Guide
Kombucha, and Why It Breaks the Rules
Sweet tea, a culture, and air. Kombucha inverts almost every rule the vegetable ferments run on, starting with oxygen.
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Everything else in the fermenting hub works by excluding oxygen. Kombucha needs it.
That single inversion changes the whole setup. No airlock, no weight, no submersion. Instead you cover the vessel with cloth and give the culture as much surface area as you can. It is the odd one out in this hub and worth understanding on its own terms rather than by analogy.
What is actually happening
Kombucha ferments sweetened tea using a mixed culture of bacteria and yeasts, usually called a SCOBY.
Two things happen at once. Yeasts consume the sugar and produce alcohol and carbon dioxide. Acetic acid bacteria then convert most of that alcohol into acetic acid, which is what makes kombucha sour and what preserves it.
The bacteria need oxygen for that conversion, which is why the vessel is open rather than sealed. Seal it and you get more alcohol and less acid, which is the opposite of the intention.
The rubbery disc that forms on the surface is cellulose produced by the bacteria. It is a by-product rather than the culture itself, though it does carry organisms and it does serve as a physical barrier. The liquid holds most of the working population, which is why a cup of finished kombucha is a better starter than a new disc on its own.
What you need
Tea. Plain black or green. Nothing with added oils, so no Earl Grey, no flavoured blends, and no herbal infusions as the base. The culture needs the compounds in true tea.
Sugar. Plain white sugar is the standard and it works best. This is food for the culture rather than for you, and most of it is consumed. Honey behaves differently and can disturb the culture balance.
A culture and starter liquid. Both. The liquid matters more than the disc, since it carries the population and it acidifies the new batch immediately, which is the main protection against anything unwanted taking hold.
A wide vessel. Surface area drives the ferment. A wide jar beats a tall narrow one of the same volume. two litre fermenting jars with airlocks and weights works if you swap the airlock lid for a cloth cover.
Cloth and a band. A tight weave cloth or a coffee filter. Not cheesecloth, which lets fruit flies through.
No metal contact for anything sustained. A stainless spoon briefly is fine. Do not ferment in metal.
The first ferment
Brew strong tea. Roughly eight tea bags or two tablespoons of loose leaf per two litres. Steep ten minutes.
Dissolve the sugar while hot. Around 100 to 130 grams per two litres.
Cool completely. This is the step people rush and it kills cultures. Room temperature, not warm. Adding a culture to hot tea ends the project immediately.
Add the starter liquid. Roughly one part starter to nine parts fresh tea, or about 200 ml per two litres. More is safer.
Add the culture, cover with cloth, secure with a band.
Leave at room temperature, out of direct sun, for seven to fourteen days. Warmer is faster.
Taste from day seven with a clean straw or spoon. It should be tangy with a little residual sweetness. Too sweet means longer. Too sharp means you have gone toward vinegar.
A new disc will form on the surface within days. That is a sign of health rather than a requirement.
The second ferment
This is where carbonation and flavour come from, and it is optional.
Strain the kombucha into bottles, leaving the disc and about ten percent of the liquid behind as starter for the next batch.
Add flavouring. Chopped fruit, fruit juice, ginger, or a spoonful of sugar. The yeasts need something to work on to produce gas.
Seal the bottles. Now you do seal them, because you want the carbon dioxide retained.
Leave at room temperature for one to four days.
Burp daily. This is not optional and it is the only genuinely hazardous part of kombucha. Sealed bottles under active fermentation build real pressure, and glass failure is a documented risk. Use bottles designed for pressure, such as swing top beer bottles, never repurposed thin glass.
Refrigerate when carbonated. Cold slows everything and stabilises the pressure.
If a bottle feels hard, open it over a sink and away from your face.
Reading the vessel
Kombucha looks alarming to a newcomer and almost all of it is normal.
Normal. A new pale disc forming on the surface. Brown stringy strands hanging beneath it, which are yeast. Sediment at the bottom. Bubbles. A vinegary smell. Discs sinking, floating, or forming at odd angles. A lumpy or uneven disc.
Mould. Dry, fuzzy, raised, and coloured: blue, green, black, or white but fluffy. It grows on top of the disc, not underneath, and it looks like bread mould because it is essentially the same thing.
The distinction is the same as everywhere else in this hub. Flat and wet is culture; dry and fuzzy is mould. When mould appears, discard the whole batch including the disc and start again with a fresh culture. Do not attempt to rescue it.
