Sweet potato (Ipomoea batatas) is a dicotyledonous plant belonging to the family Convolvulaceae. It is popularly known for its starchy and edible tubers used as a root vegetable. The young leaves and shoots are sometimes eaten as greens. Sweet potato is a stable food crop widely cultivated in most parts of Africa, especially in Nigeria, though it's believed to have originated from Central or South America.
In Nigerian markets, three varieties are in popular demand: the purple, yellow, and white potato. Though some varieties are popularly known as the Irish potato, indicating their non-nativeness, the crop is widely cultivated and has become a staple food in all parts of Nigeria.
Sweet potatoes were introduced to Nigeria by the Portuguese in the late 1600s. Today, the white-fleshed variety, with purple or beige skins, has taken root and now features in the top ten of cultivated roots and tubers in Nigeria, up there with cassava, yam, and potatoes.
Nigeria is the largest sweet potato producer in West Africa and one of the largest producers globally. Sweet potatoes are enjoyed in many ways - boiled, roasted, fried, pottaged, and incorporated into kunun, a drink-pap made from cereals and/or grains with origins in northern Nigeria.
Diversity and Characteristics
According to research, there are about 6,500 sweet potato varieties, including wild accessions, farmer varieties, and breeding lines. The skin colors of sweet potatoes come in various shades: yellow-orange, creamy white, tan, reddish-purple, and red.
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Sweet potato varieties with white or pale yellow flesh are less sweet and moist than those with red, pink, or orange flesh. The primary center of diversity of sweet potatoes is located in North Western and South America (Colombia, Ecuador, and Peru) and parts of Central America (such as Guatemala), where a great diversity of native sweet potatoes, weeds, and wild ipomoea exists. Secondary centers of sweet potato diversity outside of the Americas are in China, South East Asia, New Guinea, and East Africa.
Ipomoea batatas is not known in the wild, and plants found growing in the wild are remnants from abandoned cultivated fields or coming from sweet potato seeds, which continue growing by vegetative propagation. Several wild ipomoea species having some morphological resemblance to Ipomoea batatas have been considered potential wild ancestors of the sweet potato.
West African Sweet Potatoes
West African sweet potatoes are medium to large in size and have an oblong shape with one bulbous end tapering to a slender, pointed tip. The cream-colored skin is semi-rough, covered in brown spots and patches, and bears a few medium-set eyes that are scattered across the surface. Underneath the thin skin, the pale white to cream-colored flesh is dry, slightly sticky, firm, and starchy.
Cultivated for processing into flour, slicing into chips, or used in everyday cooking, West African sweet potatoes are favored by local farmers as a source of ground cover for gardens and for their extended storage capabilities. West African sweet potatoes are best suited for cooked applications such as boiling, pureeing, mashing, baking, deep-frying, stuffing, and steaming.
In Africa, sweet potatoes can be served at any meal of the day and are commonly boiled and consumed plain or served with a peanut-tomato sauce. They can also be cubed and mixed into soups and stews such as mafe, which is a traditional stew made with seafood, meats, or vegetables, or they can be fried, mashed, flattened into pancakes and fritters, or sliced and baked into French fries.
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In addition to cooking, West African sweet potatoes can be peeled, dried, and ground into flour for extended use. In Nigeria, the leaves are also consumed and are boiled into a tea to help reduce symptoms associated with asthma and gastrointestinal issues.
West African sweet potatoes pair well with peanuts, meats such as chicken, lamb, smoked fish, and goat, broccolini, kale, okra, green beans, eggplant, coconut, tomatoes, onions, garlic, and ginger.
Morphological and Anatomical Characteristics
A study was conducted to reveal the anatomical (leaf epidermal) and morphological similarities and differences of three varieties of sweet potato (Purple, Yellow, and White varieties) to establish the degree of relatedness or divergence among them and establish the phylogenetic relationship that exists amongst them.
Ten (10) tubers of the representative samples of three varieties of sweet potato (Purple, Yellow, and White varieties) were collected from different locations (Mararaba, Masaka, Keffi, Lafia, Makurdi, Gboko, Jos and Suleja) within the north central part of Nigeria and planted in the research farm of the University of Agriculture, Makurdi, Nigeria. At full maturity, they were characterized using macroscopic and microscopic taxonomic sources of evidence.
Epidermal peels of the leaves were prepared for microscopic studies following standard protocols. Macromorphological characters, including leaf length, leaf breadth, petiole length, plant height, root length, and stem diameter, were assessed. Qualitative and quantitative data generated were analyzed using SPSS software.
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Key Findings
The lowest number of stomata was recorded in the Purple variety (21.25) but possessed the highest number of epidermal cells (54.4). Consequently, the Purple variety had the lowest stomatal index (28.09%), which varied widely from the indices obtained in the Yellow and White varieties (40.06% and 41.5%).
The purple type differed from others in terms of stomatal indices and the number of epidermal cells. Only the Purple variety has hexagonal-shaped epidermal cells apart from the polygonal and irregular shapes common to all the varieties. Stomata types were tetracytic, anisocytic, and paracytic in the three varieties, but the Purple Variety had an additional stomatal type (anomocytic).
Macroscopically, wide variation was observed with respect to some qualitative characters (leaf apices, leaf shape, leaf margins, leaf bases, leaf color, stem color, root color) and quantitative characters (leaf sizes, petiole length, stem diameter, root sizes, and plant height).
