These black plastic caps are made to provide good torque retention and the poly cap will not rust over time. The airtight caps ensure maximum carbonation build-up and flavor development by creating an airtight seal at the opening of your bottle. They are sure to help you brew successfully, whether brewing Kombucha, Kefir, Beer, Kvass, Soda, Cider, Mead, Wine, Infused Oils, Infused Vinegars – you’re only limited by your imagination!