The purple color could be because of a potato disease caused by a fungus, specially if you notice that the purple area covers almost the entire potato. When the cause is a fungus, it usually happens at harvest or during the storage process.
Is it safe to eat potatoes with purple?
All potatoes, including purple potatoes, are quite nutritious and provide a range of nutrients in both their skin and flesh. They're especially rich in minerals and boast more potassium than a banana.
Why are my potatoes turning purple?
The effect you noticed is called "after-cooking darkening," and it can happen with any kind of cooked potatoes in any kind of pan. Typically, it happens if you boil (or fry) potatoes and then store them—they will sometimes turn black or purple over time if they are exposed to air.
Why is my potato purple under the skin?
The purple tones are probably just bruises from the harvesting or when they where being packaged, etc. Before cooking your Russet potatoes just cut the spots off. But remember that if there is a green color around the potatoes then they shouldn't be cooked at all.
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