Thermal Tips for Key Lime Pie, a Classic!

Key lime pie is easy to make with very few ingredients, and we have the temperature tips you need to be sure the flavors are on point, the crust is crisp, and the filling will hold a clean slice rather than flooding out when serving. Keep an instant-read digital thermometer like a ThermoPop handy for this sweet project.

Common Problems: Many key lime pie recipes require no baking whatsoever; but we have found that for the best results, some heat really needs to be applied. Common problems with key lime pie are when you go to serve a slice and 1) the crust crumbles apart, and 2) the filling doesn’t hold together, rather it jiggles and oozes out like cooling magma.

The crust needs to spend some time in the oven to firm up so it holds together when the pie is sliced. This is the same concept as blind-baking with traditional pie crusts; but so much easier. Check out our post on holiday pies to learn more about pie crust and custard pies. Graham cracker crusts are nearly foolproof.

Protein Denaturation: Key lime pie filling ingredients couldn’t be simpler: sweetened condensed milk, lime juice, and egg yolks. How could we possibly serve a finished pie containing eggs without cooking it? The acid in the lime juice denatures the egg proteins in the same way heat does. This is exactly what happens when lemon or lime juice is added to seafood in making ceviche. It’s a sort of “chemical cooking”. Even with the protein coagulation firming up the filling with the addition of lime juice, a key lime pie often still doesn’t hold up when sliced. A quick bake at 325°F (163°C) completely remedies that problem. The proteins will set up just enough to give you clean edges with every slice.

“Introducing an acid to egg proteins neutralizes some of their negative electrical charges. Consequently, when the proteins are heated and unwind they do so at a lower temperature. The altered electrical charges on the proteins cause them to form a weak, soft gel…thus, the strong(er) acid in lemon juice (lime juice in our case), while encouraging an egg to cook and form a solid, keeps the solid moist and creamy.”-Cook’s Illustrated, The Science of Good Cooking. Baking the pie causes the egg protein to set up firmly enough to hold its shape when sliced, and the high acid not only jump starts the protein denaturation, but keeps the texture of the filling rich and creamy.

The components are, well…easy as pie: a graham cracker crumb crust, tangy custard filling, and plenty of lightly-sweetened whipped cream to balance the strong flavor of the filling. We followed King Arthur Flour’s recipe for Classic Key Lime Pie. Here are our tips for success from bottom to top:

The Kitchen Project:

Ingredients

• 1-1/2 cups graham cracker crumbs
• 1/4 cup powdered sugar
• 1/8 tsp. salt
• 1/3 cup melted butter
• Grated zest of 1 medium lime (optional)
• 3 large egg yolks
• 2/3 cup freshly squeezed lime juice
• Sweetened whipped cream, as desired (we used 16 oz [454 g] heavy cream)

• Preheat oven to 325°F (163°C)

Crust: Melt the butter and cool to 80-90°F (27-32°C)—any time a recipe calls for melted butter, this is the temperature range it should be in (ours had cooled slightly by the time we snapped our photo). Pulse graham crackers in a food processor until it resembles a coarse flour, and place in a medium-sized bowl. Add the powdered sugar and salt, whisk to combine. Add the melted, cooled butter and mix until thoroughly combined. Press the crumb mixture into a 9 in (23 cm) pie pan evenly on the bottom and up the sides of the pan. You can use the base of a dry measuring cup or a spoon to make the process easier. Bake in a 325°F (163°C) oven for about 15 minutes. The crust will be fragrant and slightly darker in color. Set the crust aside to cool, and prepare the filling.

Filling: In a medium mixing bowl, whisk together the sweetened condensed milk, egg yolks, and lime juice until it thickens—about 3-4 minutes of whisking. Pour the thickened filling into your cooled pie shell and bake at 325°F (163°C) until the internal temperature reaches 145°F (63°C)—about 20-25 minutes. An instant-read digital thermometer like a ThermoPop is a great thermometer for spot-checking the internal temperature. Cool the pie to room temperature, then refrigerate for 3 hours, garnish with whipped cream, and serve.

This pie is simple enough to prepare from start to finish in one day so you can serve it after dinner. Applying just enough heat at the right time is really all it takes to pull the recipe off without a hitch. Happy baking, and have a slice of pie!