Myths About Sprouted Potatoes: Are They Safe To Eat Or Not?

Roughly 75% of people in the United States eat potatoes at least once a week, and it's been voted the country's favorite veggie for eight years in a row. Although potatoes have been a staple food in South America for thousands of years, they didn't really take off in the U.S. until Thomas Jefferson added what were essentially French fries to the White House menu. 

Today, fries are the most popular way to eat the humble spud — yet it is by no means the only one in the U.S. Across the Farm Belt, roasted potatoes are the regional favorite. For folks in New England, mash comes out on top, while in the Great Lakes, it's all about the perfect air fry. With more than 200 types to choose from, there's no wonder the practical, versatile veggie is a culinary fixture. What's even more surprising is: Potatoes can be poisonous. 

The potato plant's leaves contain glycoalkaloid toxins, as do new sprouts and green skin. So what happens when your tubers start to sprout or turn green, and can you eat them? Is there a way to store potatoes to stop both happening? We asked Kasia Duellman, associate professor of plant pathology and seed potato extension specialist at the University of Idaho, and Bryan Quoc Le, food science consultant.

Potatoes come in all shapes and sizes but one thing they all have in common is they don't have a tremendously long shelf life. Whether you buy them individually or by the pound in a plastic bag or a net, if they're left at room temperature and exposed to natural light, chances are (within a couple of weeks), your potatoes will have started to sprout.

That means the tubers, which lie dormant in cooler temperatures, are preparing to grow a new plant, having been triggered to do so by the warmer environment of your kitchen or cupboard. They might look a little strange, with multiple pink or white shoots emerging from the potato.

Depending on how long they have been left, they could be anywhere between a few millimeters and a couple of inches long, but it's a completely natural process, according to Bryan Quoc Le, food science consultant. "Potatoes begin to sprout after spending long periods of time in warm, humid environments, which mimic the conditions of sun-bathed soil, and initiate the growing process," he said.

Leaving your potatoes in natural light can also lead to developing green spots on their skin. That's because it triggers the production of chlorophyll, as well as alpha-solanine and alpha-chaconine in the tuber. While the former can actually be good for humans, thanks to its antioxidants, the same can't be said of both alpha-solanine and alpha-chaconine. Both are glycoalkaloid toxins which, if eaten, can make humans very sick, explained Bryan Quoc Le, food science consultant.

One study revealed that alpha-chaconine levels in potatoes stored for three days at room temperature — 72 degrees Fahrenheit — rose by 62%, described Oregon State University. After 16 days in the same conditions, the increase was 300%, though levels of glycoalkaloids do start to come down when potatoes have sprouted, scientists found. 

If you do come across any potatoes that have patches of green skin, it's wise to exercise caution, advised Kasia Duellman, associate professor of plant pathology and seed potato extension specialist at the University of Idaho. She encouraged home cooks to cut the green part out, adding that the rest of the tuber would be fine to eat.

Who hasn't gone into their cupboard looking for the bag of potatoes bought last week, aiming to turn them into a large plate of fluffy mash or a pile of golden hash browns, only to discover they've all started to sprout? Luckily, all is not lost: They can be cut out and you can still cook and eat the potato. Just make sure to follow a few simple rules.

Before doing anything, take a look at the length of the sprouts and the general state of the potato. If it looks shrunken or withered or feels spongy, toss it straight into the garbage. "Shriveled potatoes with long sprouts can have higher levels of glycoalkaloids that can potentially give you a stomach ache," warned Kasia Duellman, associate professor of plant pathology and seed potato extension specialist at the University of Idaho.

If, on the other hand, the potato feels solid and otherwise looks in good shape, and the sprouts are very small, you can go ahead and cut them out. Be sure to remove every scrap of the sprout, whether you choose to break them off by hand, dig them out with the little loop on your potato peeler, or using the tip of a sharp kitchen knife. While you're doing this, you may notice your potato has green skin, so you can cut that off too. If you have any doubts, throw out the potato.

More and more home cooks are cultivating their green fingers, either by growing their own fruits and veggies or, if they don't eat every scrap of them, turning their food waste into compost. When it comes to planting or composting sprouting potatoes, things can be complicated. Tubers initially bought for eating but that have begun to sprout might not be suitable for planting, according to Kasia Duellman of the University of Idaho.

