Sterilization steam

Given the conditions described above and the caveats below, flasks and other media-containing vessels should not be filled to capacity prior to autoclaving. As a general rule, flasks and so forth, should not be more than two-thirds full. Table B-1 makes even more conservative recommendations (Pawsey, 1974). 

Container Volume Maximum volume prior to autoclave cycle 

Container Type Volume/size Maximum autoclave volume 

Adapted from Pawsey (1974) with the kind permission of R. Pawsey. 

Upon completion of the sterilization cycle, steam is exhausted and the pressure drops to atmospheric levels. The rate of pressure decrease varies with the autoclave but most are equipped with fast and slow exhaust cycles. Fast exhaust is used for glassware, tubing, and utensils, while slow exhaust cycles are used with liquid media. Once the cycle is complete, media should be removed from the autoclave as soon as possible because prolonged heating will cause heat-sensitive components to degrade. 

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The comparison of the effectiveness of sterilization cycles at different temperatures becomes possible. Eor example, for steam sterilization,... 

Steam (qv) sterilization specifically means sterilization by moist heat. The process cannot be considered adequate without assurance that complete penetration of saturated steam takes place to all parts and surfaces of the load to be sterilized (Fig. 1). Steam sterilization at 100°C and atmospheric pressure is not considered effective. The process is invariably carried out under higher pressure in autoclaves using saturated steam. The temperature can be as low as 115°C, but is usually 121°C or higher. 

The critical parameters of steam sterilization are temperature, time, air elimination, steam quaUty, and the absence of superheating. Temperature and time are interrelated, as shown in equation 8. The success of steam sterilization is dependent on direct steam contact which can be prevented by the presence of air in the chamber. The abiUty of steam to heat a surface to a given temperature is considerably reduced by the presence of air. Air elimination, therefore, is regarded as an absolute parameter. If the required amount of air has not been eliminated from the chamber and the load, no combination of time and temperature results in complete sterilization. 

Superheated steam results when steam is heated to a temperature higher than that which would produce saturated steam. The equiUbrium between hquid and vapor is destroyed, and the steam behaves as a gas. It loses its abiUty to condense into moisture when in contact with the cooler surface of the article to be sterilized. This process resembles dry-heat sterilization more than steam sterilization and, under ordinary time—temperature conditions for steam sterilization, does not produce stetihty. 

The selection of an appropriate steam-sterilization cycle must be made after a carefiil study of the nature of the articles to be sterilized, the type and number of organisms present, type and size of each package, and type of packaging material used. Cycle-development studies may be conducted using fiiU autoclave loads. 

Biological indicators for steam sterilization utilize bacillus stearothermophilus. In monitoring industrial cycles, a sufficient number of preparations each having a known degree of resistance are added to the load and retrieved after exposure, and cultured. 

Processing in Hquid sterilants results in wet products which require highly specialized packaging. Therefore, Hquid sterilization should only be considered if the sterilized article is to be used almost immediately. Liquid sterilants or their residues can be harmful to living tissues. Therefore it is always necessary to rinse articles with sterile water or saline solution foUowing treatment. Whereas Hquid sterilization is an extremely useful method for articles that caimot withstand the conditions of steam sterilization, the problems associated with its use limit its appHcation. 

The contents of the seed fermenter were used to inoculate 1,500 liters of steam sterilized medium of the following composition in tap water. 

Fluoro-l 13,17ot-Dihydroxy-21-Acetoxy-1,4-Pregnadiene-3,20-Dione A medium consisting of 1% dextrose hydrate, 2% cornsteep liquor of 60% solids and Kalamazoo tap water was adjusted to pH 4.9 with sodium hydroxide. The medium was steam sterilized at 15 pounds pressure for 30 minutes, cooled, and then inoculated with a 24-hour growth, from spores, of Septomyxa affinis, ATCC 6737. The medium was agitated, sparged with sterile air at the rate of one-tenth volume of air per volume of medium per minute. At the end of 24 hours of fermentation at room temperature, the pH was about 7.4. 

