How to turn milk into glue

Use a simple chemical reaction and some smart mixing to turn normal milk into natural glue

1. Heat the milk

First of all, measure out around 250ml of milk, then pour it into the pan and start to warm it – ask an adult for help if you need it. Then once it’s warm, add three teaspoons of white vinegar to the saucepan.

2. Separate the curds

Continue heating the mixture, stirring it consistently as you warm it, until you start to see solids separate from the liquid. These are the solid curds separating from the liquid whey in the milk

3. Strain the mixture

The vinegar causes a chemical reaction in the milk that forces the two substances to separate in the acid. Use a sieve to strain the mixture, then gather up the curds and mould them into a ball.

4. Add some water

Put the solid curds back into the pan and add a little water and a tablespoon of baking soda. Heat the mixture up again until it starts to bubble – this is carbon dioxide that’s being released in the reaction.

5. Get the consistency

The bubbles are formed due to the reaction when the acidic vinegar is neutralised by the alkaline baking soda. You may need to add more water or baking soda to get the thick, paste-like consistency for your glue.

6. Start sticking

Leave the mixture to cool. Now that the acid is neutralised, the curds can form back into a liquid again –they are made of milk protein, or casein. This is a natural glue, so test it out on some household objects!

(Illustrations by Ed Crooks)

Summary:

Milk is a complex compound containing fat and proteins (including casein). When you add vinegar to the milk, it causes the pH to drop, and the negatively charged casein begin to attract other molecules to form lumps. This is called coagulation. When the alkaline baking soda is added, this neutralises the acid, allowing the molecules to separate again.


This article was originally published in How It Works issue 131, written by Stephen Ashby


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