This is a four(ish) ingredient "sciency" pie that is a great activity to make with (or without!) students. As a high school science teacher, I love to have my students make this pie during those (all too often) spring days when half the class is off doing things other than attending class (sports games, fine arts practices, AP exams, field trips for other classes, etc.).
A little background: While in college, I studied abroad at the University of Sussex in Brighton, England. While there, I was introduced to the delicious banoffee pie. I would like to say that my first bite of this sweet, caramely goodness was in a quaint British shop I discovered while meandering through the cobblestone streets. But alas, my first taste was in the school's lowly cafeteria. However, that certainly did not make it any less delicious. While this recipe may not be entirely authentic, it is easy and also absolutely delicious.
Step 1: Ingredients (Per Large Pie)
- 1 crust (I personally like graham cracker)
- 1 - 2 bananas (depending on size and how much you love bananas)
- 1 can of sweetened condensed milk
- 1 cup of heavy whipping cream (Optional: 2 tbsp sugar and ½ tsp vanilla to make the whipped cream sweeter)
Step 2: Prepare Your Caramel Sauce (...and Throw a Little Nerdy In)
You will need to boil your sweetened condensed milk for about 3.5 hours, flipping the can halfway through. Make sure you use caution and watch your water level! The water should stay above the can to prevent the can from overheating and exploding.
When the can of sweetened condensed milk is placed in boiling water, the sugars contract which results in a liquid with a higher density, and therefore, experiences a decrease in volume. As long as your water does not boil off, your can should not explode since the boiling point of sweetened condensed milk is higher than that of water.
That being said, WARNING, WARNING, WARNING, you are being forewarned about the potential dangers. As you all know, sometimes science forgets about what is supposed to happen and ends up doing the opposite. I have successfully NOT exploded any cans and will continue to use this method, but I am sure that at some point in time someone has had a can explode all over their kitchen.
However, the risk of a disastrous (and potentially dangerous mess) is worth it to me because of the excellent science applications that results from the simple creamy liquid miraculously turning into caramely gold. This activity provides a great opportunity to discuss gas laws and the relationships between pressure, volume, and temperature.
However, if you think the science is cool, but don’t want to risk explosions, here are some alternative options:
- You can just pour the can of sweetened condensed milk into a pie plate. Cover it with aluminum foil, and place it in a baking dish with about one inch of water in it (like you would bake a cheesecake). Bake it at 425 degrees Fahrenheit for 60-90 minutes (until golden brown).
- Or just buy a can of dulce de leche.
Your students may wonder, “When the temperature increased, shouldn't the increase in pressure and volume have caused the can to explode?”
Which leads to... an introduction to boiling point elevation and invites a discussion on why the boiling point of the caramel sauce is higher than the boiling point of the water.
Short summary: the temperature at which a liquid boils is higher when it is a solution (solute + solvent) than the boiling point of a pure solvent. Similarly, the freezing point of a solution is lower than the freezing point of a pure solute. An example would be when salt is added to water, the temperature has to be colder to freeze that salt water solution than if you were freezing water by itself. Conversely, you would have to reach a higher temperature to boil that salt water solution than with water alone. Since all the liquids that we ingest are mostly water, anything other than pure water is going to freeze at a lower temperature and boil at a higher temperature. That is why the sweetened condensed milk does not boil in the can even though the water outside of the can is boiling.
If your students ask if the can will boil eventually, or the longer that the can sits in the boiling water, you can show them a water heating curve and explain that although energy is being added to the boiling water (via the stove burner), the temperature does not change. All that energy is being focused towards separating the water molecules as they begin leaving the liquid state and entering the gaseous state.
If your students don’t believe you, you may want to have them see for themselves by monitoring the temperature of boiling water while they heat up a beaker of water on a hot plate.
Step 3: Tattoo Those Bananas
Before using your bananas, you might as well have fun with them! “Tattooing” the bananas allows you to see them oxidized right before your eyes. Instructables: How to Make Banana Oxidation Art provides a thorough explanation of the process, but basically, you stab the banana peel with sharp objects (thumb tack, safety pin, toothpick, etc.) in order to create a work of art. This activity is always a hit!
Step 4: Whip Up That Cream
You can always sub out the homemade for a bottle of store bought whipped cream or a carton of Cool Whip, but this gives you a chance to talk about emulsions and get some of your students’ extra energy out! I prefer just whipping up heavy whipping cream by itself since the pie is so sweet, but if you want your students’ next period teacher to never speak to you again, you can add 2 tablespoons of sugar and half a teaspoon of vanilla extract to one cup of heavy whipping cream. Throw it all in a bowl and whip it up! If you are relying on muscle power, it will take a while so you may want to have an electric mixer on hand as back-up.
Step 5: Layer on Up
- Spread your "sweetened condensed milk magic sauce" on the bottom of your crust
- Add a layer (or two) of banana slices
- Top it off with the whipped cream
Step 6: Eat the Pie
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