How do I make yoghurt at home?

How to make yoghurt with no special equipment, why the temperature matters, and how to keep a culture going indefinitely.

Difficulty: beginner
Time: 10 hr
Read: 3 min
Safety: caution

Short answer

Heat milk to 85°C and hold it briefly, cool to 45°C, stir in a couple of tablespoons of live yoghurt, and keep it at around 40–45°C for 6–10 hours. A flask, an oven with the light on, or a wrapped pan in an insulated bag all hold the temperature well enough.

Yoghurt is milk plus bacteria plus a stable warm temperature. The two temperatures that matter are 85°C, which denatures the milk proteins so the set is thick, and 45°C, which is where the culture works. Everything else is patience.

Safety

Use fresh milk and scrupulously clean equipment — you're deliberately encouraging bacterial growth, so anything contaminating it also grows. If the finished yoghurt smells wrong, is pink or has any mould, discard it. Refrigerate promptly once set and use within a week.

What you'll need

1 litre of milk
2 tablespoons of live yoghurt with active cultures
Thermometer
Clean jar or flask
Insulated bag or a flask (optional)

Step by step

Sterilise the jar and any utensils with boiling water.You're cultivating bacteria deliberately, so cleanliness determines whether you get the right ones.
Heat the milk to 85°C, stirring so it doesn't catch, and hold for 5–10 minutes.This denatures the whey proteins, which is what makes a thick set rather than a thin, separated one. Skipping it gives runny yoghurt.
Cool to 45°C.Standing the pan in cold water speeds this up. Too hot and you kill the culture; too cool and it works slowly or not at all.
Stir in two tablespoons of live yoghurt until fully mixed.Check the label says 'live' or 'active cultures'. Some commercial yoghurts are heat-treated after culturing and won't work.
Pour into the jar or flask and keep it at 40–45°C for 6–10 hours.A flask, an oven with just the light on, or a wrapped jar in an insulated bag. It needs stability rather than precision.
Don't disturb it while it sets.Moving it during setting breaks the gel and gives a lumpy or watery result.
Refrigerate once set — longer culturing gives a tangier, firmer result.Six hours is mild, ten is noticeably tangy. It firms up further as it chills.
Keep two tablespoons back as the starter for the next batch.It can continue indefinitely, though the culture weakens after many generations and occasionally needs refreshing from a shop-bought pot.

Tips

Straining set yoghurt through muslin for a few hours gives thick Greek-style yoghurt. The liquid whey that drains off is useful in baking and smoothies.
Whole milk gives a richer, thicker result than semi-skimmed. Adding a couple of tablespoons of milk powder thickens it further.
If it doesn't set, the culture was probably killed by milk that was too hot, or the yoghurt used as starter wasn't live.
The same principle underlies most simple ferments: clean equipment, the right temperature, and time.

Common mistakes

Adding the culture to milk that's too hot — Above about 50°C it kills the bacteria and nothing happens.
Skipping the 85°C heating step — The proteins aren't denatured, so the yoghurt sets thin and separates.
Using heat-treated yoghurt as a starter — The cultures are dead. The label must say live or active cultures.
Moving it while it sets — It breaks the forming gel and gives a lumpy, watery result.

Questions people ask

Why is my homemade yoghurt runny?

Usually because the milk wasn't heated to 85°C first, the incubation temperature was too low, or it wasn't left long enough. Straining also thickens it substantially after the fact.

How long does homemade yoghurt keep?

About a week refrigerated. Keep two tablespoons back as a starter before you finish it, so you can make the next batch.