QRP OCCUPARE VITA COMPLETE FERMENTATION STARTER KIT FOR MASON JARS (jars not included). NEW PATENT PENDING QRP OCCUPARE VITA FERMENTATION FOOD RETAINER CUP isolates and protects the airlock from the expanding fermenting food. The Cup situates just underneath the cap on the jar mouth. Slits in the cup allow the airlock to exhale oxygen while teeth and the Cup itself hold the fermenting food down meaning NO WEIGHTS NEEDED and absolutely no possibility of food contamination of the airlock which would probably cause mold! Without some form of gasket, the Mason jar plastic lids are not airtight. Quality Reliable Products Mason Jar Silicone Seals fit snug into Mason jar plastic lids. QRP seals provide a perfect airtight, anaerobic enclosure while fermenting Pickles, Kimchi (vegetables) or Kombucha (tea). Metal vs. Plastic Lids use for fermentation and storage of fermented foods? We find that whatever we are fermenting doesn't normally come in contact with the lid during fermentation, however can briefly come in contact with the lid during storage. We've done the research, which reveals that metal shouldn't be used during fermentation, nor for storage of fermented foods due to possible corrosion. QRP OCCUPARE VITA Excellent Quality 3-Piece Airlocks supersede S-Curve Bubble Style Airlocks because they are sturdier and easier to keep clean.