By Magnus von Kageneck, cellar master (who's that?) · Updated: September 2026
Kombucha isn't wine, but it belongs to the same family: yeasts and bacteria turn sugar into something else. There's no ready-made SCOBY floating in the bottle of starter, and that's intentional: it grows by itself from the active starter liquid during your first batch. If a piece of culture is floating in it after all, it goes into the jar too.
What's at work in the jar
The starter is matured, sour tea with live cultures of yeasts and bacteria, brewed on black tea. The yeasts break down the sugar and ferment part of it into a little alcohol and carbon dioxide. The bacteria, mostly acetic acid bacteria, produce acid. On top grows a rubbery skin, the so-called tea fungus, or SCOBY. It isn't a single fungus, but a mesh of cellulose in which yeasts and bacteria live together, much like a vinegar mother.
Acidity is what matters: the faster the sweetened tea turns sour, the less chance mould and unwanted microbes have. Keep the starter in the fridge until you use it; its strength fades over time, so use it before the best-before date.
The first litre: quantities and order
Besides the kombucha starter with its 100 ml, all you need is tea, sugar and tap water:
- 2 bags of black or green tea or 2 teaspoons of loose tea, ideally without added flavourings
- 50 g of white granulated sugar, no honey and no sweetener
- 900 ml of water, 200 ml of it boiling and 700 ml cold
- Thoroughly clean the jar, the spoon and your hands, and rinse the jar with hot water. No traces of washing-up liquid may remain.
- Pour 200 ml of boiling water over the tea and stir in the 50 g of sugar until nothing is left on the bottom.
- Let it steep for 5 to 15 minutes, depending on the type, then remove the bags or leaves.
- Add 700 ml of cold water. The tea is now at about 25 °C, and on no account above 30 °C. Tea that is too hot harms the culture; tea that is too cold slows it down.
- Let the starter come to room temperature beforehand, shake it vigorously and add the whole bottle.
- Cover the jar with a cotton cloth and pull the rubber band over it.
- Put it in a quiet spot at 21 to 25 °C, out of the sun and off the radiator, and then try not to move it. In winter, leave a thermometer in the spot for a few days first; the windowsill is often too cold.
That gives you one litre with 50 g of sugar, i.e. 5 per cent. Honey brings a strong flavour of its own, and sweetener gives the culture nothing to ferment; the figures here are based on ordinary granulated sugar. The jar in the kombucha starter set holds 1.7 litres, but for your first batch you still stick to one litre. The starter should make up at least a tenth of the batch. With more tea, the whole thing sours too slowly, and that gap is exactly where mould gets in.
A cloth, not a lid
As long as the batch is working, it needs oxygen; the lid only goes on after fermentation. The cloth still keeps out dust and fruit flies, which carry in unwanted microbes. Pull it taut and check the edge: a gap under the rubber band is enough for the flies. If you only have the starter, use a clean, tightly woven cotton cloth and a glass vessel; metal doesn't tolerate the acid.
Conversely, protect your wine: never put the kombucha jar in the room where wine or mead is fermenting or stored. Use separate cloths, straws, funnels and measuring instruments, and wash your hands before you go to the wine. Fruit flies carry acetic acid bacteria from jar to demijohn, and acetic spoilage in wine cannot be fixed.
How to recognise the new SCOBY
After a few days, a thin, milky film forms on the surface. Over time it gets thicker and rubbery, white to beige. Holes, bubbles, unevenness, brownish patches and brown strands on the underside are normal. On the very first batch the skin often stays thin and everything takes longer; the SCOBY gets stronger with every round.
From day 5 to 7 you can taste it, ideally with a clean straw slid in from the side under the skin. Usually the kombucha tastes pleasantly tart and only slightly sweet after 7 to 14 days. It may smell sour, slightly of vinegar and a little of yeast. The longer it stands, the sourer it gets.
Mould: no compromises here
Mould sits on top and looks different from everything described so far: dry, furry or fluffy spots, often round, green, blue, black or white. The SCOBY, by contrast, is moist and smooth; it belongs to the liquid and doesn't lie on it like dust.
If you see mould, don't skim anything off and don't try to save anything. The liquid and the culture go out completely, the jar is cleaned hot and the cloth is washed at a boil. The mould's network reaches further than you can see. A white, dry, wrinkled skin can also be kahm yeast, which you may know from wine. With kombucha you don't have to tell the two apart: when in doubt, throw it out and start again.
