Brewing Kombucha in a Stainless Conical Fermenter: Batch Size, SCOBY, and Carbonating Under Pressure

Kombucha is fermented the same way beer is, in a vessel that keeps the wrong things out and lets the culture work, and once you brew more than a gallon at a time the glass jar stops making sense. A stainless conical fermenter turns kombucha from a countertop hobby into 5, 10 or 30 gallon batches with a tap for tasting, a cone to drop the yeast out of, and, if the tank holds pressure, a second ferment you can carbonate without a single bottle.

Here is how to do it, and the three ways kombucha differs from beer in the same tank.

Is stainless right for kombucha?

Yes, if it is 304 or 316 stainless. Kombucha finishes around pH 2.5 to 3.5, more acidic than beer, which rules out aluminum, plastic that scratches, and anything with lead solder or brass in the wetted path. 304 stainless is what commercial kombucha is fermented in. A stainless conical with tri-clamp fittings and silicone gaskets has nothing the acid can attack.

One rule: dedicate the tank to kombucha, or clean like you mean it. The acetic acid bacteria in a SCOBY are exactly the bacteria beer brewers fear. If the same fermenter will see beer again, run a full hot caustic clean and sanitize cycle, replace the gaskets, and keep separate hoses and sample valves for the two.

Batch sizes and ratios

Per gallon of finished kombucha: about 1 cup (200 g) of sugar, 8 to 10 g of tea (four to five bags, black or a black-green blend), and 1 to 2 cups of strong starter liquid from the previous batch. So a 10-gallon batch in a 13-gallon tank is 10 cups of sugar, roughly 90 g of tea, and half a gallon or more of starter. Leave headspace: the SCOBY grows on the surface and you want room to lift the lid without disturbing it.

Tank	Batch	Sugar	Tea	Starter
8 gal conical	5 gal	1 kg	45 g	2 to 4 L
13 gal conical	10 gal	2 kg	90 g	4 to 8 L
26 gal conical	20 gal	4 kg	180 g	8 to 15 L

Brew the tea strong in a kettle, dissolve the sugar hot, then dilute with cold water in the fermenter to reach 75 to 80°F before adding the starter and SCOBY. Never add the culture to hot tea.

Primary fermentation: the SCOBY needs air

This is the first difference from beer. The acetic acid bacteria that make kombucha taste like kombucha are aerobic. They need oxygen at the surface, which is why a SCOBY forms a mat on top. So for the first fermentation:

• Leave the lid off, or replace it with a breathable cover: a cloth tied over the top opening, or the lid resting loose with the airlock port open and covered in cloth. No airlock, no pressure.
• Hold 75 to 82°F. Cooler and it stalls; warmer and it goes vinegary fast. A jacketed tank helps here in a cold room: run warm water through the jacket, not cold.
• Keep it out of direct sunlight and away from anything that ferments beer.

Primary takes 7 to 14 days. Taste from the sample valve daily from day five: you want a balance of sweet and tart, and a pH of 3.0 to 3.5. Sampling from the valve rather than dipping a cup is the second big advantage over a jar; the SCOBY never gets disturbed.

The cone: dump the yeast strands

Kombucha throws brown yeast strands and sediment that settle in the cone. Before flavouring, open the cone valve for a second into a bucket and drop them out. The beer-brewing habit of a quick dump applies exactly. What you keep is clearer, and the second ferment carbonates more predictably.

Keep a jar of the primary liquid, from the sample valve, as the starter for the next batch. Lift the SCOBY out with clean hands or a sanitized tray and store it in that starter.

Second fermentation: carbonate under pressure, no bottles

This is where the conical earns its keep. The traditional second ferment is bottles: fruit, sugar, cap, wait, and hope none of them explode. In a pressure-rated conical you do the same thing in the tank:

1. Dump the yeast, remove the SCOBY, add fruit purée, juice or a little sugar through the top port (about 1 to 2 cups of juice per gallon).
2. Seal the lid. Fit a spunding valve and set it to 10 to 12 psi.
3. Hold 75°F for 2 to 5 days. The residual yeast ferments the new sugar; the CO2 stays in solution instead of venting. Taste from the sample valve.
4. Crash to 34°F with the valve still set, dump the fruit sediment from the cone, and push the finished kombucha to kegs through a closed transfer kit. Serve from a tap, or counter-pressure fill bottles that are already carbonated and cold.

No bottle bombs, no guessing, and a crash that stops the second ferment exactly where you want it. Our Unitank is rated to 30 psi working with a 15 psi relief valve, so the pressures above are well inside its range.

Cleaning between batches

Rinse, recirculate hot alkaline cleaner through a spray ball for twenty minutes, rinse, sanitize. Pull the valves and gaskets and soak them. Kombucha's SCOBY fragments hide in gaskets, so inspect them; silicone tri-clamp gaskets are cheap to replace.

Troubleshooting

Problem	Cause	Fix
Vinegar in five days	Too warm, or too much starter	Run the jacket cooler; hold 75°F
No SCOBY forming, sweet after ten days	Too cold, or the culture was added to hot tea	Warm the jacket to 78 to 80°F; restart with fresh starter
Fuzzy spots on the SCOBY	Mould: not enough acid at the start	Discard the batch and SCOBY. Use more starter next time
Flat second ferment	Not enough sugar, or yeast all dumped	Add 1 cup juice or 2 tbsp sugar per gallon; dump less aggressively
Over-carbonated	Spunding valve set too high, or too long warm	Set 8 to 10 psi; crash sooner

The setup

A Unitank in 8, 13 or 26 gallons, a sample valve, a spunding valve, a transfer kit, and, for a room that swings in temperature, the temperature control kit. If you are scaling a kombucha business, the 1 BBL Unitank gives 31 gallons a batch on the same design. Ask us about pairing tanks and chillers for a kombucha cellar; several of our customers run them.