Published: 24 September, 1998

Turning milk into homemade moo glue

I have a recipe to make homemade glue. The recipe asks you to heat skim milk and vinegar until the milk curdles. Then it asks to remove the liquid from the mixture and add sodium bicarbonate to the curdles. What is sodium bicarbonate and why is it an important ingredient to make the glue?

To answer this question, let's first ask another question: "What exactly is the 'homemade glue' that you make using this recipe"? The sticky material that you are making is a chemical called casein. It has been used as an adhesive for a long time. In the late 1800's, it was first used as a wood glue in Switzerland, and is even used today to glue the seams of some grocery sacks. Casein is also used to fortify bread, and is often an ingredient of non-dairy coffee cream. You can eat it, and it is actually good for you!

In your experiment, you are separating casein from milk. Casein is found naturally in cow's milk: however, the amount of casein in a typical glass of milk would only fill a couple of thimbles! To get this sticky stuff away from the rest of the milk, which isn't very sticky, you add vinegar. Vinegar contains an acid, and it causes the casein to precipitate (in other words, it turns the casein into a solid). Now, the casein can be easily separated from the rest of the milk, which is a liquid. The problem now is that casein isn't very sticky after being reacted with acid. Plus, these curds are kind of like cottage cheese - you wouldn't want to stick paper together with this, would you? (yuck!). To make it into a glue, you must add a base. A base is the chemical opposite of an acid. In your experiment, the base is sodium bicarbonate (commonly known as 'baking soda'). When you add it to the curds, it reacts to make the casein soluble again (no longer chunky!), and it is now sticky like a glue.