In our previous ChemShorts activity, we experimented with an interesting property of polystyrene, the polymer in recyclable #6 plastic. Proteins, such as those found in milk, are natural polymers. Let’s see if you can turn milk into a “plastic.”
Materials
• Milk, one cup (any fat % milk may be used)
• Large spoon
• Microwave or stovetop
• Paper towels
• Saucepan or Pyrex® cup to heat the milk
• Strainer
• Vinegar
Optional: Food coloring, cookie or silicone molds
Be safe!
Adult supervision is required when using the microwave or stovetop.
Experiment
Gently heat the milk in a saucepan (stovetop) or cup (microwave) until you see steam coming from it. Heating the milk causes the proteins in milk to begin to open up. Add vinegar one tablespoon at a time. You should notice the milk curdle—the solids are called curds and the liquid is called whey. (Think of the “Little Miss Muffet” nursery rhyme.) Gently stir the mixture while adding the vinegar until curdling is no longer observed. Do not add more than four tablespoons of vinegar! Place a paper towel in the strainer and pour the milk contents onto the towel, allowing the liquid to run through the strainer and into a bowl. Take the curd (solid) remaining in the strainer and squeeze out more liquid. Use additional paper towels to dry the solid. You can add food coloring at this point if you want a colored product. To obtain a smoother product, soak the curd in vinegar for an hour or so and then repeat the straining/drying process. (Adding more vinegar or soaking the solid in vinegar may give the product a strong smell.) Shape the curd into shapes using cookie cutters or silicone molds. Allow the molded solid to dry for two days; it will turn into a hard “plastic.”
What’s happening?
Just like polystyrene, the proteins found in milk are very large molecules. Milk proteins, such as casein, are large enough to make milk appear cloudy, but not large enough to be seen with the naked eye. The structure of the milk protein looks like a long chain that has been folded up into a ball. Ionic, charged groups in the protein are hydrophilic (water-loving) and are on the outside of the protein, while neutral hydrophobic (water-hating) groups are on the inside where they are protected from interacting with water. As vinegar, an acid, is added to milk, some of the groups react with the acid and change how they interact with each other. This causes the protein to change its three-dimensional shape. This process is called protein denaturation. The milk curdles when the proteins are denatured. Protein denaturation also occurs when you heat an egg white.
You may have noticed that the word “plastic” appears in quotes in the title of this activity. Generally, a plastic is made from petroleum (oil) in a man-made process. The word plastic describes a polymer that is moldable. The hardened casein solid obtained from milk is a moldable polymer, but because it came from a cow, it is not officially a plastic!
Explore more at
https://chicagoacs.org/images/downloads/Chemical_Bulletin/2021_11_chembull.pdf
https://www.steampoweredfamily.com/activities/make-plastic-from-milk/
https://www.rookieparenting.com/milk-vinegar-experiment-polymerization/
https://www.mombrite.com/how-to-turn-milk-into-plastic/
To view all past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts
