Thursday 17 February 2022

Dry heat sterilization explained


The temperatures and times required for dry heat sterilization are far higher than autoclave temperatures as described before in this chapter of steam sterilization. This is because the mechanisms of microbiological inactivation as a result of exposure to dry heat are quite different and much slower than those applying to saturated steam. This results in dry heat being less suitable for the sterilization of components.


There are three forms of heat transfer, convection, conduction and radiation. With dry heat ovens, convection is the primary means of heat transfer. Convective heat transfer is the transfer of heat from one place to another by the movement of fluids and involves the combined processes of conduction (heat diffusion) and heat transfer by bulk fluid flow (sometimes called heat advection). The convection heat transfer mode comprises two mechanisms. In addition to energy transfer due to random molecular motion (diffusion), energy is also transferred by bulk, or macroscopic, motion of the fluid. This motion is associated with the fact that, at any instant, large numbers of molecules are moving collectively or as aggregates. Such motion, in the presence of a temperature gradient, contributes to heat transfer. With ovens, convection is forced convection. In this case the fluid is forced to flow by use of a pump, fan or other mechanical means.


Mode of action by dry Heat


In dry heat sterilization ‘Air’ is a good insulator, causing slow heat transfer from air to the product/items in dry heat processes. The heat conductivity of the items themselves can be somewhat slow, and stratification of air in the chamber can occur. Thus, for select, heat stable products, dry heat is the mode of choice for implementing the sterilization process.



Dry heat kills microorganisms primarily by reacting with, and oxidizing, their proteins, although other factors such as the depurination of DNA may play a secondary role. The effectiveness of dry heat as a microbicidal agent with the ability to kill a wide range of microorganisms has been well established. Dry heat microbial destruction depending upon the time- temperature combination, incinerate the microbial cell. For dry heat to be effective, the items to be sterilized should be free from soiling substances and be dry, since water droplets can interfere with the process, particularly in relation to short sterilization run times.


The microorganism generally selected as a biological indicator for use in dry heat sterilization validations is Bacillus atrophaeus (formerly B. subtilis var niger). This microorganism has been chosen for its known resistance to dry heat. Depending on the approach taken to the development of sterilization processes, i.e., if a product specific approach is taken, supplemental studies may be needed to evaluate the resistance of the naturally occurring bioburden on the product/items (validation aspects are discussed later in this chapter below).


 Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (

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