chinese yam (nagaimo)
A long, pale brown tuber with white, juicy flesh that is crisp when raw and turns into a thick, slippery paste when grated.
Caution: The skin and freshly cut flesh can make your hands itch, so it is best to wear gloves when peeling and grating.
Nagaimo is a type of yam grown in Japan, China and Korea; the tubers can reach a metre in length and are usually sold cut into sections. Unlike most yams it is also eaten raw, and its flavour is mild, faintly sweet and earthy. What matters most is texture: cut into sticks it is crisp and juicy, while grated it becomes a sticky, stringy paste that the Japanese serve as tororo over rice or noodles and add to batters such as okonomiyaki for lightness. When cooked it turns soft and slightly floury, much like potato, but loses its sliminess. You will find nagaimo in Asian grocery shops, especially Japanese, Korean and Chinese ones, often wrapped in plastic in the chilled vegetable section.
Substitute:
For raw, grated dishes such as tororo there is no substitute, because no other widely available ingredient has this slippery, stringy consistency. In batters such as okonomiyaki you can leave it out and use an extra egg or a little baking powder instead, though the result will be denser and less fluffy. Raw in sticks, jicama or kohlrabi come close in crunch, and cooked, a waxy potato works, in each case without the characteristic sliminess.