Questions are often asked concerning buying and eating potatoes that have a green skin (Figure 1) and may be green inside (Figure 2). Potato tubers, like vines, turn green when exposed to light. There are two facets to the question of green potatoes. One is the market appearance of potatoes and the other is health concerns dealing with eating a green potato.
These are two separate though related issues. Marketing appearance problems are associated directly with greenness which is due to chlorophyll biosynthesis. Health concerns are due to a parallel biosynthesis of a glycoalkaloid called solanine.
First, however, what causes greening in potato?
Exposure of potato tubers to light either in the field, in storage, on the store shelf, or at home, will induce the formation of a green pigmentation on the surface of the potato. This is called "greening" and indicates the formation of chlorophyll. This pigment is completely safe and is found in all plants, lettuce, spinach etc. It is primarily found in leaves and is responsible for a plant's ability to make food, photosynthesis. The US Standards consider greening of 5% of a lot of tubers as 'damaging' and the lot will be graded down. Therefore, green potatoes are graded out before reaching the retail market.
Greening is strongly affected by three light factors: quality, duration, and intensity. Chlorophyll is green because it reflects green light while absorbing red-yellow and blue light. Chlorophyll formation is most efficient under red-yellow light. Under green light, there is practically no potato greening and there is little under blue or ultra-violet lights. Day-light fluorescent lights are quite capable of inducing greening, more so than incandescent light. As a rule, fluorescent light above 75 foot-candles exposure at room temperature, 68F, for three to five days will start the greening process. Light intensity may be as low as 5 foot-candles and light durations as short as 12 hours can cause greening of a few potato varieties such as Kennebec.
A key fourth factor is the temperature during light exposure. This is important because greening is an enzymatic response and enzyme activity is increased with increasing temperature. There is NO greening when the temperature is less than 40F, refrigeration temperature, and is most rapid at 68F, room temperature. The difference in greening at 50 versus 68F is how long it takes to fully green.
Of itself, chlorophyll is not a health concern, it is harmless and tasteless. BUT, in potato tubers, it is like the "canary in the mine shaft." The green indicates an increase in the presence of glycoalkaloids, especially, in potato, the substance "solanine" (see structure). When the potato greens, solanine increases to potentially dangerous levels. Increased solanine levels are responsible for the bitter taste in potatoes after being cooked. Solanine biosynthesis occurs parallel but independent of chlorophyll biosynthesis; each can occur without the other. Unlike chlorophyll, light is not needed for solanine formation but is substantially promoted by it. The formation of solanine in potato is localized to the skin, usually no deeper than one-eighth inch (3 mm). In processed potatoes such as chips and fries, there is little hazard since peels are removed. It also needs to be strongly emphasized that potato breeding programs have resulted in the commercial release of only potato lines with very low levels of solanine.
[It also needs to be noted that all members of the botanical family Solanaceae produce glycoalkaloid toxins, not just potato. Two common examples are tomatine from tomato and nicotine from tobacco. Some members of this family are historically notorious such as belladonna and deadly nightshade.]
Light contains ultra-violet radiation as well as visible rays. Ultra-violet and visible light in the blue-violet region promotes the formation of glycoalkaloids, steriod-like compounds, and, for potatoes, most notably "solanine" in tubers. When tubers are exposed, the solanin content in the peel may increase as much as ten times. Toxic levels for people are about one-hundredth of an ounce for 200-pound person. This 200-lb person would need to eat about 20 lb of normal potatoes in a day to reach this level. But, with UV light-exposed tubers in which solanin had increased ten-fold, only 2 lb could be dangerous. A large baked potato frequently weighs close to a pound but common sizes are six to 11 ounces in restaurants. Potatoes containing more than 0.1% solanine (.01 oz/10 oz potato) are considered unfit for eating. Cooked potatoes cannot turn green nor produce solanine because the enzyme mechanism for their production is destroyed by heating at cooking temperatures. But, note the chlorophyll and solanine already produced before cooking will remain after cooking.