Yoghurt and soured milk, step by step
Interactive lesson: from heated milk to thick yoghurt — the culture, a warm place without a machine, soured milk in a pot, straining, labneh and signs of spoilage.
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The whole lesson as text
1. Milk and heating
Yoghurt isn't milk left to go sour, it's a controlled fermentation.
- Fresh milk — Yoghurt works best with milk no more than a day old. Old milk is already slightly sour, and the yoghurt turns out grainy or bitter.
- Milk powder — For a thicker yoghurt, stir skimmed milk powder into the cold milk: 20–30 g per 1 L for cow's milk, 30–50 g for goat's. More gives a chalky taste.
- Heat to 85–90 °C — Hold the milk at 85 °C (185 °F) for 30 minutes or at 90 °C (194 °F) for 10 minutes. Raw milk never goes to 43 °C with the culture.
- Slowly, without boiling over — Heat in a water bath or over a low flame, stirring all the time. A scorched bottom gives the yoghurt a smoky taste.
Why: Heating to 85–90 °C (185–194 °F) destroys the bacteria that cause disease and those that would compete with the culture. It also changes the whey proteins, so the yoghurt is thicker and releases less whey.
- Sheep's milk gives a very thick yoghurt, goat's milk a thin one. That's not a mistake: drink goat's yoghurt like soured milk, strain it or add milk powder.
- Heat shop-bought pasteurised milk to 85 °C anyway. UHT milk only needs warming to 43 °C.
2. The culture
The culture only goes in once the milk has dropped to the right temperature.
- Cool to 43–45 °C — Stand the pan in cold water and stir. Thermometer in the middle of the pan: at 45 °C (113 °F) add the culture, and it will be 43 °C when you pour it.
- A spoonful per litre — One tablespoon of yoghurt (15–20 g) per 1 L is enough. Two or three tablespoons give a sourer, sometimes grainy yoghurt.
- Mix the starter — Mix the starter in a cup with 100 mL of warm milk so there are no lumps. Return it to the pan and stir gently for 30 seconds; don't whisk hard.
- Into warm jars — Pour the milk into clean, warm jars or a bowl and close them.
Why: Above 46 °C (115 °F) the culture starts to die, below 40 °C (104 °F) it works slowly. The two yoghurt bacteria work best at 42–43 °C (108–109 °F).
- The starter can be plain yoghurt with live culture, yoghurt from your last batch or a freeze-dried culture used as directed on the sachet.
- A home culture weakens after 5–10 generations. When the setting time gets more than an hour longer, get a new one.
3. A warm place without a machine · Locked
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4. Soured milk the old way · Locked
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5. Thick yoghurt and labneh · Locked
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6. Storage and spoilage · Locked
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