Thermal Tips for Key Lime Pie, a Classic!

There is no season that is not excellent for Key lime pie. In the heat of summer, it is a sweet and tart way to cool down, and in winter it is a bright, sunshiny taste of warmer weather. Either way, chances are good that whenever you’re reading this, it’s time to go get some limes and make a pie.

Key lime pie is easy to make and has very few ingredients, which is nice. But making it just right can require a little care. To get a crisp crust and a filling that holds its shape you need some thermal thinking. Here, we’ll take a look at the thermal (and acidic) principles of Key lime pie to help you get perfect pie every time. Grab a Thermapen^® ONE and a citrus juicer and let’s get to it.

Key lime pie: what is it and how should it be done right?

Key lime pie is made primarily of key lime juice, egg yolks, and sweetened condensed milk. In fact, that’s pretty much the filling right there. It was, some pastry scholars think, based on a recipe for a lemon pie published by the Borden company back in the ’30’s.

Since gaining renown, Key lime pie has faced the same forces of entropy and “modernization” that affect so many foods. It’s can be easy to find a key lime pie now, but it can be hard to find a good one. That’s why you should make your own!

So what is a good Key lime pie? There are things we want and things we don’t want.

• We don’t want an artificial green color. If a restaurant tries to serve you a Surge-green slice of key lime pie, send it back.
• We don’t want a soggy crust.
• We don’t want any artificial, fake flavors. If your pie isn’t flavored with actual lime juice, you don’t need it.
• We don’t want a Key lime pie that is runny. We should be able to lift a slice out of the pan and have it hold its shape.

Making your own pie from scratch the right way does away with all of these problems, leaving you with a dessert that is bright, clean-flavored, and easy to eat (and eat, and eat).

How to make Key lime pie right

Let’s look at the list of things we want and don’t want and see how we can achieve them.

Key lime pie color and crust

First, color. Ture key lime pie is barely green. It should have the color of, well, lime juice mixed with milk—because that’s what it is. Incorporating the grated zest of a lime (Persian is fine for this) into the filling will give it accents of green and a better, lime-ier flavor, but that should be the extent of your coloration.

What about a soggy crust? No problem. Blind baking the graham-cracker crust helps to firm and crisp it before we add the wet filling.

Key lime pie flavor

When it comes to natural lime flavor there’s a firm line that has been drawn. The purist will insist that it is, in fact, fresh Key lime juice, and nothing else, that should be used.

Key limes—named for the Florida Keys—are smaller, thinner skinned, and more interestingly flavored than your standard Persian lime. They are sometimes sold as “Mexican limes” and can be yellow when fully ripe (though green ones are just fine). The flavor of the Key lime is critical to this dish, so I recommend seeking them out; they can often be found at Mexican or Latin markets. You’ll need 12–20 of them for this dish. If you just can’t find them, get bottled key-lime juice. If you can’t find that, then settle for fresh Persian limes.

Key lime pie: the perfect texture

This brings us, finally to firn, not runny, pie filling. Some Key lime pie recipes require no baking whatsoever, depending on the acids in the lime juice to denature the proteins in the egg yolks and the milk until they hold their shape—kind of a milk-and-egg ceviche. Though that process can work on its own, a two-pronged approach is best:

“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 acid in lemon [or lime] juice, while encouraging an egg to cook and form a solid, keeps the solid moist and creamy.” 

—Cook’s Illustrated, The Science of Good Cooking.

It’s hard to get the pie to set up the right way with just acid and eggs, and the acid helps the eggs cook up more smoothly, so a bake cycle is beneficial to the whole process. Baking the pie at 325°F (163°C) until it reaches an internal temperature of 145°F (63°C) is perfect. Using a Thermapen ONE to verify the temperature is far more accurate than shaking the pie to check for “wiggle,” and will help you to not overcook it. Use one and you’ll never be in doubt of whether your pie is done or not.

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If you follow these guidelines, you’ll be in pie heaven in no time. Really, the hardest part of this recipe is waiting for the pie to cool so you can eat it (it is best served chilled). You can top it with whipped cream, as we do, or with meringue, or just leave it bare, according to your like.

This pie is simple enough to prepare from start to finish in one day so you can easily 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. Give it a try and you’ll be glad you did. It will help either cool you down or warm your heart. Sweeten your life with this slice of sunshine, then share it with a friend to sweeten theirs, too.

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