- 4 pounds pitted plums cut into quarters
- 1 1/2 pounds granulated sugar
- 1 tablespoon Pomona's Universal Pectin see notes below
- 4 teaspoons calcium water see Pomona's Pectin package for directions)
- 1 teaspoon unsalted butter
- Combine the plums, sugar, and pectin in a large bowl and toss. Cover and refrigerate for at least 8 and up to 24 hours.
- Place a few metal spoons in the freezer. Transfer plum mixture to a large, wide pot and stir in calcium water. Cook over medium heat, stirring occasionally to prevent scorching. Simmer until plums are mostly softened, about 15 minutes. For a smoother jam, place a food mill over a bowl and ladle a few cups of the mixture into it. Mill the mixture into the bowl. Repeat until roughly half the plums have been milled, then stir the milled plums back into the pot.
- Continue to cook until mixture starts to foam. Scrape off and discard foam with a metal spoon. Stir in butter. Continue to cook, stirring occasionally to prevent scorching, until foaming has subsided, about 15 minutes longer. Continue to cook, stirring more frequently, until jam is glossy, about 10 minutes longer.
- To test for doneness, spoon a small amount of jam onto one of the frozen spoons and return to freezer for 5 minutes. Remove from freezer and check consistency. The jam is ready when it's spreadable, but not runny. Cook until this texture is reached.
- Transfer jam into washed and rinsed mason jars. Seal jars and process according to manufacturer instructions. Store or serve.
Pomona's Universal Pectin, uses calcium to activate jelling, precluding the need for tons of sugar.
Recipe from Serious Eats.