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I've been following this recipe for lemon bars by Claire Saffitz. The crust turned out perfect, however the lemon custard was super soft, just short of runny. Cutting the entire thing into pieces / bars was nearly impossible without creating a big mess. I'd like to have a firmer custard, but I'm not experienced enough to know how to adjust the recipe.

The recipe uses 9 egg yolks, 3 whole large eggs, 300g sugar (1.5 cups), 360ml lemon juice (1 cup) and 115g of butter (1 stick). These are stirred together, heated in a pot until slightly thickened, poured over the pre-baked crust and then put in the oven for 25-30 minutes. Can I simply add more eggs/egg yolks? Will more butter create a stiffer custard when cooled? Does the acidity of the lemon juice influence the stiffness? I don't think a longer baking time would have stiffened up the custard any more.

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    How did you ensure that the custard has fully set? Did you measure the internal temperature, and what was it?
    – rumtscho ♦
    Sep 27 at 9:47
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    Using an accurate food thermometer will be the easiest and most reliable way to troubleshoot this. I suspect you simply didn't get the curd hot enough to thicken it, but there's no way to know for sure.
    – Sneftel
    Sep 27 at 10:19
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    I haven't watched the video, if it calls for large US eggs then medium european eggs would be the right call @YPOC. If the recipe calls for XL US eggs then EU large would be the equivalent.
    – GdD
    Sep 27 at 14:26
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    As much as it appals me to ask, but have you considered stirring in some instant custard powder? It'll firm up your custard perfectly without really impacting on the flavour or colour.
    – Richard
    Sep 27 at 20:23
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    physics note: custard is mostly water. Water has an incredibly high specific heat (heat input required to raise the temperature.) If your oven is only a little bit too cold, or your custard has just a little too much liquid, the cooking time could go up by a surprisingly large amount. Five extra minutes may not be enough. I once overshot the liquid on a pumpkin pie, in a slightly crappy oven, and the cook time was almosts double the recipe. (This was before I bought a good oven thermometer.)
    – Glenn Willen
    Sep 29 at 19:56

2 Answers 2

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Saffitz's recipe is unusual in having a large number of eggs in the filling but no starch. Most lemon bar recipes have some starch in the filling to help it set, usually flour, from 1/4 to 1 cup.

So if it wasn't setting firm, the first thing I'd try is adding some flour or cornstarch to the filling.

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    ^^^This ... Most of the recipes I've used have had cornstarch in them. I also make sure that the custard is fairly thick before it is poured on the crust, at the very least I'm able to get a perfectly clean 'stripe' on the spoon or spatula when checking for doneness.
    – Sharpenologist
    Sep 27 at 21:25
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The first place to troubleshoot is the doneness of the custard.

"Time in the oven" is a very bad way to judge when to stop baking. It can be wildly off. The only certain way is to measure the internal temperature.

Make it again, using a digital oven thermometer, and take it out at maybe 85 Celsius. The optimal range for custards is 83-87, but you want to avoid overheating it, and there can be a bit of carryover, so it's best to stay in the middle of the range. Also, make sure it's fully set before cutting, so leave it overnight in the fridge.

Only if the above doesn't work, should you try finding a better-working recipe. You can also tweak on your own, but that's usually more work than trying out a few existing recipes until you find a good one. If you do tweak, the most promising direction is to increase the ratio of egg whites to egg yolks. This will change your taste away from typical lemon curd, but will also give you a firmer custard, yolky custards tend to be smeary. So maybe 5 whole eggs and 6 yolks would be a good place to start. If the egg whites aren't enough for a good tweak, I'd start reducing the sugar.

Can I simply add more eggs/egg yolks

At that high amount of eggs, I wouldn't try adding even more. The ratio of yolks to whites can be a good thing to change though, see above.

Will more butter create a stiffer custard when cooled?

No, it will create a softer custard.

Does the acidity of the lemon juice influence the stiffness?

Potentially yes, but it's complicated. You don't need any acidity to get a firm custard with eggs, they work all on their own. In a lemon curd, the acidity does change the texture, but the larger problem is in reducing its curdling properties, else you'll end up with a grainy mess. So you don't want to go that route, it will be very complicated if it works at all.

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    If op is going to change the recipe, adding starch is the way to go instead of fiddling with the yolk ratio
    – Agos
    Sep 28 at 5:52
  • @Agos yes, starch-based fillings exist too. It is a different type, that needs more tweaking, but if that's what the OP wants to go for, it will also work.
    – rumtscho ♦
    Sep 28 at 6:51

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