Dairy ferment

Milk kefir

BeginnerAlso: kefir

Milk fermented at room temperature by kefir 'grains' — a tangy, lightly fizzy drink.

Temperature: 18–26 °C; Warmer is faster and more sour; very warm kitchens may need under 12 hours.
Time: 12–48 hours (usually about 24); Thickened, tangy, perhaps separating — time depends on temperature and the grain-to-milk ratio.

Timeline

Stage 1: Add grains to milkstart · about 12 min
Hands-on — 1–2 tbsp per 250–500 ml
Stage 2: Ferment at room temperaturefrom 12 min · about 24 h
Fermenting / waiting — 20–25 °C
Stage 3: Strain & chillfrom 24 h · keeps about 7 days
Cold storage — Fridge, about a week

Ingredients

1–2 tbsp kefir grains
250–500 ml pasteurised milk

Equipment

Glass jar
Breathable cover (cloth) or loose lid
Plastic or stainless sieve

Steps

Put the grains in a clean jar and pour in the milk.
Cover loosely; leave at room temperature (about 20–25 °C) out of direct sun.
After 12–24 hours, when thickened and tangy, strain out the grains.
Put the grains in fresh milk for the next batch.
Refrigerate the kefir; an optional second ferment (a few hours with fruit peel) adds flavour.

Is it working?

Thick like pouring cream
Fresh, sour, slightly yeasty smell
Grains look plump, white to cream

Smell & texture: Tangy, yeasty-fresh; fizzy on the tongue. Separation into curds and whey just means it went a bit long — shake it.

Signs tell you it's fermenting, not that it's safe: never rely on smell or appearance alone.

Safety, mould and discard rules

Mould or yeast?

Dairy ferments should not grow anything on the surface. Any fuzz, spots or coloured patches (pink, orange, green, black) mean contamination: discard the whole batch — don't scrape it off.

Storage

Refrigerate strained kefir. Rest grains in fresh milk in the fridge for a break of up to about a week.

Shelf life: Best within about a week in the fridge. Ferments in sealed bottles build pressure — open carefully.

The science: what's happening

Kefir grains are a rubbery matrix of polysaccharide (kefiran) holding lactic acid bacteria and yeasts. The bacteria sour the milk; the yeasts add a little fizz, aroma and a trace of alcohol.

Organisms

Lactic acid bacteria (Lactobacillus kefiranofaciens, Lactococcus, Leuconostoc and others)
Yeasts (Kluyveromyces, Saccharomyces and others) — the mix varies by grain

Troubleshooting

Too sour or separates: Shorter ferment, more milk per spoon of grains, or a cooler spot.
Thin and not souring: Grains may be recovering — keep feeding fresh milk daily; check the room isn't too cold.

A dish (not a ferment) gone wrong? Try Recipe Rescue.

Cook with it

Learn more

Sources

Potential Hazards with Fermented Foods — Association of Food and Drug Officials
Raw drinking milk — UK Food Standards Agency

Guidance changes: check the source for current wording. Original text; figures quoted from the sources where stated.