Sweet potato (Ipomoea batatas), a dicotyledonous plant belongs to the family convolvulaceae (Ajani et a.l, 2016). Root crops were regarded as food mainly for the poor, and have played a very minor role in international trade. This misconception has lingered for so long because of the lack of appreciation of the number of people who depend on these root crops, and the number of lives that have been saved during famine or disasters by root crops. Root crops contain an appreciable amount of carbohydrate, vitamins and minerals and may have a competitive production advantage in terms of energy yield per hectare over cereals produced in ecologically difficult conditions (FAO, 1990).
Root crops are an important food source for one fifth of the total world population. With the escalating population, we may have to rely on root and tuber crops more. They are naturally energy rich and have been known to save lives during drought and famine. Taro, for example, has served as a staple in many countries in Asia, Africa, and the Pacific. Based on recorded global gross production data, taro is the fourteenth most consumed vegetable in the world (Lebot et al., 1991).
Root and tuber crops play multi-purpose roles in the global food system as a starch supplier, food security crop, source of cash income, raw material for feed and processed products, and as key components in small-scale agro-enterprise development. The relative importance of individual root crops varies both by region and country. For example yams are a major food crop in West Africa, the Caribbean, the south Pacific Islands, South-East Asia, India and some parts of Brazil. Cassava is particularly important in South America, west East, Central and South Africa and Oceania. Taro plays an important cultural role in the diet of the people of the pacific Islands, West Africa, Oceania and the West Indies (UNIFEM, 2002).
Sweet potato (Ipomoea batatas L.) is a dicotyledonous plant which belongs to the family of convolvulacea (Tewe et al., 2003). United Nation’s Food and Agriculture Organization (FAO) (1990, 2011) reported that sweet potato (Ipomea batatas L.) is a very important crop in the developing world and a traditional, but less important crop in some parts of the developed world. According to (FAO, 2011), sweet potato is one of the seven crops in the world produce over 105 hundred million metric tonnes of edible food products in the world annually. Only potato and cassava, among the root and tuber crops, produce more. China alone produced 80 to 85% of the total sweet potato production in the world while the remaining countries in Asia have the next highest production and then, followed by Africa and Latin America (Centro Internacional de la Papa, 2009). It is among the world’s most important and under-exploited food crops. With more than 133 million metric tonnes in annual production(Scott and Maldonado, 1999; Grant, 2003), sweet potato is the world’s seventh most important food crop after wheat, rice, maize, potato, barley and cassava (Tewe et al., 2003).
It grows under many ecological conditions, has a shorter growth period than most crops (Oke, 1990). Sweet potato is widely grown as a staple food in many parts of the tropical and subtropics, which include developing countries (Odebode, 2002). Its production is highly seasonal in most countries leading to marked variation in the quantity and quality of roots in markets and associated with price swings (Faostat, 2008; Centro Internacional de la Papa, 2009).
Sweet potato is a minor root crop in tropical Africa despite its industrial potentials as indicated by its growth in terms of production (Tewe et al., 2003). Among the root and tuber crops, it is the only one that had a positive per capita annual rate of increase in production in Sub-Saharan Africa (Hahn, 1984). Sweet potato roots are bulky and perishable unless cured. This limits the distance over which sweet potato can be transported economically. It will be established that in cases where countries are capable of generating surplus, it tends to be relatively localized but dispersed and this leads to a lack of market integration and limits market size (Katan and De Roos, 2004; FAO, 2011).
It has been a life saver for centuries in many tropical, sub-tropical and warmer temperate areas of the world, warding off famine in times of both climatic disaster and war. Sweet potato plays a major role as a famine reserve for many rural and urban households because of its tolerance to drought, short growth and high yield with limited inputs on relatively marginal soils (Tewe et al., 2003).
Despite the fact that sweet potato commonly categorized as a subsistence, “food security” or “famine relief” crop, its uses have diversified considerably in developing countries over the last four decades (Grant, 2003). Sweet potato consumption has been adjudged to decline as incomes rise - a change often linked with urbanization, partly because it is perceived as a “poor man’s food” but mostly because of the lack of post-harvest processing or storage (Faostat, 2008; Centro Internacional de la Papa, 2009). The tubers can be steamed, roasted, boiled, baked, and fried. It has the potential of bridging food gap due to diversified processing and utilization technologies that have been produced but not yet fully exploited (Nungo et al., 2007).
Sweet potato is a very efficient food crop and produces more dry matter, protein and minerals per unit area in comparison to cereals (Woolfe, 1992). Research have reported that sweet potatoes being the staple food in the developed countries account for 130 kcal of energy per person per day against 41 kcal in the developing countries where it is still considered as vegetable. Apart from being a rich source of starch, sweet potatoes contain good quantity of secondary metabolites and small molecules which play an important role in a number of processes (Friedman, 1997). Many of the compounds present in sweet potato are important because of their beneficial effects on health, therefore, are highly desirable in the human diet and functions as a functional food (Katan and De Roos, 2004).
