The ratio that keeps kombucha safe
Kombucha is sweet tea fermented by a SCOBY, a symbiotic culture of bacteria and yeast, and its safety depends less on fancy equipment than on getting the basic ratio right. Too little sugar starves the culture and stalls fermentation, too little starter liquid leaves the tea insufficiently acidic at the start, and both mistakes open the door to mould, which is the one outcome that means throwing a batch away. The widely used, well-tested ratio is seventy grams of sugar per litre of water, two tea bags per litre, and one hundred millilitres of starter liquid per litre, which is ten percent of the batch, and the calculator on this page scales those numbers to any batch size you choose.
Starter liquid is not optional, and it is not the same as the SCOBY itself. It is mature, acidic kombucha from a previous batch, or raw unflavoured shop-bought kombucha if you are starting from scratch, and its job is to drop the pH of the fresh sweet tea immediately, so that friendly microbes dominate from hour one. A SCOBY without enough starter is like a sourdough starter added to unsalted dough: it may eventually work, but the early hours belong to whatever else happens to be around. Never skip the starter to save a bottle, and never use flavoured kombucha as starter, since flavourings and oils can interfere with the culture.
Brewing the sweet tea properly
Brew the tea strong and sweet, then cool it completely before anything living touches it. Boil the water, stir in the sugar until fully dissolved, add the tea bags, and steep for ten to fifteen minutes for black tea, which is the most reliable base because its nitrogen and nutrients support the culture well. Green tea works too and gives a lighter, more delicate kombucha, but it ferments a little more slowly and produces a less robust SCOBY over many batches, so beginners should start with black tea or a black and green blend. Herbal teas, especially those with essential oils such as peppermint or Earl Grey with bergamot oil, can weaken or damage the culture and are best avoided for the main ferment.
Cooling matters as much as brewing: tea above about thirty degrees can harm or kill the SCOBY, so wait until the sweet tea feels cool to the touch, not just lukewarm. An ice bath speeds this up for large batches, and planning the brew the evening before removes all temptation to rush. Once cool, pour the tea into a clean glass jar, add the starter liquid, slide the SCOBY in gently, and cover with a clean cloth secured by a rubber band, never a sealed lid, because the ferment needs air. Keep the jar out of direct sunlight, away from the kitchen bin and houseplants, and at a steady room temperature.
| Batch | Sugar | Starter liquid | Tea bags | Jar size |
|---|---|---|---|---|
| 1 litre | 70 g | 100 ml | 2 | 1.5 litre jar |
| 2 litres | 140 g | 200 ml | 4 | 3 litre jar |
| 3 litres | 210 g | 300 ml | 6 | 4 litre jar |
| 4 litres | 280 g | 400 ml | 8 | 5 litre jar |
| 8 litres | 560 g | 800 ml | 16 | 10 litre jar |
Fermentation time, tasting, and temperature
First fermentation usually takes seven to fourteen days, and temperature is the main variable: at twenty-four to twenty-six degrees a batch may be ready in a week, while at eighteen degrees it can need two weeks or more. Taste from day five onwards with a clean straw or spoon, without disturbing the SCOBY more than necessary, and look for a balance between sweetness and tang. Early kombucha tastes mostly like sweet tea with a slight bite, ready kombucha tastes pleasantly tart like mild vinegar with a little sweetness left, and over-fermented kombucha tastes strongly vinegary, which is not ruined, it is simply vinegar you can use for dressings or dilute into a new starter.
Do not judge readiness by the SCOBY alone, because a new SCOBY layer forming on the surface, brown stringy bits, and a slightly yeasty smell are all normal signs of an active ferment. What is not normal is fuzzy mould, which looks like blue, green, or black spots sitting on the surface, not under it, and a mouldy batch must be discarded entirely, jar sterilised, and the culture replaced, since mould roots run deeper than the visible patch. If the jar smells like nail polish remover or acetone, the ferment is likely too warm or too long, and moving it cooler and tasting earlier next time usually fixes it.
Second ferment, bottling, and fizz
Once the first ferment tastes right, bottle most of it for a second ferment, keeping back your starter liquid and SCOBY for the next batch. Add a little fruit, juice, or ginger to each bottle, leave about two centimetres of headspace, seal tightly, and leave at room temperature for two to four days to build carbonation, burping the bottles daily by briefly opening them to release excess pressure. Glass bottles with swing tops or pressure-rated caps are essential here, because ordinary thin glass can break under carbonation pressure, and a bottle that has never been burped in a warm kitchen is a genuine safety risk, not just a mess risk.
Keeping a culture healthy for months
A healthy SCOBY routine is simple: brew regularly, keep the ratio consistent, and never let a jar sit untouched for months in a cold corner without feeding. If you need a break, make a fresh batch and let it ferment fully, then store the jar covered in a cool place, where it will slowly turn to vinegar but keep the culture alive for several weeks. When you return, use that vinegary liquid as an extra-strong starter, brew fresh sweet tea, and the culture usually revives within one batch. Keep notes on batch size, sugar, and tasting days, and your kombucha will become more predictable with every brew.
- Use 70 g sugar, 2 tea bags, and 100 ml starter per litre.
- Cool sweet tea fully before adding the SCOBY.
- Cover with cloth, never a sealed lid, for the first ferment.
- Taste from day 5 and bottle when pleasantly tart.
- Burp second-ferment bottles daily to release pressure.
Flavouring ideas and common troubleshooting
Second-ferment flavouring is where kombucha becomes personal, and a few combinations work reliably for beginners: fresh ginger with lemon, muddled berries with mint, apple with cinnamon, and mango with a pinch of chilli all ferment well and carbonate nicely. Use about one tablespoon of fruit or juice per five hundred millilitre bottle, taste the base kombucha first so you know how sweet it still is, and remember that sweeter bases carbonate faster because the yeast has more sugar to convert. If a batch tastes flat after bottling, the usual causes are too little residual sweetness, too short a second ferment, or bottles that do not seal properly, and checking the seal is the cheapest fix before changing anything else. If bottles over-carbonate, refrigerate them promptly, open them over a sink, and shorten the next second ferment by a day.
Kombucha brewing is a ratio, a clean jar, and patience, and the ratio is the part you never need to guess again. Use the calculator above for any batch size, keep your starter generous, and your SCOBY will reward you with batch after batch of tangy, fizzy tea.