The 5 L flask (or 5000 mL) is ideal for making very large starters for those big boy (high gravity) beers and lagers. If you want to get your starter wort up to a boil using the flask, we recommend you heat it up with a boiling water bath. Place the flask with the desired water amount, into a pot with additional water, and then bring the water up to a boil. Tada! Uses a #11.5 stopper.