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Lab Preparation: Key Lime Pie

Thanksgiving approaches for readers in the US, which means that I turn my attention around here to lab preparations that have already been through trials in my own kitchen. I’ve posted my procedure for chocolate pecan pie here in the past, and I plan to make it again this year (my kids will brook no change in this menu). And I should also note another reliable recipe from here at Pipeline HQ, chicken pot pie, which should adapt very easily to using up whatever leftover turkey you need to clear out. Personally, I can clear out quite a bit of it in the form of turkey sandwiches with mayonnaise-and-horseradish. My wife puts hoisin sauce on hers instead, in a wrap or flour torilla with fresh scallions to make a sort of turkey-for-duck Asian substitution, but you may need other options.
But I wanted to provide another well-tested prep, so for those of you looking for something a bit different, here’s one for key lime pie, adapted from the Cooks Illustrated version. I’ve checked this procedure several times, and the only defect of the resulting pie is that it tends to have a short half-life, especially if left unwatched. It is, in theory, more of a summer dessert (being served cold), but no one’s ever objected when I’ve made it at other times of the year.
First, the only tedious part of the whole recipe. You’ll need to zest some limes – if you haven’t done this before, it involves scraping the green peel off in small flakes or strips from the limes (wash ’em first), while not getting deep into the bitter white pith below. There’s a lot of flavor in the volatile oils in that layer; this step really is important. Fellow blogger Megan McArdle has long recommended a Microplane Zesterfor this, and I may have to break down and buy one myself. There are other zesters, which are basically small, shallow grating tools. Judicious use of a standard grater, or even a peeler or sharp paring knife if you have a steady hand, will do the trick. You’ll want to come up with four teaspoons of lime zest, and I have been unable to come up with a weight equivalent for that (I’ll weigh it next time I do this recipe and amend this post!) But that’s three limes worth if you’re pretty efficient, but more likely four limes of zest, if that helps. Note: I’m talking about the amount from usual-sized “Persian” limes here, not the authentic Key limes, which are smaller. If you have the latter, feel free to use them, although zesting them is more of a pain and (to my palate, anyway) there’s not too much difference in the finished product. Floridians disagree.
Take four eggs and separate out the yolks into a bowl (no use for the whites in this recipe, unfortunately). Mix the lime zest into the yolks – you’ll notice that they’ll extract some color, and they’re doubtless doing the same with the flavor constituents as well. While that stands, take some of those ugly sheared limes you’ve created and squeeze out 1/2 cup (about 120 mL) of lime juice. If you’re particular, you can run this through a strainer to remove pulp, which will give you a more homogeneous pie, but I sometimes don’t bother. Stir the juice into the yolk mixture, and then mix in one 14 oz can of sweetened condensed milk. That’s a standard can size here in North America, about 390 mL, but I have to confess that I’m less clear on the product’s availability and packaging size in the rest of the world – clarifications welcome in the comments. Leave this mixture standing during the next part of the recipe; it’ll set up a bit as the lime juice goes to work denaturing the casein protein in the milk (and probably albumin in the egg yolks). It will be an interesting yellow-green color.
Now, a crust. You’ll need the oven heating to 325F (160 to 165C). The recipe calls for 11 full-sized Graham crackers to be put into a food processor (or crushed up in a bag) and thus turned into crumbs.That quantity is easy enough to round up in North America, but I’m not sure about the rest of the world. It comes to about 154g of Graham cracker, which might help. As for substitutions, I think that British wholemeal digestive biscuits (McVitie’sor equivalent) would work out fine, although they seem more open-textured than Graham crackers, so you might want to watch and not over-process them. The idea is to produce crumbs, not dust).
Combine these crumbs with 3 tablespoons (about 37g) granulated sugar. Melt 5 tablespoons (65 to 70 grams) of (unsalted) butter, and mix this into the crumb/sugar preparation. Press this into an even layer in a 9-inch round pie pan, up the edges as well. (I’m not sure about standard pan sizes in other parts of the world, but that’s about 23 cm diameter and about 3 to 4 cm deep). Bake this for 15 minutes and let it cool to room temperature or a bit above. It’s worth taking the time to make the crust, by the way – I think this pie is much better with a crumb crust than a standard pie crust, and the homemade one, although quite easy to make, will wipe out any store-bought version.
Pour the lime/milk/egg mixture into the crust, and bake the whole thing at the same temperature for about 17 minutes – the interior will still look a bit fluid, but it should be fine on cooling. Cool to room temperature, and then chill thoroughly before serving.
As mentioned above, these things don’t last long around here – fresh limes (juice and zest) are sui generis, and if you like the flavor, you’ll eat more of this than is good for you. Of course, there is the vitamin C content, which is no doubt substantial. Lemons for limes is an obvious substitution, although I haven’t tried it – that’s the equivalent of para-chloro for para-fluoro in a med-chem SAR (although I’ve seen that one fail, while lemon-for-lime is foolproof outside of the expected flavor change). I suspect that this would work with oranges as well, although you would probably want to add some extra lemon juice to take the acidity up in the first step. And for that matter, it would probably work with grapefruit, although you’d probably run into some bitterness problems that would require experimentation. Sugar won’t cancel that out (consider the quinine-driven taste of bitter lemon, which you can hardly find in the US any more, and which I pounded down while in England the other month). I’d be glad to hear about any such research, naturally!

