Yard and Garden Column for use during the week beginning Dec. 2.
By Katie Duttweiler
Plant Pathology graduate student
Iowa State University Extension
This Thanksgiving, I was assigned to make the candied yams. I figured I got off easy since the recipe called for only butter, sugar and yams. My relief quickly changed to dismay when I was unable to find any of these so-called yams in the supermarket. The produce section left me terribly confused, confronted by a mound of what looked to be elongated potatoes with the label “sweet potatoes or yams.”
Since I had no idea how to identify a yam, I headed to the canned food aisle. There, I was pleased to find cans labeled yams but puzzled to read that the first ingredient listed was sweet potato. In fact, there were no yams in that can of yams at all.
So what is this mysterious yam and why is it often associated with sweet potatoes? Botanically speaking, a yam is a swollen underground stem from an herbaceous, climbing vine of the genus Dioscorea. Sweet potato is a swollen storage root from the herbaceous, trailing vine Ipomoea batatas.
Yams belong to the yam family, Dioscoreaceae, while sweet potatoes are from the morning glory family, Convolvulaceae. So it turns out that yams and sweet potatoes are not even closely related. All the name confusion is probably due to the fact that the word yam is used as a universal term for any orange colored tuber.
As a side note, the potato is a swollen underground stem of the herbaceous annual Solanum tuberosum. The potato belongs to the nightshade family, Solanaceae. Therefore, the potato is not closely related to either sweet potatoes or yams.
Yams are grown throughout the tropics, especially in sub-Saharan Africa, South and Central America, the West Indies and Asia. Most are grown for food, but some yam species are grown for medicinal purposes. Various culinary techniques are used to prepare yams such as baking, boiling, mashing, frying, grilling, pounding into flour etc. It is essential to cook yams before eating them, since fresh tubers contain natural substances that irritate the digestive tract. Yams are a good source of vitamin C, vitamin B6, potassium, manganese and dietary fiber.
Color and size of yams can vary widely. The skin color can range from off-white to dark brown, and the flesh can range from white to yellow to purple. Tuber size can range from a few ounces to over one hundred pounds. The shape is often similar to a baking potato. In the United States, true yams are rarities in supermarkets. This labor intensive crop requires trellising and mounding, and is not recommended for Midwest home gardens since it is adapted to tropical climates.
Sweet potatoes, however, can be cultivated in temperate climates and are commercially produced in the United States. Even so, most sweet potato production occurs in Asia, Latin America and Africa. While most Asian sweet potatoes are used for livestock feed, the crop is used most commonly for human consumption in the rest of the world. The swollen root can be boiled, baked, fried, dried, ground for flour, dehydrated, canned, frozen, etc. Sometimes the leafy greens also are consumed after cooking. Sweet potatoes are a good source of vitamin A, vitamin C, vitamin B6, manganese, copper, potassium, iron and dietary fiber.
Sweet potato plants prefer warm days and nights, and well drained, acidic soil. Like yams, sweet potatoes are grown in mounds but do not have to be trellised. Sweet potatoes take about 100-125 days to mature, so harvest generally occurs immediately before or after the first autumn frost. A curing process before storage allows the skin to develop a protective cork layer which decreases disease problems and keeps the tubers from drying out. Sweet potatoes should be stored in a cool, dark place and never in the refrigerator (chances of off-flavors and rotting increase when tubers are exposed to temperatures below 50 degrees F).
As I observed in the supermarket, the shape of sweet potatoes often resembles an elongated potato. The skin color varies from white to orange to brown and the flesh color varies from white to yellow to purple. What we find in our supermarkets are sweet potatoes with brown or red/orange skin and orange flesh, but they are often mislabeled as yams. Luckily, they are perfect for a tasty dish of candied yams.