When you reach into a sack of potatoes only to find they’ve started turning green, you’re faced with the conundrum of whether to throw them away or not.
Some cut their losses and toss the green potatoes, while others remove the green spots and use them anyway.
However, green potatoes are more than just undesirable. They can also be dangerous.
In fact, the green color and bitter taste that potatoes occasionally develop can indicate the presence of a toxin.
Some people wonder whether eating green potatoes can make you sick, or if peeling or boiling them will make them safe to eat.
This article covers everything you need to know about green potatoes and whether they pose a risk to your health.
The greening of potatoes is a natural process.
When potatoes are exposed to light, they begin to produce chlorophyll, the green pigment that gives many plants and algae their color ().
This causes light-skinned potatoes to change from yellow or light brown to green. This process also occurs in darker-skinned potatoes, though the dark pigments may disguise it.
You can tell if a dark-colored potato is greening by scratching off part of the skin and checking for any green patches underneath ().
Chlorophyll also allows plants to harvest energy from the sun via photosynthesis. Through this process, plants are able to produce carbs and oxygen from sunlight, water and carbon dioxide.
The chlorophyll that gives some potatoes their green color is completely harmless. In fact, it’s present in many of the plant foods you eat every day.
Nevertheless, greening in potatoes can also signal the production of something less desirable and potentially harmful — a toxic plant compound called solanine ().
Summary: When potatoes are exposed to light, they produce chlorophyll, a pigment that turns potatoes green. Chlorophyll itself is completely harmless, but it can signal the presence of a toxin.
When exposure to light causes potatoes to produce chlorophyll, it can also encourage the production of certain compounds that protect against damage from insects, bacteria, fungi or hungry animals (, , ).
Unfortunately, these compounds can be toxic to humans.
Solanine, the main toxin that potatoes produce, works by inhibiting an enzyme involved in breaking down certain neurotransmitters (, ).
It also acts by damaging cell membranes and can negatively affect your intestine’s permeability.
Solanine is normally present in low levels in the skin and flesh of potatoes, as well as in higher levels in parts of the potato plant. Yet, when exposed to sunlight or damaged, potatoes produce more of it.
Chlorophyll is a good indicator of the presence of high levels of solanine in a potato, but it isn’t a perfect measure. Although the same conditions encourage the production of both solanine and chlorophyll, they are produced independently of each other ().
In fact, depending on variety, one potato may turn green very quickly, yet contain moderate levels of solanine. Anther may green slowly, yet contain high levels of the toxin ().
Nevertheless, greening is a sign that a potato may be starting to produce more solanine.
Summary: When exposed to light, potatoes produce a toxin called solanine. It protects them from insects and bacteria, but it’s toxic to humans. Greening in potatoes is a good indicator of solanine.
It’s difficult to say exactly how much solanine will make you feel sick, as it would be unethical to test this in humans. It also depends on a person’s individual tolerance and body size.
However, case reports of solanine poisoning and one toxicology study in humans can provide a good idea.
It seems that ingesting 0.9 mg/lb (2 mg/kg) of body weight is enough to cause symptoms, although 0.6 mg/lb (1.25 mg/kg) could be enough to make some people ill ().
That means that eating a 16-ounce (450 g) potato that has surpassed the acceptable level of 20 mg solanine per 3.5 ounces (100 g) would be enough to make a 110-pound (50-kg) person sick.
Yet, if a potato has developed very high solanine levels or if the person is smaller or a child, consuming even less might be enough to make them ill.
The hallmarks of solanine poisoning are nausea, vomiting, diarrhea, sweating, headaches and stomach pain. Relatively mild symptoms such as these should resolve in about 24 hours (, , ).
In extreme cases, severe effects, such as paralysis, convulsions, breathing problems, coma and even death, have been reported (, ).
Summary: Potatoes that contain very high levels of solanine can cause nausea, vomiting and headaches. In extreme cases, paralysis, coma or even death may result.
Solanine levels are highest in the skin of a potato. For this reason, peeling a green potato will help significantly reduce its levels.
Studies have estimated that peeling a potato at home removes at least 30% of its toxic plant compounds. However, that still leaves up to 70% of the compounds in the flesh ().
This means that in potatoes with very high solanine concentrations, the peeled potato might still contain enough to make you sick.
Unfortunately, boiling and other cooking methods, including baking, microwaving or frying, do not significantly reduce solanine levels. Thus, they won’t make green potatoes any safer to eat ().
If a potato has just a few small green spots, you can cut them out or peel the potato. Because solanine is also produced in higher concentrations around the eyes, or sprouts, of a potato, they should be removed as well.
However, if the potato is very green or tastes bitter (a sign of solanine), it’s best to throw it away ().
Summary: Peeling a green potato significantly reduces its solanine levels, but cooking does not. It’s best to throw away potatoes when they turn green.
Fortunately, reports of solanine poisoning are rare. However, it may be underreported because of the generic nature of its symptoms.
Potatoes that contain unacceptable levels of solanine usually do not make it to the grocery store.
Nevertheless, if not handled properly, potatoes can produce solanine after they have been delivered to a supermarket or while being stored in your kitchen.
Therefore, proper potato storage is important for preventing higher levels of solanine from developing.
Physical damage, exposure to light and high or low temperatures are the main factors that stimulate potatoes to produce solanine ().
Be sure to inspect potatoes before purchasing them to make sure they have not been damaged or already started greening.
At home, store them in a cool, dark place, such as a root cellar or basement. They should be kept in an opaque sack or plastic bag to shield them from light.
Storing them in the refrigerator isn’t ideal, as it’s too cold for potato storage. Some studies have even shown increased solanine levels due to storage at refrigerator temperatures ().
What’s more, the average kitchen or pantry is too warm for long-term storage.
If you don’t have a cool enough place to store your potatoes, only purchase the amount you plan to use. Store them in an opaque bag in the back of a cabinet or drawer, where they will be best protected from light and warmth.
Summary: Potatoes containing high amounts of solanine will usually not make it to the grocery store. Still, it’s important to store potatoes properly to prevent them from turning green after you buy them.
Green potatoes should be taken seriously.
Although the green color itself is not harmful, it may indicate the presence of a toxin called solanine.
Peeling green potatoes can help reduce solanine levels, but once a potato has turned green, it’s best to throw it away.
Inspect potatoes for greening and damage before buying them and store them in a cool, dark place to prevent them from going green before you use them.