Plug your yeast starter flask like a professional laboratory. These foam plugs keep the dust, bacteria and wild yeasts out of your starter while letting air into the flask to keep your starter oxygenated. If you already have a flask, measure the opening to make sure you get the right size.
The 1 3/4 inch size fits 1L, 2L, 3L and some 5L flasks
The 2 1/2 inch size fits 5L flasks with wider opening