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Do foamed eggs whites, folded into a batter including some liquid, contribute to the thickening of that liquid?

A class of English 'puddings' are those derived from 'lemon surprise pudding'. In these baked puddings a sponge forms on top and a flavored custard below.

Here's an archetype recipe demonstrating the general approach.

Butter is creamed with sugar, egg yolks beaten in and a liquid (normally dairy), a small amount of flour and then also flavored liquid, e.g. lemon juice, orange juice, puréed fruit etc. Finally, foamed eggs whites are folded in.

In deciding the quantity of eggs required to thicken the liquid, should I consider the egg whites, or do the egg whites solely provide leavening/rising potential? Should I just consider the thickening power of the yolks?

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The egg white foam has an effect on holding the whole mass together. In a normal custard, you have a fine network of bound proteins, with an emulsion of fats in water being trapped in that network. In a custard with foamed egg whites folded in, you get a more complicated network of proteins, in which the egg whites participate too. So you cannot "just consider the thickening power of the yolks".

The egg foam will thicken the whole thing - but neither to the same degree, nor into the same kind of texture as a custard made with whole eggs. So there is no formula, you will have to finetune your recipe empirically.

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  • Thanks for a super informative answer! I'll experiment with, say, a thickening ratio reduced by 50% and go from there. I'm assuming for a sauce rather than a set custard. I'm interested in how the texture will differ, typically because fruit is used it will be high in acidity and sugar too. Sep 13, 2022 at 9:05
  • If you want a sauce, I would ditch the flour. In some custards (as opposed to your auto-layering cake) it acts an extra "insurance" that a filling will set thick enough, but introduces its own difficulties (basically, you have to boil it, which overcooks the egg proteins). And the sugar in fruit isn't enough to offset the extra acid, so the texture will be much less creamy even when made without curdling.
    – rumtscho
    Sep 13, 2022 at 10:11
  • It's not just a sauce - it's a "layer pudding" with sponge on the top, custard below. Regarding the egg temperature - it's cooked in a bain marie. Here's the archetype - "lemon surprise pudding" bbc.co.uk/food/recipes/lemonsurprisepudding_82708 but it can be made with other fruit. Note that I've seen that if making a chocolate equivalent you probably don't need the flour because of the cocoa solids. Sep 13, 2022 at 13:49
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    @DanGravell Oh, I see. I thought you were deriving a sauce recipe. If you keep the sponge layer, then yes, you will need the flour. Good luck, and enjoy your surprise pudding!
    – rumtscho
    Sep 13, 2022 at 13:52

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