Sterilization packaging — everything you need to know

Drying efficiency for pouched equipment:
During sterilization, steam must condense on the products in order to transfer its heat to them. The amount of condensate generated to achieve a temperature rise depends on the material. The transfer of heat energy is theoretically reversible, meaning that the amount of heat accumulated by the products during sterilization should be sufficient for drying if it is returned to the condensate. Drying is achieved through the principle of water vaporisation as a function of temperature and pressure reduction. The rate at which vacuum is achieved and the vacuum power are therefore critical factors. Condensate remains predominantly attached to the products from which it absorbed its heat energy.

This is unquestionable for textiles, for which a vacuum of around ten minutes is sufficient to achieve drying that meets the dryness standards (max. +1% weight gain).

For equipment, controlling the drying process is more complex (max. +0.2% weight gain). Approximately 30% of the condensate detaches from the products to which it has transferred its heat. If, by gravity, this condensate flows to the bottom of the sterilizer vessel, it will be vaporised upon contact with the jacket or extracted by the vacuum pump if the vacuum port is located in the lower part of the sterilization chamber. However, if it becomes trapped in an area lacking an immediate heat source, its excessively low temperature compromises vaporisation. Heat must then be brought to these areas to allow the condensate to warm up and potentially vaporise — this is known as fractional drying, applicable only to equipment loads.

When vacuum drying alone is insufficient, fractional vacuum cycles using carefully calibrated sterile air inlets can assist with drying by promoting heat radiation from the jacket towards the load. These fractions can also be introduced using steam or an alternation of steam and air. The mechanical effect of these variations on the load can also displace some condensate towards areas that may be more favourable for receiving heat. The flow of dry air from the ventilation cycles may be able to absorb suspended moisture if carried out below atmospheric pressure. It should be noted, however, that these complex systems only refine the drying process (influencing it by approximately 10 to 15%). It is the first drying vacuum after pressure exhaust that plays a fundamental role in the final drying result.

However, if the material is too cold before the sterilization plateau (poor pre-treatment), if the vacuum system is unable to reach the required level, or if the condensate has moved away from a heat source and the heat cannot reach it, then unwanted residual moisture must be expected.
In a properly functioning unit, controlling the drying process is only conceivable with perfectly homogeneous and reproducible loads. The specific constraints of the pharmaceutical or laboratory environment mean that we are obliged to manage deviations and accept them. Nothing can prevent one or more items from occasionally being wet. Of course, basic load plan composition rules apply, but these rules are often site-specific and cannot be transposed from one site to another. It is equally important to choose the packaging carefully based on the composition of the products to be sterilized and the capabilities of the sterilizer, as well as your own expectations.​