Sweet Potato: Part of the Yamily?

Wednesday, November 14, 2012

With fall in full swing and Thanksgiving just around the corner, it seems an apt time to address a question that arises all too often in the grocery store and at the dinner table: what’s the difference between a sweet potato and a yam?

Although sweet potatoes and yams are both angiosperms, true yams are monocots, while sweet potatoes are dicots. And – brace yourselves – neither yams nor sweet potatoes are closely related to regular potatoes at all. Regular baking potatoes come from the Solanaceae family, which includes tomatoes, eggplants, peppers and tomatillos.

The sweet potato belongs to the Convolvulaceae family, whose other famous member is the Morning Glory flower. Yams are a tropical root native to Africa, and are members of the Dioscoreaceae family. Most varieties of true yams are actually far too large to sell whole in supermarkets, and are usually sold in plastic-wrapped chunks weighing a pound or two. In the United States, real yams are only generally sold in international supermarkets, but can also be bought canned.

So what about all those “yams” you buy in the grocery stores around here? The “yams” you buy that look like sweet potatoes — but are labeled yams — are, in fact, a version of sweet potato! What you’ll find labeled as sweet potatoes in the supermarket are generally firm and starchy with a lighter orange flesh, and are a bit dry when cooked.

These were the first variety of sweet potato to be widely grown and sold in the United States. According to the Library of Congress, when the sweeter, softer, orange-fleshed version of the sweet potato began to be commercially produced here in the mid-20th century, the name “yam” was given to distinguish it from the firmer, white-fleshed variety of sweet potato. African slaves were the first to refer to these sweeter types of sweet potatoes as yams, because they resembled the true yams found in Africa.

To make things even more complicated, you’ll probably notice a wide variety of “yams” to choose from. Jewel yams are the most commonly found market sweet potato, and are my personal favorite; they have a dark, long, smooth exterior with deep orange flesh that’s tender and moist.

Garnet yams are quite similar, with a brownish exterior and a smooth sweet interior that is brilliant orange. Hannah yams are quite different from the Jewel or Garnet, with an off-white skin and light-colored meat; they’re tender and sweet when cooked, but notably more dry.

In two supermarkets on the same street here in San Francisco, all of these varieties were interchangeably labeled as sweet potatoes or yams, depending on the store – those that label them sweet potatoes are correct!

You might have also seen Japanese yams (again, just a variety of sweet potato), which have a very dark, pinkish skin and pale flesh that has a slightly nutty, dry texture. Then there is the White sweet potato, which is actually most similar to a Japanese yam, with a purplish skin and white flesh that is starchy, mild, and tastes a bit like roasted chestnut. This variety of sweet potato is very popular in Caribbean and Latin American countries. 

From a health perspective, all of these sweet potatoes, despite their sweetness, are actually quite healthy. Sweet potatoes are almost twice as high in dietary fiber as ordinary russet baking potatoes, and that fiber slows digestion and helps stabilize blood sugar.

Sweet potatoes are also a great source of vitamins A and C, copper, vitamin B6, iron and potassium. Interestingly, sweet potatoes lack nightshade alkaloids that are found in regular potatoes, which have been linked to allergy-related symptoms. Some dietitians even recommend switching from regular to sweet potatoes to help with inflammatory joint-related conditions such as rheumatoid arthritis.

I personally enjoy the “yams” sold in U.S. supermarkets because their sweet taste and smooth texture make for a satisfying meal, without the need for additional unhealthy toppings. I like to wash a Garnet, fork it, and toss it in the microwave — easy, healthy and delicious!

And so, getting back to the original question, today, the yams you buy in the supermarket are actually sweet potatoes. In fact, the USDA requires that the term “yam” be accompanied by the term “sweet potato” (although the produce guy at Whole Foods thought I was out of my mind when I asked him about it). But remember, although a yam in America is really just a sweet potato, neither sweet potatoes nor yams are potatoes at all. Oh, potatoes, yamakin’ me crazy!