The British Pharmacopoeia (1993) recognizes five methods for the sterilization of pharmaceutical products. These are (i) dry heat (ii) heating in an autoclave (steam sterilization) (iii) filtration (iv) ethylene oxide gas and (v) gamma or electron radiation. In addition, other approaches involving steam and formaldehyde and ultraviolet (UV) light have evolved for use in certain situations. For each method, the possible permutations of exposure conditions are numerous, but experience and product stability... 

Application of the F- value concept has been largely lestrieted to steam sterilization processes although there is a less frequently employed, but direet parallel in dry heat sterilization (see section 4.3). 

Table 20.2 Pressure-temperature relationships and antimicrobial efiScacies of alternative steam sterilization cycles... 

The relationship between temperature and pressure holds true only in the presence of pure steam adulteration with air contributes to a partial pressure but not to the temperature of the steam. Thus, in the presence of air the temperature achieved will reflect the contribution made by the steam and will be lower than that normally attributed to the total pressure recorded. Addition of further steam will raise the temperature but residual air surrounding articles may delay heat penetration or, if a large amount of air is present, it may collect at the bottom of the sterilizer, completely altering the temperature profile of the sterilizer chamber. It is for these reasons that efficient air removal is a major aim in the design and operation of a boiler-fed steam sterilizer. 

Steam sterilizers, or autoclaves as they are sometimes known, are stainless steel vessels designed to withstand the steam pressures employed in sterilization. They can be (i)... 

The F-value concept which was developed for steam sterilization processes has an equivalent in dry heat sterilization although its application has been limited. The Fh designation describes the lethality of a dry heat process in terms of the equivalent number of minutes exposure at 170°C, and in this case a z value of 20°C has been found empirically to be appropriate for calculation purposes this contrast with the value of 10°C which is typically employed to describe moist heat resistance. 

British Standards Institution (1991) Specification for Steam Sterilizers for Aqueous Fluids in Rigid Sealed Containers BS 3970. London BSI. 

Biological indicators (Bis) for use in thermal, chemical or radiation sterilization processes consist of standardized bacterial spore preparations which are usually in the form either of suspensions in water or culture medium or of spores dried on paper, aluminium or plastic carriers. As with chentical indicators, they are usually placed in dummy packs located at strategic sites in the sterilizer. Alternatively, for gaseous sterihzation these may also be placed within a tubular hehx (Line-Pickerill) device. After the sterilization process, the aqueous suspensions or spores on carriers are aseptically transferred to an appropriate nutrient medium which is then incubated and periodically examined for signs of growth. Spores of Bacillus stearothermophilus in sealed ampoules of cultrrre medium are used for steam sterilization morritoring, and these may be incubated directly at 55°C this eliminates the need for an aseptic transfer. 

One of the long-standing criticisms of Bis is that the incubation period required in order to confirm a satisfactory sterilization process imposes an undesirable delay on the release of the product. This problem has been overcome, with respect to steam sterilization at least, by the use of a detection system in which a spore enzyme, a-glucosidase (reflective of spore viability), converts a non-fluorescent substrate into a fluorescent product in as little as Ihour. 

G Chansiri, RT Lyons, MV Patel, SL Hem. Effect of surface charge on the stability of oil/water emulsions during steam sterilization. J Pharm Sci 88(4> 453—458, 1999. 

GC Na, HJ Stevens, BO Yuan, N Rajagopalan. Physical stability of ethyl diatrizoate nanocrystalline suspension in steam sterilization. Pharm Res 16(4) 569-574, 1999. 

Autoclave steam sterilization is a well-established and widely used procedure. Normally, steam enters through the top of the chamber (Fig. 9). Being lighter than air, it remains at the top of the chamber but steadily and continuously drives the air out of the chamber through the bottom vent throughout the sterilization cycle. The velocity of steam entering the... 

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