Lime Coconut Pie — Key lime pie with a tropical coconut edge

i. Origin & history

The pie starts with Key lime pie, one of the best-known regional desserts in the United States. It comes from the Florida Keys, where small, aromatic Key limes grew and where, before reliable refrigeration and with little fresh milk available, canned sweetened condensed milk was a staple. Condensed milk became widely available in the second half of the nineteenth century, and by the early twentieth century the combination of condensed milk, egg yolks and lime juice in a pastry or crumb crust was established in Key West. The origin story often told credits a cook known as Aunt Sally, working for a wealthy Key West family, but it is not well documented. Florida made Key lime pie its official state pie in 2006.

Coconut is just as rooted in the region. Coconut palms grow across South Florida and the Caribbean, and coconut cream pie and coconut custard pie are long-standing Southern diner favourites. Cuban, Bahamian and other Caribbean communities in Florida brought their own coconut sweets — such as Cuban coco quemado and Latin American cocadas — and lime and coconut are a natural tropical pairing in the drinks and desserts of the islands.

Lime coconut pie is the meeting of these traditions rather than a dish with a single recorded origin. It appears on the menus of Florida restaurants and bakeries and in American home baking as a variation on Key lime pie, usually with coconut in the crust, the filling or the topping — often all three. The name varies: coconut Key lime pie, coconut lime pie and lime coconut cream pie describe much the same thing.

It is distinct from a coconut cream pie, which has a cooked, cornflour-thickened custard filling and is not sharp at all, and from a lime meringue pie, which uses a cooked lime curd under meringue in the manner of lemon meringue pie.

ii. How lime juice sets the filling

The remarkable thing about Key lime–style fillings is that the acid does the thickening. Sweetened condensed milk is milk reduced by about half and heavily sugared, so its milk proteins are highly concentrated. When an acid such as lime juice is stirred in, those proteins (mostly casein) partly coagulate and the mixture thickens noticeably within a minute, turning from a pourable syrup into a soft, spoonable cream. Egg yolks add richness and, with a short bake, firm the filling into clean slices.

That makes the amount of lime juice structural, not just a matter of taste. Too little and the filling never thickens properly; the classic proportion is about 120 ml lime juice to a 397 g tin of condensed milk. Coconut complicates this: coconut cream adds fat and water but no protein to the reaction, so if you add too much the filling sets softly and can weep. Keeping the coconut cream to a few tablespoons, and adding coconut flavour through the crust and topping as well, keeps the set.

The brief bake — about 15 minutes at a low temperature — is mainly to cook the egg yolks safely and give a firmer texture. The filling should be set at the edges with a slight wobble in the centre; over-baking makes it rubbery and can crack the surface. It then needs several hours in the fridge, during which it continues to firm.

Coconut in the crust needs toasting. Raw desiccated coconut mixed into a crumb base tastes flat and can turn chewy as it absorbs moisture from the filling; baked with the crumbs and butter, it crisps and develops a nutty, toasty flavour that makes the coconut unmistakable.

iii. Ingredients

Makes 10 servings · scroll the side panel to adjust

• 160 g graham crackers or digestive biscuits, finely crushed
• 40 g desiccated coconut, for the crust
• 25 g caster sugar
• 90 g unsalted butter, melted
• 397 g tin sweetened condensed milk
• 4 egg yolks
• 120 ml fresh lime juice (Key or Persian limes)
• 1 tbsp finely grated lime zest
• 60 ml coconut cream (the thick part from a tin)
• 25 g desiccated coconut, for the filling
• 250 ml double cream, cold
• 2 tbsp icing sugar
• 25 g coconut flakes, toasted

Key limes are small, seedy and more aromatic and floral than the common Persian (Tahiti) lime, but they are hard to find outside Florida and the Caribbean; ordinary limes make an excellent pie, a little sharper. Always use fresh juice: bottled lime juice tastes flat and cooked. Take the thick coconut cream from the top of a chilled tin of full-fat coconut milk. Use unsweetened desiccated coconut; sweetened flaked coconut makes the pie too sugary. Graham crackers are the American standard for the crust, and British digestives are a close substitute.

