Nutritional Potential and Antnutritional Factors of Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica Tubers: A Review

In Ethiopia, root and tuber crops play a significant role for food and nutritional security by filling food shortage that might exist until harvest of major crops. The objective was to review the Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica tubers which are indigenous crops to Ethiopia with good nutritional value and drought tolerance. Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica are a good sources of carbohydrate, minerals and fiber. Little is known about the contribution of these crops to food and nutritional security, nutritional value and major limiting factors to their wider utilization in Ethiopia. These crops are generally regarded as poor man’s food and remain neglected compared to other food crops in human diet. The anti-nutritional factors available in these indigenous crops can be reduced by different processing methods like fermentation, boiling and cooking to tolerable level. The generated information will give insights for their development, promotion and wider utilization to support food insecurity crises and calorie gap for the fast growing population in Ethiopia. Generally, the Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica tubers crop have appreciable nutritional composition of foods, minerals, ant nutritional factors and they are also used to maintain the fractured bone of human being. Thus, the crops should be included in people’s daily diet and solve the food insecurity problems in the country. All the stakeholders should give due enphasis to these indegenous crops and process to different food products.


Introduction
Several agricultural crops are known to be originated from Ethiopia and it is also stated that Ethiopia is well known for its diversity of indigenous food plants, of which 27% are cultivated vegetables by traditional farmers in home gardens and about 29% are non-cultivated vegetable species [1]. They can contribute to food security of rural dwellers either through direct consumption or as a source of cash income. However, food insecurity crisis is still persisting being prevalent in many parts of Ethiopia [25]. This is mainly due to highly selective and restricted food consumption habit of the population as well as less exposure to important indigenous food plants such as Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinicatubers [23].
Ethiopia has abundant natural food sources that can be used to mitigate malnutrition and food insecurity among which root and tuber crops are the pillars [28]. Root and tubers crops are consumed by one third of the world population next to cereals being a good source of carbohydrates worldwide [14]. They provide a substantial part of the world's food supply, animal feed and processed products for human consumption and industrial uses [14].
Food insecure and subsistence farmers of developing countries depend highly on root and tuber crops as a principal food source of nutrition and cash income [36]. These crops are used as a major food sources during the failure of other food plants during erratic rainfall since they are drought tolerant and high yielding in harsh weather conditions [37]. Since indigenous crops like Cocciniaabyssinica, Plectranthus edulis and Dioscoreaabyssinicahave good nutritional value and also resist drought climatic conditions their potential productivity and the way of consumption of these crops should be changed. Cocciniaabyssinica, Dioscoreaabyssinica and Plectranthus edulis are very important in South and South Western Ethiopia [6]. Although these crops are drought resistant and food security crops in drought prone areas, their potential as a human food medicine has not yet been fully recognized and utilized [24].
Though this species has promising importance and easily grown in areas where it is stapled and co-stapled food item, it has been an underutilized plant by Ethiopians. Food and nutritional insecurity are the major challenges in developing countries including Ethiopia. To solve this problem, the search for alternative food that is neglected, underutilized, and wild edible food crops has the potential to address food insecurity [41]. Thus, purpose of this review is that to review nutritional composition, anti-nutritional factors of Cocciniaabyssinica, Dioscoreaabyssinica and Plectranthus edulis underutilized tubers crop indigenous/ Native to Ethiopia.

Methods
To evaluate the Nutritional Potential and Anti-nutritional Factors of Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica tubers a narrative review was applied. Recent and specific articles about Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica were exhausted. The historical background, agricultural and traditional development, antinutritional factors and processing methods to reduce antinutritional factors, current status and future perspectives of the roots and tubers as food crop have been also thoroughly assessed.

