In the first article in our Sterilization Series, we provided a broad overview of sterilization methods. We then delved into heat-based sterilization methods in our second article within this series. We'll now take a closer look at the process of sterilization by steam, including how it works, what elements are critical to the process, and why saturated steam is the ideal tool for killing microorganisms.
Before we delve into the steam sterilization process, it may help to understand the difference between vapor and condensation, as well as what makes saturated steam unique.
Steam vs. Condensation
Vaporization is the process of converting water to an invisible gas called steam, whereas condensation is the process of steam cooling and converting back into water. The two things are polar opposites that put H2O molecules in two completely different states; a single water molecule cannot be both things at once. It's either steam/vapor or condensation/water, not both. That said, steam and condensation can occur simultaneously when many water molecules are present. How?
When you boil water in a tea kettle, it's heated until the water molecules are so agitated that they begin to convert to a gas state and rise up and out of the remaining water in the form of vapor. Once the water is at a full boil, you'll see plumes shoot out of the tea kettle's spout. Those plumes are often mistakenly referred to as steam--mistakenly, because steam is actually an invisible gas that expands to 1,600 times the volume it was when it started out as water. It leaves the kettle because there isn't enough room for it inside once it's expanded and converted to vapor.
While the steam itself isn't visible, the condensation created when that vapor hits the colder air outside is. The white plumes we see shooting out of a boiling tea kettle's spout are actually the result of steam being cooled to the point that it condenses and turns back into water droplets--i.e., the state the vapor was in before it was heated and began expanding.
Left on the stove long enough, the water will eventually completely evaporate from the tea kettle, leaving it empty and dry. There may be water droplets remaining on the outside of the tea kettle and its surrounding area due to condensation of the vapor upon hitting the colder air outside the kettle, though.
Saturation is the complete meshing of two materials until they're totally united and there's no room to add more of either substance. A sponge saturated with water is completely soaked, with water filling every crevice and hole in the sponge until it can't hold a single additional drop of water. Saturated steam works the same way; it fills a contained space completely, until there isn't room for a single additional water droplet to be added to the space. Since steam is a gas, it's able to saturate in a truly complete way.
As soon as the temperature of the space containing the saturated steam drops, the steam will condense back into water and shrink to 1:1,600 the size. It therefore takes very little water to create saturated steam in an enclosed space like an autoclave. If the autoclave were even half filled with water, there wouldn't be enough room for it to completely transform into steam!
Saturated steam is typically still between 3% and 5% water. That tiny bit of water aids in the sterilization by steam process. Let's look at why that is.
Sterilization by Saturated Steam
Several elements must be in perfect balance for sterilization by saturated steam to be effective: steam, temperature, time, and pressure. Steam created at just the right temperature must be present for a precise amount of time and under a certain amount of pressure to kill all microorganisms clinging to the items being sterilized. Pressure aids in bringing the temperature up to the necessary level to kill microorganisms in a short amount of time.
In an autoclave, the pressurized chamber containing the piercing tools, body jewelry or other items being sterilized is quickly brought up to 121-124°C. That temperature must be maintained for 15 minutes to kill all microorganisms clinging to the items being autoclaved. This combination of saturated steam, pressure, temperature and time will kill viruses, bacteria, spore-forming and non-spore-forming bacteria, and protozoa--in other words, all of the different types of microorganisms that could potentially be present.
Spores tend to be dry, and so they take longer to fully saturate and cook into extinction compared to viruses, which are killed within the first few minutes of an autoclave cycle. Saturated steam is the only way to address drier microorganisms like spores. Why does it work so well? It has to do with the way a steam sterilization cycle works.
The Mechanics of a Steam Cycle
As soon as an autoclave is turned on, pressure is applied and the temperature begins to rise. As it does, the small amount of moisture inside the machine begins to vaporize and expand. As the resulting steam encompasses the objects within the chamber, an interesting thing happens. As soon as the steam comes in contact with the colder objects inside, it condenses, turning back into water that's much smaller in size than it was while in vapor form and releasing a great amount of heat. That shrinkage causes a suction-like effect, drawing even more vapor towards the objects, which steadily increases the core temperature of those objects as progressively more steam condenses on them. The cycle continues until the items within the autoclave are fully saturated and as hot as the steam being directed at them.
