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What's in a name? When to comes to differentiating between sweet potatoes and yams, the answer is actually quite a lot. If you've always thought the two tubers were one and the same, you're hardly alone—what's more, you're also not entirely incorrect. Read on for a tablespoonful of botany, a pinch of geography, and a dash of cooking advice when as it relates to the differences (and similarities) between yams and sweet potatoes.
First, the soupçon of science: Every plant has just one universal botanical name derived from Greek and Latin. By contrast, plants (and plant foods) can have a cornucopia of common names acquired in different languages, or in the same language in different regions. One plant can share a common name with another plant, which can be confusing, especially when cultures converge in a given place. To understand the difference between yams and sweet potatoes, the botanical name is extremely important. True yams belong to the genus Dioscorea. Sweet potatoes, on the other hand, belong to the genus Ipomea. The gist? Yams and sweet potatoes are different vegetables from different plants. It's not necessarily wrong to refer to sweet potatoes as yams (because the name has become normalized through history of use), but it is important to understand that the two tubers look and taste different.
What Is a Yam?
Let's start with the true yam. There are hundreds of species of Dioscorea, with plants native to the tropical parts of Africa, Asia, Central and South America, and Oceania (that's the Central and South Pacific). The tubers of many of these twining vines vary in size (they can be massive), shape, skin- and flesh-color, and texture, depending on the species. Their skins tend to be thick, and are either smooth, striated, or tufted with hairs. With some exceptions, yams are always cooked before consuming, as they may contain toxins that are only destroyed by heat; cooking makes their texture anything soft, crumbly, fibrous, or waxy, depending on their variety, but their flavor is always mild, tending towards slightly sweet, nutty, or even meaty. The common English name—yam—is thought to be derived from a Guinean word meaning "something to eat," transliterated by Portuguese slave traders to inhame, to igname in French, and to yam in English.
In most stores in the United States, we are likely to encounter only handful of different true yams. White yam (Dioscorea alata) is what you'd be most likely to find at an American supermarket that does not specialize in diverse produce. It has many English common names (and more in other languages), including Greater Asiatic yam, water yam, winged yam (alata means winged); it has been cultivated for so long that its roots have become obscured, but the white yam's place of origin is thought to be Southeast Asia. Its skin is brown and rough, its shape is very variable, and its flesh is pale. Confusingly, purple yam (the ube of the Philippines) is the same species, but with vividly violet flesh. Increasingly (and incorrectly), purple sweet potatoes are referred to as ube. The real deal is this true yam.
African yam, Guinea yam, or Ghana yam is a botanical mouthful: Dioscorea cayenensis subsp. rotundata. It has a reputation for deliciousness and is the yam of choice when making West and Central African fufu, glutinously good balls destined for dipping into the luscious gravy of stews. (Fufu is also made with cassava or green plantain.) These white-to-yellow fleshed yams are hefty and log-like, with rough, pale brown skins. Culinary journalist Elizabeth Schneider, author of the encyclopedic Vegetables from Amaranth to Zucchini ($14.25, amazon.com), observes that cutting white and African yams in half before boiling leads to "a creamier and fluffier texture" than if they are boiled whole, or even halved and peeled.
Cushcush yam (Dioscorea trifida), also sold Stateside as mapuey, is native to Central and South America and widely cultivated in the Caribbean. The tubers grow in clusters, and each is elongated—sometimes tapered at the ends, sometimes rounded—with vertically striated, rough brown skin. The texture of cooked cushcush is lighter than other yams, which tend to turn very solid as they cool.
Two exceptions to the cook-your-yams rule belong to two East Asian yams. The relatively thin-skinned Chinese and Japanese yams, Dioscorea polystachya and D. japonica, respectively, are long and relatively skinny, with distinctive, hairy tufts. Chinese yams have a golden-brown skin, while Japanese yams (also called mountain yam) are brown-skinned. Both yams can be eaten raw, julienned, or grated, or quick-pickled. Their viscous texture is an important part of their appeal, and can be used to thicken soups. Nancy Hachisu-Singleton, author of Preserving the Japanese Way ($50, amazon.com), recommends that mountain yam be "grated into a wonderful slimy mass and eaten raw with soy sauce."
What Is a Sweet Potato?
All sweet potatoes are produced by the vine Ipomea batatas, native to Central and South America, which is a member of the morning glory family. Their tapered tubers have relatively thin, edible skins and sweet flesh that ranges from white to orange and purple. Their tender leaves are a good cooked vegetable, too. Sweet potatoes cook more quickly than yams, are moister, and they taste distinctly sweet. When cool, they do not turn steadfastly glutinous and solid. Sweet potatoes are often referred to and sold as yams, especially if they are purple or orange. Why? In a word, marketing: In the 1930s, scientists at the Horticulture Department of Louisiana State University launched a national marketing campaign to differentiate orange Louisiana sweet potatoes from white-flesh sweet potatoes grown in other parts of the U.S. They called them yams. Clearly, it worked.
The canned candied yams sold in grocery stores are sweet potatoes, and just about all the results returned when you Google "candied yams" will refer to sweet potatoes. It's perfectly fine as long as you understand that sweet potatoes are not botanically related to true yams. If you are happy calling sweet potatoes yams, carry on. Common names acquire agency through history of use. As Charmaine Solomon explains in her tome Encyclopedia of Asian Food ($2.93, amazon.com), with yams, "the difference only becomes important when preparing these tubers for cooking."