Skip to content
kuazaraKuazara
Log in

Kitchen science · Fermentation and measurement

How is yoghurt made? The bacteria that set milk

A spoonful of yoghurt turns a pan of milk into yoghurt overnight. Learn how two kinds of bacteria turn lactose into lactic acid, why milk sets as the pH falls, why temperature matters so much, and the rules for making yoghurt safely at home.

  • 17 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

One evening a pan of milk is heated and cooled a little, a spoonful or two of yoghurt is stirred in, and the pan is wrapped in a thick blanket. By morning the same pan holds a fragrant yoghurt thick enough for a spoon to stand up in.

How did one spoonful of yoghurt transform a whole pan of milk? And why is the milk heated first and then cooled? Couldn't you just stir the yoghurt straight into cold milk?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Ordering

Making yoghurt, step by step

Put the cards in order by dragging them or moving them with the keyboard, then press Check.

Prediction

Predict first

When do you think you should add the starter (the yoghurt) to the milk?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Number line

How many times more acidic?

Drag the marker or move it in steps of 10 with the arrow keys, then press Check.

Prediction

Predict first

The pH falls from 6.6 to 4.6. By how much do you think the acidity (the H⁺ concentration) rises?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Share: Share to Google Classroom (opens in a new tab)

Embed the simulation on your site

Paste this code into the HTML of your blog or school site. Visitors try the simulation without an account; no progress is saved.

The embedded page sets no cookies and does no tracking. A small link back to Kuazara sits underneath.

Log in to continue

The rest of the lesson is waiting for you, with a mini quiz. Creating an account is free.

Start for freeLog in

Frequently asked

5 questions

Why is the milk first heated to 85–90 °C?

For two reasons. Most of the unwanted microbes in the milk die, so the yogurt bacteria take over without rivals. Also, the whey proteins unfold in the heat and attach to the casein, so the yogurt sets thicker and weeps less.

Will yogurt set faster if I add the starter to hot milk?

No. Yogurt bacteria work best at 42–45 °C; milk at about 50 °C or above harms them, and the yogurt may not set. In much cooler milk, fermentation slows down. The finger test varies from person to person; a kitchen thermometer is more reliable.

How does runny milk turn thick?

The bacteria turn lactose into lactic acid and the pH falls. As the pH nears about 4.6, the casein micelles lose most of their negative charge, stop repelling each other and stick together into a network that holds the water inside; the milk sets into a gel.

Is the yellowish liquid on top of yogurt a sign it has gone off?

No. That is whey that has separated from the casein network, and it is normal; you can stir it back in. Signs of spoilage are mould, pink, orange or black spots, fizzing and an unusual smell; if you see these, throw the yogurt away without tasting it.

Does adding more starter make better yogurt?

Not always. Extra starter can speed up fermentation, but the yogurt may turn out too sour or runny. One or two tablespoons of fresh, live yogurt per litre of milk is usually enough.