I devised this recipe when I wanted a variation of plum jam and thought why not use canned crushed pineapple? The result was beautiful jewel-tone jam with a pleasing fruit combo of flavors and textures. No added commercial pectin here, so if the thought of standing and stirring, oh about a half hour, scares you away, go with the standard recipes on pectin containers instead. But I enjoy cooking jam mixtures without adding pectin because once they reach that "sheet" or "spoon" test stage, there's no guesswork or split second timing - you are guaranteed the consistency is perfect for jam. Yield 6 half-pints, each jar with 16 tablespoon servings of yummy jam.
1 1/4 lb. plums (I used black plums to get the dark rich red color)
1 20-ounce can crushed pineapple undrained (either in own juice or heavy syrup, doesn't matter)
3 cups sugar
Wash and stem plums, don't peel. Cut fruit away from pit. Chop fruit fine or grind. I got about 2 1/2 cups ground from the 1 1/4 pounds of plums using my KitchenAid grinder attachment. If you chop fine you'll likely have a little over 3 cups.
Place prepared plums, undrained pineapple and sugar in large deep kettle over medium high heat (I used a 12-quart stainless steel pot to prevent hot splashes while cooking.)
Stir frequently at first then constantly once the mixture begins to boil to prevent scorching, adjusting heat or flame as needed to maintain a constant boil, until mixture becomes thick, and the color intensifies and becomes jewel-tone. Use the "spoon" or "sheet" test for jelly making to help determine doneness. The National Center for Home Preservation has a great explanation and drawings of this at: http://www.uga.edu/nchfp/how/can_07/jelly_point.html. The cooking and stirring process for a batch this size can easily take 30 or more minutes after it begins to boil - so be patient!
Remove from heat and skim foam off top if any.
Ladle into hot sterilized half pint jars, leaving a 1/4 inch headspace and seal. Any leftover can be refrigerated.
Place jars in boiling water bath and boil 10 minutes (sea level - adjust for altitude as necessary.)
Remove jars and allow to cool on padded surface away from drafts and undisturbed for 12 hours.