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Why Are Sweet Potatoes… Sweet?

The great mystery of sweet potatoes resolved. Find out what makes this potato variety sweet—and how to make the most of it.

Sweet potatoes: they’re so sweet, just looking at their name is enough to give you a toothache. But have you ever wondered why that is? What is it that gives this variety of potatoes its malty sweetness?

As curious as anyone else about this question, I scoured the Internet—and our editorial team’s vast library of cookbooks—for information. Well, I’m happy to report I’ve found the answer, and, if you take a gander below, so will you.

Sweet potatoes have a special enzyme called amylase. When they are cooked, the amylase breaks down the starches that they contain and turns them into sweet, syrupy malt sugar.

Malt sugar, or maltose, is a simple sugar formed from two molecules of glucose. Although it has a sweet taste, malt sugar is only ⅓ to ⅔ as sweet as table sugar, depending on the concentration.

The moister the potato, the sweeter it is, American culinary author Harold McGee writes in On Food and Cooking: The Science and Lore of the Kitchen. Moist or soggy sweet potato varieties, McGee says, convert as much as 75% of their starches to maltose.

Buying, Storing, and Cooking Sweet Potatoes

When selecting sweet potatoes at the grocery store, choose ones that are firm, full-bodied, and feel heavy in the hand. The skins should be intact, not bruised, and the potatoes themselves should be free from spouts.

Store sweet potatoes in a cool and dark place, away from direct sunlight and sources of heat, such as a cabinet, your pantry, or the basement. With this storage method, sweet potatoes last 3 to 5 weeks.

Keep the sweet potatoes dry and place them in a paper bag or a cardboard box for storage that allows them to breathe. Plastic bags, especially when wrapped tightly, hinder airflow, keep moisture in, and promote spoilage.

Wash sweet potatoes immediately before cooking them, scrubbing the dirt off and blotting them dry with a paper towel (the final step is unnecessary if you’re planning to boil, simmer, braise, or stew them).

Baking Sweet Potatoes

Even though there’s more than one way to cook sweet potatoes, they tend to come out their best—sweet and fork-tender—when baked on a baking sheet with a wire rack, or lined with aluminum foil or parchment paper, in a 425°F (220°C) oven. For even cooking, turn them once halfway in.

Poke each potato in five or six places before putting it in the oven. Sweet potatoes have moist flesh and tough skins. If you don’t poke the flesh, steam will build up on the inside and can cause them to swell and explode. (I’ve had this happen to me once. Not only was clean-up a pain, but it nearly broke the fan on my wall oven.)

Sweet potatoes should be baked until they are mealy and tender. At a temperature of 425° (220°C), most sweet potatoes will be done after about 1 hour.

Boiling Sweet Potatoes

Fill a pot with water, salt it with a pinch or two, then crank the heat on your burner up to high to bring it to a full, vigorous boil. While that happens, wash and peel the potatoes, cutting them up into thirds or quarters depending on their size.

When bubbles are forming and bursting aggressively and hectically in the water, turn the heat down to medium-high (for a simmer) and add the potatoes. Cook for 15-20 minutes until you can insert the edge of your knife or tines of your fork to the center with little-to-no resistance.

(Sweet potatoes are a great addition to a soup or stew when you want to thicken the cooking liquid with their starches and add a touch of sweetness the end product.)

Sweet Potato Fries

You can make French fries out of sweet potatoes.

That said, it’s best to bake the fries rather than shallow- or deep-fry them. Sweet potatoes take a long time to cook. If you fry them in cooking oil, the outside will dry out by the time the inside gets all tender.

I’ve also had success putting sweet potato fries in a 350°F (180°C) airfryer. The result were crispy, tender fries with a malty taste and no oiliness whatsoever. Try this cooking method out, and chances are you won’t be disappointed.

Are Sweet Potatoes and Yams the Same Thing?

Many of us refer to sweet potatoes with darker skins as “yams.” But a yam is not a sweet potato. Unless we’re talking about true yams (and, nine times out of ten, we aren’t), this is incorrect.

Sweet potatoes and yams are both root vegetables with a sweet flavor. They can be boiled, stewed, fried, baked, roasted, as well as broiled, or grilled. However, they are not the same thing, and the similarities between them end there.

Sweet potatoes are native to the Americas, while yams originated in Africa and Asia. The former have smooth orange skin, moist orange flesh, and hold their shape when cooked. The latter have a rough, brown skin and dry, white flesh that seeps starches in the cooking water and cooks to fall-apart tender.

Sweet Potatoes

Sweet potatoes are the edible, starchy, malty-sweet root vegetables of the Ipomoea batatas plant. Native either to Central or South America, it’s said to have been domesticated some five thousand years ago between Mexico’s Yucatán Peninsula and the mouth of the Orinoco River in Venezuela.

Yams (True Yams)

Yams are the tubers of lilies. They fall in the Dioscorea genus, comprising 600 species of flowering plants in the Dioscoreaceae family. Yams, says Michigan State University’s Saneya Moore, originated in Africa and Asia.

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Written by

Jim is the former editor of Home Cook World. He is a career food writer who's been cooking and baking at home ever since he could see over the counter and put a chair by the stove.