28 comments on “Lab Preparation: Key Lime Pie”

  1. MTK says:

    My favorite dessert.
    Just one addition to the recipe (and this is where my Florida roots come in), make sure you use the smaller, thinner skinned key limes (Citrus aurantifolia) rather than the bigger, greener, and less tart Persian limes.

  2. canchem says:

    Get the microplane. I’ve owned 5 zesters in my life (what? I like gremolata.) and it’s by far the most superior I’ve ever had. You can knock the zest of a whole grapefruit in 15 seconds with no pith removal.

  3. BV says:

    Key Limes were compared against the Persian limes in the Cooks Illustrated article, and their tasters actually preferred the Persians.
    I’ve made this recipe many times and it is always a winner. Cook’s also uses this same technique for lemon bars.

  4. TM says:

    Use the egg whites to make a meringue topping, as for lemon meringue pie. I’m sure that would work well for the lime version too.

  5. Thxgiving Gnome says:

    If you substitute lemons for the limes it would essentially result in a “Lemon Icebox Pie”. This is one of my favorite deserts made by my grandma and she would usually top it with a meringue. Food & wine gives a similar preparation of the Lemon Icebox Pie (sans Meringue):

  6. MTK says:

    Then call it Persian lime pie, not Key lime pie.
    That’s as bad as Laura Bush’s use of lemon juice in guacamole and not lime juice.
    The Floridian in me winces at both of these kitchen travesties.

  7. BV says:

    Valid point. I shall inform Cook’s of their nomenclature faux pas.

  8. James says:

    We love key lime pie (especially towards the winter months…it reminds my wife and I of summers on the beach).
    Just a couple of notes on your recipe:
    First – I like the idea of using McVities for the crust – if you try, let us know how it turns out!). We’ve substituted either brown sugar or turbinado for the regular white sugar in the crust and it adds a nice flavor profile (in addition to sweetening).
    Next – on the lemon/lime substitution, if you do that, you’ve essentially achieved a lemon meringue pie (and can use the egg whites for the meringue…reducing the non-used components of the recipe). Also – consider Myer lemons for this (yummy!)
    Finally – don’t use regular limes!!!! You should be able to find key limes at Whole Foods/Trader Joes, or the like. I much prefer the key lime (a bit tarter and much jucier – although smaller, so it takes more squeezing to get adequate amounts of lime juice).

  9. Dickweed Jones says:

    Guys- Be serious. Nellie and Joe’s Key Lime Juice (preferred, but other bends can be used–NOT lime or lemon juice). Mix juice (half cup), 3 egg yolks and 1 can of Borden’s sweetened condensed milk (14 oz). Stir until smooth, pour into graham cracker crust. Bake at 350 for 10 minutes. Allow to set overnight in fridge. Cover with whipped cream immediately before serving. Yield = 6-8 people (94% ee)

  10. luysii says:

    Ah Key Lime Pie ! My wife is about to start, et je suis le zestaire ! Happy Thanksgiving to all.

  11. Nick K says:

    Key Lime Pie – a classic of American cuisine! Even better with meringue on top. Thanks for the recipe.

  12. Brooks Moses says:

    I’ll second the comment of canchem@2 — the microplane zester really is a far superior tool.
    The suggestions of variation are intriguing, in particular the grapefruit! The bitter of grapefruit is largely in the zest, though, so one plausible substitution there would be using orange zest (or mostly orange zest with a little grapefruit) and then using grapefruit juice. I shall have to try this.

