By Nik, founder of 東歐嚴選｜kefir.link and former chef · First published 4 March 2022 · Updated 3 November 2026

You have strained a jar of plain kefir, and now you would like it to taste of mango, strawberry or vanilla. A second fermentation does exactly that. You add fruit or a flavour, close the bottle, and let the kefir work for a few more hours. It comes out fruity, tangy and pleasantly fizzy.

The short version

What it is: you add fruit or a flavour to strained milk kefir and close it in a bottle.
How long: leave it at room temperature for 6–12 hours. In a Taiwanese summer, check it at about 6 hours.
Then the fridge: once it tastes right, move the bottle to the fridge.
Drink it within 3 days of adding the fruit.
The bottle: use a thick bottle made for fizzy drinks and leave 2–3 cm of space at the top.
Opening: chill the bottle first, then open it slowly over the sink so the gas can escape.

See our live grains How to make kefir

How we checked this guide

Food-safety guidance for kefir: the BC Centre for Disease Control’s guide to making and storing kefir (BCCDC, PDF).
Kefir makers’ methods: second-fermentation times and flavours from Mad Millie (Mad Millie) and Fermenting for Foodies (Fermenting for Foodies).
Bottles and pressure: bottle choice, fill level and opening from Kombucha Kamp (Kombucha Kamp).
Taiwanese home kitchens: how a home kefir maker in Taiwan times a second fermentation in warm weather (PTT cookclub, in Chinese).
Our own kitchen: the times and steps are the ones we use and teach our customers on LINE.

What changes in the bottle

Strained kefir still holds lactic acid bacteria and yeasts. When you add fruit, they feed on its natural sugar and keep fermenting.

More bubbles: the yeasts make gas, and the closed bottle keeps it in the kefir.
A fruity flavour: the kefir takes on the taste of the fruit or spice.
A fresher tang: the kefir becomes a little sourer as fermentation continues.
A thicker texture: fruit pieces and purée make the kefir thicker.

Start with a finished batch that has fermented with the grains as usual. Add fruit only to the strained kefir, and keep your grains in plain milk. Read our kefir-making guide for the first fermentation.

What to add, and how much

These amounts are for one 500 ml bottle of strained kefir. Fruit makes up about a tenth of the bottle or less.

Flavour	Amount for 500 ml	How to add it
Strawberries, blueberries or other berries	40–50 g, about a small handful	Cut strawberries into small pieces. Add blueberries whole or lightly crushed.
Mango	40–50 g, about 3 tablespoons	Cut it into small cubes or mash it into a purée.
Banana	About a third of a banana	Mash it with a fork and stir it in.
Vanilla	Half a vanilla pod, or ½ teaspoon of vanilla extract	Split the pod open and drop it into the bottle.
Cinnamon	1 stick, or ¼ teaspoon ground	Take the stick out once the taste is right.
Cocoa	1–2 teaspoons	Stir it into a spoonful of kefir first, then add it to the bottle.
Honey	1–2 teaspoons	Measure it with a spoon and stir it in well.

Combinations we like: mango with vanilla, banana with cocoa and a pinch of cinnamon, and strawberries with a teaspoon of honey.

Berries, mango and banana keep the kefir smooth and creamy. Pineapple and citrus juice taste best blended into a smoothie and enjoyed straight away.

Step by step

Strain your kefir. Strain a finished batch and put the grains back into fresh milk.
Add the flavour. Put the fruit or spice into a clean bottle, then pour in the strained kefir.
Leave space at the top. Fill the bottle so that 2–3 cm of space stays empty below the cap.
Close it and leave it at room temperature. Stand it in a shaded spot for 6–12 hours.
Check it at about 6 hours in summer. A warm Taiwanese room makes the kefir ferment faster. Look for small bubbles and a firm-feeling cap.
Move it to the fridge. Once it tastes the way you like, put the bottle in the fridge. Take out cinnamon sticks and vanilla pods once the flavour is strong enough.
Chill it before you open it. Cold kefir holds the bubbles better and opens more gently.

Warm rooms speed up everything in kefir. Read why temperature matters when making kefir.

Bottles and opening safely

Choose thick bottles made for fizzy drinks: a sturdy round glass bottle with a swing-top or screw cap holds the gas well.
Leave 2–3 cm of space at the top: the gas needs room to collect under the cap.
Keep the bottle in a cool, shaded spot: a kitchen counter away from the stove and the window works well.
Chill it first: put the bottle in the fridge for a few hours before you open it.
Open it slowly over the sink: hold the bottle upright, loosen the cap a little at a time, and let the gas escape before you open it fully.
Stir it after opening: if the kefir has separated into curds and whey, stir it with a clean spoon.

Making water kefir too? Read water kefir second fermentation.

How long it keeps

Drink it within 3 days: count the 3 days from the moment you add the fruit.
Keep it in the fridge: store the bottle closed, at 2–4°C.
Use a clean spoon: pour or scoop with clean tools each time.
Throw away anything with fuzzy mould: coloured, fuzzy spots on the surface mean the bottle goes in the bin. Read Is the white layer on kefir mold? to see what is normal.

Recipes with flavoured kefir

When you want fruit and kefir right away, blend them and drink the glass fresh.

Ready in a minute: the any-fruit kefir smoothie and the mango kefir lassi.
For pineapple lovers: the pineapple coconut kefir colada is blended and served at once.
Ready by morning: kefir overnight oats and kefir chia pudding rest in the fridge overnight.
For hot days: strawberry kefir ice pops.

Your own fruit kefir starts with live grains.

Our live kefir grains ferment fresh milk with lactic acid bacteria and yeasts. They come with a printed care guide, pH strips and help on LINE.

Shop live kefir grains How to make kefir

Frequently asked questions

What is a second fermentation of milk kefir?

You add fruit or a flavour to strained milk kefir, close it in a bottle and leave it to ferment for a few more hours. It becomes fruity and fizzier.

How long should I leave it at room temperature?

Leave it for 6–12 hours. In a Taiwanese summer, check it at about 6 hours, then move it to the fridge.

Can I put the fruit in the jar with the grains?

Add fruit to the strained kefir only. Keep the grains in plain milk so they stay strong.

Which bottle should I use?

Use a thick bottle made for fizzy drinks, such as a sturdy round glass bottle with a swing-top. Leave 2–3 cm of space at the top.

How do I open a fizzy bottle?

Chill it in the fridge first. Then open it slowly over the sink so the gas can escape.

How long does flavoured kefir keep?

Keep it in the fridge and drink it within 3 days of adding the fruit.

My kefir separated after adding fruit. Can I still drink it?

Yes. Open the bottle and stir the curds and whey back together with a clean spoon.

Live kefir grainsSee them

Milk Kefir Second Fermentation