At their simplest, thick, sweet jams are made from crushed fresh fruit and sugar. The high sugar content retards the growth of foodborne pathogens, like bacteria and fungus, as long as the jam was properly processed and it remains sealed. Once you open the jam, its shelf life is drastically decreased.
Properly canned jams with vacuum-sealed lids stay fresh for up to one year. After a year, the texture and flavor will decline. Light-colored jams and low-sugar jams may deteriorate faster. Unopened jars should be stored between 50 to 70 degrees Fahrenheit in a dark, dry place.
After opening, store processed jams in the refrigerator at 40 degrees Fahrenheit for a month or more. Examine jars frequently for mold.
Although it was once considered safe to scrape mold off the top of jams and jellies, it is now considered harmful. So, if you see mold in a jar of jam, throw the entire jar out.