science of key lime pie

I tried my hand at making a key lime pie today and it looks like it turned out pretty well, although it's currently chilling in the refrigerator to thicken up even more. Key lime pie has to be one of my favorite desserts, and I know it's one of Komokuten's favorites.

Key lime pie actually is one of the easier pies to make as the crust is just graham cracker crumbs, sugar, and unsalted butter mixed together and pressed into the pie pan. The filling consists of lime zest, egg yolks, sweetened condensed milk, and lime juice. Sweetened condensed milk (so good in vietnamese coffee too), citrus fruit (think Jamba Juice: "Citrus Squeeze" or "Orange Dream Machine"), mmmmm. Oh yeah, there's that butter and sugar thing that makes everything good too. Everything in moderation, right?!

In traditional pie crust (apple pie, for instance), one would have to "cut" cold butter and sugar into flour in a food processor (which I don't have) to make the dough. Then the dough is really temperature sensitive (must be kept really cold so the butter doesn't melt into the flour). As a result, the pressure is on to work carefully and quickly to form the crust in the pan (and a crust to cover the pie). Of course, one could always buy a frozen pre-made crust, but where's the challenge there?! (Don't worry, I shortcut like Rachael Ray does in 30 Minute Meals all the time, so I can eat dinner before midnight on weekdays.)

The cookbook had this awesome blurb about how key lime pie actually thickens. I love my geeky cookbook, The Best Recipe, by the editors of Cooks Illustrated. So here's an excerpt for those that are curious about the science behind the art of the key lime pie.

The extrordinarily high acid content of limes and the unique properties of sweetened condensed milk are responsible for the fact that lime pie filling will thicken without cooking.

The acid in the lime juice does its work by causing the proteins in both the egg yolks and the condensed milk to coil up and bond together. This effect is similar to that of heat. The same process can be observed in the Latin American dish seviche, in which raw fish is "cooked" simply by being pickled in lime juice.

But this process does not work well with any kind of milk; it requires both the sweetness and thickness of sweetened condensed milk. This canned product is made by boiling the moisture out of fresh milk and then adding sugar. Because the milk has been evaporated, or condensed, it is thick enough to stiffen into a sliceable filling when clabbered by the lime juice. The sugar, meanwhile, plays the crucial role of separating, or "greasing," the protein strands so that they do not bond too tightly. If they did, the result would be a grainy of curdled filling rather than a smooth and creamy one. Of course, a liquidy, curdly filling is exactly what one would get if one tried to use fresh milk instead of canned, because fresh milk lacks the crucial added sugar and is also much thinner.

We discovered that cream is not a viable substitute for sweetened condensed milk either. It does not curdle the way milk does because its fat, lke the sugar in condensed milk, acts as a buffer to the lime juice. Cream is roughly 50 percent liquid, however, and thus it will only thicken, not stiffen, when clabbered.

So cool! I knew that lime juice (or any acid) would "cook" meats since I was a kid, but it wasn't until now the pieces all fit together. The steak my family makes is marinated southeast asian style (garlic, ginger, soy sauce, fish sauce), cooked medium to medium-well. I remember cutting up my steak as a kid and squeezing some lime on it (it complements the taste of the marinade) and anything pink or red in the meat immediately turned brown because it "cooked", at least on the surfaces touched by the lime juice.

Alex told me about seviche (and how good it is) not long ago and I am curious to try it once he takes me to a place around here that actually has a good dish of it. I'm guessing food safety wise it's no more dangerous than sushi, which I really enjoy, that is if it's prepared correctly.

For those of you grossed out by the notion of "cooking" by lime juice, no worries on the key lime pie. The pie, complete with filling, did bake in the oven to complete the cooking, in the traditional sense, with heat. For those grossed out by eating raw fish, there's always the vegetable rolls ::grins::