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Should I cover the tops of flasks with foil before autoclaving?   

lab autoclave procedure avoid foil

Should I cover the tops of flasks with foil before autoclaving?  

Short answer: No. (DISCLOSURE: This blog fully complies with Betteridge’s Law of Headlines)

We’re consistently surprised by the persistence of this folk custom. The justifications for covering items with foil prior to autoclaving vary. We’ve heard many—including the prevention of both explosion and implosion—but most seem to focus on “keeping things out” of the sterilised container: spores, contaminants, dust, water condensation, etc.

But, regardless of the justification, the foilers’ certainty that wrapping foil over flasks is The Right Way to Do It™ is remarkably consistent.

Let’s leave aside the fact that aluminum foil can itself introduce contaminants to your experiment (household foil is food safe, but by no means sterile or pristine; it is rolled using food-safe lubricants that can persist in the finished product).

Instead, let’s take a moment to focus on what’s happening inside the autoclave and how foil can hamper that process.

Loose Foil Prevents Sterilisation

The whole point of autoclaving is steam saturation. It’s heat that deactivates microorganisms, spores, viruses, and other biological agents. And the heat carrying capacity of steam is much greater than dry air. This is why it takes hours to bake a potato at 250ºF, but just 15 minutes to pressure-cook it at the same temperature. 

What’s true in the kitchen is true in the lab:

Set to the same temperature, a steam autoclave works hundreds of times faster than comparable dry-heat methods. (E.g., the same load processed at the same temperature takes 20 minutes in a steam steriliser, or roughly 16 hours using dry heat.)

But in order to reap the benefits of steam sterilisation, you need to make sure that steam completely permeates your load and contacts all surfaces. We all know that cold pockets decrease the likelihood of sterilisation. Cold pockets occur when something traps cold air in the load—for example, foil wrapped over the mouth of a flask.

Because foil prevents steam saturation and penetration, it invariably decreases the effectiveness of a standard sterilisation cycle. In one study, foil-covered flasks were only successfully sterilised about 40% of the time using standard gravity cycles. During one series of trials using large vertical flasks, having loose foil over the mouths of the flasks always prevented sterilisation (as shown by biological indicator test strips placed in each flask). This was even the case when the cycles included five minutes of extra freesteaming.

“You have to be able to jiggle it.”

Kelly McVey is an Instructional Support Technician for the Department of Chemistry and Biochemistry at California State University, Fullerton. She is in charge of lab equipment testing and maintenance. She also regularly trains incoming lab users in safe and effective autoclave use. 

“What I tell students,” she explains, “is that with with all of our caps or foil, you have to be able to jiggle it. So, for foil, it’s best if you do something like lay a pencil across the top of the flask and then apply the foil, so you have those two passageways for steam at the top.… If it’s sealed, you aren’t going to get good steam penetration inside. And if steam does get in, it’s not going to dry inside” which defeats one of the reasons people give for using foil in the first place.

If your principal investigator doesn’t like the idea of foil with big gaps—or just stopping with the foil wrapping altogether—please feel free to share this blog post with them, as well as these: