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Steam sterilization

Steam sterilization

We normally distinguish between two types of autoclave: laboratory autoclaves and large volume autoclaves. Liquids can be sterilized in both if the rules are observed (liquid program). An important feature of both types of autoclave is additional cooling equipment and a pressure equalization system so that liquids can be autoclaved quickly and safely. It is important that the internal pressure in the sterilizing chamber is always kept above the bottle pressure in order to avoid the liquid boiling over from the vessels. Note: the relevant regulations with regard to the sterilization of liquids in the autoclave must be complied with without fail. For the protection of personnel a thermolock is required on the door, as laid down by the German authorities in Pressure Vessel Ordinance TRB 402, irrespective of the temperature in the bottle. The sterilization of liquids in DURAN® ISO square laboratory bottles is carried out at 121 °C for 20 minutes in the liquid program. In this case the program control operates via a temperature sensor in a reference bottle containing the same volume of liquid as the liquid charge to be sterilized. This is the only way a validatable sterilization process can be carried out. Liquids can be autoclaved in loosely capped or open vessels. Open and loosely capped vessels are autoclaved using the prevacuum process with a steam/air mixture and cooling (VOVV-DLK) or the gravitation process with a steam/air mixture and cooling (GRAV-DKL). Further information about this process can be found in DIN 58 950 Part 2. The VOVV process is used when cold solutions have to be sterilized in open or loosely capped vessels whereas the GRAV process is used for warm solutions. The following special features should be noted with regard to the sterilization of liquids:

The very high specific heat capacity of water (4.18 KJ/Kg*K) Large quantities of energy are required for heating up. Great amount of energy that has to be removed by the cooling process Long charge times Jacket cooling or water irrigation for the cooling of the liquids Temperature monitoring in the vessel by temperature sensor Safety aspect when emptying: danger!!!

As can be seen from these points, sterilizing liquids by autoclaving is a process that consumes considerable amounts of energy. For that reason it is important to carry out sterilization centrally as far as possible making use of all the space available with optimum vessels. In such cases DURAN® square laboratory bottles have proven themselves through their shape and appropriate closures. Another benefit is the fact that when the sterilization process ends they can be closed tightly and thus cross-contamination is almost impossible. They can then be supplied direct to the end user. Note: when sterilizing liquids in DURAN® square laboratory bottles in an autoclave the screw caps should not be screwed tight, as this will prevent pressure equalization taking place.

How the program operates

The operation of a sterilization process breaks down into the following time segments:

Operating time: the whole period of time from the start of the sterilization process to its completion. It consists of following phases: Build-up period: this is also referred to as the preheating period. Steam flows into the autoclave chamber and air is expelled. Equalization period: referred to as thermal lag. It is the time required for every point on the material being handled to reach the temperature required. Sterilization period: here the temperature required, e.g. 121 °C, is reached and maintained for a certain period, e.g. 20 minutes. Cooling-off period: this is of great importance particularly when sterilizing liquids.

Using a temperature sensor in a reference bottle, the autoclave can only be opened when the temperature has dropped below 70 °C. If the pressure equalization is too rapid the liquid overheats resulting in delay in boiling and consequently to boiling over of the liquid.

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