I roasted golden and red beets, separately, the night before a dinner party. They were very fresh, since they came from an organic co-op delivery. Peeled, sliced, and put them in separate Tupperwares in the fridge. The next morning, the yellow ones were this disgusting black, radiating from the center. Only the beets at the top of the Tupperware were spared, and just barely. The red ones were fine, though they're so dark, I guess I wouldn't notice if they'd turned black. Does anyone know what happened and how I can avoid this in the future?