safe or not?
I consulted with another Faculty, Nellie Oehler, to assist me with your questions. Thanks for attaching the photo! We don’t think that this is mold because it is too uniform in shape and doesn't appear to be floating above the fruit, either. How much sugar did you use in the syrup? Since the jar siphoned this may be a fine line of sugar crystals that could come from the syrup or even from ripe plums. Minerals from the water are another possibility. With well water the high acid of the plums could precipitate the minerals from the water. Were the plums pitted or not pitted? The pits might also cause this if they were left in the plums. Did you process your plums in a water bath canner? Did you use the recommended processing time listed in one of the free OSU Extension publications listed on our web page http://extension.oregonstate.edu/community/food-preservation? The current Ball Blue Book would be a good resource, too. If you processed it correctly and it has a good vacuum it is probably safe. If you open the jars and it doesn’t have a good vacuum and there is an off odor then I would not recommend eating them. Plums are very high in acid about 2.8-3.2 (unless they are really overripe) so this probably is not a serious safety issue.