Key Lime Pie (Acid-Set Lime Custard in a Crumb Crust)

Variations

• The whipped cream version — which is what the photograph on this page shows: in Florida today whipped cream is more common than meringue, and the quantities are in the ingredient list. The traditional topping, and the one this page explains, is the meringue.
• Key limes and Persian limes: key limes are small, seedy, and ripen yellow, and they are more acidic and more aromatic than the common Persian lime. That said, bottled key lime juice is standard even in Florida — squeezing 150ml out of fruit that size takes more than twenty of them. It is not a shortcut that costs you authenticity.
• Why condensed milk: fresh milk did not reach the Florida Keys before refrigerated shipping. Tinned condensed milk was the dairy you could get on the islands, and it made this pie. A rare case of an ingredient being the history.
• Frozen key lime pie: the same filling, unbaked and frozen. Since it sets by acid in the first place, it holds its shape frozen too.
• Against cheesecake: both are a dairy filling in a crumb crust, set by different means. Cheesecake sets because egg coagulates with heat; this sets because acid coagulates at room temperature.
• Against panna cotta: set with gelatine. Three ways to set the same milk and sugar — gelatine, egg, acid.
• The same principle as ceviche: lime juice denaturing fish protein until it looks cooked and lime juice coagulating the protein in condensed milk are the same event. Acid does part of what heat does.
• Against rasgulla: there the point of acid-setting milk is to make curds you lift out as cheese; here the point is to leave the set alone. The difference is how much acid.
• Against the meringue in macaron: macarons use Italian meringue too, cooked and stabilised by hot syrup in the same way. Pavlova and Eton mess use French meringue and are far softer.

Tips

• The acid sets the filling, not the oven. You can watch it thicken as the juice goes in: the acid coagulates the casein in the condensed milk into a network. In principle it would set without any baking at all. The oven is there for the yolks.
• Add the juice gradually. Poured in at once, the acid acts too strongly in one spot and you get grains. Stir and trickle.
• There is no shame in bottled key lime juice. Key limes are small and full of seeds; 150ml takes twenty-odd fruit. Most Florida kitchens use the bottle.
• Do not confuse it with evaporated milk. This is made with sweetened condensed milk. Evaporated has neither the sugar nor the concentration and it will not set.
• Take it out while the centre wobbles. Baked until fully firm in the oven, it contracts as it cools and the surface cracks. The fridge finishes the job.
• The meringue goes onto hot filling. Meringue put on a cold filling is the most common reason this pie weeps: the contact surface has to cook, or liquid collects between the two.
• Seal it to the crust. Meringue shrinks a little as it bakes, and if the edges are not anchored to the crust it slides off in one piece.
• Golden beads mean overbaked. Those brown droplets on the surface are sugar weeping out of the meringue. Colour on the tips is enough, and a blowtorch makes it easier.
• Italian meringue is the safer one. Syrup at 118°C cooks the whites, so it is more stable than a French meringue made with half-raw white, and it has not collapsed by the next day.
• Chill it before cutting. At room temperature the filling is soft. Three hours is the minimum and overnight cuts cleanest. Dipping the knife in hot water and wiping it between cuts keeps the faces sharp.