  13. RKN says:

    Key Lime Pie is not only my favorite dessert, it’s my favorite food. Period. And I am pro-Key, anti-Persian in this context.
    Gobble gobble.

  14. Chris Hansen says:

    Thanks for posting this! I’ve been wondering what to bake for our faux Thanksgiving next weekend and I think this will do just fine. I will try the digestives, but there are other biscuits that have a rougher texture that I might try as well.

  15. luysii says:

    My wife informs me that je n’est pas le zestaire, malheureusement je suis le squeezaire

  16. Dickweed Jones says:

    @11- Putting meringue on Key Lime Pie is like putting Bosco on a Kobe steak. SOOOOO wrong.

  17. My 0.02 says:

    I was expecting a good deep-fried turkey recipe, instead.

  18. daveh says:

    I use the same recipe. Just cook the crust and pour the filing into it. I have no real need to cook the filing as it stiffens overnight in the fridge.
    Key limes, off the tree, taste notably different from persian here in Miami. But, I have tried the big farm, bagged key limes available in the store…. not so different. Obviously the stock was selected for it’s understated, inoffensive nature.
    If you ever get kaffir limes make this pie! You won’t need the zest. I’ve got some and I am making one tonight!

  19. Tom Womack says:

    I tend to make the crust using crunched-up ginger nut biscuits (I think the US term is ginger snaps; they’re hard cookies with a strong ginger flavour); the contrast with the lime is pleasant.
    I’ve made a couple of pies with black currants instead of the limes; they have the same sort of tartness, and the pie comes out a wonderful purple colour. However, apparently black currants were banned in the US until 2003 and are still forbidden in Maine and New Hampshire.

  20. Thomas says:

    A thorough SAR study should also include Seville oranges, though they have a much shorter season. I must try that next time they are in season here.

  21. Chris says:

    I made a pie this evening for our faux Thanksgiving tomorrow. Instead of digestive biscuits, I used Hob-Nobs, which are an oaty type of biscuit. I have (of course) not cut it yet, but I suspect that it’ll be crisp and crunchy. (Location: I’m in London, England)

  22. Jill says:

    I made your recipe after my husband saw it on your blog. However, here in Germany I couldn’t find anything like graham crackers, so substituted vollkorn Zwieback. The pie–especially the crust–got a definite thumbs-up at our expat Thanksgiving dinner today, and was the first dessert to disappear. The only difficulty with the Zwieback was to get the right consistency since when crushed it tends to go straight from Brocken into Staub.

  23. Derek Lowe says:

    Tom, #19 – ginger snaps are definitely an idea worth trying. I’ll put that on the list.
    Chris, #21 – let me know how it turns out! I think that Hob-Nobs would work pretty well. They’re another part of the McVitie’s cookie/biscuit empire, aren’t they?
    Jill #22 – it’s weird how much better the home-made crust is, isn’t it? I think it’s the butter, personally, instead of the industrial shortening that I assume is used in the store-bought ones.

  24. Anonymous BMS Researcher says:

    I strongly agree with the microplane recommendation — best graters I have ever used. Available many places, including Bed, Bath & Beyond.

  25. ech says:

    The microplane zester is great, but for long zest strips a dedicated zester is better. The one I have has a row of 5 small loops at the end of the handle and will produce inch long strips of zest with little effort. You can then mince with a knife if needed.
    The microplane nutmeg grater (essentially a fine microplane grated bent into a U) is also excellent. Turns out nutmeg dust in no time.
    For those not up on Key Lime Pie lore, there are four camps in Florida among the pie lovers, based on crust (graham cracker or pastry) and topping (meringue vs. nothing).

  26. Onyx says:

    My favourite dessert!

  27. Torquill says:

    For the record, 4 teaspoons of zest came out to 6-7g on my little kitchen scale; I’d really like more sigfigs on that, but aim for about 6.5 if you can. The mass may depend on the hydration of the lime peels, of course… mine were a trifle dry. I suspect you can’t go wrong by erring a little on the high end.

  28. Marluxia Bliss says:

    I used crushed gingernut cookies instead of Graham biscuits for the base and it was delicious *A* I find key lime pie a little sweet for my taste even with the citrus, and the ginger offset that nicely and added a little bit of a kick to the flavour to boot.

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