Activity: Another sticky solution - making glue from milk

You will need

• 2 cups
• a spoon for stirring
• coffee filter paper or piece of thin rag
• 1 packet of powdered non-fat milk
• vinegar
• baking soda
• set of measuring spoons
• measuring cup
• hot water
• paper or another material to test your glue

Warning: This experiment involves hot water. Please ask an adult to help.  

What to do

1. Measure about ¼ cup of hot water into a cup.
2. Add 2 tablespoons of powdered milk to the water.
3. Stir until dissolved.
4. Add 1 tablespoon of vinegar to the mixture, and stir. You should see the milk begin to separate into solid chunks of curd and a watery liquid called whey.
5. Stir until the milk is well separated.
This separation works best when the milk is very warm or hot. That is why powdered milk and hot water are used. If your milk does not separate well, try a little more vinegar or try heating it. If it still does not separate after a few minutes, try it again with warmer water.
6. Separate the curd from the whey. To do this, line the clean, empty second cup with the coffee filter, and then pour the curd and whey mixture into the filter.
7. Lift the filter slowly. The liquid whey should drain through the filter, leaving only the curd. The curd is what you want to keep.
8. Squeeze the filter containing the curd to remove as much of the whey (liquid) as possible. Drop the lump of curd back into the plastic cup that you first used.
9. Use the spoon to break the curd into small pieces. Breaking the curd is important in making good glue.
10. Add one teaspoon of hot water and 1/8-1/4 teaspoon of baking soda to the chopped curd and mix thoroughly. You should see some slight foaming or bubbling. Keep mixing until the curd becomes smoother and more liquid. The curd has now become glue.

    Tips: If the mixture is too thick, add a few drops of water. If it is too lumpy, add another pinch of baking soda, and stir. You should see some foaming. Don't be afraid to experiment by adding more water or baking soda to improve the consistency of your glue. The finished product can vary from a thick liquid to a thick paste. This depends on how much curd there is, and how much water and baking soda are used.

11. Use glue to paste pieces of the paper together. It may take 15-30 minutes to dry depending on how much you use, but it should work as well as a traditional white school paste.
Cover the cup of glue with plastic wrap and let it sit for a few hours, or even overnight. The consistency should become smoother and clearer. Dispose of the wet glue in the rubbish within 24-48 hours or it will start to smell like spoiled milk.

What's happening?

Milk reacts with an acid like vinegar to make a new substance. The vinegar curdles the solids in milk and then the rubbery solid is separated from the liquid part of milk. The curds dry to form a hard plastic like substance called casein plastic. After the casein is separated from the whey by filtering, baking soda is added to neutralize the acid. When the curd no longer has acid in it, it returns to a more liquid form. The foaming you see when the baking soda is added to the curd is carbon dioxide gas, which is made when the baking soda reacts with the acid in the vinegar. The resulting liquefied casein protein is our natural glue.

Plastic are made from polymers. Polymers are very large molecules, formed by repeated patterns of chemical units strung together. Although "polymer" might bring to mind rubber or slime, did you know that there are polymers all around us, including inside our bodies? The protein DNA, which is the "blueprint" for cellular reproduction, is a naturally-occurring polymer. The protein, casein, is a polymer as well. Other natural polymers are cellulose and starch. Bone, horn, cotton, silk, rubber, paper, and leather all come from naturally-occurring polymers!

There are human-made polymers, as well. Fabrics such as rayon and polyester, polystyrene (used in styrofoam coffee cups), and PVC (used in pipes) are common examples of these artificially-occurring polymers.

Applications

These curds of casein protein are used to make cheese. In cheese making, an enzyme called rennin is used to separate the curd from the whey, instead of using an acid such as vinegar. The curd is then washed, sa lted, and active cultures (particular micro-organisms) are added. This mixture is then aged until it turns into cheese. The type of cheese it becomes depends on the specific cultures added, and how it is stored during the aging process.

A legendary story has it that cheese was 'discovered' by an unknown Arab nomad. He is said to have filled a saddlebag with milk to sustain him on a journey across the desert by horse. After several hours riding he stopped to quench his thirst, only to find that the milk had separated into pale watery liquid and solid white lumps. Because the saddlebag, which was made from the stomach of a young animal, contained a coagulating enzyme known as rennin, the milk had been effectively separated into curds and whey by the combination of the rennin, the hot sun and the galloping motions of the horse.

Casein is great stuff. Simple milk curds, like the ones you just made, can be combined with chemicals and then hardened into plastics. Casein is used to make combs, jewellery, even some house paints. Casein glue in paint is what makes it stick to your walls and ceilings. Casein has been made into fibre for clothes, too.

Casein was used by the Ancient Egyptians as a fixative for pigments in wall paintings. It has also been used as a part of various glues but it appears not to have been used as the basis of a solid plastics material until the end of the 19th century when casein was used to make a washable white board for replacing the slates used in school - paper was too expensive at that time for use by children to practice writing.

Britain , the United States and Russia all manufactured casein products from the early 1900's. The major producer of casein plastics in the UK only stopped manufacturing in about 1980.