In countries across the world, people plant and harvest types of yams that are indigenous to their regions. Yams are climbing plants with vines that bear tubers on an annual or perennial basis, and some varieties can remain in storage for several months without spoilage. The United States generally uses the terms yam and sweet potato interchangeably, though the sweet potato, which is grown in southern states, is not in the same family as true yams.
There are four types of yams that grow in Africa. The aerial yam, or air potato, contains tubers that grow off thick winding vines in the air, not underground as with most other types of yams. The aerial yam weighs one to four pounds and sometimes contains a toxic substance.
West Africans process the white Guinea yam to make their national dish, called "fufu." The white yam may be stored up to seven months before using it. The yellow Guinea yam is prevalent in West African forest zones. This type of yam does not store as well as the white variety and takes an additional four months to mature. African bitter yams contain a toxin that is bitter to taste, but repeatedly boiling them in water can remove the toxin so they are safe to eat.
Asia grows six types of yams for consumption. The Chinese, or cinnamon, yam is most common in Southeast Asia and is difficult to harvest because the tubers grow directly down into the ground. This downward growth is paired with a length of more than 5 feet, so digging up the yams is extremely time-consuming. Asians use this type of yam to make a cooking starch that will store for months, because the vegetable itself does not store well and is considered a lesser yam.
The Asian greater yam is the oldest variety in the region and can be stored five to six months before use. This type of yam needs a large amount of rain--at least 60 inches per year--to reach its mature size of up to 130 pounds. There are hundreds of varieties of the greater yam that consistently produce tubers weighing eight to 22 pounds, including a fingered yam. This variety has small finger-like areas that grow off the main tuber.
The Japanese yam is slim and brittle, reaches 6-foot lengths and contains a naturally occurring digestive aid. Harvesting this type of yam requires skill to not break the vegetable. Asia also produces a bitter yam that may have the toxin removed by boiling repeatedly so that it is safe to eat.
South American Yam
South America grows only one type of yam, the cush-cush. It is mass-producing, bearing tubers in clusters of up to a dozen. This yam is small and long and releases the odor of bacon and eggs when it is cooking. This yam has the lightest fluffy texture of all yams.