There are a few common reasons why lemon pie filling may not thicken properly:
Not enough cornstarch or flour
Cornstarch or flour is essential for thickening the filling. If you didn’t use enough cornstarch or flour in your recipe, the filling will remain too thin. Most recipes call for 2-3 tablespoons of cornstarch or flour per 1 cup of lemon juice. If your filling is too thin, try adding a bit more next time.
Cooking the filling too long
Overcooking the lemon filling can cause the cornstarch or flour to start to break down, preventing it from thickening properly. The filling should only be cooked until it comes to a boil and starts to thicken. Boil for 1 minute then remove from heat.
Not cooking the filling enough
On the flip side, if the filling isn’t cooked long enough, the cornstarch won’t have time to thicken it. Be sure to bring it to a full boil and cook for at least 1 minute while stirring constantly.
Added liquid was too hot
If you add the lemon juice or other liquids to the filling when they are boiling hot, it can cause the cornstarch to overcook and lose its thickening power. Make sure to temper the liquids by stirring a bit into the cornstarch before adding the full amount.
Sugar was added too early
Adding sugar too early in the process can compete with the cornstarch for the available liquid and prevent it from fully thickening. Add the sugar after tempering the cornstarch with a bit of the lemon juice.
Using old cornstarch
Cornstarch can lose its thickening power over time. Make sure yours isn’t past its expiration date or clumping up, which indicates staleness. Use fresh cornstarch for the best results.
Not fully combining ingredients
It’s important to fully incorporate the cornstarch into the liquid ingredients to activate its thickening abilities. Make sure to whisk vigorously to prevent lumps.
Too much lemon juice
Using more lemon juice than the recipe calls for can throw off the ratio and prevent proper thickening. Stick to the amounts in the recipe.
Heat was too low
The filling needs to reach a full boil in order to fully thicken. If the heat is too low, it may not boil properly. Bring to a boil over medium-high heat.
Filling cooked too long after thickening
If the filling is cooked for too long after it has thickened, the cornstarch can start to break down again. Once it reaches the desired consistency, remove from heat.
Curdled filling
If the filling curdles or separates, the cornstarch has likely broken down too much. This can happen if the filling is overcooked, the heat is too high, or too much lemon juice is used.
Incorrect ratios
Using the wrong ratios of liquid to cornstarch/flour can prevent proper thickening. Follow a trusted recipe with the proper ratios.
Conclusion
With lemon pie filling, the thickness comes down to having the right ratios of ingredients and not overcooking the filling past the point of thickening. Let the filling come to a boil, then remove from the heat once it has thickened to avoid breaking down the cornstarch. Use fresh ingredients in the proper amounts called for in the recipe and you will have delicious, thick lemon pie filling.