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Dry Heat Sterilization Processes

Dry heat is primarily used to sterilize instruments and heat-resistible glass and instrument parts. The heat transmission by dry heat is less effective than by airless, saturated steam, because steam is able to transport the heat better, than just air. Apart from that, proteins can be easier denaturized in a wet milieu than in a dry one. This is the reason why higher temperatures and longer times are needed. The dry heat sterilization is carried out in a closed chamber. Bigger apparatusses work with circulating air, i.e. a ventilator circulates the air in order to generate a constant temperature and a faster heat exchange. According to pharmacopies and WHO recommendations the temperature should be between 150 and 250°C with a sterilization time between 30 and 180 min. The following process parameters are to be chosen due the heat sensitivity of the goods sterilized:

3 hours, 150 °C
2 hours, 160 °C
30 minutes, 180 °C

These values just refer to the actual duration of sterilization. The heat-up and cool-down times must be added accordingly.To monitor the dry heat sterilization process, biological indicators according to EN 866-6 should be used, but their resistance values reached nowadays in practice are not ideal to monitor dry heat sterilization processes. There are two reasons for this:

When sterilizing with dry heat the germ B. subtilis 106 (D160°C = 2,5 min) stops growing at 160°C / 30 min, so monitoring at 160°C / 2 h becomes impossible.

Biological and chemical indicators heat up much faster than other sterilization goods, because of their low weight. It is recommended to monitor the sterilization process with thermo-electric measurement and to put sensors on the outside, or if possible into the heaviest sterilization goods, to monitor the heat-up process properly.