Versatile stopper that fits anything that uses a #7 stopper, including our 1000 ML Erlenmeyer Flask. 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.