<Questions and Answers about Sweetpotatoes>

The Japan Society for Root and Tuber Crops (JRT) maintains a Questions and Answers Corner on its Website which accepts questions from the general public. General questions are answered by JRT staff, and more complex ones are fielded by experts via the Sweetpotato E-mail Mailing List maintained by JRT.

Below is a compilation of the most frequently asked questions.

Physiology/Ecology

Q 1-1 Does the sweetpotato flower?

A: Yes, sweetpotato normally flowers and produces seeds only in tropical and subtropical areas. Therefore, it is unusual for it to flower in Japan outside of Okinawa Prefecture. When such a rare event does occur in fall, cold prevents those flowers from producing seed.

To produce improved sweetpotato varieties requires crossing varieties having desirable characteristics. Sweetpotato is forced to flower and produce seed by grafting sweetpotato shoots to the rootstock of a dwarf morning glory variety.

For more information:

Laboratory of Sweetpotato Breeding, Kyushu National Agricultural Experiment Station, maintains an Internet site "Sweetpotato Web." Click on "breeding" to view sweetpotato flowers, and learn more about how new sweetpotato varieties are bred.

http://duf.mykz.affrc.go.jp/sweetpotato/index.html

Q 1-2 It is said sweetpotato is high in dietary fiber. Just how much does it contain?

A: According to the Estimated Dietary Fiber Content chart prepared under guidance of the Science and Technology Agency, sweetpotato's values are compared with burdock root and soybeans. Though the latter two foods contain more dietary fiber per 100 g than sweetpotato, their serving sizes are smaller. Looking at the amount of dietary fiber contained per normal serving of these three foods, sweetpotato is by no means inferior.
To learn more about dietary fiber, also refer to "Sweetpotato Facts, 8) Functional components of sweetpotatoes."

Q 1-3 Why is baked sweetpotato sweet?

Baked sweetpotato becomes sweet through the action of the enzyme beta-amylase which breaks down cooked starch into maltose. Since this enzyme is most active at 70 degrees C, the longer this temperature is maintained during baking, the sweeter the baked sweetpotato will become.

<Additional information about amylase>

1. The amylase enzymes hydrolyze starch, a polysaccharide, and are of two types: alpha-amylase (found in animal saliva, and so on) and beta-amylase (found in plants).

2. Alpha-amylase randomly hydrolyzes starch, while beta-amylase cuts off maltose molecules in an orderly fashion from oxidized ends of starch. Maltose is composed of two glucose molecules making it a disaccharide, a sugar composed of two smaller sugar molecules. (Table sugar, or sucrose, is also a disaccharide, but composed of glucose and fructose.)

3. Beta-amylase cannot act on starch granules till they are gelatinized by heat and water. The enzyme will then continue to break down starch into maltose till it is destroyed by excess heat.

4. Starch granules begin to gelatinize at 65-75 degrees C. Sweetpotato beta-amylase is more sensitive to temperature than that from other plants or bacteria. The sensitivity varies according to sweetpotato variety, but the beta-amylase is destroyed at about 70 degrees C.

5. It is not well understood what role beta-amylase plays in the growth of sweetpotato.

Miscellaneous

Q 5-2 Where is sweetpotato most consumed in the world?

A: According to the Food and Agriculture Organization of the United Nations (FAO), world average annual per capita sweetpotato consumption is 11.6 kg (1996-98 average). Individual countries with the highest annual per capita consumption are the Solomon Islands (173.8 kg), Rwanda (117.4 kg), Burundi (96.7 kg), Papua New Guinea (86.3 kg), Uganda (79.4 kg), China (41.1 kg), and so on. Especially in the case of China, sweetpotato is mostly consumed as starch used in various food products.

Kenya, Madagascar, Comoro Islands, Vietnam, and so on each have an annual per capita sweetpotato consumption of about 20 kg. Japan at 7.1 kg is 32nd in the world in sweetpotato consumption, and South Korea at 4.3 kg is 42nd. (Part of Japan's sweetpotato consumption is in the form of starch which is used in various food products.)

It must be kept in mind that the accuracy of statistics varies widely from country to country. Also, in developing countries there is often a big difference in diet between the city and rural populations.

Additional material from JRT's "Questions and Answers Corner" is to be translated later.