Last Updated on September 9, 2022

If you’re relatively new to the world of autoclaving (a.k.a. steam sterilization), you might have some basic questions about how lab autoclaves work to sterilize things like glassware and culture media and what the optimal autoclave temperature is.

Fundamental Questions About Autoclaves

•  Why is the required temperature typically 121 degrees Celsius (121°C)? What’s so special about that temperature for autoclaving lab media?
•  How does the autoclave temperature get up to 121°C? — doesn’t water simply boil at 100°C?
•  Why can’t I use other techniques to sterilize my lab materials, such as bleach, microwaving, or incineration?
•  What happens to all those microbes when I autoclave them?

These are great questions as you look to start up a lab. In this article, we’ll give you the answers. We’ll explain why 121°C is a “magic” temperature for your autoclaving needs, and a lot more.

Killing Pesky Lab Contaminants with Autoclaves

The common thread among all these questions is the fact that contaminated materials—such as glassware, Petri dishes, depleted agar, gloves, pipette tips, bioreactors, and more—get contaminated with living things.

We’re talking about bacteria, fungi, and viruses. When you’re, say, breeding a yeast to develop a new variety of beer, you want that yeast to thrive. But you don’t want bacteria invading your bioreactor. (We wrote an entire blog about autoclaving for brewery labs. Check it out here.)

In other words, your job, to put it bluntly, is to kill things. Sterilization, in this context, means efficiently and decisively killing those microscopic creatures that would otherwise contaminate your lab materials. You want these critters to be “inactivated.”

Technically speaking, you want to use the high heat from the lab autoclave to denature the protein and inhibit its ability to perform an enzymatic reaction. Think of the heat as “melting” the protein. Once the microbes are dead/inactivated, they’re basically harmless. You’re then free to safely dispose of or reuse the materials they’d once inhabited.

The Magic Sterilization Temperature in Labs

For most lab applications using autoclaves, the basic sterilization setting is “121°C for 20 minutes.” But where do these numbers come from? And how do you get to 121°C, if water boils at 100°C? Wouldn’t it just boil away to steam?

In your kitchen, yes. Inside an autoclave, no. The autoclave is sealed. So when the water inside it boils and turns to steam, it doesn’t simply vent into the air. It’s trapped inside, and as the temperature goes higher, it pushes ever more energetically, trying to get out. Thus, the pressure increases. (Specifically, Tomy SX-700 Lab Autoclave reaches 0.25 mega-pascals of pressure, while our SX-500 gets up to 0.263, at this temperature.)

With the rising pressure, this is how the temperature exceeds 100°C. One-hundred twenty-one degrees Celsius, by the way, isn’t so much a “magic” number as it is a “blanket” number. Just like “20 minutes,” it’s a good safe number that accommodates a wide range of commonly used lab media. However, there are outliers. To be sure, you can simply look up the autoclaving needs of your particular medium, and program the autoclave accordingly.

Working Under Pressure

The pressure that builds up as the steam exceeds boiling temperature isn’t just a random byproduct of the heating. It’s also essential to the autoclave’s function.

Think of it this way: Everything you place inside the autoclave has little recesses and cavities, whether it’s within the structure of a complex bioreactor or even the uneven surface of depleted agar. You want that extreme high heat to reach inside every single nook and cranny.

The high-pressure steam does that for you. It literally pushes its way into every available void—killing every exposed microorganism.

This process is efficient. And it’s fast. (Shameless plug: Tomy autoclaves feature a dual-fan design, so they cool down 42% faster than non-fan autoclaves, allowing you to increase your throughput that much more.)

And that’s why autoclaving is simply a safer and more effective option for sterilizing things. Do you want to destroy things by incinerating them? Would you trust bleach to get into those minuscule voids that only high-pressure steam can penetrate? Would you trust a microwave to reliably sterilize used media? No, no, and no.

If you have any questions, don’t hesitate to contact us directly or leave a message in the comments below.
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