We need to talk about your autoclave sterilization rituals and aluminum foil.
Specifically: Do not use aluminum foil when sterilizing empty beakers and flasks!
Loosely crimping foil over the mouths of empty flasks prior to autoclaving them is a widespread practice in many research labs. Justifications for this practice vary. Some claim it keeps water from condensing in the glassware, others cite the need to keep spores from drifting into glassware once it’s sterilized. Most will admit it’s just what they were taught to do.
Whatever the reasoning, it’s a bad idea.
Autoclave Sterilization Demands Steam Penetration
“People are religious about why they do use foil the way they do it,” notes Philip Berriman, an instrumentation technician at California State University (Fullerton). “People have a certain logic, but they don’t necessarily do experiments to validate that.”
As an example, Berriman points to one enormous variable introduced by the practice:
“How loose is ‘loose’? Oooh, you should see the differences [I’ve seen]!” These range from foil barely crimped enough to stay put to foil mashed down like a beer bottle cap.
Berriman has done his own series of tests, using one of Fullerton’s 150L front-loading Priorclave research-grade autoclaves. Among his findings:
Foil always decreases the effectiveness of a standard sterilization cycle—and can even prevent sterilization altogether. This is especially the case when when you use a gravity cycle with no vacuum stage. Berriman found that both standard beakers and Erlenmeyer flasks failed to sterilize fully when their mouths were covered in “loose” foil.