There are many justifications for crimping foil loosely over the mouths of containers before autoclaving. The most common—and seemingly most reasonable—is that this common autoclave procedure keeps viable spores from drifting onto the sterilized items.
That seems to make sense: If I want to keep dust off the the sofa stored in my garage, I drape it with a sheet, don’t I?
But Philip Berriman doesn’t buy it. Phil is an instrumentation technician at California State University (Fullerton), and spends all day working with autoclaves. To him, the “spore shield” foil rationale highlights the futility—and outright folly—of using foil like this.
Putting foil over the mouths of containers decreases the likelihood of those items being properly sterilized to begin with—and does nothing to protect them from spores.
Bad Autoclave Procedure — Foil Decreases the Efficacy of Sterilization
“When you say ‘loose’,” Phil explains, “you really need to mean ‘loose’—there have to be air gaps.” Without air gaps, the steam cannot get access to the interior of the vessel you’re trying to sterilize. The likely result: a non-sterile load.
The only reason a steam autoclave can sterilize a load at 121ºC in just 15 to 20 minutes is because steam is remarkably effective at transferring thermal energy (compared to dry air, which is an excellent insulator). A dry-heat sterilizer set to 121ºC will take upwards of 12 hours to sterilize the same load.
In order to assure sterilization, your foil needs to be crimped on loosely enough to provide nice air flow. (People rarely do so; the picture above is indicative of what Phil often sees.)
But as that flask cools, it’s going to draw in air—regardless of how tightly you mash the foil around the lip of the flask. In effect, the cooling flask hoovers up a nice big sample of all the spores floating around your lab.
“One of the reasons that the ‘loose foil’ habit seems somewhat futile to me,” Phil concludes, “is that the practice means the user is more paranoid about what might land on their ‘sterile’ items than about whether the items ever became sterile in the first place.”