effectively employed for the removal of microorganisms from air or other gases. In the case of liquid solutions, it is used with thermolabile medium or products, that is, those easily destroyed by heat, such as human and animal serums and enzymes. 

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Milk has been a source for food for humans since the beginning of recorded history. Although the use of fresh milk has increased with economic development, the majority of consumption occurs after milk has been heated, processed, or made into butter. The milk industry became a commercial enterprise when methods for preservation of fluid milk were introduced. The successful evolution of the dairy industry from small to large units of production, ie, the farm to the dairy plant, depended on sanitation of animals, products, and equipment cooling faciUties health standards for animals and workers transportation systems constmction materials for process machinery and product containers pasteurization and sterilization methods containers for distribution and refrigeration for products in stores and homes. 

It is an axiom of sterilization technology that appropriate conditions must be estabUshed throughout the material to be sterilized. The time-at-temperature conditions are critical when considering any sterilization method using heat. 

It is necessary to determine the bioburden and make cycle verification studies when ethylene oxide sterilization is used, as it is for other sterilization methods. The manufacturer of hospital sterilization equipment provides cycle recommendations based on the expected bioburden and the consideration of an appropriate safety factor. In ethylene oxide sterilization, it is necessary to determine if residues of the stefilant are absorbed by the sterilized article, and to examine the possible formation of other potentially toxic materials as a result of reaction with ethylene oxide. 

Other Sterilants. Sterilization methods, developed in response to the requirements of a low temperature, noncorrosive stedlant and rapid turnaround time required by most hospitals, include use of hydrogen peroxide vapor, hydrogen peroxide plasma, and peroxy acetic acid. Acceptance of these methods was not universal as of this writing (ca 1996). 

The most widely used sterilization method ia the food industry is moist heat. The heat is usually suppHed by high pressure steam, but because most foods already contain moisture the role of steam is to heat the food to the required temperature. The cooking and sterilization processes can frequendy be combined into one. The food may be sealed into impervious containers of glass, metal, or plastic film and undergo terminal sterilization, or it may be presterilized in batches or in a continuous operation and then filled into a presterilized container. The latter process is called sterile filling. 

In conclusion, the editors thank most sincerely the contributors to this book, both for complying with our strictures as to the length of their contribution and for providing their material on time, and our publishers for their friendly courtesy and efficiency during the production of this book. We also wish to thank Dr H. J. Smith for his advice on various chemical aspects, Dr M. I. Barnett for useful comments on reverse osmosis, and Mr A. Keall who helped with the table on sterilization methods. 

A sterilization process may thus be developed without a full microbiological background to the product, instead being based on the ability to deal with a worst case condition. This is indeed the situation for official sterilization methods which must be capable of general application, and modem pharmacopoeial recommendations are derived firm a careful analysis of experimental data on bacterial spore survival following treatments with heat, ionizing radiation or gas. 

In the following sections, factors governing the successful use of these sterilizing methods will be covered and their application to pharmaceutical and medical products considered. Methods for monitoring the efficacy of these processes are discussed in Chapter 23. 

Thermostability. The choice of sterilization method depends on the thermostability of the achve ingredient, autoclaving being applied only to dmgs that are heat stable in aqueous solution. 

Sterilization methods have been discussed in Chapter 20 and the various types of sterile products have been described in Chapter 21. For manufacturing purposes an important distinction exists between a sterile product which is termirrally sterilized and one which is not. Terminally sterilized means that, after preparation, the produet is transferred to containers which are sealed and then immediately sterilized by heat (or radiation or... 

Sterilization method Principle Device Parameter(s) monitored... 

Type of dextran Molecular weight (average) Product sterilization method Clinical uses... 

Five sterilization processes are described in the USP steam, dry-heat, filtration, gas, and ionizing radiation. All are commonly used for parenteral products, except gas and ionizing radiation, which are widely used for devices and surgical materials. To assist in the selection of the sterilization method, certain basic information and data must be gathered. This includes determining... 

The USP also recommends the use of biological indicators, whenever possible, to monitor all sterilization methods except sterile filtration. Biological indicators are generally of two types. If a product to be sterilized is a liquid, microorganisms are added directly to carefully identified representative samples of the product. When this is not practical, as with solids or equipment to be sterilized, the culture is added to strips of filter paper. The organism chosen varies with the method of sterilization. 

When drug solutions and containers can withstand autoclaving conditions, this method is preferred to other sterilization methods because moist heat sterilizes quickly and inexpensively. However, judgment must be exercised and experiments run to ensure that the solution and container are permeable to steam. Oils and tightly closed containers, for example, are not normally sterilizable by steam. 

The choice of the primary packaging should be justified. This should include consideration of the safety of the packaging for the patient and user. Account should be taken of the optimal sterilization method for the finished product. 

For sterile products, particular attention should be paid to the choice of an appropriate method of sterilization. Wherever possible a terminal sterilization process should be applied to the product in its final container-closure system, as suggested in the Ph Eur. The preferred options include steam sterilization, dry heat sterilization, and irradiation using the Ph Eur listed conditions (saturated steam at 121°C for 15 minutes dry heat at 160°C for 120 minutes irradiation with an absorbed dose of not less than 25 kGy). Where these cannot be used, the application must include justification for the alternative procedure adopted on the understanding that the highest achievable sterility assurance level should be achieved in conjunction with the lowest practicable level of presterilization bioburden. There is guidance in the form of decision trees as to the preferred options for sterilization method to be applied ... 

Where terminal processing is not possible, the justification for alternative sterilization methods will be included in the EPAR, or at least a statement to the effect that sterile filtration/aseptic processing will be used. Presterilization bioburden issues that arose during the assessment will be included in the EPAR. 

Reissinger, A., Vilich, V., and Sikora, R. A. (2001). Detection of fungi in planta Effectiveness of surface sterilization methods. Mycol. Res. 105,563-566. 

A wide variety of equipment is used for chromatography and, in general, such equipment should be dedicated to the purification of one product and should be sterilized or sanitized between batches. The use of the same equipment at different stages of processing should be discouraged. Acceptance criteria, lifespan and sanitation or sterilization method of columns should be defined. 

Of the possible sterilization methods, dry heat seems to be the only practical and certain method. The procedure established in 1964 by the Jet Propulsion Laboratory, Pasadena, Calif calls for three successive 36-hour heatings at 145°C. The purpose of this work is to establish which of known explosives and pyrotechnic compns can withstand sterilization without being decomposed... 

Investigate sterility test procedures and room sanitation and sterilization methods to eliminate cause. 

The method for sterility testing of (product name) USP is manufacturing site SOP and the microbiological sterility method validation summary report is provided in (provide reference attachment number). The USP bacteriostasis/fungistasis test was performed to validate the sterility test method... 

Sterility testing of (specify name) USP microbiological sterility method validation summary report... 

If sterility test contaminant is same as routine environmental contaminant the sterility test is voided. Investigate sterility test procedures and room sanitation/sterilization methods to eliminate cause. If media-fill environmental contaminant is same as routine environmental contaminant increase the number of media-fill vials in media hll to determine the product risk potential. Review monitoring technique for possible problem. Review personnel practices, gowning, sanitation, and sterilization. 

Some of the chemical classes of antiseptics, disinfectants, and sterilants are described briefly in the text that follows. The reader is referred to the general references for descriptions of physical disinfection and sterilization methods. 

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