I need some help in selecting sweet potatoes at the store. This Christmas dinner, I tried a recipe that called for sliced garnet sweet potatoes, microwaved until soft and then baked with craisins and apricots and other good stuff. Some of the sweet potatoes 'waved nicely: becoming soft and a nice golden orange color. Other sweet potatoes were tougher, more fibrous and had a brown ring about a half inch in from the exterior. They also had brown spots that extended far into the potato.

My only guess was that some of the potatoes were old. I made an emergency trip back to the market to get some replacements, carefully picking those that looked young and unbruised. After taking these home and 'waving them, I had the same result: some perfect, mixed with some discolored. Any idea how come?