The pH test for those who want to know exactly
Smell and taste are enough most of the time. If you want to be sure, measure with pH test strips from a chemist or pharmacy. The acidometer from the wine cellar measures something else, namely total acidity in grams per litre (Measure, don't guess); the pH value tells you how strongly the acid acts.
Two numbers are enough. Right after setting up, with the starter stirred in, the value should be no higher than 4.5. Finished kombucha sits at around 2.5 to 3.5. If the first value is higher, there was usually too much water or a flagging starter, and the acid isn't protecting the batch yet. Don't wait and hope for the best; start again straight away with fresh starter, at least a tenth of the batch. The same applies if no film at all has appeared after 14 days, or if the date on the bottle had passed. Only drink kombucha that clearly smells and tastes sour.
Harvesting and the second round
Once you like the taste, take out the SCOBY with clean hands and keep back about a tenth of the kombucha, around 100 ml from the first batch. That's your next starter. The rest goes into bottles and into the fridge. It keeps fermenting slowly there too: drink it promptly and briefly open sealed bottles every few days.
The second round follows the same recipe, with the SCOBY and the reserved kombucha instead of the starter bottle. Now the 1.7-litre jar can be filled higher, to 1.5 litres:
- 3 tea bags and 75 g of sugar
- 1,350 ml of water, 300 ml of it boiling for the tea and sugar and 1,050 ml cold
- 150 ml of kombucha from the previous batch, plus the SCOBY
The SCOBY grows by one layer with each round; you can separate off surplus layers and use them for a second batch or give them away.
Second fermentation in swing-top bottles: fizz in moderation
If you like it fizzy, fill the finished kombucha into pressure-resistant swing-top bottles, add a few slices of ginger or at most 5 per cent apple juice or another fruit juice, but no grape juice, and leave the bottles at room temperature for 1 to 3 days, briefly opening each one once a day. Then they go into the fridge.
Why so cautious? In the sealed bottle the yeasts keep fermenting. As a rule of thumb: 4 g of fermented sugar per litre gives about 1 bar of overpressure at 10 °C, and roughly a third more at room temperature. Apple juice has about 100 g of sugar per litre; 50 ml of it in 950 ml of kombucha brings about 5 g into the litre. Fermented out, that's around 1.25 bar at 10 °C and 1.7 bar at 20 °C, plus about 0.3% vol more alcohol: semi-sparkling wine territory, which is what a pressure-resistant swing-top bottle is built for. A tenth of juice, i.e. 10 g of sugar per litre, would already give more than 3 bar at 20 °C. That's sparkling-wine strength, and it doesn't belong in any swing-top bottle, only in sparkling wine bottles with a stopper and wire cage, at no more than 24 g of sugar per litre.
On top of that comes the residual sugar in the kombucha, which you don't know without measuring. That's why the pressure isn't limited by the 5 per cent alone, but also by the short time, the fridge and the daily opening. Only use bottles built for pressure, not thin-walled decorative or juice bottles. Never leave them out of the fridge for more than three days, and open them over the sink, pointing away from your face.
What kombucha is and what it isn't
Kombucha is a sour, lightly sparkling tea drink that you may or may not like. I won't promise you more than that. It isn't alcohol-free: the yeasts always produce some alcohol, and a little more is added in the sealed bottle. During pregnancy and breastfeeding, leave it alone as you would any alcoholic drink; the same goes for children and for anyone who can't or doesn't want to drink alcohol. If you have a weakened immune system, check with your doctor first whether a live, unheated ferment is right for you.
If you're then looking for a ferment that takes more patience: the vinegar booklet among the Maischewerk booklets leads you to your own vinegar.
Source: manufacturer's information on the kombucha starter (100 ml) and the kombucha starter set; batch quantities from Maischewerk customer service. Pressure and alcohol derived from 4 g of sugar per litre ≈ 1 bar at 10 °C and 0.47 g of alcohol per g of sugar.
Magnus von Kageneck is Maischewerk's cellar master character. The texts in this series are written with the support of artificial intelligence and are checked for technical accuracy before publication.
Related in the shop: Vinegar & Ferments and Starter Sets