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Tsukune imo are very large generally round roots that are bumpy and gnarled in shape and reach up to pound (500grams) in weight. They are typically covered in dirt and require a good scrubbing. The surface of the root is dark brown to black and is rough and scaly. Beneath the thin layer of skin is a dense, bright white interior. The flesh is very sticky with a crisp yet starchy texture and offers a rich, slightly sweet flavor.
Tsukune imo are available from the late fall to early spring months.
Tsukune imo, pronounced ‘soo-koo-nae ee-mo,’ is a variety of Dioscorea oppositifolia or D. batatas, also knowns as Japanese yam or Yamaimo which translates to “mountain potato.” Another common name for the root is Yamato imo. The giant roots are used much like potatoes and are often grated and used in traditional Japanese dishes like tororo-jiru or okonomiyaki. The name comes from the root’s round and lumpy appearance that resembles a Japanese chicken meatball called “tsukune.”
Tsukune imo are rich in carbohydrates, fiber, calcium, and phosphorus. They are a source of vitamins A, B1, B2, and C, as well as iron and zinc. The roots contain several amino acids, including lysine, leucine, and tryptophan. They are rich in diastase, a digestive enzyme that breaks down carbohydrates making them easier to digest. The roots are ideal for those managing diabetes, high-blood pressure and high cholesterol. The flesh of Tsukune imo may cause irritation to the skin of the hands while preparing due to the presence of calcium oxalate. Wearing gloves will prevent irritation.
Tsukune imo can be enjoyed raw or cooked. It is often used as a binder in miso-based soups, grated for egg dishes, soba noodles, and Japanese nimono. People in Japan use it to make a confectionary buns or the frittata-like dish okonomiyaki. It can be sliced into thin chips and fried or made into isobe-age or pan-fried and made into oyaki. Look for tubers that are heavy and have a more uniform shape with a moist surface and no scratches. After peeling the skin, put them in a bowl of cold water with a splash of vinegar for fifteen minutes to neutralize the oxalates and to preserve the white hue of the flesh. To store, wrap them in a newspaper and keep in a cool dark place. Cut pieces can be wrapped in plastic and stored in the refrigerator for a few days. For long term storing, they can also be parboiled and frozen.
Tsukune imo and other Japanese yam varieties have been used for culinary and medicinal purposes for centuries. During the Edo Period, in the 17th through mid-19th centuries, men were the only ones who used roots like Tsukune and were known to grate the tuber into their bath water for vitality. They believed that regularly eating the roots would increase their physical strength. The roots are also said to be helpful against ‘natsubate,’ the tiredness experienced during Japan’s long, hot summers.
Tsukune imo are native to the mountainous regions of China. Botanically, their classification has confused researchers who have changed the scientific name of this variety several times. In Japan different names are used for Tsukune imo in different regions, making them somewhat difficult to identify. In Kyoto Prefecture they are called Tabayama no imo, in Ishikawa they are Kaga Round Mom or Kagamaru imo, and in Mie they are referred to as Ise imo or I Momento. A developed cultivar, Kisa No. 1 was introduced during the late 1980s followed by Takashiro and Aoyama, and later in the late 1990s, an improved cultivar called Hirokei was released. Tsukune imo are cultivated in the southern Mie, Nara, and Hiroshima Prefectures and in the northern Aomori Prefecture. They are most often spotted in the Kansai region which includes Kyoto, Osaka and Wakayama Prefectures.