Yams versus sweet potatoes

I made this page for people bickering over whether that orange tuber at the grocery store is a yam or a sweet potato. Short answer: it’s a sweet potato. You’ve probably never even seen a yam. Look at the photograph below. Now you’ve seen a yam.

Photograph of cut Okinawan purple sweet potato, Beauregard sweet potato, oriental sweet potato, and a yellow yam.

Unless you live in West Africa, Asia, or the Caribbean (the places where yams are grown) chances are extremely high that you haven’t eaten one. The exception would be foodies who eat at unusual restaurants and shop at places like H-Mart. The only real exposure most Americans have to yams is when females pop progesterone-containing birth control pills — the progesterone is made from a yam saponin (diosgenin).

One way to recognize an actual yam is by its large size. For example, look at the yams in the photograph below (by Jeff Haskins, for Global Crop Diversity Trust). It’s from yam festival, apparently in the hours before things got festive. Those are some nice looking yams.

Yam festival celebrations

But they can get much bigger. Here’s a 304 lb yam in Fiji. And a 606 lb one in India.

Another easy way to determine whether you have a sweet potato or yam is to look for oozing latex when you slice it. Here’s a photograph:

White latex from sweet potatoThere are other differences, too. Sweet potatoes have smoother skin and usually are tapered at both ends. If you’re worried of forgetting, just print out a wallet-sized reference card:

You can print extra cards and send to your family before they come for Thanksgiving, too, just in case you have ignorant, opinionated relatives. 

In case you’re curious, yams and sweet potatoes are not closely related. At all. Yams (there are multiple species, all in the family Dioscoreacea) are monocots, like onions and grass. Sweet potato (just one species, Ipomoea batatas, in the family Convolvulaceae) is a dicot. In case you skipped high school biology class, monocots and dicots diverged some 200 million years ago. Yams and sweet potatoes are not related to the potato (Solanum tuberosum, in the Solanaceae, with tomatoes and eggplants), although all are, indeed, plants. Their locations on an evolutionary tree of all plants is shown on the illustration below. The Botanist in the Kitchen has more details.

yams-and-sweet-potatoes-tree

You might not want to know, but the confusion about yams and sweet potatoes started because Europeans enslaved people from Western Africa. A lot of them. Yam (the word) is West African in origin (anyinam means yam; nyami means “to eat”). When Africans were forcibly taken to areas planted with sweet potatoes (like on George Washington’s farm) most likely the slaves called them yams, even though they weren’t. As a result, some varieties of sweet potatoes were sometimes called yams (e.g., see this book from 1900), especially in the South and especially in regards to sweet potato cultivars that were soft and extra sweet. They were often given racist, offensive names that I really can’t even repeat on this blog. This caused confusion early on, of course, since most people knew they weren’t really yams. Indeed, I found a 1921 publication (The Sweet Potato: A Handbook for the Practical Grower) that advised that the habit of referring to sweet potatoes as yams “best be dropped.”

Unfortunately, the marketing of sweet potatoes as yams was formally adopted by growers in Louisiana:

The Louisiana industry coined the term ‘yam’ in 1937 as part of a national marketing campaign …” — La Bonte and Smith [pdf]

The USDA eventually started to require that growers indicate, somewhere, that the tubers are actually sweet potatoes. But growers usually shrink the font of the actual ingredient so that is it all but invisible. As a result, many U.S. citizens don’t know what a yam actually is.

Photograph of box of sweet potatoes, not yams

Who cares? I’m a botanist, so I care. But there are other reason, too. Allowing companies to label sweet potatoes as yams is like selling prunes as dates. Moreover, mislabeled sweet potatoes are a health problem for people who are allergic to them

Thanks for reading the end, and please consider sharing this with your relatives this Thanksgiving.

11 Responses to Yams versus sweet potatoes

  1. Steve W. says:

    Hi Colin! Great article; as a big fan of sweet potatoes and/or yams, I had been wondering about this distinction for some time. (And I do shop at H-Mart, which I recognized from your photos, so I might be one of the relatively few Americans who actually do eat yams.) Thanks for clearing this up.

    • H-Mart is the best. I try to buy something unusual each time I go, so it’s educational, too. I try to ask the employees or other shoppers how to prepare the mystery item, and it’s always an experience even if taste-wise it’s not a do-again. Thanks for stopping by.

  2. Stacie Reeves says:

    I’m an avid sweet potato eater, and it always annoys me when people get yams and sweet potatoes mixed up! I’m not sure why.

    • Sweet Poatato Pete says:

      I’m with you on that, Stacie. It bugs me when people can’t be accurate on certain things, especially what they’re eating, and refuse to change their wording even when presented with definitive evidence. Like saying “candied yams”, when it’s really sweet potatoes. Around 2004(?) I read an article in Cook’s Illustrated magazine explaining the difference between sweet potatoes and yams. Since then I’ve been gently (really) trying to educate folks about the difference when the opportunity comes up.
      It doesn’t help that food markets still erroneously label some varieties of sweet potatoes as “yams”.

  3. Oh Colin, and then there is the wealth of folklore associated with yams. Anancy’s ‘Tickie, tickie, boom, boom’, what a story! Gotta look for it to share with you :)

  4. H Wildey says:

    Well, that has answered a question for me. But, I guess the next step is removing the “potato” from sweet potato, since they aren’t related to potatoes either. Then what should we call it?

    • Ideally, yes, but it’s not as bad as the sweetpotato/yam problem. Few people go to store and are confused about sweetpotato/potato difference. Some, of course, but the latter is mainly an ignorance about the _degree_ of botanical relatedness. Thanks for your comment, and happy holidays.

  5. MaryAnn Hardy says:

    OK THEN!! I’ve been eating “SWEET POTATOES” my entire life, no matter what color they were inside or out. THANK you!! I’ve had a hunch about this for a long time… having read some novels about the South Pacific and “yams. Thanks for an excellent article! Now I’m going to make a poster to put in our kitchen for the education of one and all :)

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