Contents
Lime juice, egg yolks and sweetened condensed milk in a graham crust. It is a Florida Keys invention and it is a direct product of nineteenth-century food technology: before refrigeration reached the islands, canned condensed milk was the only milk there was, and the acid of the lime sets the filling without heat.
That chemistry is the whole recipe, and it is worth understanding.
What makes it work
- The acid sets the filling, not the oven. Lime juice at pH 2 denatures the proteins in the condensed milk and the yolks, and the mixture thickens on the counter within minutes of mixing. Baking for 15 minutes is a modern food-safety step for the raw yolks, not a setting step.
- Key limes are smaller, more acidic and more floral than Persian limes. Bottled key lime juice is entirely acceptable and is what most Keys restaurants use. See lime and Lime juice.
- Never use green food colouring. A real key lime pie is pale yellow, because it is made of egg yolk and milk. Green is a tell.
- Zest goes in, and it matters more than the juice for aroma.
- Chill it hard. Four hours minimum; it firms considerably.
Ingredients
One 23 cm pie. Serves 10.
Crust:
- 250 g graham crackers or digestive biscuits, crushed
- 100 g unsalted butter, melted
- 40 g granulated sugar
- 2 g fine salt
Filling:
- 4 egg yolks
- 400 g sweetened condensed milk
- 150 ml key lime juice, about 20 key limes or 6 Persian limes
- zest of 4 limes
- 2 g fine salt
Topping: 300 ml double cream whipped with 30 g powdered sugar, or a meringue
Method
Heat the oven to 175 °C / 350 °F.
Mix the crust ingredients, press firmly into the tin, and bake 10 minutes. Leave the oven on.
Whisk the yolks with the zest for two minutes until pale and slightly thickened — this small step noticeably improves the set. Whisk in the condensed milk, then the lime juice and salt. The mixture will thicken visibly as you whisk.
Pour into the warm crust and bake 15 minutes, until barely set with a slight wobble.
Cool to room temperature, then refrigerate at least four hours.
Top with whipped cream before serving.
Notes
- Do not substitute the condensed milk. The sugar concentration is part of the chemistry and part of the preservation.
- A meringue top, made from the four leftover whites and browned for eight minutes at 175 °C, is the older Keys presentation.
- Keeps four days refrigerated and freezes well — frozen key lime pie is a legitimate dessert in its own right.
Elsewhere on the site
Method and theory: Pie and tart recipes · pies and tarts · lime
Nearby: lemon meringue pie · banana cream pie · lemon tart