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F1 makes it sour. F2 makes it drinkable. It is also the only stage with a real hazard.

The method

1. Strain the finished F1 kombucha, holding back 10–15% as starter for the next batch.
2. Fill swing-tops to about 4 cm below the rim. Headspace is where the CO2 goes.
3. Add flavouring: 10–20% by volume of fruit purée or juice, or 1 tsp of sugar, or a few centimetres of grated ginger.
4. Seal and leave at room temperature 2–5 days.
5. Refrigerate. Cold stops fermentation almost completely.

Pressure, honestly

Sealed fermentation in glass builds real pressure, and over-carbonated bottles do burst. Three habits remove the risk entirely:

• Use one plastic test bottle per batch, filled at the same time. When it is rock hard, every glass bottle is ready. This is the only reliable gauge.
• Burp daily in warm weather — open, hear the hiss, reseal.
• Chill before opening, and open over a sink. Cold liquid holds CO2; warm liquid gives it up all at once.

Never use a screw-cap bottle that once held a still drink. Wine bottles and juice bottles are not pressure-rated.

Flavour combinations that work

• Ginger and lemon — the reliable one; ginger feeds the yeast, so it fizzes hard.
• Raspberry and mint.
• Hibiscus and apple.
• Second-flush green tea with peach, in summer.

Fruit pulp will settle. Strain again at bottling if a clear drink matters to you; the pulp is fine in compost.

Reusing bottles well

Rinse immediately after pouring — dried kombucha residue is stubborn. Wash in hot water with a bottle brush; skip detergent inside where you can, and rinse thoroughly where you cannot. Sanitise only if a batch has gone wrong. Replace gaskets when they stop sealing, sold individually for cents. A well-kept swing-top outlasts most kitchen appliances.

The waste ledger

Home kombucha replaces bought bottles almost one for one. Tea leaves and fruit pulp compost. The excess SCOBY that forms every batch can be composted, dried into a leather-like sheet, or given away — but be honest about how many of these projects you will finish. Composting it is not a failure.