Short answer: cover your brew with a clean, tight-weave, breathable material such as cotton cloth or a coffee filter, and secure it with a rubber band. Avoid cheesecloth, which lets fruit flies through. Don’t seal the vessel with an airtight lid during the first fermentation, because the culture needs air.
What the cover is for
The SCOBY grows at the surface of the brew, and it needs air exchange to do that. A good cover does two jobs. It keeps flies, dust and other contaminants out, and it lets air in. It also has to stay put, because a cover that slips lets flies in.
Good covers
- Tight-weave cotton cloth. A clean, unbleached cotton cloth works well. An old cotton T-shirt cut to size is a common choice, and so is a bandana. Tea towels are woven more tightly than cheesecloth and are a good option.
- Unbleached cotton muslin. Works well when it’s secured with a rubber band.
- Coffee filters. They’re lint-free and have a fine pore size. They must cover the whole opening.
- Fitted reusable covers. Some have elastic edges sized for gallon jars. Check the size against your vessel before you buy.
Wash new cloth before you use it, and avoid cloth treated with bleach or chemicals. Those residues can end up in the brew.
What to avoid
- Cheesecloth. Its loose weave lets fruit flies and their eggs slip through. One Food52 writer covered a SCOBY with two layers of cheesecloth, found fruit flies and larvae in it, and had to discard it.
- Airtight or solid lids. A solid metal or plastic screw lid blocks the air exchange the culture needs during the first fermentation. Save hard lids for transport or for finished bottles. Plastic wrap pulled tight over the top has the same problem.
- Chemically treated cloth, as noted above.
- Loose or stretchy covers that slip off or leave gaps.
Securing the cover
Use a rubber band wide enough to fit around the vessel, or a bungee cord. Colostate’s extension guide describes the aim: keep insects and contaminants out, but allow airflow. Make sure the cover overlaps the opening on all sides, and check it each time you taste or refill. Replace it if it tears.
Covering a carboy or a spigot vessel
- Carboys have a narrow neck, so a cloth can be hard to secure. A wide-mouth vessel is easier to cover and to clean. Food52 recommends a wide-mouth jar for air circulation and for easier SCOBY removal. Brewing kombucha in a 5-gallon carboy covers the rest of the set-up.
- Spigot vessels need the same cover on the top. Keep the spigot clean and make sure it’s food-safe. Continuous brew kombucha explains the draw-and-refill routine.
Where to put the covered brew
- Out of direct sunlight.
- Away from ripening fruit. Food52 advises keeping the jar away from ripening stone fruit, which draws fruit flies.
- Somewhere with enough air movement, but where the cover stays in place.
Dealing with fruit flies
- Check the cover, the vessel rim and the SCOBY for flies, eggs or larvae.
- If larvae have got into the SCOBY, many brewers discard the batch and start again.
- Clean the vessel and look for the gap that let flies in before you rebrew.
Covers for second fermentation
Bottles for the second fermentation use airtight caps, not cloth. The cloth is only for the first fermentation. Best bottles for kombucha covers bottles and caps.
For a wider look at equipment, see best kombucha starter kits. For what to do with the SCOBY between batches, see what is a kombucha starter?