SAVE ON SECONDARY FERMENTATION BOTTLES If you've been saving your store-bought kombucha bottles for your secondary fermentation, then these lids will be the answer to your success in re-using them. These lids have a built-in gasket that will form an airtight seal and prevent carbonation from escaping during your secondary fermentation. The original lids from your store-bought kombucha aren't fully airtight causing aerobic fermentation in your secondary when it should be anaerobic. This allows carbonation to escape and can create some funky off-flavors in your kombucha. Remember - these caps fit MOST standard-sized store-bought kombucha bottles, but not all companies use the standard bottle size.