Agricultural Performance of Plectranthu edulis, Cocciniaabyssinica and Dioscoreaabyssinica
Ethiopian potato (Plectranthus edulis) is one of the underutilized tuber crops indigenous to Ethiopia [43]. Eating underutilized edible plants has been looked as a sign of poverty which is largely due to lack of knowledge on their nutritional benefits. The crop is widely grown in South and South Western parts of the country. The tubers of Plectranthus edulis are clearly stem tubers with the pairs of "eyes" (that is compound auxiliary buds) being arranged in the same alternating pattern as the axillary buds on stolon and stem [35]. P. edulis is an old crop according to the information from the growers. Plectranthusedulisis important to the cultural, social and economic life of the household. It is particularly important in local diets mainly between September and November [28]. It is highly valued for its contribution to food security in these periods since other food crops will not be ready for consumption.
Plectranthus edulis is an indigenous plant to Ethiopia growing in mid and high altitude areas [46]. It is known to Ethiopia due to its horticultural uses since it is fast growing, produce lovely flowers which are resistant to most pests and plant diseases [46]. Though plectranthus edulis is mainly cultivated for its edible tuber, the leaves are also eaten after cooking like vegetables in some parts of Ethiopia, particularly important in local diets mainly between September and December since other food crops are not ready for consumption [52].
Coccinicaabyssinica is an annual trailing vine belonging to the curcurbitacceae family. It is native to Ethiopia and local known as Anchote [1]. The crop is widely grown and utilized in Western, Southern and Southwestern parts of Ethiopia [17]. Coccinicaabyssinica produce a single tuber and the size of the tuber gets bigger as the plants are maintained longer in the field (personal communication with farmers). The tubers could remain in the soil with little quality loss until the next planting season [39]. This shows that the crop can be harvested at any time of the year once it attains full maturity. Coccinicaabyssinica is the highest yielding root crop [15]. Thus, the crop demands comprehensive research attention so as to develop a high yielding variety using the existing germplasm through breeding.
Different Studies revealed that Coccinicaabyssinica is high yielding crop compared to the other root and tuber crops found in Ethiopia [29]. The plant has been grown over a wide range of environments for a long time, and its cultivation and utilization have been passed from generation to generation through oral tradition with very little record information [1,44]. Coccinicaabyssinica is produced on several hectares of land in Western Oromia zones, with an average yield varying from 100-180 quintals per hectare [2,22].
Dioscoreaabyssinica, belongs to the family Dioscoreaceae and the genus Dioscorea, is a climber plant twining to the right, with an herbaceous stem and a large tuber [13,26]. It is cultivated during the raining season in the South, West and the South-West highlands of Ethiopia [39]. Tubers of Dioscoreaabyssinicahave long been used as food as a major source of starch in human nutrition [39]. Locally, Dioscoreaabyssinicais commonly known by its vernacular name "Boyna" which is equivalent to the common English name "Yam". The plant is serving as a source of food within a restricted region of the country (Southern Nations Nationality Peoples Regional State) and few neighboring cities and rural areas to this region [10]. This might be due to lack of information on its potential as a source of important nutrients such as starch which are the main source of calorie. It is also known that Ethiopia is one of sub-Saharan countries with various indigenous resources but poorest country in the Horn of Africa.

