Breathable foam stoppers will withstand boiling or autoclaving temperatures, making them ideal for use in preparing a yeast starter. The honeycomb structure of the foam stoppers will act as a filter for any airborne bacteria or wild yeast that might enter the flask. That's important 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 large foam stoppers are 46-60 mm wide and fit our 2000 and 5000 ml borosilicate glass flasks.