How do you make a pie filling that holds its shape when you slice it? Learn the science-y secret to perfect pie slices in this edible experiment.
Make 3 masking tape labels. Use marker to write “Control” on 1 label, “Sugar” on second label, and “Cornstarch” on third label. Place labels side by side across top of large plate, leaving 1 inch between each label. (In an experiment, the control is the sample in which you don’t add or change anything.)
Use 3 more pieces of masking tape and marker to create second set of “Control,” “Sugar,” and “Cornstarch” labels. Place 1 label on each microwave-safe bowl.
Add sugar to bowl labeled “Sugar” and use spoon to stir until well combined. Add cornstarch to bowl labeled “Cornstarch” and use clean spoon to stir until well combined.
Make a prediction: Do you think the 3 bowls of blueberries will look the same or different after you cook them in the microwave and let them cool? Why do you think so?
Place bowl labeled “Control” in microwave and heat until blueberry mixture bubbles, about 1 minute. Use oven mitts to remove bowl from microwave (ask an adult for help). Use clean spoon to stir blueberries. Repeat heating and stirring with bowls labeled “Sugar” and “Cornstarch.” Let all 3 bowls of blueberries cool completely, about 20 minutes.
(Don’t read until you’ve finished the experiment!)
As you can see, without some help in the thickening department, most pie fillings would end up soupy (like your “Control” blueberries)—it would be impossible to cut neat slices of pie! Mashing the blueberries releases some of their watery juice. Heating the mashed berries causes their tiny cells to collapse, releasing a flood of juice.
In the “Sugar” sample, the sugar actually pulled even more liquid out of the berries (check out the bright-pink juice in the photo on the previous slide). Sugar makes pie fillings (and this berry mixture) taste sweet, but it doesn’t help when it comes to making things thick and sliceable.
The best thickening solution? Starch. When you add cornstarch (or tapioca, another kind of starch) to a liquid, such as juicy mashed blueberries, the tiny starch granules absorb some of the liquid and expand (think: tiny sponges). This makes the liquid thicker. And cooking your filling is key: As the mixture gets hotter and approaches its boiling point (212 degrees) the starch granules absorb even more liquid, eventually forming a nearly solid mesh of starch molecules with water trapped inside, like in your “Cornstarch” sample. That’s why you heated your berries in the microwave in this experiment, but the same science is at work when you cook a pie filling before adding it to the crust, or when a pie bakes in a hot oven.