Mould in kombucha almost always means insufficient starter liquid, too little sugar, tea that was too weak, or a cold slow start. All four leave the batch insufficiently acidic during its most vulnerable days.
Safety, honestly
Kombucha is generally low risk when made with enough starter liquid and kept clean, and two points are worth stating plainly.
Acidity is the protection. A batch that starts with adequate starter liquid is acidic from day one and stays that way. Skimping on starter is the main cause of problems.
It contains alcohol. Small amounts, typically well under one percent in a finished first ferment, and higher if fermentation is prolonged or the second ferment is vigorous. Worth knowing rather than assuming it is zero.
Guidance on home brewed kombucha varies by country and some authorities advise caution for pregnant, immunocompromised, and very young drinkers. Check your own national food safety authority rather than relying on any website, including this one.
Do not brew in ceramic that may be lead glazed, and do not use metal vessels.
Keeping it going
Kombucha is a continuous practice rather than a batch project, and it settles into a rhythm quickly.
Keep the disc and ten percent of each batch as starter for the next. Brew the next batch of tea while the current one is finishing so there is no gap.
Discs multiply. A new one forms each batch and they stack. Peel off and discard or give away the older, darker ones, keeping the newer pale ones.
A hotel is the storage solution for a break: discs in a jar covered with finished kombucha, at room temperature, with a little sweet tea added monthly. They survive for months this way.
Very sour kombucha is not wasted. It is essentially a light vinegar and works well in dressings and marinades.
Flavouring the second ferment
The second ferment is where kombucha becomes a drink people actually want, and the options are wider than most recipes suggest.
Fruit, chopped or puréed. Berries, mango, peach, and pineapple all work. Roughly ten percent by volume. Purée gives more carbonation than chunks because the sugars are more available.
Fruit juice. Simplest and most consistent. A splash per bottle.
Ginger. Grated fresh, and it carbonates enthusiastically. Ginger and lemon is the reliable combination that most people like.
Herbs and spices. Mint, basil, cardamom, cinnamon, or a chilli. These add aroma and no sugar, so add a little fruit or juice alongside if you want fizz.
Herbal tea. Brewed strong and cooled, added at bottling. This is where flavoured teas are safe to use, since they are nowhere near the culture.
Two practical points. Strain the fruit out before refrigerating if you dislike sediment, and expect a more vigorous ferment from anything high in sugar, which means burping more carefully.
Start with less flavouring than you expect to need. Kombucha is assertive on its own and a heavily flavoured bottle can taste muddled.
RUN THE NUMBERS
Illustrative figures. Times vary considerably with temperature.
| Detail | |
|---|---|
| Tea | 8 bags or 2 tbsp loose, per 2 litres |
| Sugar | 100 to 130 g per 2 litres |
| Starter liquid | About 200 ml per 2 litres, minimum |
| Vessel | Wide mouth, cloth covered |
| First ferment | 7 to 14 days at room temperature |
| Second ferment | 1 to 4 days, sealed, burped daily |
| Then | Refrigerate |
| Cost per 2 litres | Tea and sugar, very little |
Against retail kombucha, which is priced as a speciality drink, the saving is among the largest on this site. The inputs are tea and sugar.
WHAT GOES WRONG
Mould on the surface. Not enough starter liquid, weak tea, too little sugar, or a cold slow start. Discard everything and restart with fresh culture.
No new disc forming. Not a problem on its own if the liquid is souring. Discs are a by-product.
Nothing happening at all. Too cold, or the culture was killed by hot tea. Move somewhere warmer, around 22 to 26 degrees.
Tastes like vinegar. Fermented too long. Still usable in dressings, and start the next batch shorter.
No carbonation after a second ferment. Not enough sugar or fruit added, bottles not properly sealed, or too cold.
A bottle that gushes or bursts. Over-carbonated. Burp daily, use pressure rated bottles, and refrigerate sooner.
Fruit flies. Cheesecloth rather than a tight weave. They are attracted to this specifically.
DO THIS NOW
- Brew strong tea, dissolve the sugar while hot, and then cool it completely before anything else happens.
- Add at least ten percent starter liquid by volume, which is the single best protection against mould.
- Cover with a tight weave cloth, not cheesecloth, and leave at room temperature for a week before tasting.
- If you bottle for carbonation, use pressure rated bottles and burp them daily without exception.
Next
Kefir is the other culture based ferment here and it is considerably simpler, with no tea, no sugar, and no second ferment. Yoghurt and kefir, which work differently covers both.
Kombucha keeps developing in the refrigerator like everything else in this hub. Storing what you ferment covers realistic timescales.