what is difference in using egg yolks vs. whole eggs in lemon tart recipe?
I recently made a lemon tart recipe that is very, very simple - for the filling, 3 eggs, 1 can sweetened condensed milk, 2 tsps. lemon rind, 1/2 cup lemon juice, whisk ingredients together, bake at 325 degrees for 30 minutes. The recipe calls for a vanilla wafer crust but I used a ready made graham cracker crust just to see if I liked the recipe (I didn't want to waste good cookies on a crust if I didn't like the filling). SO, I made the tart and it is very good. However, I find the consistency a little stiff. I've seen other recipes calling for egg yolks rather than whole eggs. Would the filling be more tender if I used only yolks? Is there some other easy way to lighten up the texture of the filling? If you have a better recipe than what I've written, please share, but only if it's fairly simple.
If you didn't over cook it, the reason for it's stiffness could be lack of egg yolks. I might try using two egg yolks and two whole eggs in this recipe and beat the egg whites with the sugar until glossy, then fold it into the batter before pouring into the tart pan. Or, I might try it again as published but separate the eggs and use only the yolks.
It appears to be a sort of lemon curd, minus the butter. The egg yolks supplies some of the yellow color, along with the fat for creaminess. I don't know what the whites would add to this filling, except some leavening and volume, and perhaps the toughness you allude to. Egg whites contain proteins which harden when exposed to heat. Egg yolks retain a creamy texture even when firm-cooked (thanks to the fat).
I'd rec trying it with yolks only and seeing which texture you like the best. Also, don't be afraid to double the zest amount, or use 1/2 tsp lemon oil for more assertive lemon flavor.
Beating the whites with sugar and folding them into the batter would create a chiffon-like batter, which may collapse, but it might be very interesting!
The filling is essentially a custard, and including the whites will indeed make it stiffer. Think of the texture of yolk vs white in a boiled egg, or flan (made with whole eggs) vs creme brulee (yolks only). You should increase the number of yolks (maybe 6?) or the filling will be too runny.