Dairy ferment

Yoghurt

BeginnerAlso: dahi, curd, yogurt

Warm milk set by lactic bacteria into a soft, tangy curd — the easiest ferment to start with.

Temperature: 40–46 °C; Below about 37 °C it sets slowly and may grow other bacteria; above 50 °C the culture dies.
Time: 4–12 hours to set; Shorter is milder; longer is tangier and firmer.

Timeline

Stage 1: Heat & cool milkstart · about 1 h
Hands-on — 82–85 °C, then cool to 43–46 °C
Stage 2: Incubatefrom 1 h · about 8 h
Fermenting / waiting — 40–45 °C, don't stir
Stage 3: Chill & eatfrom 9 h · keeps about 10 days
Cold storage — Fridge ≤5 °C, best within about 1–2 weeks

Ingredients

1 litre pasteurised milk (whole for thickest set)
2 tbsp plain live yoghurt from a fresh, unopened pot (or a freeze-dried starter)

Equipment

Heavy saucepan
Thermometer
Clean jar or bowl with lid
Somewhere warm: oven with the light on, insulated box, yoghurt maker or Instant Pot

Steps

Heat the milk to 82–85 °C, stirring, and hold a few minutes (longer gives a thicker set).
Cool to 43–46 °C — warm on the inside of your wrist.
Whisk the starter with a ladle of the warm milk, then stir it all back in.
Pour into a clean container, cover, keep at 40–45 °C undisturbed.
Check from 4 hours: it's set when it pulls away cleanly from the side when tilted.
Refrigerate at once; it firms further as it chills.

Is it working?

Set like custard; tilting the jar shows a clean edge
A little clear yellowish whey on top (normal)
Clean, fresh, tangy smell

Smell & texture: Fresh and pleasantly sour; smooth or slightly lumpy. Stir the whey back in or pour it off.

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 at 5 °C or below as soon as it sets.

Shelf life: Home yoghurt is best within about 1–2 weeks in the fridge. Save starter for the next batch from a fresh batch within a week.

The science: what's happening

Heat-loving lactic acid bacteria turn milk sugar (lactose) into lactic acid. As the acid rises, milk proteins (casein) lose their charge and knit into a gel, so liquid milk sets into a spoonable curd.

Organisms

Streptococcus thermophilus
Lactobacillus delbrueckii subsp. bulgaricus (plus other cultures in many starters)

Troubleshooting

Didn't set: Starter too old or killed (milk above 50 °C), or kept too cool. Use fresh starter and check the temperature. If it sat warm for hours without setting, discard it.
Runny: Use whole milk, heat longer at 82–85 °C, or add 2 tbsp milk powder. Strain for thicker (Greek-style).
Too sour: Incubate for less time and chill promptly.
Grainy: Milk was heated too fast or the culture was mixed in too hot; whisk starter in gently at 43–46 °C.

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

Cook with it

Learn more

Sources

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

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