How to Thicken a Lemon Filling in Baking


Lemon filling for cakes, pies and pastries has a reputation for being difficult to make. Essentially a custard, lemon filling or curd gets thickened with eggs, cornstarch and sometimes butter. If yours seems too runny, the problem is likely caused by one of three things: not enough cornstarch, inadequate cooking time or cooking the filling for too long. In most cases, you can fix the problem simply by adding more cornstarch and cooking the filling again. Just watch it carefully so it doesn't scorch.

Things You'll Need

  • Cornstarch
  • Covered containers or plastic wrap
  • Put equal amounts of cornstarch and cold water in a bowl and stir it into a smooth slurry. Drizzle the mixture into the custard as you reheat it to simmering over medium heat on the stove. Simmer it for one minute, stirring constantly.

  • Pour the custard into a bowl to cool for 20 minutes. Assess the thickness; add more cornstarch and recook the filling as needed, pulling it off the heat after one minute at a simmer.

  • Refrigerate leftovers within two hours of cooking the custard. Cover the pie or custard with plastic wrap or place it in a covered container. Refrigerate it at 40 degrees and use the custard within three or four days.

Tips & Warnings

  • Cover the cooling custard with plastic wrap or a lid to prevent a skin from developing on the surface.
  • Raw eggs must be heated to at least 160 degrees Fahrenheit to destroy bacteria, such as the salmonella bacteria, that can cause food poisoning. A simmer raises the temperature of the filling adequately to make it safe to eat.
  • Cornstarch must be heated to simmering to thicken, but if you cook it for more than a minute or two, the starches break down and the filling becomes loose again.

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