Autoclaves, Market Forge, Wisconsin
Why is an autoclave such an effective sterilizer? An autoclave is a large pressurecooker; it operates by using steam under pressure as the sterilizing agent. High pressures enablesteam to reach high temperatures, thus increasing its heat content and killing power. Most of theheating power of steam comes from its latent heat of vaporization. This is the amount of heatrequired to convert boiling water to steam. This amount of heat is large compared to that requiredto make water hot. For example, it takes 80 calories to make 1 liter of water boil, but 540 caloriesto convert that boiling water to steam. Therefore, steam at 100º C has almost seven times moreheat than boiling water. Steam is able to penetrate objects with cooler temperatures because oncethe steam contacts a cooler surface, it immediately condenses to water, producing a concomitant1,870 fold decrease in steam volume. This creates negative pressure at the point of condensationand draws more steam to the area. Condensations continues so long as the temperature of thecondensing surface is less than that of steam; once temperatures equilibrate, a saturated steamenvironment is formed.
Achieving high and even moisture content in the steam-air environment is important for effectiveautoclaving. The ability of air to carry heat is directly related to the amount of moisture present inthe air. The more moisture present, the more heat can be carried, so steam is one of the mosteffective carriers of heat. Steam therefore also results in the efficient killing of cells, and thecoagulation of proteins. When you cook beef at home, for example, it can become tough whenroasted in a covered pan in the oven. But just add a little water in the bottom of the pan, and you will find that the meat will be tender! The temperature is the same and the time of roasting is thesame, but the result is different. Now (as in an autoclave) add another parameter, pressure. Byputting this same roast in a pressure cooker you can reduce the time it takes to cook this roast byat least three quarters, and you still get just as tender a finished product.How does killing occur? Moist heat is thought to kill microorganisms by causing coagulation ofessential proteins. Another way to explain this is that when heat is used as a sterilizing agent, thevibratory motion of every molecule of a microorganism is increased to levels that induce thecleavage of intramolecular hydrogen bonds between proteins. Death is therefore caused by anaccumulation of irreversible damage to all metabolic functions of the organism