Native to Southern China this yam has been cultivated in Japan from very early times. Known in the U.S. mainly as an invasive weed, this vine with arrowhead shaped leaves grows large tubers below ground and small tubers above ground. It spreads mainly by dropping these small "air potatoes" and rarely flowers.
Nagaimo is roughly cylindrical and may be two or three feet long, but the photo specimen was 24 inches long, 2-7/8 inches diameter and weighed 3-1/4 pounds. The skin is tan and the flesh is white and, unlike other yams, sticky and more than a bit slimy. The two Japanese names, nagaimo and yamaimo (shorter, thicker), are applied depending on the root shape.
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Uses: In China this yam is widely used in herbal medicine but in Japan it is more used for food..While most yams require at least cooking to detoxify them, this yam is eaten raw and shredded after being soaked whole in a vinegar-water solution to neutralize oxalic acid in the skin. It is also used to make yamaimo soba noodles. Tororo, sometimes served over sushi or with rice, is grated yam mixed with dashi, shoyu and mirin. Some say this yam is an "acquired taste" outside of Japan, but considering the quantities sold in Southern California's Chinese and Southeast Asian markets that taste seems to have been rather widely acquired
Buying: These are found in markets serving East Asian and Southeast Asian communities. Most often they are cut into segments and packaged on foam trays covered with clear shrink wrap but some markets have them loose, full length or cut. They should be a very light tan, a reddish color indicates they have been on the shelf too long.
Storing: For a yam nagaimo is rather perishable. It needs to be kept dry, cool and in the dark. For shipping whole nagaimo is often packed in sawdust but you can wrap it in newspaper and keep it in the refrigerator or another cool place for about 2 weeks. Cut it should be kept refrigerated and used in less than a week. Raw grated nagaimo can be kept frozen in a form that allows it to be broken or cut off as needed.
Cooking: For most traditional Japanese recipes nagaimo is shredded or grated and used raw or lightly cooked. I find the fine shredding side of my grater does a good job. You end up with a gooey mush that's slimier than snail snot but do not be discouraged - it cooks quite well by various methods and the result is not slimy.
Medicinal Uses: This yam has long been important in Chinese herbal medicine and also in Japan. It is used for a variety of digestive and respiratory problems, diabetes, lowering blood pressure and cholesterol, and male sexual disfunction. It has been reported that in Japan during the Edo period slippery grated nagaimo was used in the men's baths for purposes beyond the scope of this page.