It is a familiar sight, that when potatoes exposed to light, turn green. The potato is a ‘growing-machine’ - and, with or without the presence of light, it will grow. It is, however, perhaps the only species of tuber that has the ability to form chlorophyll (green pigmentation) when exposed to light, and actually begin the process of photosynthesis! Tubers are specialized stems, and are quick to do, what they do best -grow! In fact, when in storage, they are often very ‘eager’ to get the season going!
In association with this green pigmentation (chlorophyll) on the tuber, is a compound known as solanine (the name derived from the solanaceous family, of which the potato belongs). Solanine is considered to be a neurotoxin. It will definitely make you sick if ingested. Another plant, commonly known as ‘deadly nightshade’ also belongs to this family of plants. It is a slender vine and all parts of the plant are toxic when eaten - especially the green fruit before they ripen to a red colour.
Dr. Ambrose Zitnak, formerly a research chemist in the horticultural sciences department, University of Guelph, could attest to the deleterious effects of solanine, first-hand. He was alerted to the possible adverse effects of solanine, and with both courage and curiosity (all in the name of science), he experimented on himself by ingesting different quantities of the green potato tissue. He lived. But later gave-up on further self-imposed ‘human guinea pig’ experimentation. We are all grateful!
The potatoes that grow at the top of a potato hill can be especially affected by chlorophyll, which develops the green tissue of the tuber. The more light, the greener the potato. Potatoes grown and hilled on sandy soils, are more subject to being exposed to light, especially after a heavy rain and the accompanying erosion of soil. The greening of potatoes protects the potato from herbivory (plant-eating animals) as it is objectional to them. This herbivory ‘distaste’ in other solanaceous crops include peppers, tomatoes, and eggplants, for example.
The green part of any potato should be discarded. It is bitter, and contains solanine (an alkaloid), and when eaten in ‘excess’ can cause severe illness. Symptoms of solanine poisoning include: diarrhea, fever, abdominal pain, and vomiting. These same symptoms will often occur within a few hours, but may ‘wait’ for a full day or two, after eating. A 16-ounce (450 g) fully green potato is enough to make an adult very ill, and even more so - if a small child. While the chlorophyll is harmless, the associated presence of solanine is not. And cooking will not destroy the green potato toxin.
Potatoes are generally stored in the dark to prevent sprouting and greening of the tissue. Supermarkets usually sell potatoes in opaque paper bags to keep the light out, but where there are ‘windows’ in the packaging, greening can sometimes occur, and these purchases should be avoided.
Thankfully, the greening of potatoes can be mostly avoided, however, growers do need to take meticulous care on the sorting line. It is one of the few cases, in this world, where ‘green’ is not good, and no amount of ‘green-washing’ will help!