The associate professor of plant pathology said they would not fall under the strict guidelines required for certified seed potatoes, and "may harbor potato pathogens, such as viruses, that may lead to plants that produce disappointing yields or otherwise grow poorly." Duellman recommended planting only certified seed potatoes to ensure home cooks got a good crop at harvest time.

In contrast, Bryan Quoc Le, food science consultant, had no qualms about planting sprouting potatoes, and said cutting them up would enable each sprout to grow into a new plant. As for composting them, if you don't want your sprouted potatoes to grow inside the composter, chopping them up into small pieces — rather than individual sprouting plants — can ensure they will break down properly.

Potatoes can also be affected by dark spots and blemishes, both inside and out. There are several explanations for them: Some marks on the outside could have been caused by pressure during handling or storage. This is bruising and, just like an apple with a softer spot, could be a warning sign but it isn't always visible when choosing fresh produce. That said, a bruised potato is still fine to eat.

Potatoes can also have little black spots on their skins which are hard to wash off. According to Kasia Duellman, associate professor of plant pathology and seed potato extension specialist at the University of Idaho, the scientific name of this fungus is Rhizoctonia solan, but in her field, it's also known as the more dramatic-sounding "black scurf." While she doesn't recommend eating skin with the black spots, as they might cause an allergic reaction, Duellman also said they were superficial and could be easily removed by peeling the potato.

But it's not just blemishes on the outside that can affect potatoes. The centers of some might be irregularly hollow with a brownish color, but it doesn't mean it's rotten. Known as "hollow heart," it can be caused by some sort of environmental disruption or stress during the growing season. These discolorations can also be triggered if they're stored in conditions that are too cold, causing the starches to break down into sugars, according to Bryan Quoc Le, food science consultant. In both cases, the potato is perfectly edible.

If there's one thing we love about potatoes, it's their culinary flexibility. They can be baked, air, pan and deep-fried, mashed, roasted, steamed, and boiled. They are delicious on their own or as an added ingredient, thickening and enriching soups and stews, and are the shining star in every summer's potato salad. The one way they're not eaten very often is raw — but that doesn't mean it never happens. One Redditor wrote: "Wash the potato well, slice, then salt. I've been eating them that way for 50 years."

Unsurprisingly, the potential danger posed by those pesky glycoalkaloids raised its head on the social media thread and also for Kasia Duellman, associate professor of plant pathology and seed potato extension specialist at the University of Idaho. She said eating a little raw potato that wasn't green was fine, but added: "Peels also have more of the toxins than the flesh, so it may be wise to avoid eating raw potato peels." Bryan Quoc Le, food science consultant, agreed, and pointed out that the uncooked starch in raw potato could cause other unpleasant health issues: "The rigid structure will cause significant digestion upset," he warned.

People in the United States eat around 117 pounds of potatoes per year, of which 50 pounds are frozen and 35 pounds are the fresh variety, according to the Idaho Potato Commission. Although the levels of glycoalkaloid toxins do vary between potato types several incidents of poisoning affecting multiple people have been reported around the world, most notably in the UK and Canada. 

Alarmingly, it doesn't take much to feel the effects of alpha-solanine and alpha-chaconine: Just 2 milligrams per kilogram, according to this assessment by Integrated Laboratory Systems. If you do eat parts of a potato that contain the glycoalkaloids, such as green skin or sprouts, the symptoms can vary depending on how much you consume and your sensitivity to alpha-solanine and alpha-chaconine.

According to food science consultant Bryan Quoc Le, the main symptoms to be aware of are: "Diarrhea, stomach pains, vomiting, headaches, confusion, fever, and flushing of the skin." However, previous case studies have also documented people who have experienced hallucinations, alongside other impacts on their nervous system. 

There are lots of foods that need to be thoroughly cooked to ensure they're safe to eat: Shellfish, eggs, and chicken, to name but three, can lead to vibriosis and salmonella. When it comes to potatoes, unfortunately cooking doesn't eliminate the risk posed by any glycoalkaloids that could be present in the sprouts or green skin. But there are other ways to prevent them from making you ill.

"The peel has more glycoalkaloids than the tuber flesh, so peeling is the first step to reduce risk if there is concern," said Kasia Duellman, associate professor of plant pathology and seed potato extension specialist at the University of Idaho. She also recommended getting rid of any sprouts and "eyes": The buds that grow into longer sprouts, to minimize the levels of alpha-solanine and alpha-chaconine. Once you've done that, you're free to fry, boil, or roast your potatoes as you wish.