iv. Method

1. Heat the oven to 175 °C (350 °F). Mix the biscuit crumbs, 40 g desiccated coconut, caster sugar and melted butter until it resembles wet sand, then press firmly over the base and up the sides of a 23 cm pie dish.
2. Bake the crust for 10–12 minutes, until fragrant and the coconut is lightly golden. Leave to cool for 15 minutes and lower the oven to 160 °C (320 °F).
3. Whisk the egg yolks and lime zest together for 1 minute until slightly pale. Whisk in the condensed milk, then the lime juice; the mixture will thicken noticeably within a minute. Whisk in the coconut cream and stir in the 25 g desiccated coconut.
4. Pour the filling into the crust and smooth the top. Bake for 15–18 minutes, until set at the edges with a slight wobble in the centre and no colour on top.
5. Cool on a rack for 1 hour, then refrigerate, uncovered at first, for at least 4 hours or overnight, until cold and firm.
6. Shortly before serving, whip the double cream with the icing sugar to soft, billowy peaks. Spread or pipe over the pie, leaving a border of filling showing if you like.
7. Scatter with toasted coconut flakes and a little extra lime zest. Slice with a knife dipped in hot water and wiped between cuts.

v. Tips & common mistakes

• Measure the lime juice. It is what thickens the condensed milk. Short on juice and the filling stays soft; about 120 ml for a standard tin is right.
• Keep coconut cream modest. Coconut adds fat and liquid but does not help the filling set. More than a few tablespoons and the pie will slump when cut.
• Toast the coconut in the crust. Baking the crust develops the coconut flavour and keeps it crisp under the moist filling.
• Zest before juicing. Zest carries the lime's fragrant oils, which is where most of the aroma comes from; it is almost impossible to grate a squeezed lime.
• Bake low and briefly. Pull the pie when the centre still wobbles. Over-baking turns the filling rubbery and cracks the top.
• Chill properly. At least four hours, ideally overnight. A pie cut too soon is loose in the middle and the slices collapse.

vi. Variations

A classic Key lime pie leaves out the coconut entirely and is sometimes topped with meringue rather than cream, as many Key West cooks prefer. A coconut lime meringue pie uses the filling here under a toasted Swiss meringue. Passion fruit and lime replaces a quarter of the lime juice with strained passion-fruit pulp. A no-bake version uses pasteurised egg yolks, or omits the yolks and relies on the acid set alone, giving a softer pie. Lime coconut bars bake the same filling over the crust in a square tin.

For more coconut pies and tarts on DessertCat, see the coconut tart, chocolate coconut tart and pineapple coconut tart. For citrus, the tangerine tart and the British lemon posset use acid to set cream in a similar way.

vii. Serving & storage

Serve lime coconut pie well chilled, in neat wedges, as a summer dessert after grilled fish, barbecue or a spicy Caribbean meal; its sharpness makes it lighter than it looks. A few slices of fresh mango or pineapple on the side suit it well.

Keep the pie covered in the fridge for up to 3 days; add the whipped cream on the day of serving, as it weeps after a day. The baked, unfilled crust can be made a day ahead, and the filled pie the day before serving. Without its cream topping, the pie freezes well for up to a month: freeze until firm, wrap tightly, then thaw overnight in the fridge. Some people like it eaten semi-frozen, like an ice-cream pie.

ix. More from North American

• Coconut Tart — golden baked coconut filling
• Pineapple Coconut Tart — tropical fruit tart
• Lemon Meringue Pie — lemon curd under meringue
• Lemon Posset — cream set with citrus
• Cocadas — Latin American coconut sweets
• Fruit Pizza — cookie crust, cream cheese and fruit