Introduction: Key Lime Pi

Picture of Key Lime Pi

In this guide, I'm going to show show you how to make a key lime pie celebrating everyone's favorite transcendental number, π. Well, all right, it may not be your favorite, but it is the one we're commemorating with this pi. Er, pie.

Here's what you'll need:
at least 1 disposable pizza pan (9" or bigger)
printer
circle cutter (optional)
hobby knife
tape

eggs
limes
sweetened, condensed milk
graham crackers
butter
sugar

cooling rack
pie plate
spatula
knife
drinking straw

The way in which we're going to commemorate π is with something I'm going to call a "pie circle". In other words, we're going to create a pie based on the design of a crop circle found in England in the middle of 2008.  You can read all about that crop circle at this link:
http://www.dailymail.co.uk/news/article-1027178/Easy-pi-Astrophysicist-solves-riddle-Britains-complex-crop-circle.html

Step 1: Start the Template

Picture of Start the Template

The first step in making this happen will be to make a template. Start by printing out a few sheets of polar graph paper; I used the generator at http://incompetech.com/graphpaper/polar/ with 10 primary spokes and 14 concentric circles.

Cut an 8.5" circle from the center of the pizza pan. I used a circle cutter to "score" the rim of the circle, then a hobby knife to finish off any spots that weren't cut all the way through. Also cut the circle out of the graph paper; it should be about 8".

Tape the two circles together concentrically. I found it easier to cut the sections out of the template if I shaded in the parts to be cut first. Start by shading in the entire center two circles. Skip the next circle, then fill in three segments of the fourth circle, representing the 3 at the beginning of π. Skip the fifth circle, then fill in 1 segment, representing the 1 after the decimal. Continue in that fashion, skipping one circle between successive digits, until you hit the end of your graph paper. In my case, this was after the 9.

Step 2: Cut It Out!

Picture of Cut It Out!

Next, cut out the shaded parts of the template. Be sure to cut all the way through the aluminum as well. I found this easiest to do with a hobby knife, cutting out one segment at a time. When you're finished, remove the remaining paper.

Step 3: Making the Crust

Picture of Making the Crust

Now we're going to put together the pie.  First, we'll need a graham cracker crust. Grind 10 graham crackers in a food processor for about a minute, until they're fine crumbs. Stir in 3 tablespoons of sugar and 5 tablespoons of melted butter.

Press the crust-to-be into a 9" pie plate. Be sure to go up the edges of the plate, but try not to get any crumbs on the rim. Bake the crust at 325°F for about 15 minutes, then cool on a rack for 20.

Step 4: Breaking Both Eggs and Rules

Picture of Breaking Both Eggs and Rules

Next, we'll put together the filling. In order to do that, we're going have to break both eggs and rules (4 eggs, 2 rules).  And they're not just any two rules, either; they're what key lime pie fans probably think of as the two cardinal rules for making key lime pie. First, we're going to use green food coloring, and second, we're not actually going to use key limes.

For the filling, mix four egg yolks together with the zest of two limes (regular, Persian limes) in a medium bowl. Stir in one can of sweetened, condensed milk (14 oz). Add 3 drops of green food coloring, and stir that in too. You can leave it out if you really want to, but I think it makes the pie look more like a barley field. Surprisingly, that's a good thing in this case.

Stir in 1/2 cup of lime juice, then let it sit for about 20 minutes. During that time it will get somewhat thicker; that's the acid from the lime juice thickening the proteins.

Step 5: Assembly

Picture of Assembly

Next, pour the filling into the crust, then even it out with a spatula. (Don't do what I did #1: try not to drip filling on the edge of your pie plate.)

Spray your template with cooking spray. This will help when we remove the template later. (Don't do what I did #2: try not to spray vegetable oil all over your camera.)

Press the template gently into the pie. Don't press it in too much, but you do want to keep the template from bending up instead of sitting flat on the pie. Bake the pie at 325°F for about 15 minutes; it should be fully set, but still a bit jiggly in the center.

Step 6: The Reveal

Picture of The Reveal

Cool on a rack for 10 minutes, then dust with graham cracker crumbs to highlight the ridges left behind by the template. Try not to use much more than you need; if there are too many crumbs, they won't all stick to the cooking spray on the top of the template, making it hard to remove the template without spilling. You can also skip the crumbs entirely if you prefer; the template will actually leave subtle but still visible ridges in the pie filling.

Now comes the hard part. Using the tip of a knife, pull up the edge of the template. It should release from the pie without much effort. Continue pulling up the template, working evenly from both sides towards the center. The excess graham cracker crumbs will probably stick to the cooking spray on the top of the template, as long as there aren't too many, but do try to be careful not to spill them where they shouldn't be.

Lastly, we'll add a decimal point to our π. Dip the end of a drinking straw into a dish of graham cracker crumbs; you want to pick up enough crumbs to make a dot, but not so many that the dot will overflow into the rings. Keep the crumbs from falling out by covering the bottom end of the straw with a finger, then position the straw over the pie between the 3-segment ring and the first 1-segment ring. Remove your finger, letting the crumbs out.

Refrigerate the pie for a few hours or overnight, then serve and enjoy!

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