Why You Should Never Make Key Lime Pie Filling In Advance

If you're suffering from a busy schedule, you're probably looking to get a leg up on your cooking practices. Every little hack you find can prove to be a super convenient boon, especially if you've got a big meal or celebration to plan for. One of the biggest favors you can do for yourself is to learn to prep ahead ingredients and fillings that may feature in your dish. And while there are plenty of tips for prepping foods at home, there are certain dishes where you should avoid this make-ahead method at all cost.

When you're using an ingredient like citrus, time is most definitely of the essence. According to Saura Kline, Pastry Chef at Local Jones, a Key lime pie filling is subject to this time sensitive rule. While you might see some recipes stating to leave your Key lime pie filling for a few hours or even overnight, Kline urges you to avoid this. "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," she says. 

Understanding how lime functions in your pie goes way beyond simply finding out the difference between Key limes and their standard brethren. In fact, once your limes are added into your recipe, that's where the real culinary alchemy begins. When you add citrus to dairy, you're effectively neutralizing the milk proteins present in the dairy ingredient, causing them to clump together when they would otherwise drift around the dairy unperturbed. And, as Saura Kline stated, your egg protein is also affected by the presence of citrus. The acidity in your lime juice denatures the proteins in the egg, a similar reaction to what happens when you cook proteins with heat. This is why you'll hear of the fish being "cooked" in ceviche, and can also spur the argument of whether citrus makes eggs safe to eat.

But before you throw your Key lime pie into the oven in a panic, don't worry — you do have a little bit of wiggle room. While you should definitely be aware of the denaturing and curdling processes going down in your pie, an ingredient like sweetened condensed milk can help counteract the curdling process. Unlike milk or heavy cream, sweetened condensed milk has more protein and sugar, protecting it from the acid's denaturing qualities. As an emulsifier, it also helps bind the filling together. So, if you have sweetened condensed milk in your pie, you should have a little extra time on your hands. Still, your safest bet would be to just make the pie filling when you need it.