What it shows
Adding vinegar to hot milk makes the milk protein casein clump into curds. Pressed and dried, the curds set into a hard, mouldable plastic. Buttons and ornaments were made this way before petroleum plastics existed.
The science
Casein is a protein that floats in milk as tiny clusters held apart by their electric charge. Acid from the vinegar cancels that charge, so the clusters stick to each other and form curds, a process like making paneer. When the curds are squeezed and dried, the long protein molecules tangle and lock together, giving a hard solid. Because the material is made of linked long molecules, it counts as a natural polymer.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 1 cup full-cream milk
- 4 teaspoons vinegar
- A saucepan and a stove (adult only)
- A sieve or clean cloth
- Paper towels
- Cookie cutters or bottle caps as moulds
- Food colour, optional
Safety
For this project: An adult must heat the milk and do the pouring; the child works with the curds only after they are cool. Do not boil the milk over, and do not eat the plastic.
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.
Ask an adult to heat the milk until it is steaming but not boiling.
The adult adds the vinegar and stirs gently; white lumps appear at once and the liquid turns watery.
Let it cool for ten minutes, then pour through the sieve to collect the curds.
Press the curds between paper towels to squeeze out as much liquid as possible.
Knead the lump for a minute until it is smooth like dough; add a drop of food colour if you like.
Press the dough into the moulds and poke two button holes with a toothpick.
Leave the buttons on a plate in a dry place for two days, turning them once a day.
Test the hardness by tapping and compare a thick button with a thin one for drying time.
What you should see
Hard, slightly glossy buttons that keep their shape and holes, which can be sewn onto cloth; thicker pieces take longer to dry and may crack if dried too fast in the sun.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What makes the milk form lumps?
The vinegar acid makes the casein protein clump together into curds.
2Is the plastic made from the fat or the protein?
From the protein casein; the fat and water mostly drain away.
3Why is this called a polymer?
It is made of long chain molecules linked together.
4How is this like making paneer?
Paneer is made the same way, by adding lemon or vinegar to hot milk.
Take it further
Make one batch from toned milk and one from full-cream milk, and compare the amount of plastic each gives.
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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