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I'd like to create a denser/thicker/firmer key lime pie filling than my usual result (using lime juice, egg yolks, and sweetened condensed milk). What is the best way to thicken this kind of custard-based filling which thickens at least in part due to the interaction of citric acid and egg yolks? Would clear jel be an option? If so, which type (instant vs. cook-type)? If not, what other agents might do the trick?

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    what do you mean by jel? Gelatin or jelly?
    – Luciano
    Commented Jun 25 at 8:15
  • Regardless of what particular solution you go with here, I strongly recommend you make a few small test batches of the filling with varying amounts of the ingredient you choose to figure out what proportion you need. It’s very easy with most options to end up either with a filling that is far too stiff or tastes ‘off’. Commented Jun 25 at 20:41
  • My key lime pie uses Greek yogurt, tastes great. Commented Jun 25 at 21:26

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I'd suggest being wary of overdoing stiffening it up particularly given that you seem to be using standard ingredients that are rarely considered too runny if made normally.

That said, various starches can provide more stiffness on up to way too much. I happened to take a stab at Vegan Key Lime Pie once upon a time and cornstarch worked out the best of the ones I tried; but too much of that was objectionable from a texture as well as taste point of view, so tuning the amount to get an acceptable set that didn't slump into where a slice had been cut while in the fridge without being overly stiff took a series of trials.

The standard key lime pie custard already doesn't slump, so small amounts would probably be advisable unless you are really sure you want a rubbery texture, and you might change your mind once you try (taste-test) that.

If you want it to set up very hard, agar agar will do that, but it's very easy to go too far with that and achieve unpleasantness.

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  • Something along the lines of a mix of tapioca or sago and rice starches/flours a la Kueh Lapis might work, would need to watch for being too firm though.
    – bob1
    Commented Jun 25 at 21:59
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Your best bet is gelatin. You can control the firmness and it will not be a flavor thief.

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  • Is Knox "Gelatine" what I need or do I need something else?
    – Joel Cure
    Commented Jun 24 at 17:15
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    Gelatin types comes in a different "bloom" strengths. What you use depends on how firm you want the outcome to be. Knox brand is on the higher end of the scale. You'll want to do some research on how to use and control. Plenty of info online.
    – moscafj
    Commented Jun 24 at 18:38
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I make key lime pie with sweetened condensed coconut milk when I can't use dairy for various reasons. The pie has some thickening power from the eggs, but not from the milk's reaction with acid, since coconut milk doesn't react to acid the same way dairy does, so the non-dairy version needs some help from starch. The recipe I used recommended 4 tablespoons of tapioca starch in the pie to add structure. I used 3 tablespoons of potato starch with excellent results (a bit firmer than the pie I made with dairy). Since you are using dairy and your pie texture should be firmer to start with, I would use 1 or 2 tablespoons of starch and see if that has the results you're looking for.

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  • Now there's one I didn't try. Next time...
    – Ecnerwal
    Commented Jun 25 at 19:12
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Xanthan gum might do what you want. It's vegetarian, gluten-free and is clear. In the UK at least, you can get it in major supermarkets, usually in the Free-From section.

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  • Alternatively, you can also try guar gum, locust bean gum, carob bean gum, or carrageanan, etc.
    – Arctiic
    Commented Jul 1 at 5:12
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Arrowroot can be used to thicken sauces, juices, jellies and syrups and does not affect the taste (unlike cornstarch).

I've used it before with other citrus juices (orange and lemon) to make a thick gel for topping desserts and this recipe for vegan key lime pie (not personally tested) uses arrowroot instead of eggs.

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  • I trialed cornstarch, tapioca, and arrowroot; of the three, cornstarch appeared to work best, but that was for pretty much all the thickening, not just adding a bit.
    – Ecnerwal
    Commented Jun 25 at 19:11

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