1. Cassava, Yams, and Aroids
    2. Potato
    3. Sweet potato
    1. Ethnic Markets
    2. Development of New Value-added Products
    3. Sweet potato in Breads and Flours
    4. Use as Animal Feed or an Animal Feed Supplement
  7. Table 1

Vegetables grown for their edible roots or tubers encompass a wide range of starchy root crops, some of which are true botanical roots and others which are tubers or corms. The most economically significant root crops globally include potato (Solanum tuberosum L.), sweet potato (also spelled sweet potato) [Ipomoea batatas (L) Lam.], cassava (Manihot esculenta Crantz), yams (Dioscorea spp.), and aroids [principally Colocasia esculenta (L.) Schott var. esculenta and Xanthosoma spp.]. Although traditionally, root vegetables have been considered low status and generally unimportant crops by consumers, governmental organizations, and researchers, on a global scale they account for three of the seven most important food crops in the world (FAO 1989).
Of these five majors root crops, only potato and sweet potato are grown to any extent in the United States, and of these two, sweet potato has the greatest potential for increased usage and consumption. However, there are other starchy root vegetables grown in various areas of the world where they are of local economic and cultural importance and which could conceivably be considered potential new crops for domestic consumption (O'Hair 1990a, b). Among the most promising may be some of the Andean root crops (Sperling and King 1990). In addition, apios (Apios americana), has received attention as a potential new crop. Apios is unique among the root and tuber crops mentioned in that it fixes nitrogen and also produces edible tubers, fleshy roots, and seeds (Putnam et al. 1991). Tubers are high in protein and carbohydrates (Walter et al. 1986) and are preferred by some to the domestic potato, Solanum tuberosum. The potential of apios as a new crop has been described by Reynolds et al. (1990).

Most root vegetables are produced in developing countries. In developed countries, 99% of root production is in potato (Rhoades and Horton 1990). Fresh weight production of the five major root vegetables (potato, sweet potato, cassava, yams, and aroids) is listed in Table 1 and compared with the other major crops which are produced at a level of 100 million tonnes globally. Potatoes, cassava, and sweet potato rank among the seven major, global food crops which are produced at that level. Yams and aroids are produced at a much lower level, but are important staple food crops on a regional level.
Sweet potatoes and potatoes are produced in the United States (Table 1). Cassava, yams, and aroids are long season crops requiring one to two years for maturity which precludes their being grown in any significant amount in the continental United States. Although some short-season types are available, they still require six months or more for maturity. There are some experimental results indicating that yams could be successfully produced domestically (Marsh 1991).

Cassava, Yams, and Aroids
There is little consumption or consumer familiarity with cassava, yams, or aroids in the United States outside of ethnic populations. This status is not likely to change except for increased consumption by steadily increasing ethnic populations. They are, however, imported into the United States from the Caribbean and Latin America and are available for consumption. Quality of imported products is generally poor unless purchased immediately from a local market. Once the product reaches grocery shelves, it is generally well below fresh quality standards.
Studies in Missouri have shown that yams can be produced in that state by starting them in a greenhouse for six weeks followed by about 22 weeks in the field (Marsh 1988). Yield per plant was much lower than those obtained in tropical growing areas (3.5 kg/plant as opposed to 20 to 30 kg/plant) but yams were of good marketable quality.

Potatoes have been the most widely used root vegetable grown in the United States; current per capita consumption is about 56.4 kg. Quality is excellent, consumption is high, and there are a multitude of products available to the consumer. Potatoes are used in many ways as a vegetable and as a snack food. Even so, new types of potatoes are being introduced to the fresh market for consumers to try. Among these are yellow potatoes and blue potatoes. The unique characteristics of these potatoes are novelty traits which may capture a very small portion of present markets. They do represent a potential for specialty markets at very high prices.

Sweet potato
Sweet potatoes are consumed in the United States on a much smaller scale than potatoes with the per capita consumption now about 5.8 kg. Quality is excellent but few products are available. Normally sweet potatoes can be purchased fresh, canned (whole or cut), as a reconstituted patty, and in baby food. They are most often used in pies or as a candied vegetable. Use is seasonal revolving around Thanksgiving, Christmas, and Easter. This seasonality has been a major problem in marketing efforts to increase consumption. In the past, various products have been developed from sweet potatoes, such as chips and fries, but none have been successful.
Sweet potato does have the potential for increased production and consumption. Sweet potato germplasm, like that of potato, is diverse with many types with different flesh colors, skin colors, textural properties, and nutritional components. There is excellent potential for the development of new products and new types of sweet potatoes to fit various demands of different consumer groups. Breeding efforts in the United States have focused almost exclusively on a single phenotype: a sweet vegetable type (luxury type) which has copper skin, deep orange flesh (high carotene), sweetness, and moist texture (Collins 1987). The American public, through surveys, has expressed a flexibility of choice for some attributes, but the desired marketing type has remained the same. Although this type can still be greatly improved through breeding efforts, it is unlikely that the demand for the luxury type of sweet potato will increase significantly unless new uses and products are developed.

