The Direct Flame:
One of the simplest methods of dry heat sterilization is direct flaming.
Mode of Action :
Dry heat probably does most of its damage by oxidizing molecules. Moist heat destroys microorganism mainly by denaturing proteins; the presence of water molecules helps disrupt the hydrogen bonds and other weak interactions that hold proteins in their three-dimensional shapes. A simple analogy is the slow charring of paper in a heated oven, even when the temperature remains below the ignition point of paper.
To effectively sterilize the inoculating loop, one must heat the wire to a red glow. The flame of the Bunsen burner is employed for a few seconds to sterilize the bacteriology loop before removing a sample for a culture tube and after preparing a smear as shown in (figure A) below.
Figure A: Indicating Dry Heat Sterilization Using Bunsen Burner
Perhaps the most rapid sterilization method, to sterilize and dispose of contaminated paper cups, bags, and dressings, is the use of a direct flame in the process of incineration. It is still common practice to incinerate the carcasses of cattle that have died of anthrax and to put the contaminated field to the torch because anthrax spores cannot adequately be destroyed by other means. Indeed, British law stipulates that anthrax-contaminated animals may not be autopsied before burning.
The Hot-Air Oven:
Mode of Action:
The effect of dry heat on microorganisms is equivalent to that of baking. The heat changes microbial proteins by oxidation reactions and creates an arid internal environment.
Items to be sterilized by this procedure are placed in an oven. Generally, a temperature of about 170oC maintained for nearly 2 hours ensures sterilization. The longer period and higher temperature (relative to moist heat) are required because the heat in water is more readily transferred to a cool body than is the heat in air. For example, imagine the different effects of immersing your hand in boiling water at 100oC (212oF) and of holding it in a hot air oven at the same temperature for the same amount of time.
The hot-air oven utilizes radiating dry heat for sterilization. This type of energy does not penetrate materials easily, and therefore, long period of exposure to high temperatures are necessary. For example, at a temperature of 160oC, a period of 2 hours is required for the destruction of bacterial sores. Higher temperatures are not recommended because the wrapping paper used for equipment tends to char at 180oC.
Uses of Hot Air Ovens:
The hot-air method is useful for sterilizing dry powders and water-free oily substances, as well as for many types of glassware, such as pipettes, flasks, and syringes. Dry heat does not corrode sharp instruments as steam often does, nor does it erode the ground glass surfaces of nondisposable syringes.
Important Factors to Determine the Exposure Time:
Thereby burning microorganisms removed from the materials, because organic matter insulates against dry heat. Moreover, the time required for heat to reach sterilizing temperatures varies according to the material. This factor must be considered in determining the total exposure time.