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XI Sweetpotato Nutrition and Dietary Culture

1. A nutritional perspective on sweetpotatoes

(1) Compared to other types of tubers, sweetpotatoes contain less moisture and have more calories per 100 g when dried (387 kcal for sweetpotato, 376 kcal for potato, and 353 kcal for taro). Thus, sweetpotatoes feature large energy per unit weight (when raw). Conversely, they have only one-third the protein of conventional potatoes and two-thirds that of taro. Sweetpotatoes contain hardly any fat. .

(2) Since sweetpotatoes contain abundant sugar and (beta-amylase, which decomposes maltose into amylose), they yield large quantities of maltose when steamed or baked, causing their sweetness to increase. When a sweetpotato is consumed, some starch fractions not digested in the intestine are not absorbed at all; instead, they become nutrients for intestinal bacteria and are decomposed to produce intestinal gas. This is why excessive consumption of sweetpotatoes can cause belching, heartburn, and flatulence.

(3) One feature of sweetpotatoes is their high calcium content, the highest of all tuber types. The skin contains especially high calcium concentrations - five times more than the flesh - so sweetpotatoes are an excellent source of calcium when eaten unpeeled. Yellow-fleshed varieties contain about 50 μg/100 g of carotene (provitamin A), which is less than in green and yellow vegetables. On the other hand, people tend to eat larger quantities of sweetpotatoes at a time, giving them comparable carotene to these vegetables. Sweetpotatoes contain roughly equal levels of vitamin C to conventional potatoes, and this vitamin is not destroyed during cooking.

(4) Jalapin (white latex), which oozes out when a sweetpotato is cut, has amylase-activating, microbial-growth-inhibiting, and laxative functions. In fact, Jalapin is one of the factors in sweetpotatoes for eliminating constipation. Moreover, there are reports that sweetpotatoes possess a specific factor which reduces blood urea, i.e. causes protein to be stored.

2. Sweetpotato history and dietary culture

(1) Sweetpotatoes originally spread across Japan due to their characteristic as an emergency crop. Credit for recognizing this ability in sweetpotatoes should go to Konyo Aoki, author of the "Bansho-ko." In this text, he praises sweetpotatoes for their versatility, noting that they can be eaten as a substitute food or confectionery, or consumed in the form of "sake" or mochi. This versatility distinguishes sweetpotatoes from other types of tubers.

(2) Given their powerful sweetness, sweetpotatoes can be eaten simply by steaming or baking them. To ensure steamed potatoes do not taste soggy, various methods have traditionally been employed. "Imomeshi" (boiled rice or wheat mixed with steamed sweetpotato) was a general meal. In seasons where raw sweetpotatoes were not available, people ate kankoro (sliced and dried, raw sweetpotatoes). Kankoro and dried sardines eaten together form a well-balanced Japanese-style diet. Only the elevated salt content of this diet is less than ideal.

(3) In the Edo era, an entire cuisine based around sweetpotatoes began to evolve. People began to eat baked sweetpotatoes as a snack, and baked sweetpotatoes won sweeping popularity. The subsequent richness of sweetpotato-based cuisine in the Edo era is explained in the book "Kansho Hyakuchin" [One Hundred Unique Sweetpotato Dishes], written in 1789 by the owner of the shop "Chinkoro" in Osaka. The book actually covers 123 different dishes popular in those days. Reading the text today, one can realize that there was a great deal of creativity in the cuisine of the period, despite limitations of cooking utensils and seasonings.

○ Sweetpotato recipes from "Kansho Hyakuchin"

Unique dish

Mitaraiimo

Grate raw sweetpotatoes. Add flour to the grated sweetpotato, and mix to form round, kumquat-sized balls. Steam the balls, then put onto bamboo skewers 5 at a time. Bake the skewers while applying soy sauce with added sugar.

