Prepare the jars ahead of time by washing them in the dishwasher or by hand in hot soapy water, rinsing well. Set the jars aside on a clean tea towel and cover them with another clean tea towel until ready to sterilize them.
Wash the plums thoroughly, cut them in half and remove the pit. Place in a food processor. Pulse for about 10 seconds, long enough for the plums to break down and forms a thick liquid. Refer to my photos. Pour the plum mixture into the Dutch oven or sauce pot. On medium heat, pour the plum mixture into a Dutch oven or large sauce pot and once it starts to boil, reduce heat to a simmer for 2½-3 hours. Stir occasionally to avoid the jam from sticking to the bottom of the pan. Add the sugar during the last 5-10 minutes of cooking time, enough time for the sugar to melt. Test the jam and depending on the ripeness of the plums, you may need to add a little more sugar.
Wash jars in a dishwasher or by hand in hot soapy water and rinse well to remove any soap. Place them upside down on a clean towel until ready for sterilizing.
To sterile jars, place clean jars in a 250°-275°F (120°-130°C) oven for 20 minutes. Sterile lids the same way, either in the oven or in a sauce pot with boiling water, leaving them in for at least 5 minutes. Then transfer them to the oven to dry, before adding to the jars. Make sure the lids are dry before adding to the jars to avoid them from rusting.
Once the jars and lids are ready, add the jam to the jars, and wipe the rim with a paper towel. Place the hot lids on top and tightly seal each jar. NO bath required. Once you hear them pop, they have sealed.
Store in a cool dark place, usually in a cold cellar or basement for up to 12 months.
