These breathable foam stoppers canstand up toboiling temperatures, making them greatto use whenpreparing a yeast starter. Thefoam stoppers will act asa filter for any airborne bacteria or wild yeast that might enter the flask. This isimportant because a lot of air is sucked into the flask while it is cooling. You can also leave the stoppers on the flask instead of a fermentation lock. Some people use foam stoppers to replace fermentation locks in their carboys, too.

The small foam stoppers are 35-45mm wide and fit our 500 or 1000 ml flasks, or a glass carboy.