Hoshi-imo (dried sweet potatoes) (干しいも)
Hoshi-imo is made from sweet potato through the processing of steaming and then drying. While hoshi-imo is produced nationwide in Japan, more than 80 percent of hoshi-imo is industrially produced in Ibaraki Prefecture. It is also called, 'kanso-imo' (dried sweet potato), 'mushi-kirihoshi' (steamed, cut and dried sweet potato), 'kibboshi,' 'imokachi,' and so on.
Its color is brown inclined to dark yellowish green. Although the size of hoshi-imo varies depending on the person who makes it, the piece generally sold at shops has a long and narrow tubular shape with length around 10-15 cm and width around 5 cm. Its gummy texture and sweet taste peculiar to the sweet potato are unique, both of which are produced by the moderate moisture content. Moreover, hoshi-imo is an excellent alkaline food with a virtue of keeping the bowels in good condition, containing various nutrients.
You can not only eat it raw but also eat it after toasting, because toasting makes hoshi-imo softer and sweeter. Furthermore, slightly scorching provides you with pleasant aroma. Although hoshi-imo is frequently made at home due to its simple recipe, some tricks are required in steaming.
As stated above, making hoshi-imo is not complicated, but it takes time and labor. Therefore, some manufacturers offer a 500-gram pack of hoshi-imo for about 1000 yen. Moreover, a group of hoshi-imo left over after cutting fine slices is called 'sekko,' sold at about half the usual price. Despite its low price due to sekko being categorized in the second class due to the irregularity of the hoshi-imo contained in the pack, the taste of the hoshi-imo is the same as that sold in the usual pack. In addition, you may sometimes find the surface of hoshi-imo covered with white powder. This white powder is not mold but crystallized sugar coming to the surface after being formed through the autolysis process in potato.
Fresh sweet potato in its skin is steamed for about two hours. After being put out from the steamer, the steamed sweet potato is peeled and expanded on bamboo screens, and dried in the sun for about a week utilizing chilly wind peculiar to the winter. While some steamed sweet potatoes are dried entirely, most of them are dried after being sliced to one centimeter thickness using such tools as piano wires. The steamed sweet potato dried entirely without being cut is called 'maru-hoshiimo' (uncut whole piece of dried sweet potato), which requires about 20 days to dry.
Various technique are adopted, including the saccharification method of the fresh sweet potato, which enhances sweetness.
From the hygienic viewpoint, plastic greenhouses and nets are used when sweet potato is dried these days. Moreover, some manufacturers have introduced machines for drying the sweet potato, aiming at mass production. Some sweet potatoes are dried after being boiled. Hoshi-imo made from the boiled sweet potato becomes harder than that from steamed sweet potato, because starch contained in the sweet potato is not changed into an adhesive material.
Sweet potato sorted into Tamayutaka-kind (agricultural and forestry No. 22) and Izumi-kind are frequently used as a raw material. The Tamayutaka-kind is larger than other kinds with whitish skin and substance, tasting sticky rather than flaky.
Hoshi-imo is produced in winter through early spring as a logical consequence of using fresh sweet potato. However, hoshi-imo kept in freezers are sold throughout a year.
Besides Ibaraki Prefecture (Tokai-mura, Hitachinaka City), which boasts the largest output among all prefectures in Japan, Gunma Prefecture, blessed with downdraft blown down from the top of mountains in winter, and Shizuoka Prefecture and Nagasaki Prefecture, where the manufacture of the hoshi-imo was industrialized in the Meiji period, produce lots of hoshi-imo. Under the technical instruction of manufacturers in Ibaraki Prefecture, hoshi-imo is also produced in the People's Republic of China, which is coming onto the market in Japan. However, there are differences in sweetness and texture between China-made hoshi-imo and domestic.
How to preserve
Hoshi-imo can be preserved at room temperature; however, it easily gets mildewed because of its manufacturing method without using preservatives and other chemical products, in addition to the latest technique for preventing hoshi-imo from being over dehydrated, aiming at better texture. Therefore, hoshi-imo is cited as one of foodstuffs suffering from lots of complaints. It is recommendable to keep it in refrigerator. Frozen hoshi-imo can be preserved for a long period.
Hoshi-imo sold at stores is usually packed with deoxidizer into a bag made of wrapping material with high shielding properties for long-term storage.