When you’re buying potatoes, avoid the green ones. Sometimes you’ll see patches of green on potatoes or the whole potato may have a greenish tinge. Exposure to light, stress, or the process of aging can bring about a transformation of a potato's amyloplasts to chloroplasts, followed by the synthesis of chlorophyll. Light, stress, and aging cause the potato to produce chaconine and solanine. The appearance of chlorophyll is a warning that there is something wrong with the potato. Solanine is a natural insecticide which is poisonous in large quantities and gives the potato a bitter taste. If there are small patches of green on your potatoes, cut them out and throw them away!