Sterilization(redirected from dry heat sterilization)
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An act of destroying all forms of life on and in an object. A substance is sterile, from a microbiological point of view, when it is free of all living microorganisms. Sterilization is used principally to prevent spoilage of food and other substances and to prevent the transmission of diseases by destroying microbes that may cause them in humans and animals. Microorganisms can be killed either by physical agents, such as heat and irradiation, or by chemical substances.
Heat sterilization is the most common method of sterilizing bacteriological media, foods, hospital supplies, and many other substances. Either moist heat (hot water or steam) or dry heat can be employed, depending upon the nature of the substance to be sterilized. Moist heat is also used in pasteurization, which is not considered a true sterilization technique because all microorganisms are not killed; only certain pathogenic organisms and other undesirable bacteria are destroyed. See Pasteurization
Many kinds of radiations are lethal, not only to microorganisms but to other forms of life. These radiations include both high-energy particles as well as portions of the electromagnetic spectrum. See Radiation biology
Filtration sterilization is the physical removal of microorganisms from liquids by filtering through materials having relatively small pores. Sterilization by filtration is employed with liquid that may be destroyed by heat, such as blood serum, enzyme solutions, antibiotics, and some bacteriological media and medium constituents. Examples of such filters are the Berkefeld filter (diatomaceous earth), Pasteur-Chamberland filter (porcelain), Seitz filter (asbestos pad), and the sintered glass filter.
Chemicals are used to sterilize solutions, air, or the surfaces of solids. Such chemicals are called bactericidal substances. In lower concentrations they become bacteriostatic rather than bactericidal; that is, they prevent the growth of bacteria but may not kill them. Other terms having similar meanings are employed. A disinfectant is a chemical that kills the vegetative cells of pathogenic microorganisms but not necessarily the endospores of spore-forming pathogens. An antiseptic is a chemical applied to living tissue that prevents or retards the growth of microorganisms, especially pathogenic bacteria, but which does not necessarily kill them.
The desirable features sought in a chemical sterilizer are toxi-city to microorganisms but nontoxicity to humans and animals, stability, solubility, inability to react with extraneous organic materials, penetrative capacity, detergent capacity, noncorro-siveness, and minimal undesirable staining effects. Rarely does one chemical combine all these desirable features. Among chemicals that have been found useful as sterilizing agents are the phenols, alcohols, chlorine compounds, iodine, heavy metals and metal complexes, dyes, and synthetic detergents, including the quaternary ammonium compounds.
(1) The method by which a substance, object, or food product is completely freed of live microorganisms. The most common sterilization techniques are heat sterilization and filtration sterilization; the latter is used with liquids and is characterized by the removal of microbial cells with filters. The vegetative cells of most bacteria, yeasts, and microscopic fungi die at temperatures of 50°-70°C within 30 min, whereas the spores of many bacteria can withstand prolonged boiling. This explains why high temperatures are used in sterilization. The simplest sterilization method is heating metal and glass objects on a flame burner. Hot-air sterilization is conducted in hot-air sterilizers at temperatures of 160°-165°C for two hours (hr). This method is used to sterilize laboratory glassware, metal objects, some powder-like materials, and substances that are not damaged by heating. Moist-heat sterilization is carried out in autoclaves with steam under pressure. Microorganic nutrient mediums are sterilized at 4 atmospheres (atm) and 121°C for 20–30 min or at 0.5 atm and 112°C for 20 min. Surgical instruments, dressings, and sutures and various canned foods are usually sterilized at 1 atm for 30 min. Soil may be sterilized only at 2 atm and 134°C for 2 hr.
Some liquids and solutions cannot be sterilized at high temperatures because the temperatures cause the evaporation or inacti-vation of vitamins and other biologically active compounds, the decomposition of drugs, the caramelization of sugars, and the de-naturation of proteins. Under these conditions, heat is not used, and liquids are passed through bacteriological filters having fine pores. Chemical sterilization is used on solid objects that may be damaged by heat, for example, some plastics and electronic apparatus. Chemical sterilizing agents include gases (ethylene oxide mixed with C02 or methyl bromide), alcohol, and mercuric chloride solutions. Radiation sterilization, with doses of ionizing radiation usually at 3–10 million rads, can also be used on solid objects that may be damaged by heat. The number of microorganisms present in the air of enclosed areas, including operating rooms and plants where antibiotics are packaged, can be reduced by ultraviolet radiation, which is bactericidal.
Sterilization is widely used in microbiological and other scientific research, in medicine, and in the food-processing industry. Spacecraft are sterilized in order to prevent the possible contamination of other planets by microorganisms from earth. Sterility is demonstrated by the complete absence of live microorganisms within an object. For this purpose, liquid or solid nutrient-rich mediums are inoculated to allow for the growth of cells that have been damaged but not completely destroyed.
A. A. IMSHENETSKII
(2) The surgical procedure by which an individual is made incapable of reproduction. Unlike castration, the hormonal regulation of sexual function is maintained.