Supplies List for Turning Milk Into Plastic

• Gather a variety of items that are made from plastic as demonstration objects
• 1 cup of milk 
• Stovetop and pan or microwave and microwaveable container 
• Thermos or another insulated container
• Heat-resistant cup or mug
• 4 tsp white vinegar
• Paper towels
• Optional: food coloring, glitter, and markers

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Step 1: Let's Talk About Plastics

Start by asking kids to grab some examples of plastic items from around the house; it could be anything: a toy, a kitchen utensil, or a plastic bottle. Next, discuss how all of these objects are made of plastic. Most plastics are made from synthetic chemicals with materials that do not occur in nature. Ask your young scientist if they think plastic could instead be made using natural ingredients.

Step 2: Start with the Vinegar

Add 4 teaspoons of white vinegar to the heat resistant mug. 

Step 3: Add the Milk

Have a grown-up heat 1 cup of milk until steaming on the stove or in the microwave. Pour the hot milk into a thermos and let the child add the hot milk from the thermos to the mug with the vinegar. When the hot milk is added to the vinegar, the milk should form white clumps (curds). Mix the mug of hot milk and vinegar slowly with a spoon for a few seconds. 

Step 4: Remove the Curds

Use a spoon to scoop out the curds once the milk and vinegar mixture has cooled a bit. Tilt the spoon against the inside of the mug to let the excess liquid drain out while retaining the curds in the spoon. Collect as many curds as possible and drop them on top of a stack of two paper towels.

Step 5: Soak Up the Liquid

Fold up the sides of the paper towels and press down on the curds to absorb excess liquid. Two more paper towels can be pressed down on top of the curds to soak up any remaining liquid.

Step 6: Time to Knead

Knead all of the curds together into a ball of "dough." This is your fancy casein polymer. 

Step 7: Shape Your Plastic

Now kids can color, shape, or mold their plastic (make sure to do this step within an hour of making the dough). Food coloring, glitter, or other decorations can be added to the wet casein plastic dough.

To shape the plastic, students must knead the dough well before shaping it. The dough can be sculpted by hand or with cookie cutters.

Let it dry for at least 48 hours. The dried casein plastic should be hard. Drying times will vary depending on the thickness of the final item (thicker pieces take longer) but plan on leaving it for at least two days. Once dry, it can be painted or colored with markers.

Find dozens more science experiments for kids in our STEM Guide.

Photos by the author