🧪 Ages 8–14 (with an adult)⏱ 30 min + 6–12 hours🧽 Mess: Low
The question: How do tiny living bacteria turn milk into yogurt?
Make a prediction
What do you think will happen to the milk overnight? Will it taste sweeter or more sour? Write your guess on the worksheet before you start. Scientists call this a hypothesis.
What you need
4 cups milk
2 tbsp plain yogurt with "live and active cultures" on the label (your starter)
A saucepan and a thermometer
A clean jar or container with a lid
A warm spot: an oven with just the light on, or a cooler with a towel
Steps
An adult heats the milk to about 180°F, stirring so it doesn't scorch, then lets it cool to about 110°F (warm, not hot).
Stir the starter yogurt into a little of the warm milk, then mix it all together.
Pour into a very clean jar, cover and keep it warm and still for 6 to 12 hours.
Check it: it should be thick and tangy. Refrigerate for a few hours to firm up.
Compare a spoon of milk and a spoon of your yogurt. How are they different?
🔬 Strain some yogurt through a coffee filter in the fridge overnight to make thick Greek yogurt, then compare.
⚠️ Safety: An adult handles the hot milk. Use very clean equipment. If it smells bad or looks pink, orange or fuzzy, throw it out.
What's happening? The science
Yogurt starter contains helpful bacteria. In warm milk, they eat the milk's natural sugar (lactose) and make lactic acid. The acid makes the milk proteins tighten and thicken, and it gives yogurt its tangy taste. This is another kind of fermentation.
Heating the milk first helps the yogurt turn out thicker and smoother. Cooling it to about 110°F is important, because too much heat would kill the bacteria.
Think further
What other foods are made by fermentation? (Hint: bread, cheese, pickles.)
