Sweet potato. さつまいも、薩摩芋。Ipomoea batatas. Other names are "kansho," "Kara-imo" and "Ryukyu-imo."
Sweet potatoes originated in Central and South America. The Spanish brought the starchy root tuber to the Philippines, from whence it reached China (Fujian) - and from there is was in 1605 brought to the Ryukyu Islands (now Okinawa). In the seventeenth century (from 1611) it spread throughout southern Kyushu - the tuber fitted very well in the volcanic soil where other plants had difficulty growing. As that was the domain of the Satsuma clan, the potato became known as Satsumo-imo, "Satsuma potato."
According to another theory, by the way, Satsuma-imo were brought directly to Nagasaki and Hirado, without following the route via Okinawa (there is a record that the British brought sweet potatoes to Hirado in 1615 - multiple routes are indeed likely).
Although the sweet potato reached Japan a little later than the ordinary potato (jagaimo), it became more popular thanks to its sweetness. After the 1730s, Satsuma-imo reached the Kansai and after that also the Kanto area. They are easy to cultivate and as they provide a lot of carbon hydrates, they were very welcome in times when the rice harvest failed. They were promoted by agricultural scientist Aoki Konyo (1698 - 1769) and instrumental in warding off large-scale starvation.
Over the centuries, the Satsuma-imo potato has been improved in Japan to become very sweet and soft. The skin is a bright, reddish purple. The inside is white when raw, creamy yellow when cooked.
Sweet potatoes can be simmered (as in the picture above, where they have been simmered in a sweet sauce containing dashi, soy sauce and mirin or sugar - this is the most common way to use them in the home), baked and fried. In that last case, they are sliced thin, and used as a popular ingredient in tempura. Baked sweet potatoes are a popular snack in winter (yaki-imo). Daigaku-imo is a snack made from baked and candied sweet potato. Puree from sweet potatoes is used in imo-kinton and in other wagashi, Japanese sweets.
Satsuma-imo also form one of the ingredients from which shochu, Japanese distilled, can be made (imo-jochu).
Preservation: store in a cool dark place outside the refrigerator. Keeps for about five days. Look for firm potatoes without brown spots.