For centuries, fairy tales and folklore have warned children and adults about the dangers of deadly nightshade (aka belladonna), but it turns out the stories have their roots firmly in scientific fact. The nightshade species — also known as genus Solanum — takes in around 2,300 different plants, including the humble potato. But it's not the only food in the family.

Other members of the nightshade clan include tomatoes, peppers, eggplants, and tobacco — they all contain some amount of glycoalkaloids. "The actual nightshade plants have so much of these natural chemicals that they are not safe to eat," said Kasia Duellman, associate professor of plant pathology and seed potato extension specialist at the University of Idaho.

Given the popularity of tomatoes, eggplants, and peppers — and their connections to this poisonous plant family — it begs the obvious question: "Why don't we hear of people being poisoned by them?" Bryan Quoc Le, food science consultant has an answer. "The specific type of alkaloid compounds found in these other plants are different from alpha-solanine and do not cause reactions in most people," he said.

Canned foods may have brought convenience to our lives in the 19th century, but millions of us still prefer to buy fresh produce, especially vegetables and fruit. Preventing them from spoiling can be a full-time job, especially fresh fruit, which is why many people ripen theirs at home. There are all sorts of tricks and tips for making sure your peaches, mangoes or tomatoes reach their peak at the right time but would keeping your potatoes next to bananas prevent them sprouting?  

"Sprouting could be triggered by warmer temperatures or if near other veggies and fruits (like onions and apples) that produce a lot of ethylene gas," said University of Idaho's Kasia Duellman. The ethylene hormone prompts fruit and vegetables to naturally ripen, while exposure to ethylene gas can accelerate that process in other produce that is kept alongside them. 

But Bryan Quoc Le, food science consultant, has some reservations. He said, "However, this effect of ethylene is a highly debated topic, and likely requires more research for greater certainty."

To prevent sprouting and greening, the solution lies in how your potatoes are kept. For food science expert Bryan Quoc Le, the answer is simple: Keep them in a cool, dark, and well-ventilated area. No leaving them out on the countertop or on a shelf where they are exposed to lots of warmth and light. But what if you don't have a suitable pantry or cupboard that's both cool and well-ventilated?

Kasia Duellman, associate professor of plant pathology at the University of Idaho, has a suggestion. As far as she's concerned, how you store potatoes depends on how they'll be used. Duellman said companies who manufacture French fries store their potatoes at warmer temperatures, usually around 45 degrees Fahrenheit. This prevents the natural starches turning to sugars which "make French fries turn dark and taste bitter when fried," she said. And it can lead to awful French fries when cooked.

"I like to store mine in the refrigerator (38-40 degrees Fahrenheit) because they last longer, and I don't mind if starches get converted to sugars at that colder temperature," Duellman added. Not everyone agrees with her: Some people believe potatoes are among several items that should never be kept in the fridge, as they become unpleasantly sweet and lack their traditional, earthy flavor. 

The more than 200 types of potato sold in the United States break down into seven main groups: Russet, white, blue and the equally colorful purple varieties, as well as yellow, and red, alongside fingerling and petite. That variety means their behavior can be very different, including how quickly or slowly they sprout and go green.

Some potato varieties store for a longer period of time (without sprouting) compared to others, and it's the same for greening, according to University of Idaho's Kasia Duellman. Scientists at Norwegian food research institute Nofima looked into the problem of greening and how it impacts food waste. They found that the Celandine yellow variety was the best at withstanding light exposure, becoming "unacceptably green" after 60 hours.

Potatoes are harvested from roughly June to October in the United States,and buying spuds in the colder months is the best time to minimize their chances of sprouting, explained to Bryan Quoc Le, food science consultant. However, it's not possible to prevent it entirely.

"Storing potatoes in the dark will prevent greening," said Kasia Duellman, associate professor of plant pathology and seed potato extension specialist at the University of Idaho. "As for preventing sprouting completely, that is not really feasible unless sprout inhibitors like CIPC are used." That said, European researchers found that Verdi, Lady Claire and Kiebitz varieties kept for up to four months without sprouting at around 39 degrees Fahrenheit.

Whenever you buy potatoes, try to keep them in ideal conditions, such as a cool, dark and dry basement but, as Duellman points out, some potato varieties only store well for a shorter period of time. The longer they are kept after being harvested the more likely they are to sprout, so the easiest answer of all is to cook and eat them.