Lemon Curd Filling

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The other day I copied from Internet this Lemon Curd Filling recipe

 1 1/2 Cups Granulated Sugar
1/4 Cup cornstarch
1/4 TSP Salt
1 1/2 Cups Cold Water
6 Egg Yolks, slightly beaten
2 TSP Finely Grated Lemon Peel (optional)
1/2 Cup Juice from 2-3 Lemons
3 TBS Unsalted Butter

 We like this recipe but the next day the top of the filling became like a rubber

 Anyone knows why and what I have to do to avoid this problem?

 

The recipe seems to have way too much cornstarch. Classic lemon curd doesn't have any starchy thickeners. My  go to lemon curd recipe from the Australian Commonsense Cookery Book has the following ratios:

  • 1 lemon--juice and rind
  • 1 egg yolk
  • 1/4 c sugar
  • 1 oz butter

I think the surface of the curd was exposed to air. The top surface becomes leathery. This happens with vanilla pudding or creme patisserie. Cover the curd with plastic wrap. The wrap must touch the top surface of the curd to prevent air from getting in. I also heard that brushing melted butter on top also helps to prevent forming a skin, but I haven't tried that, so I don't know if it works.