Virtually all yams sold locally are actually kinds of sweet potatoes, but not all sweet potatoes are yams. The yams that most Southern Californians buy in their stores are, in fact, yam-type sweet potatoes.
Confused? The botanical classification of these edible underground plant parts are enough to bewilder a biology student.
True yams are not a commercial harvest in the United States--they are a tropical crop imported from South America and sold in specialty stores, particularly in Florida and New York, said Bob Scheuerman, a farm adviser in Merced for the University of California's statewide agricultural extension service.
Like white potatoes, true yams are tubers--the enlarged, underground portion of a plant stem. They range in size from a few ounces to 40 pounds, and measure as much as three feet in length, he said.
3 Types Common
Unlike true yams, sweet potatoes are not part of the plant stem but are roots. Nutrients, energy and water are stored in the specialized root. "It's like people, you have a band of fat around the middle, and you call on it when you need it," said Aziz Baameur, a farm adviser in Riverside.
Three types of sweet potatoes are commonly sold in the United States, including the sweet potatoes that we have come to know as yams. The three are garnets, jewels and jerseys. Garnets are marketed as red yams. Jewels are sold as either sweet potatoes or jewel yams.
Garnets are typically thick, about eight to nine inches long and maroon on the outside. Jewels are chunky, about six to seven inches long and copper in color. Jerseys are fat, about six to nine inches long, white on the outside and creamy on the inside, and are only sold as sweet potatoes.
Both garnets and jewels are yam-type sweet potatoes. They are sold as yams because they do not dry out when cooked. "It's a marketing term to denote the moisture after it's cooked," Scheuerman said. Jerseys are dry when cooked, and less sweet than garnets and jewels.
Few jerseys are sold in Southern California; most go to Oregon and Washington, Scheuerman said. "Los Angeles really likes the garnets. We have trouble marketing the jewels down there in the Los Angeles area."
Jewels are sometimes sold as canned yellow yams. Only 5% of California's production is canned, compared to about 75% of Louisiana's output, Scheuerman said.
Garnets and jewels are considered very nutritious. A medium-sized red yam has more than twice an adult's recommended daily allowance of Vitamin A, and is rich in Vitamin C and iron, Scheuerman said.
Sweet potatoes have been eaten in South America for millennia. They were planted in colonial Virginia in the early 1600s. Sweet potatoes became the South's most important food crop when it was cut off from grain supplies in the North and Midwest during the Civil War.
The vegetable is widely associated today with Thanksgiving, probably because it is harvested as late as the end of October and is known as a food of colonial times. Yet it is unlikely that sweet potatoes were eaten at the first Thanksgiving in the fall of 1621, when Massachusetts Bay Gov. William Bradford invited the neighboring Wampanoag Indians to join the Pilgrims for three days of feasting and festivities.
Massachusetts is too cold for sweet potato cultivation.