The names yam and sweet potatoes are often used interchangeably, but in reality they are two very different plants.
Yams are monocots that are native to Africa and closely related to lilies and grasses. While sweet potatoes are dicots that are native to Central and South America and are part of the morning glory family. Yams are starchier and drier when compared to sweet potato varieties grown in the United States.
There are over 600 different species in the genus Dioscorea, which contains yams. Yams grow underground from a vine. They produce a tuber, which vaguely resembles sweet potatoes, that are long cylindrical, and often have “toes” sprouting from them. Yams can range in size from the size of a potato to extremely large, up to five feet long and weigh up to 150 pounds. Their flesh can be white to bright yellow, and the tuber is covered in a very tough skin that is difficult to remove. Yams contain chemicals including oxalates that can have adverse health effects if eaten. Therefore, yams must be prepared properly before they are safe to eat. Typically, yams go through cycles of boiling, pounding, and otherwise leeching out these harmful compounds before they are eaten.
Sweet potatoes (Ipomoea batatas) produce storage roots that have smooth thin skin that can range in color from white to yellow, red, purple or brown. These roots are short and blocky and have tapered ends. The flesh also ranges in color from white to yellow, orange, or orange-red. Sweet potato varieties are classified as either ‘firm’ or ‘soft’. When cooked, those in the ‘firm’ category remain firm, while ‘soft’ varieties become soft and moist.
According to the Library of Congress the confusion between yams and sweet potatoes began when soft varieties of sweet potato were introduced to the United States. “In the United States, firm varieties of sweet potatoes were produced before soft varieties. When soft varieties were first grown commercially, there was a need to differentiate between the two. African slaves had already been calling the ‘soft’ sweet potatoes ‘yams’ because they resembled the yams in Africa. Thus, ‘soft’ sweet potatoes were referred to as ‘yams’ to distinguish them from the ‘firm’ varieties.”
Today the USDA requires that when the term ‘yam’ is used to describe sweet potatoes it must be accompanied by the term ‘sweet potato.’ Unless you specifically search for yams, which can often be found at ethnic markets, you are probably eating sweet potatoes.