How to turn milk into plastic

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Experimental Procedure

Making Casein Plastic

This experiment uses hot liquids, so an adult’s help will be needed throughout.

  1. Using the masking tape and pen, label the four mugs: 1, 2, 4, and 8.
  2. Use the measuring spoon to add 1 teaspoon (tsp.) of white vinegar to the mug labeled “1,” 2 tsp, to the mug labeled “2,” 4 tsp. to the mug labeled “4,” and 8 tsp. to the mug labeled “8.”
  3. Heat 4 cups of milk (1 quart) in a large measuring cup in the microwave.
    1. The exact amount of time needed will depend on your microwave. Start by warming the milk at 50% power for five minutes. The 50% power will help you avoid scalding (burning) the milk.
    2. Have an adult check the milk with a thermometer to make sure it is at least 49°C (120°F). If it is not heated enough, put it back in the microwave for another two minutes at 50% power. Repeat this step until the milk is hot. Warmer than 49°C is fine.
    3. In your lab notebook write down the total number of minutes it took you to warm the milk and the final temperature of the hot milk. When you repeat these steps later you should try to get as close to these numbers as possible. 1 or 2 degrees warmer or cooler is fine as long as the milk is at least 49°C.
  4. Carefully pour 1 cup of hot milk in to each of the four mugs with vinegar in them. (You may need to ask an adult to pour the hot milk for you.) What do you see happening in each mug? Write down your observations in a data table, like Table 1 below, in your lab notebook. In at least one of the mugs you should see that the milk has separated into white clumps (called curds).
    1. Make sure to pour the milk in to all four of the mugs at the same time so that the milk is the same temperature across all four vinegar amounts.
Number teaspoons of vinegar Forms curds? (yes/no) Describe liquid after sieve Weight of casein plastic (in grams) Write down any other observations
1
2
4
8

Table 1. Make a table like this in your lab notebook to write down your data. Make a new table for each repeat of this experiment, for a total of three tables.

  1. Mix each mug of hot milk and vinegar slowly with a spoon for a few seconds. That will help make sure the vinegar reacts with as much of the milk as possible.
  2. Meanwhile, take one of the cotton-cloth squares and attach it with a rubber band to the top of one of the clear cups so that it completely covers the cup’s opening. This will make a sieve as shown in Figure 4 below.
    1. Make sure the cloth hangs down a bit inside the cup so that you have room to pour liquid in.
    2. Repeat this step with the other three clear cups.
    3. Label the clear cups 1, 2, 4, and 8 with the tape and pen.
  3. Once the milk and vinegar mixture has cooled a bit, carefully pour the mixture from mug “1” into the cotton cloth sieve on cup “1.” If there are any curds, they will collect in the cloth sieve. The leftover liquid will filter into the clear cup. Figure 4 below shows what the setup looks like. Where do you think the casein is, in the liquid in the cup or the curds in the sieve? Tip: You may want to do this step over a sink just in case any of the liquid spills.
A cheesecloth secured over a glass jar with a rubber band filters curds from whey
Figure 4. A piece of cotton cloth and a rubber band are used to make a sieve at the top of a clear glass. Once the milk and vinegar mixture is poured into the sieve, the curds will gather on the top of the sieve, and the liquid will drain through into the clear cup.
  1. In your table in your lab notebook, write down what the leftover liquid in the clear cup looks like. What color is it? How clear is it? Be sure to write the information down for each cup on the corresponding line on the table (for instance, cup “1” for the cup with 1 tsp. of vinegar, and so on).
  2. Over a sink, carefully remove the rubber band sieve on cup “1.” With your hands, squeeze all the extra liquid out of the curds. Scrape the curds off of the cloth and knead them together, as you would bread dough, into a ball. This is your casein plastic. Before it dries, the ball of dough will look similar to Figure 5 below.
A lumpy white ball made of curds that have been squeezed of excess liquid
Figure 5. The wet casein plastic will form a lumpy ball of whitish dough like the one shown here.
  1. Weigh the ball of casein plastic on a kitchen scale (set for grams) using a piece of wax paper to keep the scale clean. Record the weight in your table.
    1. When weighing, remember to turn on the scale and first make sure it reads zero with nothing on it. This will help make sure your measurements are accurate. Also, use a new sheet of wax paper each time you weigh a different ball of casein plastic. This will give you exact weights (without crumbs and liquid from the last ball)
    2. The amount of casein plastic each recipe makes is called the yield for that recipe. The more plastic, as measured by weight in this case, the greater the yield.
  2. Repeat steps 7-10 for the other three mugs of milk and vinegar.
  3. If you want to make your casein plastic into something, you can color, shape, or mold it now (within an hour of making the plastic dough) and then leave it to dry on some paper towels for at least 48 hours. See the “Ideas for Fun with Casein Plastic” for more suggestions.
  4. For your science project you will want to repeat steps 1-11 again two more times. This will give you enough data to see whether one recipe reliably yields more casein plastic than another.

Analyzing Your Data

  1. Calculate the average yield (amount in grams) of casein plastic made from each recipe. If you do not know how to average, ask an adult to show you.
  2. Make a bar graph showing the average yield for each recipe. You can make the bar graph by hand or use a website like Create A Graph to make the graph on the computer and print it.
    1. On the left axis (the y-axis) write the average yield of casein plastic. Make a bar along the x-axis for each of the four recipes you tested.
  3. When you look at your observations about the liquid left over after straining out the curds, do the weights of the yields make sense? Why or why not?
  4. Which recipe yielded the most casein plastic on average? Was any other recipe a close second? Based on this data, which do you think is the “best” recipe in terms of yield?

Ideas for Fun with Your Casein Plastic

Try making beads, ornaments, or figurines out of your casein plastic. You should do the molding and coloring steps (except for paint and/or marker) within the first hour of making the plastic or it will start drying out.

  1. Shaping the plastic:
    1. Knead the dough well before shaping it.
    2. Molds and cookie cutters work well on the wet casein plastic.
    3. You can also sculpt the wet casein plastic into figures, but it takes a bit more patience.
  2. Coloring the plastic:
    1. Food coloring, glitter, or other decorative bits can be added to the wet casein plastic dough. The beads in Figure 3 above were made from casein plastic dough that had yellow food coloring and multicolored glitter kneaded into it.
    2. Dried casein plastic can be painted or colored on with markers. The smiley face in Figure 3 is on uncolored casein plastic and was drawn on using a black permanent marker.
  3. Hardening the plastic:
    1. Casein plastic will be hard once it has dried.
    2. Drying time varies depending on the thickness of the final item (thicker pieces take longer), but most casein plastic requires at least two days to become hard.
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