Also, it is a low input crop and it is used as a vegetable, dessert, source of starch and it is eaten as a substitute for yam due to its lower cost of production. Sweet potato is comparatively a nutritional heavy weight; rich in complex carbohydrates, vitamins C and E, and also contains good quantities of vitamins A and B, calcium and iron (Huang et al., 1999).
Processing root crops especially sweet potato into convenience foods will improve their being accepted by the urban population. This will lead to expand markets and thus encourage the increase production of root crops and also the use of processed foods based on local products to replace imported foodstuffs will also conserve foreign exchange (Olapade and Ogunade, 2014). There are wide ranges of snack items including potato chips, maize chips, puffed dough, cookies and crackers (Fazzolare et al., 1997). Sweet potato skin colors come in various shades of creamy white, yellow-orange, tan, reddish-purple and red. Sweet potato has also been used in the production of purees and these can be used as an ingredient in various products including baby food, casseroles, puddings, pies, cakes, bread, restructured fries, patties, soups and beverages (Truong et al., 1995).
1.2 Proximate composition
Sweet potato (Ipomoea batatas L.) has played an important role as energy and a phytochemical source in human nutrition and animal feeding. This tuberous root is a rich source of carbohydrates, dietary fiber, vitamin A (as β-carotene), vitamin B6, vitamin C (Anbuselvi and Balamurugan, 2014).
1.2.1 Nutritional Composition of Sweet Potato
Sweet potato are rich in dietary fiber and have high water content and also provide 359 kJ energy with low total lipid content, which is only about 0.05 g per 100 g. In addition, sweet potatoes also are high in minerals such as potassium, calcium, magnesium, sodium, phosphorus, and iron (USDA, 2009). Because of the various roles that sweet potatoes play around the world, the concept of nutritional quality and its contribution must transform to meet specific roles in human diet. For instance, staple type diets could require high vitamin C, iron, potassium, protein and as well as high fibre. Similarly, supplemental types of sweet potato must have many of the same characters as staple types in terms of nutritional components. However, as they will not be major food component, the level of components may be more flexible and good (Oke and Worknehm, 2013).
Sweet potato is a minor root crop in tropical Africa despite its industrial potentials as indicated by its growth in terms of production. Among the root and tuber crops, it is the only one that had a positive per capita annual rate of increase in production in Sub-Saharan Africa. Sweet potato has a high yield potential that may be realized within a relatively short growing season and adaptability to a wide ecological range of 0 to 2000 meters above sea level and 30° N to 30°S. It has been a life savior for centuries in many tropical, sub-tropical and warmer temperate areas of the world, warding off famine in times of both climatic disaster and war. Sweet potato is the world’s seventh most important food crop after wheat, rice, maize, potato, barley and cassava. It grows under many ecological conditions, has a shorter growth period than most crops and shows no marked seasonality (Olapade and Ogunade, 2014).
Sweet potato skin colors come in various shades of creamy white, yellow-orange, tan, reddish-purple and red. Sweet potato has also been used in the production of purees and these can be used as an ingredient in various products including baby food, casseroles, puddings, pies, cakes, bread, restructured fries, patties, soups and beverages (Truong et al., 1995; Walter et al., 2001).
Lack of industrial or village-level processing of the crop and low levels of commercialization are major constraints to increased production of sweet potatoes. The major avenue left for preservation of sweet potato is processing into secondary products. There are wide ranges of snack items including potato chips, maize chips, puffed dough, cookies and crackers. Processing root crops especially sweet potato into convenience foods will improve their being accepted by the urban population. This will lead to expanded markets and thus encourage the increased production of root crops and also the use of processed foods based on local products to replace imported foodstuffs will also conserve foreign exchange (Olapade and Ogunade, 2014).
1.2.2 Antinutritional Factors
Compounds, which act to reduce nutrient utilization and/or food intake, are often referred to as antinutritional factors. These toxic compounds may occur in all parts of the plant, but the seed is normally the most concentrated source. Food crops regularly eaten have many beneficial nutrients but there are traces of antinutritional components such as cyanoglucosides, oxalates, phytic acid, phenolics, protease inhibitors, heavy metal etc. These antinutritional factors when consumed in foods may have adverse effects on health through inhibition of protein digestion, growth, and Fe and Zn absorption (Omoruyi et al., 2007). Taro contains antinutritional factors such as: oxalates, tannins, phytates, trypsin inhibitors, amylase inhibitors and in some cultivars cyanide (FAO, 1990).
Boiling is effective method in reducing water soluble antinutrients. For example boiling of root crops such as taro and cassava could lead to significant reduction of oxalates and cyanide respectively. Boiling also found to decrease some amount of soluble phytate (Agbor et al., 1995).Both taro and yam in their raw forms are toxic. The toxin is however destroyed by processing techniques such as cooking, soaking, ensiling and drying (FAO, 1999). In Yeki woreda, sweet potato has been widely cultivated and consumed. About 15 hectars of the woredas farm land has been covered by this vegetable. From this sweet potato farm land of the woreda 250 tonnes of sweet potato has been produced annually. The varieties of sweet potato predominantly cultivated in the woreda are yellow and red colored (Yeki woreda agriculture office, 2017). The present study aimed to assess selected proximate composition and antinutritional factors content of sweet potato tuber.