Key lime pie is, like so many regional specialities, very much the product of its environment – in this case, the Florida Keys, a chain of coral islands that trail lazily into the Gulf of Mexico and that were only linked to the US mainland in 1912. This splendid isolation fostered a reliance on local ingredients such as limes and the cans of condensed milk popular in tropical climates before widespread refrigeration. Put the two together, and you have most of a key lime pie.
Though it probably dates from the late 19th century, the dish became famous when the tourist industry took off in the Keys following the construction of the Overseas Highway – in a 1948 travel piece on Key West, the Washington Post includes it, along with turtle steaks and black bean soup, as one of the “epicurean pleasures not to be overlooked”. That said, there have been more recent claims that it’s simply an adaptation of a 1930s recipe for lemon pie created by a condensed milk manufacturer 1,500 miles north in New York City, but I won’t go there. Instead, here’s a recipe.
Filling and cooking
Most key lime pie recipes make use of the aforementioned condensed milk, beaten with egg yolks to enrich it and turn it into a kind of quick custard. The mixture is further thickened by the addition of lime juice, whose acidity “cooks” the yolks, coagulating the proteins to give the whole thing a stiffer consistency. The more yolks you use, the richer and firmer the result – chef Brad McDonald’s pie, from his book Deep South, is undoubtedly delicious with just two, but it’s tricky to cut into neat slices. Four is more usual, and safer. (Interestingly, I have some trouble getting hold of condensed milk; the man behind the counter in the third shop I try tells me demand has soared this summer. If anyone can explain why, please do let me know in the comments below; my theory is no-churn ice cream.)
Many American recipes express caution on the subject of raw egg yolk; that’s less of a concern in the UK, where hens are vaccinated against salmonella, but that’s no reason not to try a baked recipe when Cooks Illustrated magazine makes such a compelling case for heat yielding a “thick, creamy and unctuous” (though I suspect they mean rich) filling with a pronounced lime flavour – “the difference was remarkable”. My panel, however, feels that it also gives the pie a texture more reminiscent of a baked cheesecake, particularly if, like Pepe’s Cafe in Key West (“the oldest eating house in the Keys!”), you whisk egg whites into the mixture to make it really fluffy. Better, if you’re really keen to serve up perfect wedges, to go down the freezer route, as Jennifer Steinhauer’s recipe for the New York Times suggests. Not only is this the dish that Meryl Streep pushes into Jack Nicholson’s smug face in the film adaptation of Nora Ephron’s Heartburn, but the icy temperature brings out the custardy smoothness of the filling in a way that’s extremely satisfying to eat. I haven’t suggested this step in the recipe below, because it’s not always practical, especially if you want to take the pie to a picnic, but I would recommend giving it a whirl if you are serving it in the vicinity of a freezer: leave it in there for at least three hours, then get it out 10 minutes before serving.
Two of the recipes I try take a rather different approach. The Chez Panisse lemon meringue pie, which the New York Times cookbook suggests can be easily adapted to make a key lime variation, is made with a citrus curd that utilises eggs yolks and butter, thickened over heat, to make a very rich filling (“Are you sure this isn’t a lemon meringue pie?” someone queries in the gathering dusk of our socially distanced outdoor taste test), while photographer Stephen Shore’s key lime pie supreme has a cornflour and water base, with just a little butter. Mild and gently wobbly, it’s my favourite, because I must confess I find ordinary key lime pie rather intense – so sweet! So rich! So tangy! – but this is a democracy, and the panel vote for the traditional version.
Some recipes assert that this pie is only worth making with key limes, the local name for a distinct species of thin-skinned, acidic, south-east Asian lime thought to have been brought to the Keys by Spanish sailors, and rhapsodise over their gorgeously aromatic qualities. In blind taste tests, however, this claim doesn’t always stand up, but whether or not they have a point, the fact remains that even on the Keys themselves, the limes used tend to be grown in Mexico, and even these are very rarely found outside the Americas, so I’ll be using standard fruit here. A good way to create a more well-rounded, perfumed flavour is to add zest, as most recipes suggest, whisking it with the yolks to release the volatile oils before stirring in the condensed milk and juice. Half a cup, or 120ml, juice is fairly standard – when you learn this is about four standard or 20 key limes, you may well feel relieved they’re so hard to come by.
Though most older recipes for key lime pie use a graham cracker crust (graham crackers being a popular American biscuit) of the type popularised by Nabisco to promote its wares in the Great Depression, some more recent ones recommend pastry – Chez Panisse’s version uses a flaky version that, while delicious, is far too delicate for this unapologetically unsubtle dessert (I’d also question whether it’s enough for the size of tin called for in the recipe, but that’s an argument for the New York Times). The chunkier, rougher, more robust texture of a biscuit crust works much better.
McDonald makes his own crackers first, adding honey and cinnamon to the dough, too – excellent on their own but, once whizzed to fine crumbs and then mixed with melted butter, my panel, after hacking at my pie tins, pronounce the fruits of my labour “too solid, like a rock”, voting instead for a crust made with shop-bought digestive biscuits. I’m rather taken with Bon Appetit’s suggestion of substituting the butter with coconut oil for a suitably tropical taste, but the panel aren’t convinced, so I’ll leave that one up to you, like the cinnamon I’m not so keen on. Do add a pinch of salt, though – it will be very welcome with the intensely sweet and sour filling.
You could leave your key lime pie plain, as Pepe Cafe and McDonald do, but it looks more impressive with a topping; meringue is one option, but though that’s a handy way to use up those excess egg whites, and my testers are vociferous in their support for it, when questioned, they admit that, much as they like meringue, it is “a lot” in combination (egg whites are great in daiquiris, by the way). Whipped cream, meanwhile, proves a soothing foil for the tangy filling and crunchy base – serve on the side, if you prefer (this makes it easier to transport), and garnish with slices of lime, if you’re playing to the camera.
Perfect key lime pie
Prep 20 min
Cook 15 min
Chill 3 hr+
For the base
200g digestive biscuits (about 14)
75g butter, or coconut oil, melted
1 pinch salt
For the filling
4 egg yolks
400g tin condensed milk
For the topping (optional)
300ml whipping or double cream
1 tbsp icing sugar
Heat the oven to 160C (150 fan)/325F/gas 3 and grease a 20cm tart tin. Crush the biscuits to a fine crumb (this is easiest in a food processor, if you have one).
Stir in the melted butter or oil and salt, tip into the tin and press down firmly into the base and around the sides.
Bake for 15 minutes, until dry to the touch, then remove and leave to cool before making the filling.
Finely grate the zest from all four limes into a large bowl, add the egg yolks, then squeeze the juice from the fruit into a jug until you have 120ml.
Beat the yolks and zest for a couple of minutes, until the yolks thicken and turn a pale green, then stir in the condensed milk.
Finally, beat in the lime juice and taste – it should be shockingly tangy and sweet, though you can add more juice if you like, bearing in mind that chilling will make the flavour more subtle.
Pour the filling into the cooled biscuit shell and chill for at least three hours, until set. Just before serving, whisk the cream and sugar in a large bowl until the mix holds soft peaks, then dollop on top. Garnish with extra lime slices, if you have them.