plum jam is one of my favorites…

"The skin of the damson is heavily acidic, rendering the fruit unpalatable to some for eating out of hand (for which the "President Plum" variety is better suited). Because of this acidic, tart flavor, damsons are commercially grown for preparation in jellies and jams." (source: Wikipedia)
Well it must be true because the first ingredient is sugar, yet the jam isn't very sweet.I had no clue....
delicious on hot buttered toast