Why Potatoes Have Black Spots (Are They Safe to Eat?)

Potatoes with black spots on them

Is it a bruise or a sign of something more troubling? No matter what the potato tries to tell you, don’t believe it.

When cutting and peeling potatoes for frying, roasting, or stewing, every now and then one comes across a spud or two with black spots on the inside. These spots can take up a small or large portion of the potato. Depending on the size, color, and pattern, they can appear to be… dramatic.

This can’t help but beg the question: What causes these black spots to appear on the inside of potatoes in the first place? And, when peeling a spud reveals black spotting on the inside, is that spud safe for you and the family to eat?

The black spots on potatoes are basically bruises from handling. Potatoes with black spots are edible, but it is best to cut the bruised parts away as they tend to taste bitter.

Compared to other edible plants, potatoes—the starchy tubers of the plant Solanum tuberosum, originally grown in the Americas and brought to the Old World by Columbus—are quite hardy.

Stored in a cool and dark place, such as a closed cabinet or the pantry, most varieties of potatoes will keep for weeks and, as long as you don’t store them past their prime, will get more fragrant and flavorsome with time.

That said, like all edible plants, potatoes do not like being mishandled and bruised. When they are thrown into baskets or stacked on top of each other, the mechanical force of the throw or the pressing weight of the other spuds can damage their cell walls, causing black spots to form in the damaged areas.

These bruises can occur at any stage of harvest, storage, packaging, shipping, and retailing. You may have even contributed to them yourself if you handled the potatoes ungently when you put them in the cart at the store, packed them in the truck of your car, or placed them in your pantry; it is important that you do so with care.

When a potato is bruised, some of its cell walls rupture and release chemical compounds called phenols inside the tuber. The phenols are oxidized by enzymes to form melanin—the same dark-brown pigment that colors our hair and gives us our suntan.

According to Sastry Jayanty of Colorado State University, who explains this process in detail in a fact sheet on the academic institution’s website, it takes about 24 to 48 hours after the bruising for this discoloration to occur.

The discoloration is, as he describes it, “cosmetically undesirable.” Alas, a few chemical compounds get formed as a byproduct of the bruise that can also give the bruised areas of the potato an unpleasantly bitter taste.

Although potatoes with black spots are generally safe for you to eat, we recommend that you discard the bruised areas so that they don’t alter the flavor of your dish in undesirable ways.

Can You Recognize Bruised Potatoes at the Store?

When you fill your cart with fruits and vegetables at the supermarket, it’s easy to spot bruised apples, bananas or tomatoes.

Unfortunately, that’s not the case with potatoes: their thick, dirt-colored skin makes it hard to tell the good ones from the bad. But there are some rules of thumb that you can follow to shop for potatoes without fuss—and we have good tips for you on how to choose only the best ones from the basket.

“Choose firm, unblemished potatoes,” American culinary author Harold McGee writes in his book, [amazon link=”0143122312″ title=”Keys to Good Cooking: A Guide to Making the Best of Foods”], “without bruises or cut marks.” In addition to that, McGee recommends avoiding potatoes with green color, as “green surfaces and sprouts contain bitter and toxic alkaloids.”

If you see a store staffer pouring potatoes into the basket from a height, bear in mind that some of the spuds are likely to get bruised, and black spots will form on their insides within one to two days.

As soon as the potatoes are in your cart, you want to be as gentle with them as possible. Remember: They will reward you with formidable aroma and flavor if you treat them right.

Things to Know About Discoloration on Potatoes

Slice a potato and leave the slices out, and they will sooner rather than later brown on the surface. The same reaction between the phenols, enzymes, and oxygen that causes darkening on the inside is what causes the potatoes to brown on the surface.

Called “enzymatic browning,” this reaction is common to quite a few plants, including apples, avocados, and bananas, to name a few. You can counteract and prevent it by rubbing the surface with anything that contains ascorbic acid, a.k.a. vitamin C, like freshly squeezed lemon juice.

Don’t confuse bruising and enzymatic browning with spoilage. A potato can have dark spots and will brown on the surface when cut in its prime. A spoiled potato is… er, spoiled, and it should be discarded. You will recognize it by the presence of fuzzy mold, mushiness, and/or off odor.

How to Store Potatoes

Potatoes should be stored at room temperature, in a cool and dark place—such as a root cellar, basement, pantry, or inside a closed cabinet—where they will typically keep for a few weeks.

Ideally, the temperature of the storage space should be somewhere from 68°C to 71.6°F (20°C to 22°C), and the space shouldn’t be overly humid or poorly ventilated (which is why cellars, basements, and pantries are better than confided spaces such as cabinets).

Do not store uncooked potatoes in the refrigerator. The cold triggers a series of biological processes and chemical reactions in the plant that lead to the buildup of sugars. This sounds tempting, but in fact leads to darker cooked potatoes that soak up too much oil.

According to the Oregon State University, the potato varieties that last the longest include Elba, Katahdin, Red Chieftain, Yukon Gold, Burbank Russet, German Butterball, Yukon Gem, Rose Finn Apple Fingerling, Russian Banana Fingerling, Red Pontiac, All Blue, and Kennebec.

The food experts at OSU recommend cleaning and brushing the soil off potatoes before storing them, especially if that soil is coarse and sandy. The key to a long shelf life is to ensure that the tubers are completely dry after washing for storage.

Published
Categorized as Food

By Jim Stonos

When Jim isn't in the kitchen, he is usually spending time with family and friends, and working with the HCW editorial team to answer the questions he used to ask himself back when he was learning the ropes of cooking.