It's this cyclical nature of saturated steam that allows it to penetrate any microorganisms clinging to the objects inside an autoclave so deeply and thoroughly. Once the microorganisms are fully saturated with steam, heat is conducted through them at optimal efficiency, cooking and deactivating the microorganisms in a short amount of time that wouldn't be possible without the pressure element. Without the pressure element, it would take 30 minutes to kill all microorganisms if the same 121-124°C temperature range was maintained. In a pre-vacuum sterilizer held at 132°C, the microorganisms can be killed in just 4 minutes!
How a Typical Tattoo & Piercing Shop Autoclave Works
Although every machine is somewhat different from the next, the autoclaves used in tattoo and piercing shops are usually equipped to handle several common situations: sterilization of glassware, unwrapped tools, and wrapped packages (i.e. sterilization pouches), which have to be sterilized longer and at a greater temperature of 132°C to be fully penetrated. Most of these autoclaves also include a drying cycle, as well as monitors that keep a constant check on the amount of water in the machine, it's inside temperature, and other settings. The setup is almost foolproof. You simply add water to the machine, insert the items to be sterilized, select the appropriate setting based on what you're sterilizing, and wait.
Potential Issues With the Efficacy of Sterilization by Steam
The one downside of sterilization by steam is that the steam has to pass through tubing on its way into the sterilization chamber. There are a few opportunities for problems to crop up en route. One set of issues relates to there being too much water present in the steam; the other stems from over-heating steam.
- Too Much Water - If there's over 5% water in the steam being passed into the sterilization chamber, it reduces the steam's penetrating powers. Steam transfers heat much better than water, so adding too much water to the equation can make it take longer and/or require higher temperatures to kill all microorganisms in the sterilization chamber. Too much moisture can also clog the pores in sterilization pouches that steam normally passes through easily, making it harder for the steam to sterilize the contents of the packages. The packages are more likely to come out of the sterilizer wet, which is less than ideal since bacteria takes root in moist environments more easily and will thrive there. Left in that wet packaging for too long, metal contents may corrode or develop rust spots, too. Finally, overly-wet steam can lead to inconsistent temperatures throughout a device, if the vapor cools and condenses at points along its path. Bottom Line: Too much water (a.k.a. "wet steam") brings a host of problems and should be avoided at all costs.
- Over-heated Steam - Super-heated steam is the result of vapor being passed through the pipes too quickly and/or a malfunction at a point where the steam passes through a pressure-reducing valve. If vapor is over-heated, it loses its ability to condense, which is a critical part of the steam sterilization cycle. Without condensation, an autoclave is no better than a dry heat sterilizer, which is 4 times less efficient than a steam sterilizer. Even at higher temperatures, it takes much longer to fully sterilize items in a dry heat sterilizer.
Fortunately, these issues can be easily avoided by properly using and maintaining your autoclave. It's important to check it for spores periodically, too, using a spore test kit. If at any point your tools or other autoclaved items come out of the machine wet or you have an error due to over-heating, contact your machine's manufacturer for servicing.
Why Steam Is King of the Sterilization Methods
Science has yet to find another sterilization method that kills microorganisms as quickly and efficiently as steam, because no other method conducts heat as well. Water in its liquid state holds a semi-close second, but dry air falls way behind. Without moisture to conduct heat, it takes 4x longer for a dry heat sterilizer to fully penetrate microorganisms. Steam is the King of the Sterilization Methods because it's the fastest, most efficient, and cost-effective option available and because steam sterilizers tend to be less problematic than other sterilization tools. Plus, the few issues that may arise can be minimized, if not completely avoided, with proper care and maintenance of your autoclave.
Learn More About Sterilization
If you're interested in learning more about sterilization alternatives and the different sterilization methods, like sterilization by heat, check out these other articles from our Sterilization Series: