Step 1. Prepare the plums.
Begin by pitting the plums, chopping them, and coating them with sugar. For every pound of medium-sized chopped plums, you’ll need about ½ cup of granulated sugar and 1 teaspoon of lemon juice. The lemon juice enhances flavor but is optional. If your plums are fully ripe and sweet, you may want to reduce the sugar slightly; if they’re firmer and less ripe, you’ll need the full amount.
Step 2. Let them rest (optional).
If you’d like to make the jam later, place the plum and sugar mixture in a sealed container and allow it to soften for up to 24 hours (refrigerate if longer than one day, up to two days). If you plan to cook the jam the same day, you can pulse the plums in a food processor first to create a smoother texture, then mix in the sugar.
Step 3. Cook the jam.
While you could simmer the plum mixture on the stovetop, it requires constant attention to prevent scorching. Instead, I prefer the oven method: transfer the mixture into a casserole dish and bake at 325°F for about 2 hours, stirring occasionally. Continue baking until the jam has thickened and most of the liquid has evaporated. For a thicker consistency, simply extend the baking time.
Step 4. Fill the jars.
Sterilize your jars and lids while the jam cooks. Once the jam is ready and still hot, pour it into the hot jars, leaving about ¼ inch of headspace. Wipe the rims clean, place on the sterilized lids, and screw the rings on tightly.
The National Center for Home Food Preservation recommends water-bath processing for jams. Their current canning guidelines can be found here.
Personal Note:
For many years, I’ve followed a more traditional method that doesn’t include a water bath. After filling the jars with boiling-hot jam and sealing them, I place the jars on the counter, cover them with a blanket to retain heat, and leave them undisturbed for 12–24 hours. Most jars will seal during this time (you’ll hear the “pop” as they do). Any that don’t seal are refrigerated and used right away.
According to the National Center, “the high sugar content of fruit jams, jellies, and preserves adds an extra measure of safety and barriers to even spoilage.” While this method has worked well in my kitchen, I recommend using your own judgment about which process best fits your needs and comfort level.
Cook’s Note
The oven-baked method makes jam-making far less stressful than standing over the stovetop. It reduces the risk of scorching, gives you time to step away, and results in a jam with deep, caramelized flavor. Plus, this recipe uses no added pectin — the natural fruit and sugar do all the work.
Curious about why some traditions skip the water bath altogether? Read our blog post: Canning Jam Without A Water Bath: The French Method.
