Yamaimo is a variety of yam cultivated in Japan. It has rough skin, but is white inside.
Above ground, it is like a climbing vine. It can propagate via its tubers, or by seed. Plants are either male or female.
Yamaimo is used as a binder in dishes because when you peel and grate it, a juice is released making the grated Yamaimo quite slimy and gloopy, though still a bit crunchy. The juice has a good deal of starch in it.
A starch powder is also made from dried, grated Yamaimo.
There are several varieties, including Nagamio and Jinenjo. The names for the varieties are often confused.
Jinenjo is slightly sweeter and starchier than Nagamio.
Another variety, called "Teimo" or "Te-Imo" ("hand potato") gets its name from its shape, somewhat hand-shaped.
Other varieties include "icho-imo" (triangular-shaped) and "tsukune-imo" (round.)
Yamaimo tubers tend to be long, so are often sold cut into pieces, with the cut-ends coated to prevent oxidization.
Starchiness varies by variety. Nagamio is the least starchy and gloopy when grated.
All varieties will oxidize when peeled, so use straight away or soak in acidulated water.
The juice is an irritant to some people's skin, so wearing gloves is often suggested.
Store in fridge in plastic bag for up to 3 days.
In Japanese, the name "Yamaimo" comes from the word "yame" for mountain, and "imo" meaning "potato."