Natural Preservative
Pure maple syrup contains sugar, which acts as a natural preservative. This ensures that it generally does not go bad. Refrigeration is recommended for long-term storage of one year or longer.
Storage
The only chance that bacteria could damage the syrup occurs if you store it in a dirty refrigerator or other type of cool storage space. Then, mold can appear within the syrup. To get rid of the mold, simply boil the syrup until a scum appears on top, and then scrape it off. Return the syrup to the refrigerator once it cools down.
Time Frame
The only other incident that might lead to syrup becoming moldy is if you store it for several years. Again, a mold could appear within the syrup. Remove it by bringing the syrup to a boil until the mold appears on top in the form of a thick scum. Remove the scum with a knife or a similar utensil.