Autoclaves for medical use employ high-pressure steam to sterilize medical instruments/products/waste. I have heard before (and the Wikipedia article also asserts) that hot steam (AKA “moist heat”) is far superior to hot air for sterilizing things.
Why?
[sub]And yes, “Moist Heat” certainly would be a great band name.[/sub]
using moist heat there is fast heat transfer and shorter time needed.
Water vapor has a much higher specific heat capacity, therefore higher energy, than air does.
There’s two reasons.
The first has already been given: water has a much higher heat capacity and a much higher conductivity than nitrogen. You can prove this to yourself quite easily. Stick your hand inside an oven at 80oC for 30 seconds. Then stick your hand inside a saucepan of water at 80oC for 30 seconds. Actually, that’s not easy. Well, the first is easy. Your hand will hardly get warm. The second is hideously painful and likely to be permanently crippling. The thing is that even though both the oven and the saucepan are at the same temperature, the water is actually hotter. It contains more energy and it transfers that energy to your flesh much faster. Because of that it denatures proteins much faster, which is what you are aiming for with sterilisation.
The second reason is that dry material is much harder to sterilise. Sterilsiation relies largely on denaturing proteins, literally applying so much heat that they bend out of shape. When a protein is surrounded by water, it’s much easier to denature it because the water acts as both a solvent and source of ions that assists the denaturation. A dry protein is much more stable, and many organisms, including microbes, have added features that stabilise themselves in a dessicated state to the point that they are damn near indestructible. You can also see this at work yourself. Your hair is very easy to “set” into a new shape when it is wet because the water assists the proteins in folding into new configurations. OTOH dry hair is extremely difficult to shape because the protein bonds are basically locked into place. Ditto for cooking beans. Soak dry beans for an hour, and yo can cook them in 20 minutes. Roast dry beans at the same temperature for an hour, and they will make you ill because the proteins are locked into place while they are dessicated.
You’re comparing properties of liquid water with those of air, which is not the situation we’re dealing with in an autoclave.
Steam (water vapor) actually has a somewhat lower thermal conductivity than air: 0.02744 W/(mK) for saturated steam at 120C, 0.03301 W/(mK) for air at the same temp.
However, as it turns out, yes, steam (water vapor) does have higher specific heat than air, about twice as high.
And you’ve clued me in to another useful property of steam. An autoclave specifically uses saturated steam (steam which is on the verge of condensing into liquid), so it’s able to transfer additional heat to a cold object by condensing on it (similar to the way sweat cools you off by evaporating). So yes, steam will heat cold objects faster than air at the same temperature.
So it sounds like part of the use of steam is to speed up the sterilization process through more rapid heating, and the other part of the use of steam is to facilitate the denaturing of proteins at a lower temperature - which further speeds up the sterilization process and also makes it safe for items that might not tolerate the high temps that would be required without steam.
Thanks.
Liquid water may feel hotter than air of the same temperature because of the more rapid heat transfer, but it is not actually hotter.
Heat and temperature are not the same thing. Heat is thermal energy which includes both the kinetic and potential energy of a substance. Temperature only tells the kinetic energy or speed of the particles (molecules or atoms) in a substance. Particles differ in mass- therefore, water molecules at 212 F have a lot more energy than air particles at 212 F, which explains why 212 F air won’t cook your hand as fast as 212 F water.
The water may not be hotter, but it has a lot more heat energy at 212 F than air does.
things are sterilized in autoclaves that aren’t naked as well. closed containers will keep steam from contacting the contents and diluting, contaminating or wetting it.