Statistical significant differences were recorded in the two sources of taxonomic evidence used in this study (p < 0.05). As a result, the dendrogram classified the three varieties clearly.
The study suggests that the Yellow and White varieties could be described as two separate species under the Genus Ipomoea, while the White type could be seen as a variety of the Yellow type based on their close relationship. This is substantiated by their root color differences.
The synergistic information obtained from both morphological and anatomical sources of evidence in this report has yielded reliable results that may call for appropriate nomenclatural assignments. Hence, the authors suggest the following nomenclatures:
- Ipomea batatas var makurdi (Yellow variety)
- Ipomoea batatas var lafia (White variety)
- Ipomoea aguoru (Purple variety)
The first two are named based on the location of collection, while the third is named based on the author’s name.
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Orange-Fleshed Sweet Potatoes (OFSP)
Nigeria’s National Root Crops Research Institute (NRCRI) - Umudike and the International Potato center (CIP) have released a new orange-fleshed sweetpotato (OFSP) variety. Solo Gold-UMUSPO4-was officially released in July 2018, bringing the total number of OFSP varieties in Nigeria to three.
Solo Gold has special characteristics preferred by farmers, such as a higher dry matter when compared to Mothers Delight, and a much higher beta-carotene content when likened to King J. Also, it’s tolerant to sweetpotato weevil and resistant to sweetpotato viral disease, and matures in three to four months.
OFSP is a nutritious type of sweetpotato that is additionally rich in beta-carotene, a precursor of vitamin A. The orange color of OFSP is indicative of the level of beta-carotene present; the more intense the color, the more vitamin A present.
OFSP as a staple food in Nigeria can serve as an affordable and sustainable source of Vitamin A, especially for vulnerable populations. It is commonly consumed as a vegetable (boiled, fried or roasted) as well as in different products through processing and value addition for improved household food intake. These foods include amala (swallows), puff-puff, chips, cake, gari, vegetable soup, doughnut, pottage, bread, chin-chin, juice and kunu.
Sweet Potato in West African Cuisine
Sweetpotato (Ipomoea batatas (L) Lam) is of increasing importance for food and nutrition security in sub‐Saharan Africa (SSA). It is a versatile climate‐resilient crop that is easily propagated and can grow with few external inputs on degraded soils under a range of rainfall patterns. This is an advantage for poor households dependent on diverse livelihood strategies.
Sweetpotato varieties grown in SSA include diverse landraces (selected by farmers) and several varieties that are superior with respect to production and nutritional value developed by breeding programs which have emphasized pro‐vitamin A‐rich orange‐fleshed types during the past two decades.
In West Africa, sweetpotato is mostly used as a snack food, commonly sold in urban centers and rural markets as a fried product in the form of large slices or ‘chunk fries’. An estimated 30-80% of total sweetpotato production (mostly white or yellow‐fleshed) is fried in parts of Nigeria, Ghana, Burkina Faso and Côte d’Ivoire.
Consumer Preferences and Variety Selection
Households in each community grow from four to eleven sweetpotato varieties. The most preferred variety by women in Kano State was the local landrace Dan Izala (white‐fleshed) because of its high yield, short maturity period (<4 months) and better market price. Men grew Dan Izala for its early maturity and drought tolerance.
White‐fleshed local landrace Dan Bakalori was the most preferred variety by men in Kano because of its short maturity period, but women reported growing it for its good taste and flesh softness. Another variety regarded highly by both men and women in Kano was the white‐fleshed local landrace Dan China.
In individual interviews in Kwara, women preferred the white‐fleshed variety Pakurumon and the yellow‐fleshed variety Alausa, and men preferred the yellow‐fleshed variety Alausa and the orange‐fleshed variety, Mother’s Delight. Similarly, the FGD data revealed that Alausa was the most grown variety for both genders. Men chose growing Alausa because of its marketability, short maturity period, and low sugariness, whilst women desired its marketability.
In Bawku, the white‐fleshed local landrace Obare was the most preferred variety, irrespective of the producer’s gender. The orange‐fleshed varieties Kuffour (landrace) and Apomuden (bred) were also regarded highly by men and women, respectively.
Both men and women farmers in the study areas reported growing sweetpotato varieties that are high yielding, early maturing, meet their taste preferences regarding sugariness, are easily marketable, and store well.
Global Production and Consumption
In 2020, global production of sweet potatoes was 89 million tonnes, led by China with 55% of the world total.
The annual per capita consumption of sweet potatoes in the United States is only about 1.5-2 kg (3+1⁄2-4+1⁄2 lb), down from 13 kg (29 lb) in 1920.
Nutritional Value
Cooked sweet potato (baked in skin) is 76% water, 21% carbohydrates, and 2% protein, and contains negligible fat. In a 100-gram reference amount, baked sweet potato provides 359 kilojoules (86 kilocalories) of food energy, and rich contents (20% or more of the Daily Value, DV) of vitamin A (120% DV), vitamin C (24% DV), manganese (24% DV), and vitamin B6 (20% DV). It is a moderate source (10-19% DV) of some B vitamins and potassium.
The young leaves and vine tips of sweet potato leaves are widely consumed as a vegetable in West African countries (Guinea, Sierra Leone and Liberia, for example), as well as in northeastern Uganda, East Africa. According to FAO leaflet No. 13 - 1990, sweet potato leaves and shoots are a good source of vitamins A, C, and B2 (riboflavin).
