Yam or sweet potato? That is the question

Mike Mozart/Flickr

Mike Mozart/Flickr

Popeye ("I y’am what I y’am...") was never troubled by ambiguity.

But with the holiday feasting season at hand, I’m trying to come to terms with a point of perennial confusion: What exactly is the difference between yams and sweet potatoes? Would candied yams by any other name taste as sweet?

Botanically speaking, yams and sweet potatoes turn out to be entirely different plants, with different personalities, histories and social values.

Sweet potatoes, technically known as Ipomoea batatas, are a member of the morning glory family, and hail from Peru and Ecuador. Yams, properly Dioscorea batatas, are of African origin, and quickly spread eastward to Asia and the Pacific.

In the raw, they look different. True yams are rough, scaly and brown. Sweet potatoes are smooth-skinned, with an exterior that ranges from yellow to dark red. 

Yam, also known as cinnamon vine, is one of those frightening tropical plants that grow at an alarming rate — as much as 4 inches in 24 hours. Sweet potatoes are subtropical, but with proper timing can be grown as far north as southern New York.

Harvested in July and August, yams have superior keeping qualities and are available all year. Sweet potatoes, collected in late fall, are more fragile, and last about 10 weeks with care.

Of the two, the yam has the longer record in human history. Its botanical name commemorates Pedanios Dioscorides, a first-century Greek physician who write a book on medicinal herbs.

Don’t laugh at the "medicinal" bit — modern scientists side with Dioscorides. A component of birth control pills (progesterone) is derived from the wild yam, as are products to offset the symptoms of menopause and osteoporosis. It also produces a substance that may relieve carpal tunnel syndrome — good news for all you computer junkies.

Jon Lebkowsky/Flickr

Jon Lebkowsky/Flickr

But back to food value. Yams were eaten from very early times in Asia, Africa and other tropical locales with a long growing season. While there are 200 or more species of Dioscorea, the edible batatas is also known as Chinese yam, which suggests a venerable association.

According to Reay Tannahill, author of "Food in History," ancient Chinese liked their yams. "The Summons of the Soul," a poem dating to the third century BC, goes on about dishes to make the mouth water: "Ribs of the fattened ox cooked tender and succulent; Sour and bitter blended in the soup of Wu...Stewed turtle and roast kid, served up with yam sauce..."

The sweet potato didn’t reach China until the 16th century, but was a big hit; today, China leads the world in sweet potato production.

From China, the yam traveled to the Pacific Islands, where it became a dietary staple. It’s so important in Fiji that the islanders’ traditional 11-month calendar is based on its growing cycle. In the Philippines, Camotes Island is named for — you guessed it — the yam.

While the sweet potato was slower to make it to human kitchens, it’s no Johnny-come-lately. Paleontologists have determined (don’t ask me how) that the sweet potato was a favorite food of plant-eating dinosaurs, presumably those that lived in the Americas.

The big guys were on to something. The Center for Science in the Public Interest, based in Washington, D.C., ranks the sweet potato as No. 1 in nutritional value, exceeding the next best veggie (the carrot) by more than 100 points. Sweet potatoes have your beta-carotene, your vitamins A and E, B1 and B2, your nourishing complex carbohydrates.

Are they the food crop of the 21st century? Maybe. NASA is experimenting with sweet potatoes in its Advanced Life Support program for long-term space travel and, ultimately, to sustain permanent colonies on Mars. See? In the yams-vs.-sweet potato mess, the rock band Yams from Outer Space, got confused along with the rest of us.

Ultimately, we can pin the blame on American produce wholesalers and retailers. There are two kinds of sweet potatoes: those with dry, mealy flesh like "Yellow Jersey" and those with moist meat, like "Jewel" and "Centennial," first developed in Louisiana.

The produce folks started using "yam" as a trademark name for the moist-flesh type to avoid confusion in their minds (and promote it in ours). So, virtually all of the orange tuberous things we eat — microwaved, baked or roasted — are, in fact, sweet potatoes.

Your welcome. And Happy Thanksgiving.