Root vegetables are a comfort-food staple, lending substance and sweet, earthy flavors to meals. Likeable sweet potatoes grace a plate with vivid yellow and orange as well as their fine, sweet flavor. Although sweet potatoes are sturdy vegetables that store well, they'll occasionally show black spots before or after cooking. Neither affect the vegetable's edibility.
Mold Spots and Physical Damage
Occasionally, you'll find black spots on the outside of the sweet potato before it's cooked, especially if you've had it in storage for a while. Sometimes it's because the potato was damaged in harvesting or storage, allowing mold spores or bacteria to enter. These typically create dark or black spots, visible from the outside, which can simply be cut away before cooking. If you miss these damaged areas before cooking, cut them out as soon as the sweet potatoes are cool enough to handle. The mold can give an unpleasant flavor to the rest of the potato -- especially if boiled -- but if you're lucky, cutting out the spot and a generous area around it leaves the rest unaffected and ready to eat.
External damage is relatively easy to see and understand, but sweet potatoes are also prone to another, more mysterious form of blemish. Sometimes a sweet potato that appears perfectly fine before you cook it will come out of the pot or oven with large blackened areas inside. The combination of black and orange is fine when you're making Halloween ornaments, but it's rather disconcerting on your plate. Although it affects the appearance of your sweet potatoes, this is a natural phenomenon that doesn't make them unsafe to eat.
What's Really Happening
Sweet potatoes contain a large number of phenolic compounds, complex organic molecules that provide much of the vegetable's color, aroma and nutritive value. One of those phenols, called chlorogenic acid, is the culprit that makes sweet potatoes -- and sometimes, regular potatoes -- darken when they're cooked. The chlorogenic acid combines with iron ions in the potato and oxygen absorbed from the atmosphere and cooking water, creating the dark pigment. It looks rather ugly but has no effect on edibility.
Improve Your Chances
If sweet potatoes play a starring role in your plans for an upcoming meal, you can call on chemistry to help you minimize the risk of discoloration. If other acids are present, they'll compete with the chlorogenic acid and bind up some of the iron and oxygen in unpigmented combinations. Wait until your sweet potatoes are half cooked and beginning to soften, then add a splash of lemon juice or a spoonful of cream of tartar to the cooking water. If you add the acid ingredient before your sweet potatoes start to soften, it will strengthen bonds in their cell walls, and give them an unpleasantly firm texture.