Blackberry Sweet Tart Filling

Makes about 2 quarts servings Prep: 10 m Cook: 35 m Total: 45 m Beginner
5.0/5 (1)
Blackberry Sweet Tart Filling
Sweet and thick blackberry filling perfect for tarts, cobblers, or pastries. This Mawmaw recipe uses cornstarch to create a jammy, spoonable consistency that holds its shape beautifully in baked desserts. Works with fresh or frozen berries.

Ingredients

Makes about 2 quarts servings
  • 1 gallon fresh or frozen blackberries
  • 2 1/2 lb granulated sugar
  • 1/2 glass of water (ONLY if using fresh berries)
  • 6 tbsp cornstarch

Step-by-Step Instructions

  1. In a large cooking pot, combine the berries and the sugar. Since frozen berries have a fair amount of liquid, don’t add any water. However, if you are using fresh berries, add half a glass of water.
  2. Cook over medium heat, stirring occasionally until mixture is somewhat thick, about 25-30 minutes.
  3. Mix the cornstarch with just enough water to make a soft paste that can be poured.
  4. Turn your burner down low and very slowly pour the cornstarch into the blackberry mixture, continue stirring for a minute or two.
  5. The mixture will get real thick. Turn the burner off but keep stirring for a few minutes longer. The mixture will look like a thick jelly.
  6. Use to fill tarts or make a fresh cobbler. Can be stored in the refrigerator for up to 2 weeks or frozen for up to 6 months.

Common Problems and Solutions

Q: Why is my filling too runny?

A: Make sure to cook the berry mixture until it's somewhat thick before adding the cornstarch. The cornstarch paste needs to be stirred constantly as you add it, and keep stirring even after removing from heat to let it fully thicken.

Q: Can I reduce the sugar?

A: You can reduce it slightly, but sugar helps preserve the filling and balances the tartness of blackberries. Try reducing by no more than 1/2 pound if you prefer less sweet.

Q: Why did my filling get lumpy?

A: The cornstarch wasn't mixed into a smooth paste before adding, or it was added too quickly. Always make a pourable paste with the cornstarch and water, then add it slowly while stirring constantly.

Tips and Techniques

Stir continuously after adding the cornstarch even after turning off the heat—the residual warmth will continue to thicken the mixture. The filling will thicken further as it cools, so don’t worry if it seems a bit loose right off the stove.

Ingredient Substitutions

  • blackberries: raspberries, blueberries, or mixed berries
  • cornstarch: 4 tbsp tapioca starch or arrowroot powder

Equipment Needed

  • Large heavy-bottomed pot (at least 6 quarts to prevent boil-over)
  • Long-handled wooden spoon for stirring
  • Small bowl for mixing cornstarch paste

Historical Context

Sweet tart fillings like this have been a Louisiana tradition for generations, especially when blackberries are in season. Mawmaws across the state would make large batches to preserve the harvest for year-round desserts.