Why key lime pie filling shouldn't be made in advance
Key limes are so acidic that they can curdle dairy products and cook the eggs in the filling, so it's best to prepare it just before using it.
“Key lime pie is one of those pies that should be made and cooked at once. There is a lot of acid in Key limes that can cook your eggs or curdle whatever dairy is in your filling,”
When working with citrus fruits, timing is crucial. According to Saura Kline, pastry chef at Local Jones, the filling for Key lime pie is a prime example. Some recipes suggest letting it rest for a few hours or overnight, but Kline advises against it: “Key lime pie is one of those pies that needs to be prepared and baked immediately. Key limes are very acidic, which can cook the eggs or curdle the dairy in the filling,” she explains.
Understanding how lime works in a tart goes beyond distinguishing Key limes from common ones. When citrus is added to dairy, the milk proteins clump together instead of remaining dispersed. According to Kline, egg protein is also affected: the acidity of the lime juice denatures it, in a reaction similar to that caused by heat.
It's the same effect that makes the fish "cook" in ceviche. There is, however, some leeway: sweetened condensed milk has more protein and sugar than milk or cream, which protects it from the acid's denaturing properties. As an emulsifier, it also helps bind the filling, giving the tart a little more time to set.
Even with that margin, it's best to prepare the filling just before using it.
