Key Lime Pie on a Graham Cracker Crust

Before you start

Key lime pie is built on a small piece of kitchen chemistry. Acidic lime juice thickens sweetened condensed milk and egg yolks almost on contact, so the filling turns creamy before it ever sees the oven. Older recipes relied on that alone; baking the pie briefly is the modern standard because it cooks the yolks and gives a firmer, cleaner slice.

Key limes are small, seedy and more aromatic than the common Persian lime, with a floral sharpness. Fresh ones take patience to juice, and good bottled key lime juice is a reasonable shortcut. Regular limes work too; the pie is a little more sour and less fragrant. The graham crust should be pressed firmly and baked until it smells toasted so it stays crisp under the filling.

Ingredients

• Graham crust150 g (about 1½ cups, from 10 full sheets) graham cracker crumbs
• 25 g (2 tbsp) granulated sugar
• pinch fine salt
• 85 g (6 tbsp) unsalted butter — melted
• Filling2 cans, 396 g (14 oz) each sweetened condensed milk
• 4 large egg yolks
• 180 ml (¾ cup) key lime juice — about 20 to 25 key limes, or bottled
• 1 tbsp finely grated lime zest
• Topping240 ml (1 cup) cold heavy cream
• 20 g (2 tbsp) powdered sugar
• ½ tsp vanilla extract

Method

1. Heat the oven to 175°C (350°F). Pulse or crush the graham crackers into fine, even crumbs. Mix with the sugar and salt, then stir in the melted butter until the crumbs look like damp sand and clump when squeezed.
2. Tip the crumbs into the pie plate. Press them across the bottom and up the sides with the base of a measuring cup, packing firmly so the crust is even and about 5 mm thick.
3. Bake for 10 to 12 minutes, until fragrant and a shade darker. Cool on a rack for about 15 minutes. Lower the oven to 160°C (325°F).
4. Whisk the zest into the egg yolks for 1 minute, until slightly paler. Whisk in the condensed milk, then the lime juice. The mixture will thicken noticeably within a minute or two.
5. Pour the filling into the crust and smooth it. Bake for 18 to 22 minutes, until the edges are set and the center has a slight, uniform wobble. The center should reach 71°C (160°F).
6. Cool at room temperature for 30 minutes, then cover loosely and refrigerate for at least 4 hours, until cold and firm.
7. Shortly before serving, whip the cream with the powdered sugar and vanilla to soft peaks. Spread or pipe it over the pie and slice with a hot, dry knife.

Tips for success

• Roll key limes firmly on the counter before cutting; they give up more juice. A garlic press or small hand juicer also helps.
• Zest the limes before you juice them. Zest gives the pie its aroma; juice gives the sourness and the set.
• For neat slices, chill overnight and wipe the knife between cuts.

Substitutions

Key lime juice

Use fresh Persian (regular) lime juice in the same amount. The pie is sharper and less floral; some bakers add 1 tsp lemon juice to round it out.

Graham crackers

Use the same weight of digestive biscuits or vanilla wafers; reduce the sugar to 1 tbsp since both are sweeter.

Whipped cream

Top with a Swiss meringue cooked to 71°C (160°F) and toasted with a torch for a more traditional Florida finish.

Variations

Lime and coconut

Add 40 g toasted shredded coconut to the crumbs and sprinkle more on the whipped cream.

Mini pies

Press the crust into a 12-cup muffin pan lined with paper cups; bake the crusts 7 minutes and the filled pies 12 to 14 minutes.

Troubleshooting

Filling stayed soft and loose

The pie was not chilled long enough or too little juice was used. Measure the juice and chill at least 4 hours, ideally overnight.

Crust crumbles apart when sliced

It was not packed firmly or needed more butter. Press hard with a flat cup and bake until toasted.

Grainy or curdled filling

It overbaked. Pull the pie as soon as the center reaches 71°C and wobbles slightly.

Storage

Refrigerate the pie, covered, for up to 3 days; add whipped cream the day you serve it. It contains eggs and dairy, so keep it out of the refrigerator no more than 2 hours. The pie without topping freezes well for up to 1 month.

Safety notes

The filling contains egg yolks; bake until the center reaches 71°C (160°F), or use pasteurized eggs. Refrigerate leftovers. Contains wheat, egg and milk.