Potatoes turn black mainly because of oxidation.

What’s happening

• When you cut or peel a potato, you expose its inside to air
• Oxygen reacts with natural compounds in the potato
• This causes a dark gray or black discoloration

Other causes

• Cold damage: storing potatoes too cold can darken them when cooked
• After cooking: cooked potatoes can darken if left out too long
• Metal reaction: contact with iron or aluminum can sometimes cause discoloration

How to prevent it

• Keep cut potatoes submerged in cold water
• Add a little lemon juice or vinegar to the water if storing longer
• Cook soon after cutting