Despite being one of the oldest methods for sterilizing materials, modern “dry-heat” sterilization is often misunderstood. While it offers certain advantages–and is the only option for some materials–it’s a poor fit for most small research and education labs, which are best served by a general-purpose steam autoclave. As a general rule:
- Use dry-heat sterilization for: fats, oils, powders, metal instruments highly likely to rust
- Use steam sterilization for everything else
Dry-Heat Sterilization in a Nutshell
In a contemporary lab or medical setting, “dry-heat sterilization” simply means raising the temperature of an item to 170°C under normal air pressure, and holding the load at that temperature for roughly one hour. When you roast a chicken in the oven at 325°F to kill off the naturally occurring bacteria, you are essential “dry-heat sterilizing” your dinner.
Dry-Heat and Steam Sterilizer Efficacy
While both dry-heat sterilization and moist-heat treatment can get similar results with many loads, there are some tasks that are not practical with a steam autoclave. These include loads that are hydrophobic (such as fats and oils) or will be damaged by moisture (such as powders). A dry-heat process is vital for sterilizing instruments that are at risk of corrosion. Dry-heat also works well with glassware.
But dry-heat sterilizers are by no means an all purpose solution. Liquids cannot be dry-heat sterilized (they’ll boil off), nor can laboratory growth media, nor many flammable loads. Dense loads are also often a problem, as the convection/conduction method used to heat the chamber does a poor job of penetrating them.
“Dry heat is a far less efficient sterilization process than moist heat, but is nevertheless a useful procedure for sterilizing certain products such as powders and oils.”
[source: Sterilization Technology: A Practical Guide for Manufacturers and Users of Health Care Products, Robert F. Morrissey & G. Briggs Phillips, eds., 1993, p. 10]
Dry-Heat Sterilizer Vs. Steam Autoclave Efficacy
Although many loads could be sterilized by either dry-heat or steam autoclaves, steam sterilization, as a rule, is far less energy and time consumptive than dry-heat methods.
In order to kill the most resistant spores, a dry-heat sterilizer must bring its load to 170°C and hold it at that temperature for one hour. By contrast, a steam autoclave needs only to be raised to 121°C for 15 minutes, because the moisture in the steam is a much more efficient conductor of heat and much more adept at fully permeating a load. Using two thirds the heat for one quarter the time translates into a significant cost savings, not just in the energy bill, but also in terms of productivity and staffing–even in a small facility.
Use Dry or Moist Heat for:
- most metal instruments
Dry-Heat Sterilization Only:
- anhydrous fats
- metal instruments at risk for corrosion
Steam Sterilization Only:
- culture media
- flammable and heat-sensitive items
- dense loads