Theory of Sterilization

Uniform sterilization ultimately depends on the contact of the sterilizing agent with all surfaces of an object. Which sterilisation medium you use depends on the type of object. As this guide refers to the usage of the CertoClav, we will deal with moist heat as a sterilization medium.

Heat itself is known to destroy microorganisms, but the addition of moisture accelerates the process. However, moisture in the form of steam at normal air pressure is actually insufficient for sterilization. Higher pressures are required to increase the temperature of the steam to kill microorganisms. Perhaps you are familiar with the term denaturation. Death of a microorganism by moist heat in the form of steam occurs by denaturation. Steam in the form of hot water vapour causes proteins to clot, i.e. it makes them inactive. The protein can take on a new form or be fragmented into small pieces. You have probably seen this often when cooking an egg. The clear, jelly-like part of the egg, which consists of protein (egg white) and water, turns white and solid when heated. This is the albumin, which changes its molecular form due to the presence of heat. Remember that you had water in the clear part of the egg, which has now evaporated, leaving behind the denatured albumin.

It is important for the autoclaving process that the autoclave chamber is filled with saturated steam and all surfaces of the autoclaved material are fogged with saturated steam. Each fibre and surface to be sterilised must have reached a certain temperature over a certain period of time. When steam enters the autoclave under pressure, it begins to condense on objects that are cold. The condensation of steam releases heat. Water turns from steam into liquid, which is a lower energy state, with the result that heat is released to the environment. This simultaneous wetting and heating of the elements in an autoclave causes sterilisation.

Studies show that no microorganism can survive direct exposure to saturated steam at 250 °F (120 °C) for more than 15 minutes at 15 Psi (pounds per square inch). Remember that you can increase the temperature and time of the sterilization cycle. Ultimately, cycle times will depend on the size and contents of your items. When the autoclave cycle is performed, it is important to allow the condensed water to re-evaporate to maintain sterility. In other words, the items in autoclaves should be dry before removal.

Pressurized steam is the most reliable method of destroying microorganisms and their spores, and the method of choice if your items are not damaged by heat and/or moisture.

If it has been a while since you studied physics, you may have forgotten the relationship between the pressure of a gas and its temperature. When the pressure of a gas increases, its temperature also increases and vice versa. Remember, the gas gets hotter, the gas molecules collide with each other and with the sides of the container. The kinetic energy causes a heat accumulation, which in turn causes the pressure to increase. The actual heat output of water vapour comes from the transition from a liquid to a gas, or what is known as latent heat of evaporation. The easiest way to consider this is to know that 1 calorie (unit of heat) can increase 1 gram of water by 1 degree Celsius. However, it takes 540 calories to convert 1 gram of water into vapor at 100°C. As you can see, steam contains much more heat energy than boiling water. Therefore, combustion by steam is more difficult than combustion by boiling water. The nice thing is that steam has the ability to condense and become water again on objects that are cooler than the ambient temperature. This creates an immediate loss of steam volume, which creates a negative pressure inside the autoclave. This causes more steam to be introduced into the chamber as long as the steam condenses on cooler surfaces. Finally, the air and objects in the autoclave reach the same temperature. At this point the autoclave environment becomes saturated with steam. At sea level (1 atmosphere) steam has a temperature of about 100 °C. If you have a pressure of 2 atmospheres, 1 atmosphere above the standard pressure, the temperature of the steam rises to 121 °C. If you increase the pressure to 3 atmospheres, the temperature of the steam would be 130 °C. If the pressure increases, the temperature of the steam increases.