What it shows
Milk turns into curd because friendly bacteria in a spoon of old curd turn milk sugar into lactic acid. Keeping the milk at different temperatures shows how heat changes the speed of this change.
The science
Milk contains lactose and the protein casein. Lactic acid bacteria in the starter curd feed on lactose and make lactic acid, which makes the milk acidic. The acid makes the casein proteins stick together and form a soft gel, so milk sets into curd. The bacteria work best at around 35 to 42 degrees Celsius, so milk sets fastest when warm, slowly at room temperature and hardly at all in the cold. Too hot, and the bacteria are killed.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Half a litre of milk, boiled by an adult and cooled
- A teaspoon of fresh curd as a starter for each jar (3 teaspoons in all)
- Three clean small steel vessels or glass jars with lids
- A thermometer (non-mercury)
- A warm insulated box or a pot with a towel
- A refrigerator or a cooler corner
- A watch and a notebook
Safety
For this project: An adult must boil the milk and pour it. Check that the milk feels only lukewarm on your wrist. Use clean spoons and jars, and throw away milk that smells bad. Wash hands.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Pour the same amount of cooled milk, about 150 mL, into three clean jars and test that it is around 40 degrees Celsius.
Stir one teaspoon of fresh curd into each jar and put on the lids.
Place jar A in the warm insulated box, jar B at room temperature and jar C in a cool place at about 15 to 20 degrees Celsius or on a refrigerator shelf.
Write the starting time and temperature near each jar.
Tilt each jar gently every hour, without stirring, to check whether the milk has set and does not flow.
Record the time at which each jar sets and the colour and the taste (only if clean and fresh, by an adult), or smell.
Compare the times and draw a bar graph of the setting time against temperature.
Write what the bacteria did and why temperature changed the time.
What you should see
The warm jar sets in about 4 to 8 hours, the room-temperature jar in 8 to 12 hours or more, and the cold jar sets very slowly or not at all. The bar graph shows this.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What changes milk into curd?
Lactic acid bacteria turn lactose into lactic acid, which curdles the milk proteins.
2Why is a spoon of old curd added?
It carries the bacteria that start the process.
3Why must the milk not be hot when you add the starter?
Hot milk would kill the bacteria.
4Why is curd set in cold weather harder to make?
The bacteria work slowly at low temperatures.
Take it further
Compare the setting time with different amounts of starter (half a teaspoon, one and two) in the same warm place.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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