Milk kefir is milk fermented by kefir grains: small, rubbery clusters of bacteria and yeasts held together in a polysaccharide they
make themselves. You put the grains in milk, leave the jar at room temperature for about a day, strain the grains out and start again.
The three things that decide the result are how many grains you use, how warm the room is, and how long you leave it. This page gives
the numbers for each. To buy kefir instead, see our guide to the best kefir brands.
What’s in the Grains
A kefir grain is a community, not a single culture. A research team that rebuilt kefir from its main microbes used five bacteria,
Lentilactobacillus kefiri, Lactobacillus kefiranofaciens, Lactococcus cremoris, Leuconostoc
mesenteroides and Acetobacter pasteurianus, and four yeasts, including Saccharomyces cerevisiae and
Kluyveromyces marxianus (Mudoor Sooresh et al., 2025). The bacteria turn lactose into lactic acid and sour the
milk; the yeasts add a little carbon dioxide and alcohol. That mix is why kefir tastes fizzier and more complex than
yogurt, and why it ferments at room temperature.
Ratio, Temperature and Time
| What | Range used in studies | What it changes |
|---|---|---|
| Grains per milk | 2–10% by weight (20–100 g per litre) | More grains sour the milk faster and leave less lactose |
| Temperature | 20–25°C | Warmer is faster; cooler is milder and slower |
| Time | About 24 hours | Longer is thicker, sourer and more likely to separate |
| Finished pH | 4.2–4.6 | The point where it has set and soured |
These are the figures summarised in a study of kefir made under household conditions, which itself used 10% grains at 20°C for
24 hours (Alves et al., 2021). In practice, start with about 20–50 g of grains per litre (roughly 1–3 tablespoons)
and adjust: if your kefir is too sour or separates before a day is up, use fewer grains or a cooler spot; if it’s thin after a day, use
more grains or a warmer spot. The same study found that kefir made with ultra-heat-treated (UHT) semi-skimmed milk kept its expected
composition, so long-life milk works too (Alves et al., 2021).
Method
- Put the grains in a clean glass jar and pour pasteurized milk over them: whole, semi-skimmed or goat’s milk. Raw milk isn’t made safe by the grains; see kefir or yogurt with raw milk.
- Cover the jar loosely (a cloth, a coffee filter or a lid resting on top). Kefir makes a little gas; don’t seal it tight.
- Leave it at room temperature, out of the sun, for about 24 hours.
- It’s ready when it has thickened and smells clean and pleasantly sour. A few pockets of whey are fine.
- Strain it through a plastic or stainless-steel sieve, stirring gently so the kefir passes and the grains stay.
- Put the grains straight into fresh milk for the next batch, and refrigerate the strained kefir.
Strained kefir keeps well in the fridge: artisanal kefir in one study stayed within quality limits and was still well accepted after
30 days (Ströher et al., 2025), though it gets thinner and sourer as it ages. For storage times and signs it has gone
off, see how long kefir lasts.
How Fast Kefir Grains Grow
Slowly. In the household study, grains fermenting at 20°C gained about 6% of their weight a day, and about 60% over eight days of
daily batches (Alves et al., 2021). At that rate grains take roughly two weeks to double, not “every few batches”.
Remove some when the ratio creeps up (the kefir will tell you by souring faster), and share or compost the extra. To take a break,
grains can rest in milk in the fridge or be frozen; how long kefir grains last covers both.
How Much Alcohol Is in Homemade Kefir?
A little. A review of kefir’s composition gives a typical figure of about 0.48% alcohol, and more grains mean more alcohol: in goat’s-milk kefir fermented to pH 4.5, 1% grains gave about 0.3% alcohol
and 5% grains about 1% (Farag et al., 2020). If you avoid alcohol entirely, use fewer grains.
Grains or a Powdered Starter?
Freeze-dried kefir starters are sold as an easier way in. Grains are a living community you keep feeding; a powdered starter is
used for a batch or a few. In research, a freeze-dried mix of the nine traditional kefir microbes listed above produced kefir that matched
kefir made with fresh culture (Mudoor Sooresh et al., 2025). Shop sachets vary, and the label tells you which strains are
in them. Our own first kefir, in 2016, was made with a powdered starter; the notes and photos are below.
When It Goes Wrong
- If it separates into curds and clear whey, it fermented too long or too warm, or there were too many grains. Shake it and drink it, or
use it in baking; next time shorten the time or cut the grains. - Still thin after 24 hours? The room is too cool, there are too few grains, or the grains are still recovering after a rest or a move. Give it a few
batches to settle. - Very sour or yeasty kefir has fermented too long or too warm, so strain it sooner.
- Pink, orange or fuzzy growth, or a rotten smell, means contamination. Discard the kefir and the grains and start with new
ones.
Kefir is lower in lactose than milk and is often easier to digest for people with lactose maldigestion
(Hertzler and Clancy, 2003). For uses, try overnight kefir oats; for a dairy-free ferment with
different grains, see water kefir; for other room-temperature dairy ferments, see
cultured buttermilk, sour cream and crème fraîche.
