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 denatured in a wet environment 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 apparatuses work with circulating air, e.g. a ventilator circulates the air in order to generate a constant temperature and a faster heat exchange. According to pharmacopeias 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 to heat sensitivity of the goods being 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 dry heat sterilization processes biological indicators according to EN ISO 11138-4 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. athrophaeus 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.