According to the Massachusetts Maple Producers Association, unopened maple syrup will keep indefinitely, but it must be refrigerated once opened. As for mold, the MMPA says, "If any harmless mold should form on the surface, merely bring the syrup to a slight boil, skim the surface, and pour into a clean container and refrigerate." We haven't tried this ourselves, and are reluctant to recommend it unless you know exactly what kind of mold you have and whether it is truly harmless. Do any of our readers have experience with this?
In addition to refrigerating opened maple syrup, the Vermont Maple Sugar Makers' Association recommends storing unopened maple syrup in a cool place. For long term storage, the Association says that maple syrup retains its flavor best when kept in the freezer (it will not freeze solid).
StillTasty.com indicates that 100 percent pure maple syrup should keep for a year unopened in the pantry, a year opened in the refrigerator, and indefinitely in the freezer.
(Image: Matt Pettengill)