While certain coffee shops loudly and persistently proclaim that pumpkin spice season has arrived, there’s another flavorful autumn veggie that is often overlooked and underrated this time of year: the humble sweet potato.
Sweet potato dishes are Thanksgiving crowd-pleasers, especially in the southern U.S., where you’ll find tables laden with sweet potato casserole, sweet potato pie, and candied yams. But, wait … why aren’t sweet potatoes, or yams, for that matter, used for all three of those recipes? You’ve probably noticed that sweet potatoes and yams are quite similar — suspiciously similar, in fact. Maybe you've even mixed them up, linguistically or in your kitchen. But are sweet potatoes and yams interchangeable or not?
A yam by any other name …
Despite their similarities, there is a difference between yams and sweet potatoes. They are two distinct vegetables that come from completely different botanical families — similar to how peanuts and almonds can be used for many of the same purposes, but peanuts are more closely related to beans and other legumes, while almonds, as tree nuts, are more similar to stone fruits like peaches and plums. But it gets a little tricky if you live in the U.S., where the produce you see labeled as yams at the grocery store are almost always orange-fleshed sweet potatoes, not real yams. To find a real yam, you’ll probably have to stop at an international grocery store or market.
This misleading misnomer can be traced back to the 1930s, when Louisiana sweet potato farmers began labeling their crops “yams” as a way to make them stand out from the competition. According to the Library of Congress, farmers may have gotten the idea for the terminology from the passed-down tradition of enslaved Africans, who had referred to some varieties of sweet potatoes that way because of their resemblance to the yams found in their home countries. Another possibility, mentioned in a 1896 book called Sweet Potato Culture for Profit, is that the term could be connected to the Chinese yam, which had been introduced to the United States in the early 19th century, but which grew in more temperate regions farther north.
So what’s the difference?
Let’s start with sweet potatoes. Sweet potatoes come from the Convolvulaceae family, which is also where the Morning Glory flower/weed comes from. It’s a dicot, which means it has two embryonic seed leaves. Sweet potatoes can be grown in tropical and warm climates such as the southern United States, the Caribbean, and the Pacific Islands. They come in both firm and soft varieties: When you cook firm sweet potatoes, they remain firm. (These are best for roasting or making sweet potato french fries.) When you cook soft sweet potatoes, they become mashable, which is best for pies and, well, mashed potatoes. It’s the soft type of sweet potatoes that are typically sold as yams in the U.S. (think: “candied yam” recipes).
Yams, real ones, belong to the Dioscoreaceae family and are most closely related to lilies and grasses. They are cultivated mainly in West Africa and New Guinea, where they’re the top agricultural commodity. Unlike sweet potatoes, yams are monocots, meaning they have one embryonic seed leaf. True yams can weigh up to 150 pounds and grow to 7 feet long, but they can also be the size of a normal potato. There are also some telltale differences in the skin: Yams have textured brown skin that somewhat resembles tree bark, while sweet potatoes are smoother with reddish, copper, or even gold-hued skin.
In addition to the distinct taxonomy differences, sweet potatoes and yams also taste different. Yams are drier, starchier, and usually not as sweet. And you’d never want to eat a piece of yam raw — they’re toxic until they’ve been cooked. If a recipe calls for yams, can you use sweet potatoes or vice-versa? It depends. If you’re looking at a typical Thanksgiving-esque recipe, sweet potatoes will probably do just fine, and it’s probably what the recipe is referring to anyway. But if you’re making something like a yam stew or African fufu, you really do need the starchiness of the yams to hold up to the spices and flavors of the other ingredients.
In addition to their fairly similar appearances, yams and sweet potatoes provide somewhat similar nutritional benefits. Both are great sources of fiber, potassium, iron, zinc, and magnesium, but sweet potatoes contain significantly more beta carotene, which you can tell by the vibrant orange and sometimes purple coloring of their flesh. (They also come in a white variety.) Like carrots — also known for their high beta carotene levels — sweet potatoes are good for your eyes and your immune system. Sweet potatoes also have higher levels of antioxidants, which help reduce inflammation. Yams, however, contain even more fiber and potassium than sweet potatoes and are often used as a base in stews and other hearty meals. The flesh can be white or varying shades of purple.