Canned plums yesterday, they were ripe and sweet. When I tried a little this morning they were bitter. What can I do?

Did any of the plums or the sauce they are in, happened to burn, singe or caramelize? This is one of the leading reasons for canned foods to turn a batch bitter.

I'd also wait a few weeks before making any decisions on these. It often takes a while for flavors in preserves to settle in. If they still taste too sour, consider blitzing them in the food processor to use for savory glazing / basting sauces, great for grilling or serving with meat; or adding to a chutney with bold flavors such as fresh ginger, garlic and Indian spices; or blitz, cook the pureed plums down and make ketchup. You get the idea. I'm not sure how many jars you have on hand, but if this happened to me, that's what I'd do. Plums are so versatile. ;o)

Can you give us the recipe you followed, Sammy, and tell us what variety of plum you used? If anything, the plums should have been even sweeter due to the sugar in the recipe, so I'm mystified without more information.