The Science Behind Custards and Whipped Cream is the Secret to Making a Perfect Banana Cream Pie
Find out how the same cup of cream can become an airy whipped cream or silky custard
Over the past few months, I’ve been making banana cream pie after banana cream pie trying to figure out how to get the custard as smooth as velvet without overloading it with so much butter that it feels like the button on your pants is going to pop off after the second bite.
Even though banana cream pie is one of my favorite desserts, until a couple of years ago the idea of making one from scratch never crossed my mind. Banana cream pies are one of those desserts that look complicated enough when they’re spinning around in one of those refrigerated cases at a diner. When you get up close and try to figure out what it takes to make layer after layer of complicated deliciousness they can be downright intimidating.
It wasn’t until around my tenth pie when I was whisking small pieces of butter into the custard trying to figure out how many it took to get to velvety that it hit me how much alchemy there is in a cream pie.
How a tiny cup of cream can puff itself up into a billowing cloud, how a bowl of milk and some egg yolks can go from viscous liquid to thick porridge to igneous rock in a few seconds, and how pulverizing a bunch of crackers that have trouble supporting a marshmallow hold the whole thing together.
After a few incantations and enough bananas to get a troop of monkeys through the winter, I decided that to get to the pie of my dreams I needed to learn more about the science behind the pie.
Why is Whipped Cream So Full of Itself
Ask most people if they’ve ever made a foam at home and they’ll probably say no. But that’s exactly what happens when someone makes whipped cream. As the cream is whipped, it’s easy to see the transition from liquid to foam, as soft white peaks start to form the cream develops a more rigid structure.
Technically speaking, whipped cream is a colloid, which is what happens when one substance is suspended throughout another.
As the cream is whipped the fat molecules in the liquid create a network that traps the air bubbles. The trapped air allows the cream to double its volume and is what gives the whipped cream its light, airy texture. Unfortunately, adding air doesn’t reduce the number of calories.
Whipped cream can be made with whipping or heavy cream, which aren’t the same thing but can be substituted for each other, as long as the fat content is at least 30 percent. In general, the higher the fat content in the cream the more stable the foam. Of course, if you let the mixer keep running, you’ll eventually turn the cream into butter, which is probably not the best thing to top a pie with.
A quick tip if your whipped cream has gotten too stiff is to add a couple of tablespoons of whipping cream to the bowl and slowly hand whisk it until it softens back up.
Let’s Tie Everything Together with an Emulsion
There’s so much going on in the custard that it feels like it could be its own chemistry course. At its core, a custard is an emulsion, which is what happens when oil and water are brought together with an emulsifier and force. Emulsions are used as the basis for salad dressings, mayonnaise, margarine, and more.
The force used to create an emulsion is usually delivered through a whisk or blender in a way that disperses the oil throughout the liquid.
Emulsifiers are particles that play well with both oil and water, things like eggs, mustard, and honey. They contain compounds that typically have one end that is hydrophilic or water-friendly and one that is hydrophobic or oil-friendly. This allows the emulsifier to bridge the gap between the oil and water and since the hydrophobic ends repel each other, the emulsion is able to keep the oil suspended in water.
There’s a Lot Going on in a Silky Smooth Custard
A custard is a mixture of milk or cream and eggs that have been cooked and is often used as a sauce or filling in desserts. Custards are usually categorized by how they’re thickened. Types include pure or basic that are thickened by eggs alone; starch custards that use a starch such as flour or corn along with the eggs; and gelatin custards that are thickened with some type of gelatin and eggs.
The reason I chose to use cornstarch and egg yolks for the custard in our Banana Cream Pie recipe is that I wanted a custard that could hold up for a few days, so we wouldn’t have to eat the whole pie at once.
The advantage starch thickened custards have is that the addition of cornstarch slows “protein coagulation, making them more resistant-though not immune too overcooking and curdling.” (Fine Cooking)
There’s a moment when you’re whisking the cornstarch, egg yolks, and cream together when it feels like it’s going to go on forever. It’s the moment when you want to turn up the heat because that’s what you do when something isn’t cooking fast enough. It’s worth noting that turning up the heat while you’re trying to make an emulsion almost always ends badly.
The reason this type of custard goes from all liquid to feeling like you’re trying to stir a bucket of concrete is that the cornstarch doesn’t start to thicken the custard until it gets hot enough.
As the starch molecules start to come to temperature, the granules swell allowing them to absorb the water in the cream, creating a mesh that thickens the liquid. The starch’s ability to absorb water in the presence of heat also means that this type of custard can be undercooked if it doesn’t reach a low simmer. An undercooked custard might appear to be thick but will turn soupy as the custard breaks down over time.
Overcooked custards are a different problem. Because the process used to make a custard is essentially the same process used to make scrambled eggs an overcooked custard can have an eggy taste and clumps of eggs that need to be strained out. This can also happen if the eggs aren’t tempered correctly. If your custard tastes like scrambled eggs, you’re better off tossing it and starting over than trying to mask the off-putting flavors.
The advantage cornstarch has is it can stand up to the heat of the stove, and unlike flour, it’s gluten free and produces sauces with a translucent shimmer. Cornstarch’s ability to thicken sauces while keeping them clear makes it our go-to choice for fruit desserts where we want to keep the beautiful colors of the fruit without having everything turn into a sloppy mess.
To give the custard a silky smooth mouthfeel, we take it a step further and add some cut up bits of butter and a mashed up banana to ensure that every bite has a pronounced banana flavor.
Why Does Baking Something Make It Solid
Holding everything together is a graham cracker crust that’s three ingredients, graham crackers, sugar, and butter. Somehow during the process of pulverizing the crackers, adding in the sugar and melted butter and pressing everything together with a measuring cup, the crust comes together with more holding power than the graham crackers ever had by themselves.
One of the strangest things about graham cracker crusts is that they work whether they’re baked or chilled. It’s as if the melted butter is acting as a mortar, binding and strengthening the tiny bits of graham cracker together.
Better Pie Through Science
Digging into the science behind the pie helped me understand what was going on as I changed the proportions between the eggs, milk, and cornstarch, helping me come up with a banana cream pie that has deep banana flavor in every bite while maintaining a nice balance between the firmness of the filling, the texture of the crust, and the airiness of the whipped cream.