Use this foam stopper with your Erelenmeyer flask when making a yeast starter. Aeration is an important aspect of making a strong, healthy starter. This is why we recommend using a stir plate, which provides continuous aeration. This foam stopper will allow air to flow freely into your flask while keeping pesky bugs and other contaminants out. A quick spray down with sanitizer is all you need. Suitable for flasks or other vessels with an inner diameter opening of 35-45mm (1-3/8" - 1-3/4").