Kombucha, del primer SCOBY a la segunda fermentación

Kombucha is sweetened tea fermented by a SCOBY — a symbiotic culture of bacteria and yeast. The yeast eats sugar and makes alcohol; the bacteria eat the alcohol and make acids. What you drink is the point where those two are in balance.

What you need

• A SCOBY with at least 150 ml of mature starter liquid. Get it from someone brewing already, or from a supplier — never start from a bottle of flavoured shop kombucha.
• A wide glass jar, 2–3 litres. Wide matters: fermentation happens at the surface.
• Woven cloth and an elastic band. It needs air, not a lid.
• Tea, sugar, water.

Avoid metal for anything that sits in contact with the brew, and avoid soap residue. Rinse everything with hot water and vinegar.

First ferment

For 2 litres:

1. Brew 8 g of plain black or green tea in 500 ml of just-boiled water for ten minutes. No oils, no bergamot, no herbal blends — Earl Grey and chamomile will weaken a culture over time.
2. Stir in 160 g of white sugar until dissolved. The sugar feeds the culture, not you; most of it is gone by the end.
3. Top up with 1.5 litres of cold water and check it is below 30 °C. Heat kills a SCOBY faster than anything else.
4. Add the starter liquid, then the SCOBY. The starter's acidity is what protects the jar from mould in the first days.
5. Cover with cloth. Leave somewhere dark, 20–26 °C, undisturbed.

Taste from day 6, with a straw or a spoon, every day or two. It is ready when it is pleasantly tart with a little sweetness left — usually 7 to 14 days, faster in summer. Left too long it becomes vinegar, which is useful but is not a drink.

Second ferment

This is where carbonation and flavour come from.

1. Reserve 300 ml plus the SCOBY for your next batch.
2. Bottle the rest in flip-top glass bottles, leaving 4 cm of headspace.
3. Add flavouring: 5–10% fruit juice, a few slices of ginger, a strip of citrus peel. A little residual sugar is what the yeast turns into fizz.
4. Seal and leave at room temperature 2 to 4 days.
5. Burp the bottles daily — open briefly to release pressure. This is a safety step, not a suggestion; sealed bottles can build enough pressure to break.
6. Refrigerate. Cold stops the ferment.

Reading the jar

• A new pale layer forming across the surface is a healthy new SCOBY.
• Brown stringy strands hanging below are yeast. Normal.
• Bubbles and a vinegary smell are what you want.
• Fuzzy dry mould — blue, green, black, raised and dry-looking — means discard the whole batch, culture included. Do not rescue it. This almost always follows too little starter liquid or too cold a room.

Sugar, alcohol and honesty

A finished kombucha typically retains 2–6 g of sugar per 100 ml, well below a soft drink, and contains around 0.5% alcohol — enough to matter if you are avoiding alcohol entirely. Longer ferments lower the sugar and raise the acidity, not the other way round.

Waste, which is the reason to bother

A home brew replaces bottled soft drinks with tea, sugar and a culture that reproduces itself indefinitely. The jar is the packaging. Surplus SCOBYs go to friends, to compost, or into vinegar. It is one of the few kitchen practices where doing it more produces less waste, not more.