Is it safe to brew kombucha at home?

The conditions that make a brew safe rather than risky, what a healthy SCOBY looks like, and the second-ferment pressure hazard nobody mentions.

Difficulty: intermediate
Time: 20 min
Read: 5 min
Safety: warning

Short answer

Brew sweet tea, cool it, add a SCOBY and at least 10% of the volume as mature starter liquid from a previous batch, and keep it at 20 to 26°C covered with a tight cloth for one to three weeks. The starter liquid is the safety step — it acidifies the brew immediately so nothing else can establish. Never brew in ceramic or metal.

Kombucha is fermented sweet tea, and it is safe for the same reason a pickle is: it becomes too acidic for anything harmful. The difference from vegetable fermenting is that it starts at neutral pH, so the acidity has to be imported at the beginning in the form of mature starter liquid. That is the step that gets skipped, and it is the one that matters.

Safety

Kombucha is safe when the pH drops quickly, which requires enough starter liquid, enough sugar and a warm enough room. Brew in glass or food-grade plastic — never in ceramic or metal vessels, since acid can leach lead from glazes and react with metal. Bottles left too long in a second ferment can explode: use bottles rated for pressure, burp them daily and refrigerate. Kombucha contains a small amount of alcohol and is not recommended in pregnancy.

What you'll need

A SCOBY and mature starter liquid from an existing batch
Black or green tea — not flavoured or oiled teas
White sugar
A wide glass jar, at least 2 litres
A tight-woven cloth and a band
pH strips (optional)
Pressure-rated bottles for the second ferment

Step by step

Brew strong sweet tea and cool it completely.Roughly 8g of tea and 70g of sugar per litre, brewed, then topped up with cold water and cooled to room temperature. Hot liquid kills the culture, so it must be genuinely cool before the SCOBY goes anywhere near it.
Use plain tea and real sugar.Black or green tea without added oils or flavourings — Earl Grey's bergamot oil and many herbal teas do not suit the culture. White sugar is what the culture eats; honey, sweeteners and low-sugar versions do not work and honey brings its own organisms.
Add mature starter liquid, and enough of it.At least 10% of the total volume, taken from a finished, sour batch. This is the safety step: it drops the pH immediately so mould and stray bacteria cannot establish while the culture gets going. A SCOBY alone in sweet tea is not enough.
Use glass, never ceramic or metal.Kombucha is acidic enough to leach lead from ceramic glazes and to react with metal vessels and lids. Glass or food-grade plastic only, with a plastic or wooden spoon.
Cover with a tight-woven cloth.Not muslin, which fruit flies get through, and not a sealed lid. A tightly woven tea towel or coffee filter with a band. It needs air and it must not get insects.
Keep it at 20 to 26°C, out of direct sun.Below about 18°C the brew is slow and the acidification lags, which is when things go wrong. A warm cupboard is better than a cold worktop.
Leave it seven to twenty-one days and taste to judge readiness, not safety.Draw a little off with a straw or a clean spoon and taste for balance — it should be tart with a hint of sweetness left. Tasting is how you decide it is ready; it is never how you decide it is safe. pH strips reading 2.5 to 3.5 are a useful confirmation.
Know what a healthy SCOBY looks like.Cream to tan, rubbery, with brown stringy yeast strands hanging beneath it — all normal. Bumps, holes, dark patches and layers are normal. What is not normal is dry, fuzzy, raised growth on the top surface, blue, green or black. That is mould, and the SCOBY and the whole brew go in the bin.
Second-ferment in pressure-rated bottles, and burp them.Bottled with a little fruit or juice and sealed, kombucha carbonates fast. Use swing-top bottles designed for pressure, never ordinary screw-top bottles or repurposed jars, keep them out of the way, burp them once a day, and refrigerate after one to three days. Bottles genuinely do burst.
Refrigerate and drink it within a few weeks.Cold effectively stops the ferment. Keep a jar of the sour liquid back as the starter for the next batch.

Tips

Keep a 'SCOBY hotel' — a jar of spare SCOBYs in mature kombucha in a cupboard. It is your insurance if a brew goes wrong.
Fruit flies are the most common practical nuisance. A tight cloth and a clean rim solve it.
Over-fermented kombucha that has gone vinegary is not wasted — it is a decent drinking vinegar and a fine salad dressing.
Kombucha contains a small amount of alcohol, typically under 1% but higher in a long second ferment. Bear that in mind for children and for anyone avoiding alcohol.

Common mistakes

Starting a brew with a SCOBY and no mature starter liquid — The brew sits at near-neutral pH for days while the culture wakes up, and that is exactly the window in which mould establishes. The starter liquid acidifies it from hour one and is the single most important safety measure in kombucha brewing.
Second-fermenting in ordinary bottles and forgetting them — Sealed carbonating kombucha builds real pressure, and a non-pressure-rated bottle can shatter. Use proper swing-top bottles, burp daily, and put them in the fridge on schedule.
Brewing in a decorative ceramic crock — Acidic liquid held for weeks against a glaze that may contain lead is a genuine risk, and there is no way to know from looking at a crock whether its glaze is food-safe for that use. Glass removes the question.

If it doesn't work

Dry fuzzy growth on the SCOBY surface

Cause: Mould, usually from too little starter liquid or too cold a brew — Fix: Bin the SCOBY and the whole brew. Do not try to wash or salvage it. Restart with a fresh SCOBY, more starter liquid and a warmer spot.

Brew tastes like vinegar

Cause: Fermented too long or too warm — Fix: Use it as drinking vinegar or a dressing, and cut the time next batch. Keep some as starter liquid — sour is exactly what you want there.

Still sweet after two weeks

Cause: Too cold, or a weak culture — Fix: Move it somewhere at 22 to 26°C and give it another week.

No fizz in the second ferment

Cause: Not enough sugar left, or bottles not sealed — Fix: Bottle it slightly earlier while it is still a little sweet, add a spoonful of fruit juice, and use swing-top bottles.

Questions people ask

Is kombucha safe in pregnancy?

It is generally advised against. It is an unpasteurised live product with a small alcohol content and, when home-brewed, no verification of what is in it. NHS advice is to avoid unpasteurised products in pregnancy, and home-brewed kombucha sits squarely in that category. Check current NHS or Food Standards Agency guidance if you are unsure.

How much starter liquid do I really need?

At least 10% of the total volume, and more is safer — 20% for a first brew or in a cold house. Running short of starter is the most common cause of a mouldy batch, and it is entirely avoidable by keeping a jar of mature kombucha back every time.