What is the impact of repeated cooling and reheating of maple syrup?

The Cornell Sugar Maple research program website has a couple of relevant points:

If you find mold inside a newly-bought, previously unopened bottle of syrup, it is probably spoiled because of improper packaging.

What causes syrup to have a musty or moldy flavor or smell? Syrup that is improperly packed will mold, sour, or ferment. Syrup must be packed at 180°F and at a minimum of 66 Brix to avoid spoilage.

(Brix is the measure of sugar in the syrup.)

In your particular case where you see mold and want to reheat it, it seems that yes, you can reheat the syrup but it may crystallize a little:

If a consumer finds bacteria, mold, or yeast growth on syrup he or she has purchased, he or she should remove the visible growth and reheat the syrup to a minimum of 180°F (do not boil), skim any visible growth, filter, and repackage the syrup. If syrup still has an off-flavor, it should be discarded. Also, the sugar content may increase causing sugar crystals to form.

Flimzy,

If you use a lot of maple syrup, you won't need to refrigerate it; it'll keep at room temperature (depending on the temperature of your room) for a few weeks.

Repeated heating and cooling, in my long experience with pure maple syrup at home, does not affect the flavor or color of the syrup. This makes sense when you realize that maple syrup is boiled for hours in its manufacture.

However, it can cause the syrup to crystallize, and I haven't found a good easy way to decrystallize syrup (the hard way is dissolving it in water and boiling it down again). For this reason, I only heat up the amount of syrup I intend to use at a time.

I am not a food safety expert; I'm just speaking from my experience at home.

Going off of what Laura said, no, heating the maple syrup multiple times won't cause any change in the structure of the syrup, but do refrigerate the syrup after your done if it says so on the bottle.

I produce maple syrup from the sugar maples on my property. I make about 8 gallons a year, so not a huge commercial endeavor. This past year, several quart jars of our syrup grew a powdery grey mold. I processed the syrup until it reached the 219F and 67% sugar, as measured with a hydrometer. Many jars grew sugar crystals, indicating high enough sugar content. I cleaned and sanitized the jars and lids with care, as we always do. When canning - the syrup reached 180F and was then put in the sterile jars, and lids applied. Today, I reheated the syrup to the boiling point to be absolutely sure it is sterile - and also sterilized the jars in a pot of boiling water. I'm sure to grow sugar crystals again, but I'm just hoping the 3 gallons of syrup I recanned still taste alright.