Mastering Water Kefir Second Fermentation: Control Fizz, Flavor, and Bottle Pressure

By Nik, founder of 東歐嚴選｜kefir.link and former chef · First published 15 May 2026 · Updated 4 November 2026

Your first batch of water kefir tastes lightly sweet and gently sour. The second fermentation turns it into a fruity, sparkling drink, a little like a homemade fruit soda. All it takes is a plastic bottle, a splash of juice and about a day. Here is the method, the amounts, and how to time it in a warm Taiwanese kitchen.

The short answer

• The bottle: pour strained water kefir into a plastic sparkling-water bottle or a PET bottle, 70–80% full.
• The flavour: for every 500 ml, add 50–100 ml of fruit juice, or 2–3 tablespoons of fruit purée, or 2–3 thin slices of ginger.
• The time: close the cap and leave it at room temperature for 12–24 hours. In Taiwan’s summer, 8–12 hours is usually enough.
• The fizz check: press the side of the bottle. When it feels hard, the fizz is ready.
• The finish: chill the bottle for a few hours, then open it slowly over the sink so the gas escapes. It keeps in the fridge for up to 45 days.

Start with live grainsTry our ready water kefir

How we checked this guide

• Fruit and juice amounts: water kefir guides from Cultured Food Life (Cultured Food Life) and Freshly Fermented (Freshly Fermented).
• The squeeze test: plastic-bottle checks from Casa de Sante (Casa de Sante) and from a kombucha guide written for hot kitchens (InstaCuppa).
• Opening bottles: bottling advice from Kombucha Kamp (Kombucha Kamp) and a Taiwanese home maker’s water kefir notes (凱熙, in Chinese).
• Taiwan’s temperatures: monthly averages from the Central Weather Administration (CWA, in Chinese).
• Our own kitchen: the method, the amounts and the 45-day storage time come from the guide we print for our grain customers.

The method, step by step

Start with a finished first batch. Read our water kefir guide for the first fermentation.

1. Strain out the grains. Put the grains straight into fresh sugar water. The fruit goes only into the strained drink.
2. Fill a plastic bottle. Pour the strained water kefir into a clean plastic sparkling-water bottle or a PET bottle. Fill it 70–80% full, so the gas has room at the top.
3. Add the flavour. Add juice, purée, fruit slices or ginger. The amounts are in the table below.
4. Close the cap tightly. A closed bottle keeps the gas in the drink, and that gas becomes the fizz.
5. Leave it at room temperature. Stand it in a shady spot for 12–24 hours, or 8–12 hours on hot summer days.
6. Let the gas out once or twice. Open the cap slowly, let the gas escape, and close it again.
7. Press the bottle. When it feels hard, move it to the fridge.
8. Chill it, then open it slowly over the sink. After a few hours in the fridge, the bubbles are finer and the bottle opens more gently.

Plastic is the safest bottle for this step. It gives a little under pressure, and you can feel how much gas has built up.

How much fruit or juice to add

These amounts are for one 500 ml bottle of strained water kefir.

Other water kefir makers use similar amounts. Cultured Food Life adds 60–120 ml of juice to a 470 ml bottle. Freshly Fermented adds a few pieces of fresh fruit to a 500 ml bottle.

Juice ferments faster than whole fruit, so a juice bottle is usually ready first.

• Pineapple and ginger: sweet, sharp and very lively. Chill it well before you open it.
• Lemon and rosemary: light, fresh and gently sparkling.
• Mango and mint: round, fruity and cool.
• Passion fruit on its own: fragrant, tangy and bright.

Our ready canned water kefir is flavoured the same way, with dried fruit or flowers in a second fermentation. Taste the blueberry, osmanthus or strawberry can for ideas.

How Taiwan’s heat changes the timing

Warmth makes the yeasts work faster, so the gas builds up sooner.

Many second-fermentation guides from Europe and America give one to three days. They are written for rooms of about 18–22°C.

In Taipei, the average temperature is about 28–30°C from June to August. In Kaohsiung, it is still almost 20°C in January.

• Pick a calm spot: a shady cupboard or counter away from the stove, the window and the top of the fridge.
• Trust the bottle more than the clock: the squeeze test tells you when it is ready.
• Keep your grains cool too: they ferment best at 20–25°C and stay healthy below 35°C.

Checking the fizz with the squeeze test

A plastic bottle tells you how much gas is inside.

1. Press the side of the bottle with your thumb. Do this when you first close it, so you know how soft it feels at the start.
2. Press it again every few hours. In summer, start at about 8 hours.
3. Soft and easy to press: the fizz is still building. Give it a few more hours.
4. Firm, like a new bottle of soda: the fizz is ready. Put the bottle in the fridge.

Fermenting several bottles at once? Press one of them as your test bottle. When it feels firm, move them all to the fridge.

Opening and storing your bottles

• Chill it first: put the bottle in the fridge for a few hours. Cold water kefir holds the bubbles better, and the bubbles become finer.
• Open it straight from the fridge: a cold bottle opens more gently than a warm one.
• Open it slowly over the sink: hold the bottle upright and press a clean cloth over the cap. Turn the cap a little, let the gas escape, then open it fully.
• Keep it closed in the fridge: water kefir keeps for up to 45 days. It slowly becomes more sour.

A Taiwanese home maker once opened a warm pineapple bottle, and half of it sprayed out. Chilling first keeps every drop in the bottle.

Fine-tuning fizz and taste

Change one thing at a time, and write down what you did. After a few bottles, you will know your kitchen.

Check each bottle with your eyes and nose. A fresh, fruity, sour smell means all is well. If you see fuzzy mould, throw the bottle away.

Making milk kefir as well? Read milk kefir second fermentation for fruit amounts and timing with milk. Following a plant-based diet? Read Vegan probiotics: why try water kefir?

Your own fruit soda, batch after batch.

Our water kefir grains starter kit has 100 g of living grains, 1 kg of organic cane sugar, a plastic strainer, a plastic sparkling-water bottle for the second fermentation, pH strips and a printed guide. The grains are full of lactic acid bacteria and yeasts, and they keep growing with every batch.

See the grains kitTry our ready water kefir

Frequently asked questions

Live water kefir grainsSee the kit