Equal parts pitted beach plums and sugar. Our batch- 28 cups stemmed beach plums=14 cups of cooked and pitted beach plums=10 1/2 pints jam. I cooked the above in 2 batches because I did not have a large enough pot to do the whole thing at once.
14 cups washed and stemmed beach plums (pink to purple to take advantage of the natural pectin in the unripe ones)
1/2 cup water
7-8 cups sugar (sweetness will vary with ripeness of the beach plums)
5-6 pint jars (or 11 half pint jars)
Place a few small plates/bowls in the freezer to use to check that the jam is set.
Prepare jars (not lids and rings) by boiling in water for 10 minutes to sterilize and let sit covered in warm water bath until ready to fill. If you have hard water like I do add a tablespoon of white vinegar to the water to keep the glass clear. Bring a medium pot of water to boil, remove from heat and place lids and rings in pot, cover and set aside until ready to use.
Wash and de-stem plums. Place prepared plums and water in a large heavy bottomed pot and heat to a simmer over medium heat, stirring occasionally. Cook for 10 minutes until the plums are easily crushed using the back of a wooden spoon against the side of the pot. Place a large colander over an equally wide bowl, pour the cooked plums and juices into the colander, spread out the cooked plums to cool enough to handle. Set up a system where there is a discard/pit bowl and a reserve/pulp bowl near the colander. Remove the pits (discard) from the pulp (reserve). Measure the amount of drained plum juices and pulp then return both to the pot. Add an equal amount of sugar and stir. Slowly, with lid on over medium high heat, bring to a boil- stirring occasionally. Reduce heat if needed to maintain a slow rolling boil. Taste for desired sweetness and add more sugar if needed/desired, stir and return to a slow rolling boil. Cook for 10 minutes, stirring occasionally. Check that the jam is “set” (place a teaspoonful of not jam on cold plate, place briefly in freezer, tip the plate, if the jam doesn’t run and “behaves like jam” it is set), cook and check to see that the jam is set every 5 minutes. Once set, pour into the hot prepared jars filling to 1/2 inch below the rim, wipe the rim and edges with a damp paper towel, place warm lids in place and secure with rims. Place filled jars in water bath, bring to a boil and boil for 10 minutes. Turn heat off, leave jars in the hot water for 24 hours. Take jars out, wipe dry and label. Ready to serve when you are ready to eat.