Continued on next page 282 KNOWLEDGE STERILISATION Sterilisation means killing or irreversibly inactivating all fertile micro-organisms. Basically there are two procedures available for thermal inactivation - steam sterilisation and hot-air sterilisation. Why steam sterilisation? The principle of steam sterilisation is based on the thermo instability of micro-organisms and is about 100 years old. The high heat storage capacity of water, the additional heat released during the condensation of steam and the humidity of the steam are the basis of an outstanding germicidal effect. Different from the chemical sterilisation or the sterilisation with hot air, which are only suitable for thermo-stable devices (glass, instruments, etc.), steam sterilisation established itself in micro-biological laboratories because hot air or steam are significantly more efficient than dry heat of the same temperature. Its heat content is higher and the germs, especially arid spores, swell up and thus become more heat-sensitive. The steam "under pressure", which is generated by heating up water in a closed container, condenses at the sterilising good and the released energy leads to cell death or inactivation of viruses by means of protein coagulation. From sample preparation to destruction sterilisation, a steam steriliser (autoclave) can be used in an environmentally friendly manner. Steam can be well controlled in a technical way, it is not toxic, does not strain the environment, does not leave residues on the sterilising good and can be generated at relatively low costs. Selection criteria • Which statutory regulations have to be met? Law on Genetic Engineering, Federal Epidemic Law, etc. • Are there any other regulations that have to be met? GLP, GMP, etc. • Available space (floor-mounted or table-top unit), which chamber volume is necessary (also for the next 1-2 years)? • Mode of use: sample preparation, liquids, highly infectious material (waste sterilisation) - equipped with exhaust air filter and/or thermo-lock... • Record keeping: integrated printer or software? • Calibration, validation, qualification – which requirements have to be met? • Structural conditions, power, fully desalinated water, disposal, etc. • Operational costs, follow-up costs or customer service? • Charging baskets or charging buckets? • Wheels for mobile use? Procedure - Exhaust The gravitation procedure is one of the procedures that are used. It is a flow procedure during which the saturated steam replaces the air from the sterilisation chamber via an exhaust valve. Generally, the sterilisation always takes place in a saturated steam atmosphere without residue air inside the pressure vessel. Depending on the sterilising good and the micro-organisms, which have to be decontaminated, the work temperature lies at 121°C or 135°C which is about 1.1 respectively 2.2 bar overpressure. In order to attain a safe pressure-temperature correlation, the residue air has to be evacuated carefully (exhaust). During the exhaust phase the chamber insides are not yet sterile (sterilisation temperature and residence time have not been reached yet) and therefore a contaminated steam-air mixture leaves the autoclave. In this case the use of an exhaust air filter is reasonable and even mandatory for some applications. Sterilisation is a function of temperature and time Only sufficient exposure time of the destruction temperature can ensure sterilisation success. In microbiological laboratories, the sterilisation time is usually 15 to 20 minutes. It should be noted that liquids and media take much longer to reach the sterilisation temperature due to delayed heat conduction. The sterilisation can only start when the "core sterilisation temperature" has been reached. For liquids this is done by a temperature probe in a reference flask inside the autoclave's chamber. After the residence time is over, the materials in the chamber are sterile. Glass and instruments can be removed quickly once the steam and the pressure have been removed. Liquids should be cooled quickly and without losses. The laboratory staff must never be endangered by delayed boiling (explosion-like over-boiling). Safety Steam sterilisers should be manufactured in modern production facilities according to the most recent state-of-the-art technology. The Pressure Container Regulation makes it mandatory to check the following safety elements: • safety valve • pressure monitoring of the max. pressure • automatic door lock • safety thermostat for working space and heating elements
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