How long does kefir keep?

Does kefir go off in the fridge?

Not overnight. The cold slows fermentation right down, but it does not stop it completely.

Kefir is alive. Even strained, it still holds yeasts and bacteria that carry on working, very slowly. That is why the same bottle tastes different today and in three days' time: less sweet, sharper, often a touch fizzier. That is not spoiling, it is maturing. There is no best-before date like on a pot of shop yoghurt, so your nose and your tongue are the best judges. The real question is not "has it gone off?" but "do I still like it?".

Strained milk kefir

A few days in a lidded jar or a closed bottle, tasting best in the first couple of days.

In the fridge, milk kefir thickens slightly and its sourness creeps up. Plenty of people find it even nicer the day after straining, creamier and rounder. As the days go on it may split into two layers: a thick curd and a thin, pale yellow whey. Nothing to worry about, it has simply kept fermenting. Shake the bottle or give it a stir with a spoon and it comes back together.

If it gets too sharp to drink on its own, it still has plenty of uses. Older, tangier kefir is exactly what you want for a batch of scones or a stack of Pancake Day pancakes, where it does the job of buttermilk, and you can strain it overnight in a cloth to make strained kefir cheese. To get the texture right from the start, see Milk kefir too thin or too thick.

Bottled water kefir

This keeps for a few days too. It is never as fresh and fruity as straight after the second fermentation.

Water kefir usually goes into the fridge after its second fermentation: 24 to 48 hours with fruit in a closed swing-top bottle at room temperature, then into the cold. From there it slowly becomes drier, less sweet and sharper. After a week it is still drinkable, but it has lost some of its fruit. Bottles of half a litre or a litre are handy: they chill quickly and are finished in a day or two. How to get that step right, with which fruit and for how long, is all in Water kefir second fermentation.

The summary

How long can the grains wait?

The cold almost stops them, which gives you a few days off without losing your culture.

• Water kefir grains: make up sugar water as for a normal batch, put the grains in, close the jar and put it at the back of the fridge. They will wait like that for up to two weeks, as long as you renew the sugar water after a week. For longer holidays, everything is explained in Caring for water kefir grains.
• Milk kefir grains: a jar of fresh milk, roughly the amount of a normal batch, lid on, in the fridge. A weekend is no problem at all. For anything longer, the method is in Caring for milk kefir grains.

When you come back, the first batch is often a bit sluggish, the second is better, and by the third everything is back to normal. The resting liquid is not for drinking: the sugar water goes down the sink, and the milk can go into a batter if it still smells fresh.

When should you throw it out?

Good kefir smells fresh and sour. Water kefir sometimes reminds you of a sweet cider or bread dough, milk kefir of cream and a firm natural yoghurt. All of that is normal. On the other hand, throw it out without a second thought if you see:

• fuzz on the surface, especially green, black, blue or pink;
• coloured spots on the curd of milk kefir;
• a rotten smell, or any smell that really makes you pull back;
• grains that fall apart into mush and smell bad.

A thin, smooth white film on water kefir is usually just a layer of yeast that you lift off with a spoon. And milk kefir that has split into whey and curd has not spoiled, it has just fermented further. If in doubt, Milk kefir problems helps you tell the difference.

Good habits for kefir that keeps

• Bottles and jars washed in hot water and rinsed well, with no trace of washing-up liquid.
• Store them at the back of the fridge rather than in the door, where the temperature jumps every time someone opens it.
• A label with the straining date, so nobody has to guess which bottle is the oldest.
• Small bottles that you finish in a day or two.

The simplest approach is not to make more than you drink. If the kefir is piling up, cut back the milk or the water, or pick up ideas from Too much kefir?, from overnight oats to a dip for the evening.

Frequently asked questions

The best way to always have fresh kefir is to make a little every day with your own live milk kefir grains, from £14. The complete milk kefir guide shows you how.