What's the difference between sweet potatoes and yams?

Yams and sweet potatoes are often confused, and the names are sometimes used interchangeably in the U.S. However, they're two different crops. So what sets them apart?

The differences lie mainly in these vegetables' classifications, plant growth, and nutritional profiles, experts told Live Science.

Despite both growing underground and appearing somewhat similar, "Botanically, sweet potatoes and yams are not closely related, as they belong to different plant families," said Phillip Wadl, a research geneticist at the U.S. Vegetable Laboratory in Charleston, South Carolina. Sweet potatoes (Ipomoea batatas) are in the morning glory family (Convolvulaceae). Yams (Dioscorea), by contrast, belong to the Dioscoreaceae family, and there are 11 yam species cultivated for food and income, Wadl told Live Science in an email.

Yams and sweet potatoes also belong to different plant groups, or "cotyledon types," said Jeffrey Cole Denis Gregorie, a sweet potato specialist at the Sweet Potato Research Station in Chase, Louisiana. Yams are monocots, plants whose embryo has only one cotyledon, or seed leaf, like grasses and lilies. Sweet potatoes are dicots, plants with two embryonic seed leaves, which include many fruits and vegetables, such as grapes, squash, strawberries and tomatoes.

In addition, yams and sweet potatoes store food in different parts of the plant. "Sweet potatoes form storage roots which are enlarged parts of the root system," Wadl explained, "whereas yams form tubers, which are enlarged underground stems for nutrient storage."

A single sweet potato plant typically produces about four to 10 sweet potatoes, while a yam plant yields about one to five yams. Sweet potatoes generally take about three to five months to mature, compared with about six months to a full year for yams, according to Craig Yencho, a professor of horticultural science and a sweet potato and potato researcher with North Carolina State University.

How do you tell sweet potatoes and yams apart?

Visually, sweet potatoes and yams are easy to tell apart when you know what to look for. Sweet potatoes tend to be shorter and blockier, with tapered ends. Yams are usually long and cylindrical, sometimes with "toes," or branching shapes.

Sweet potatoes come in various flesh colors, including orange ‪—‬ the most common color in the U.S. ‪—‬ as well as yellow, purple and white. Their skin is typically thin and smooth, whereas "yams often have a rough, bark-like skin and their flesh can be white, purple or reddish," Wadl said. One well-known purple yam, known as ube (Dioscorea alata), is a popular ingredient in treats such as ice cream, cookies and pound cake.

Sweet potatoes and yams also have noticeable differences in their culinary and nutritional profiles, Wadl said. "Sweet potatoes are generally sweeter and have a moist, creamy texture when cooked, although dry types are available," he explained. "Yams, on the other hand, tend to be starchier, drier, and have a more earthy, sometimes less sweet, flavor compared to sweet potatoes. Their texture can be more like a regular potato."

Nutritionally, the two crops are quite different, too. Sweet potatoes, particularly the orange-fleshed varieties, are an excellent source of beta-carotene (a precursor to vitamin A), vitamin C, dietary fiber, potassium and some B vitamins. "While nutritious, [yams] typically contain less beta-carotene than orange sweet potatoes," Wadl noted. Instead, yams are a good source of potassium, vitamin C, manganese and dietary fiber.

Sweet potatoes are generally considered safe to eat both raw and cooked, Wadl said. By contrast, many wild varieties of true yams, especially those not cultivated for commercial consumption, contain compounds like dioscorine that could make them toxic to humans if eaten raw. "These must be thoroughly cooked to remove the toxins and make them safe for consumption," Wadl said.

Thanksgiving confusion around yams and sweet potatoes

If yams and sweet potatoes are so different, why do so many Americans call sweet potatoes "yams"?

Part of the answer lies in history. The yam was a key provision on voyages of enslaved people from West Africa to the Americas, "used to feed captives a familiar food," said Judith Carney, a professor of geography at UCLA. "In mainland plantation societies of North America where the African yam could not be grown, the sweet potato served as a substitute dietary staple." Because the two crops shared some characteristics, "the word 'yam' came to refer vernacularly to both tubers in mainland America, as we see in the U.S. today," Carney told Live Science in an email.

The U.S. sweet potato industry also leaned into the confusion, adopting the term "yam" for marketing reasons, Gregorie told Live Science in an email. "In the United States, many orange-fleshed, moist varieties of sweet potatoes are often mislabeled as "yams" to distinguish them from the drier, white-fleshed sweet potato varieties," Wadl explained.

As a result, the "yams" at most U.S. supermarkets are probably sweet potatoes. "Garnet yams" and "jewel yams" aren't actually yams, despite those labels; instead, they're softer, sweeter varieties of sweet potatoes, according to the University of California, Davis.

Although genuine yams are sold in many international markets in the U.S, "if you're buying a "yam" in a typical U.S. supermarket, you're almost certainly purchasing a sweet potato," Wadl said.

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