Ordinary dishes

Senbeiimo

Cut sweetpotatoes into about 3-mm hick slices. Dry the slices in the sun and bake them evenly over a fairly low flame. Alternatively, fry the slices in oil - also delicious.

Sozoimo (sweetpotato pickled in sake lees)

Cut sweetpotatoes to adequate size, immerse the pieces in nukamiso (salted rice-bran paste) overnight, remove, and immerse in sake lees.

　

Imozosui (Sweetpotato porridge)

Grate sweetpotatoes, and add to well-boiled rice. Season with miso or salt, then add chopped greens or strained baked sweetpotato as desired.

Rare dishes

Castella imo

Grate sweetpotatoes and mix them with a small amount of sweetpotato starch. Add an equal amount of egg and sugar and form cakes, then roast both sides in a frying pan. Finally, sprinkle poppy seeds over the finished product.　

Yokan-imo

Mix 180 cc of sweetpotato starch, 270 cc of adzuki bean powder and 540 cc of honey. Strain the mixture, boil and knead it in a pot, put into a tier of boxes, cool and cut.

Exquisite dish

Dengakuimo

Grate sweetpotatoes, then add to a thin-walled container, and steam. After steaming, cut pieces to adequate size, slide onto skewers, apply miso to them, and toast over a fire. Use any kind of miso you like, from nut or sansho (Japanese pepper) to horseradish miso.

3. The Future Outlook for Sweetpotato Consumption

(1) With the improvement of living standards, use of sweetpotatoes as a substitute for staple foods has decreased greatly not only in Japan but also overseas. Within Japan, however, sweetpotato consumption per capita has actually been trending upward since 1975.

(2) This trend is supported by strong health consciousness among consumers. A variety of health-related functions for sweetpotatoes have now been identified, including the ability to inhibit lipid peroxide reactions and radical generation, which cause aging and cancer. (More formally, sweetpotatoes possess antioxidant ability and radical scavenger ability). Such functions are especially strong in "Ayamurasaki," which contains an anthocyan pigment giving it purple flesh.

(3) The belief that "medicine and eating habits are based on the same source" can be confirmed by adding these scientific findings to various traditional ways of preparing sweetpotatoes. Our task for the future is to establish a new dietary culture based on new concepts of health and safety, and to pass this down to subsequent generations.

Unique dish	Mitaraiimo	Grate raw sweetpotatoes. Add flour to the grated sweetpotato, and mix to form round, kumquat-sized balls. Steam the balls, then put onto bamboo skewers 5 at a time. Bake the skewers while applying soy sauce with added sugar.
Ordinary dishes	Senbeiimo	Cut sweetpotatoes into about 3-mm hick slices. Dry the slices in the sun and bake them evenly over a fairly low flame. Alternatively, fry the slices in oil - also delicious.
	Sozoimo (sweetpotato pickled in sake lees)	Cut sweetpotatoes to adequate size, immerse the pieces in nukamiso (salted rice-bran paste) overnight, remove, and immerse in sake lees.
	Imozosui (Sweetpotato porridge)	Grate sweetpotatoes, and add to well-boiled rice. Season with miso or salt, then add chopped greens or strained baked sweetpotato as desired.
Rare dishes	Castella imo	Grate sweetpotatoes and mix them with a small amount of sweetpotato starch. Add an equal amount of egg and sugar and form cakes, then roast both sides in a frying pan. Finally, sprinkle poppy seeds over the finished product.
Rare dishes	Yokan-imo	Mix 180 cc of sweetpotato starch, 270 cc of adzuki bean powder and 540 cc of honey. Strain the mixture, boil and knead it in a pot, put into a tier of boxes, cool and cut.
Exquisite dish	Dengakuimo	Grate sweetpotatoes, then add to a thin-walled container, and steam. After steaming, cut pieces to adequate size, slide onto skewers, apply miso to them, and toast over a fire. Use any kind of miso you like, from nut or sansho (Japanese pepper) to horseradish miso.