Ethnic Markets
Populations of ethnic cultural groups in the United States are substantial and continue to increase (Marsh 1991). Opportunities exist for American growers in the United States to produce different types of sweet potatoes for those markets. Unlike cassava, aroids, and yams, importation of sweet potatoes into the United States is prohibited by law. Hence, there is little or no supply for potential ethnic markets because the type of sweet potato demanded by those markets is different from the normal phenotype which is now the major focus of domestic commercial sweet potato production. Preferred types are red or white skinned, cream to white flesh, non-sweet or very slightly sweet, and dry in texture. Consumers in these groups are more accustomed to eating sweet potato as a carbohydrate energy source (staple or supplemental staple type) than as a dessert or sweet vegetable (luxury type).
Staple, supplemental staple, and luxury types of sweet potatoes were described by Villareal (1981) and further characterized by Collins (1987). Desired traits for each type are very different and breeding programs must have specific programs for developing acceptable cultivars of these types.
Some efforts do exist to meet the demands of ethnic markets. In Florida, "boniatoes," white-fleshed, dry, firm textured sweet potatoes, have been produced for many years for the Hispanic market. Cultivars used in this production have been brought into Florida from nearby Caribbean islands, such as Cuba (O'Hair et al. 1983), and have not been improved for United States growing conditions. A new Japanese clone recently entered the domestic market and is being grown in California and North Carolina. This cultivar may not be ideally suited to domestic growing conditions. There is ample opportunity for breeders to respond to the need for new phenotypes to meet new market opportunities. No surveys have been conducted to assess potential demand, but it could equal as much as 25% of current production if the crop is properly marketed. Market feasibility studies would establish the extent of the potential market, and should they be positive, convince growers of the profitability of producing this type of sweet potato.
The North Carolina State University sweet potato breeding program has a small project underway to develop the type of sweet potato described. The variability in sweet potato is great enough so that almost any type desired can be constructed with proper breeding procedures. Several clones with different characteristics are in various stages of testing. There are also several research projects underway to study specific characteristics which are important in breeding clones with the necessary traits, such as studies on the heritability and factors affecting ß-amylase activity which lead to non-sweet types. Non-sweetness has proved to be one of the most difficult traits to incorporate in these clones even though non-sweet types exist in the breeding materials. Results of preliminary tests indicate that sweetness and the perception of sweetness is very complex and that lowering ß-amylase activity alone may not necessarily lead to non-sweet sweet potatoes.

Development of New Value-added Products
The range of variability in sweet potatoes is so great that many different phenotypes can be made available for special product development depending on the characteristics needed. Often it is difficult to determine, even through trial and error, what the best characteristics are for particular products. Value-added products such as french fries, chips, and flakes, have been developed from sweet potato but none has been successfully marketed for any length of time. Much effort has been devoted to sweet potato fries. However, consumer comments often refer to the sweetness, texture, and oil content as problems. The products developed, such as fries, have always been developed from the existing single phenotype grown in the United States today and this may be a major reason for the disappointing results with such products.

Sweet potato in Breads and Flours
There have been efforts in the private sector to capitalize on the enormous potential for sweet potato products. At least one patent has been granted for producing sweet potato bread and flour composed of 100% sweet potato. These products are marketed as hypoallergenic for people who cannot tolerate grain breads and flours. The price is quite high (approximately $12/lb. for flour and up to $17 for a loaf of bread) as is demand. The inventor in this case has surmounted the problems normally associated with increased amounts of sweet potato flour in breads. A "boniato" type sweet potato from Florida is used to make these products. As the process is patented, it is not available for general production at present. Several other patents are pending for other processes involving sweet potato (K. Slimak pers. commun.).
Where wheat has to be imported or can be grown only with extensive inputs (such as in developing countries), sweet potato is a viable alternative component for breads and flours. There are disadvantages such as reduced loaf volume and decreased storage life (Keya and Hadziye 1990), but the reduced costs and decreased dependability on imports may outweigh the disadvantages. The acceptability of component breads and flours is excellent with as much as 10 to 15% content of sweet potato flour (Sammy 1984). Some researchers have reported using up to 25% sweet potato flour with no adverse consumer reaction. In the United States, where wheat is plentiful and relatively inexpensive, this alternative use of sweet potato does not appear to be economically justified except as a specialty product.

