Dry heat sterilization processes

Dry heat is primarily used to sterilize instruments and heat-resistible glass and instrument parts. The heat transfer by dry heat is less effective than by airless, saturated steam, because steam is able to transport the heat quicker than by condensation 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 in dry heat in comparison to steam sterilization processes. The dry heat sterilization is carried out in a closed chamber. Larger chambers work with forced air circulation, 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 recommended 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 (plateau time). The heat-up and cool-down times must be added accordingly.

The effective sterilization temperature and time (F-value) has to be calculated when the defined bioburden is known during a validation procedure according EN ISO 20857.

To monitor dry heat sterilization processes biological indicators according to EN ISO 11138-4 should be used, but their resistance values are not ideal to monitor dry heat sterilization processes, because of two reasons:

  1. When sterilizing with dry heat the germs B. athrophaeus 10^6 (D160°C = 2 min) are killed at 160°C / 30 min latest, so monitoring at 160°C/2 h becomes impossible.
  2. 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.