Dry heat sterilisation is widely used for glassware and materials that are not suitable for sterilisation using saturated steam. A range of temperatures and times are used. Currently a temperature of at least 170°C for 30-60 minutes is widely used. The term is not particularly precise since variable concentrations of water may be present in the oven used (Sandle, 2013).
In the absence of water, microbial destruction is relatively slow and it is generally accepted that oxidative effects including protein coagulation have a major role in microbial destruction.
While microbial death can be modelled in a similar way to that described previously using saturated steam for microbial destruction the F value obtained is generally referred to as FH and a Tref of 170°C and a Z value of 20ºC are used. While a minimum FH of 30 minutes is generally accepted, higher values typically a FH of 60 minutes is more frequently used. The bioindicator strain proposed for validation of the sterilisation process is spores of Bacillus subtilis var niger (ATCC 9372 or CIP 77.18), for which the D-value is 5–10 minutes at 160°C (Sandle, 2013).
Dry heat depyrogenation uses high temperatures to sterilise glassware and other materials and to destroy the biochemical activity of the pyrogen and endotoxin by products of microorganisms. While the destruction of pyrogens and endotoxins can be modelled in a similar way to that described previously for microbial destruction using saturated steam, the kinetics are more complex and the lethality time curve is typically biphasic (Sandle, 2013). The F value obtained is generally referred to as FD or FP and a Tref of 250°C and a Z value of 46.4ºC are used. The FDA (FDA, 2012) requires any heat treatment process to reduce bacterial endotoxins by a factor of 1000, 3-log cycles. This corresponds to a minimum FD or FP of 30 minutes.
As mentioned earlier the kinetics of dry heat depyrogenation are complicated and use of temperatures lower than 180ºC are likely to give significantly lower estimates, and hence inaccurate predictions, of the time required for the destruction of endotoxins.
FDA (2012). Guidance for Industry Pyrogen and Endotoxins Testing: Questions and Answers, U.S. Department of Health
and Human Services Food and Drug Administration: Bethesda, MD, USA.
Sandle, T. (2013). Sterility, sterilisation and sterility assurance for pharmaceuticals. Technology, validation and current regulations. Woodhead Publishing Limited, Oxford.