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Bacteria doing the thickening

How Does Milk Become Yoghurt?

Friendly bacteria eat milk sugar, make acid, and the acid sets the milk.

1 Warm milk2 Bacteria eatthe milk sugar3 Acid sets itinto yoghurt
The bacteria are the recipe, not a contaminant.

  1. Step 1 of 5

    Milk carries a sugar called lactose.

    Milk sugar Bacteria eating itTheir waste product is acid

    It is the bacteria's food. Two particular species eat it and produce lactic acid as their waste, which is what does everything else.

  2. Step 2 of 5

    Milk proteins stay apart because of charge.

    Fresh milk Bundles repel — thinliquidAcidified milk Bundles clump — a softset

    In fresh milk the protein bundles carry a charge that keeps them repelling each other, so the liquid stays thin and pourable.

  3. Step 3 of 5

    Acid switches the repulsion off.

    1 Acid builds up2 Protein bundleslink3 The milk sets

    As acidity rises the charge is neutralised. The bundles can finally touch, and they link into a delicate net that traps the liquid.

  4. Step 4 of 5

    Warmth sets the pace.

    30 °Cslow — manyhours43 °Cideal — four tosix hours55 °Ctoo hot — thebacteria die

    The bacteria work fastest at around 43 degrees. Colder and it takes all day; hotter and they die before the job is done.

  5. Step 5 of 5

    The acid also preserves it.

    Milk Spoils in daysYoghurt Keeps for weeks

    The same acidity that sets the milk stops most spoilage microbes, which is why yoghurt keeps far longer than the milk it came from.

    A spoonful of shop yoghurt is a working starter culture — it carries billions of live bacteria.

The short version

Bacteria turn milk sugar into acid, and the acid makes milk proteins link into the soft set we call yoghurt.

Try it yourself

Stir a spoon of live yoghurt into warm milk in a flask. Leave it overnight and it will have set by morning.

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