I've been eating sweet potatoes for years but have a relatively new problem with them. I usually peel them, cut them into large chunks and pop them in the microwave for 4-7 minutes depending on the size. I stick them in the fridge, bring them to the gym and eat them after a workout with some protein. Lately, though, after I nuke them, they have multiple black spots on the sides of the chunks where the skin was. It doesn't matter if I let them cool down in the microwave before sticking them in a bag, if I let them cool out on the counter, if I stick them in the fridge on the same plate I nuked them in, or if I just stick them directly into a plastic bag when they're still hot/warm (not a good idea, I know).
This used to never happen. What's going on? Are they still edible? Sometimes the spots look like they're just on the surface, some look like they're sinking into the flesh. It looks kind of gross and makes me not want to eat them, but I love them!
Likely just oxidation from the air - nothing to worry about. You can test this by placing them into water before any brown spots form - as long as they're in the water, no spots should form.
I'm assuming from the way you phrased things that the spots aren't apparent until well after removal from the microwave, in which case caramelization would be ruled out.