Use as Animal Feed or an Animal Feed Supplement
Sweet potato is used as an animal feed and supplement in developing countries (Gohl 1981). Both vines and roots are used. Starch and protein digestibility of raw sweet potatoes has been cited as an obstacle to increased use for animal feed. Trypsin inhibitor has been implicated in poor protein digestibility. Genetic variability does exist for level of trypsin inhibitor activity and genotypes with no measurable activity are available. In the United States, competition with maize (Zea mays L.) is a major impediment to increased usage of sweet potato for animal feeds and supplements although interest is growing in the potential use as a component in chicken feed.

Recently, much attention has been focused on minor crops which are very important to the culture and subsistence of local farmers in many parts of the world. Concern has been raised about the future security of the genetic resources of these minor crops. In particular, recent attention has been focused on the Andean root and tuber crops. In addition to potatoes, several roots and tubers have played a significant role in the Andean culture and are cultivated in that area. Two of these which are more well known than others could be domestically grown. A third is lesser known but could also have some potential for production.
Oca (Oxalis tuberosa Molina, Oxalidaceae) is probably the most well known root vegetable other than potato in the Andean region. It is quite unusual in appearance with a brightly colored edible tuber and a pleasant mild flavor. Oxalic acid is a component of the tubers, but levels are generally not higher than those found in some other popular crops (S. O'Hair pers. commun.). Oca is commercially produced in Peru and has been commer-cialized in New Zealand (Yamaguchi 1983). A number of clones are grown and many accessions have been collected (S. O'Hair pers. commun.).
A second Andean root vegetable, ulluco or olluco or ullucu(s) (Ullucus tuberosus Caldas, Bassellaceae) is also grown commercially in the Andean regions of Peru. This is also a crop of wide genetic diversity with perhaps 50 to 70 clones being grown in the Andean region (S. O'Hair pers. commun.). External colors of tubers can be white, yellow, green, or magenta; the plants are frost resistant and require 140 to 150 days for tuber development (Yamaguchi 1983).
Mashua or anu (Tropaeolum tuberosum Ruiz & Pav. Family Tropaeolaceae) is a third Andean root vegetable and is related to the ornamental nasturtium. It is not commercially produced on the same scale as the other two crops. Consequently, it is not as well known as the other two. It is said to have medicinal properties and some cultivars may have pesticide properties (S. O'Hair pers. commun.).
Each of the three Andean root vegetables can be eaten fresh or dehydrated. All are used in the Andean region, along with potato, to make chuo, a dehydrated and frozen product which can last several years and serves as a secure food source when necessary. Each has the potential to be grown in certain areas of temperate climates.

There are several opportunities for increased production and usage of root and tuber crops in the United States. Of the five major root crops, sweet potato has the greatest potential for new uses and increased production. As sweet potatoes cannot be imported in the United States, there is no significant current supply for specific types of sweet potatoes suited to ethnic cultural preferences. Growers in North Carolina, who produce over 30% of the sweet potatoes in the United States, have been reluctant to begin production of new types for ethnic markets. Market feasibility studies need to be undertaken to determine the potential markets.
With respect to new and unknown root crops, three Andean root crops, oca, ulluco, and mashua, offer potential for new production. However, a lack of funds to investigate the possibilities may preclude any development.

  • Collins, W.W. 1987. Improvement of nutritional and edible qualities of sweet potato for human consumption, p. 221-226. In: Exploration, maintenance and utilization of sweet potato genetic resources: Report of the first sweet potato planning conference, International Potato Center, Lima, Peru.
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  • Rhoades, R. and D. Horton. 1990. p. 8-19. In: R.H. Howeler (ed.). Proc. 8th Symposium of the International Society for Tropical Root Crops. Oct. 30-Nov. 5, 1988. Bangkok, Thailand.
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Table 1. Production of major world food crops and root cropsz.
(000 tons)
United States
(000 tons)
Sweet potato
Root crops total
zSource: FAO 1989 Production Yearbook (FAO 1990)
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