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Versatile stopper that fits anything that uses a #7 stopper. Use as an airlock for yeast culturing, and even more useful as a sterile shield when preparing media. The proprietary foam is heat-resistant, and will withstand autoclaving or pressure cooking, making it an ideal flask plug when preparing sterile wort and agar. The honeycomb structure of the foam lets the plugged flask breathe, yet bacteria and wild yeast are filtered out and cannot reach the media. This means you can put your media in an Erlenmeyer flask, plug with a Foam Stopper, and pressure cook to sterilize, fully confident that bacteria and wild yeast will not enter the flask, even during the cooling and later storage period outside the pressure cooker.
I've used these for years for yeast starters without any problems. Sure they discolor, but by putting the stopper atop my flask with the water / DME mix at room temp. They have plenty of time to sanitize by the steam generated by boiling the mix for 5 - 10 minutes.
I do wash them by hand before and after to minimize the gunk factor. No work at all.
I haven't used one as an air lock though. I use a blow off tube on my primary fermenter and I like to use an air lock and count bubbles to determine when secondary fermentation is complete.
Pros: Autoclavable, if you need to. Fits nicely into a 500ml narrow mouth erlenmyer flask and looks better than aluminum foil!
Pros: I can see the advantages of using these stoppers while boiling culture media -- I guess. Cons: Don't plan on more than one or two uses out of these. After that first use they discolor, deform, and don't clean very well. Chlorine sanitizer turns them into a putty-like goo. I'm going back to a rubber stopper and traditional air lock. I don't see sufficient advantages to make up for having to toss these after every culture.
Pros: These stoppers are the best...especially for my starters on the stir plate. O2 exchange is great and have never had any infections using them...Easy to sanitize (Soak and Sqeeze). Cons: None