Though sweet potatoes and yams are both root vegetables, they taste different, have different nutritional qualities, and come from different plant families. The sweet potato is from the Convolvulaceae family; it is a dicot that is related to flowering morning glory plants. The yam is a monocot from the Dioscoreaceae family that is related to lilies and grasses. In Canada and the U.S., sweet potatoes and yams are sold in most grocery stores and are very popular during both American and Canadian Thanksgiving; however, what is labeled as a yam is often just a soft, orange-fleshed sweet potato.
Most yams have a drier, potato-like starchiness than most sweet potatoes, which tend to have a rich flavor and, in some cases, creamy texture. However, with a wide variety of both yams and sweet potatoes in existence, there is considerable overlap when it comes to flavor and texture.
Sweet potatoes, especially orange-fleshed sweet potatoes, are almost always sweeter than yams, but both vegetables are versatile and can have their flavors and textures altered depending on the method of cooking and ingredients used with them. It is common for both to be baked, roasted, grilled, boiled, fried, or cooked in a variety of other ways. They can be made into main dishes, side dishes, or desserts.
The following video gives a brief history of yams and shows purple yams being used in Jamaican home cooking.
Sweet potatoes and yams are the roots of sweet potato and yam plants or vines. As there are many different kinds of sweet potatoes and yams, these plants and roots' appearances vary considerably.
In Canada and the U.S., two main varieties of sweet potato are sold: one that is golden-skinned and white on the inside and one that is pink-skinned and orange on the inside. Both are around the size of regular potatoes, though usually skinnier and with tapered ends. Firm, white-fleshed sweet potatoes are starchier and more potato-like than soft, orange-fleshed sweet potatoes, which are more likely to be mislabeled as yams. (When both varieties of sweet potato were first commercially sold in North America, producers wanted a simple way to differentiate between the two, and so began calling the soft, orange-fleshed variety a "yam," as some West African slaves did.)
Yams have greater variance in their appearance than sweet potatoes do. Some yams are the size and shape of small potatoes, while others can grow up to 1.5m (nearly 5ft) in length and weigh over 100lbs (70kg). On the outside, their skins may be dark brown or light pink, while on the inside, their flesh is usually white or yellow, with some occasionally having purple or pink flesh.
Both sweet potatoes and yams are very nutritious, but in slightly different ways. They differ most when it comes to sugar content (sweet potatoes have more), potassium (yams), vitamin C (yams), vitamin A (sweet potatoes), and beta-carotene (sweet potatoes). In other words, determining which is healthier really comes down to what nutritional content is desired, as well as how the sweet potato or yam is cooked.
(per 100g serving)
Like many root vegetables, yams and sweet potatoes are important in food insecure areas, especially those affected by seasonal droughts or rains, because they can be stored for long periods of time without needing to be refrigerated. The nutritional value of the root vegetables makes them important as well.
Sweet potatoes and yams can be eaten by vegans and paleo dieters, but paleo dieters who are also trying to lose weight by cutting carbs are told to eat both vegetables in moderation or not at all. Having lots of carbs can be helpful to others, though, such as those who are bodybuilding.
Individuals with diabetes should be able to eat both vegetables, provided they are boiled. The cooking method used changes the glycemic index rating, as do additional condiments. Boiled sweet potatoes and yams have relatively low GI ratings (44 and 35, respectively), while peeled and baked sweet potatoes have a high GI rating (94), as do peeled and roasted yams (77).
edit Where Yams and Sweet Potatoes Are Grown
Sweet potatoes are thought to have originated in Central or South America, but the vast majority of today's sweet potato production comes from China. In the U.S., nearly 40% of the country's sweet potatoes are grown in North Carolina.
There are hundreds of yam varieties, but most were originally native to Africa and Asia. Today, yams are grown around the world, but West Africa is still where most yam crops—nearly 95%—are grown. Yams are less commonly grown or even sold in North America, where sweet potatoes are prominent.
edit Confusing the Terms
There is a long history of confusing yams for sweet potatoes and vice versa.
- West African slaves in the U.S. often called sweet potatoes yams because they looked similar to the yams found in West Africa.
- In Scottish English, yams have often simply been called potatoes.
- In New Zealand, sweet potatoes are known by their Maori name: kumara.
- In Okinawa, Japan, purple "yams" are actually sweet potatoes.