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.
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.
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:
There 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.
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.
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.