Benefits of Plectranthusedulis, Cocciniaabyssinica and Dioscoreaabyssinica
The importance of root and tuber crops as staple foods is linked to particular agronomic advantages; they are well adapted to a diverse soil and environmental conditions and a wide variety of farming systems [30]. They are also a high efficient source of edible carbohydrates when compared with other food crops. With diversification of conducive agro-ecologies that enable all year round production, reliable water source the country has will enable to produce these root and tubers.
Plectranthusedulisis good sources of carbohydrates, proteins and also used as a source of income in regions of Ethiopia where it is cultivated. It also contains secondary metabolites including phenolics, terpenoids, cardiac glycosides and flavonoids that are needed to combat various kinds of diseases like asthma [52]. Eating underutilized edible plants has been looked as a sign of poverty which is largely due to lack of knowledge on their nutritional benefits.
Plectranthus edulis is used for household consumption. Women farmers play major role in food preparation and food preparation of these crops involves boiling peeled or unpeeled tubers. The peeled tubers are also used for preparing stew, which is consumed with bread or enjera (thin bread prepared from grains cereals). Plectranthus edulis, apart from regular uses as food the crop is used also as cultural and medicinal uses.
Coccinicaabyssinicawith its potential productivity has advantages that encourage its development and utilization. The diversified advantages of the crop are nutritional, economic, medicinal and social values [21,29]. The crop contains relatively high protein with good amino acid composition, and provides good composition of important nutrients like calcium, potassium and iron beside the abundant carbohydrate and fiber content [1,11].
Coccinicaabyssinica has a special place in the traditions and customs of the oromo people in the crop growing areas. According to Abera Hora [1], the inclusion of Coccinicaabyssinica in dishes served at ritual ceremonies is prestigious. The crop dishes in different forms are usually served in special occasions such as 'masqal' celebration in September, Weddings, Marriage ties, Circumcision, birth days and thanks giving days at the start of a New Year or harvest time. During such occasions and/or at times of physical injuries, a neighbor that has no Coccinicaabyssinica for that season may get a present of Anchote tubers from those who have it. This is done to share their happiness and strengthen social relationships. Anchote (Coccinicaabyssinica) has considerable social importance in the Anchote growing societies.
Apart from their regular uses as food, Coccinicaabyssinica have cultural and medicinal uses. For example, the crop is used to maintain bone fracture in humans [28,3]. This could be associated mainly with high calcium content of the crops [39]. The crop is known to cure various human diseases in different areas of Wollega zones [16]. PremKumar Singh et [34] reported that the effective uses of traditional food plants that meet the nutritional requirements also used as a remedy for various diseases.
Study carried out by EHNRI [19] on the nutritional content of the edible portion of both raw and cooked tubers of root tuber crops showed that the crops are rich in various micro macro nutrients. For instance, Amsalu et al. [8] reported that Coccinicaabyssinica help to alleviate protein sources. Thus, growing indigenous root and tuber crops will have significant roles in ensuring food and nutrition security, for diversification of local food bases, for income generation and maintaining people's cultural practices.
Dioscoreaabyssinic a tuber have a dual agricultural function. It is used as source of food and as a planting material [27]. The most common use of the crop is as boiled vegetable with some kind of sauce, but the skin is not eaten. It may be removed before or after boiling. In West Africa, Dioscoreaabyssinica is often pounded into a thick paste after boiling and is eaten with soup [47]. It is also processed into flour and baked, fried, roasted or mashed to suit regional tastes and customs [10]. There are various ways of Dioscoreaabyssinica preparation for food in respect to differences in its growing regions of the country. At Sheko districts, South West of Ethiopia, cooking methods relate to the gender of the cooker to prepare for consumption [20].

Nutritional Composition of Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica
The nutritional composition ofPlectranthus edulis tuber were; the ash content of the tuber on dry matter basis varied from 4.47 to 7.03% [57], which was higher than the range reported (4.45 to 5.68%) (forSolanumtubersum [7] and Manihotesculenta (1.44 to 3.19%) on dry matters basis [61]. The crude fat content of the tuber was varied from 5.01 to 10.87% as reported by [57]. Their findings were higher than white unpeeled Solanum tubersum (0.43%) Colocasiaesculenta (0.69%) and Manihotesculenta (0.12 to 0.69%) on dry matter basis (Charleset al., 2005; USDA Food Composition Databases) standard refernces release. This relatively higher fat content might be related with the presence of a high amount of essential oils (>0.05% volatile oil on dry matter basis) in the genus Plectranthus [5]. The crude protein and crude fiber were ranged from 9.85 to 15.8% and 1.15 to 4.16% as reported by Aberaet al., [57]. Allemann and Hammes [7] reported in which the protein contents were higher than Solanum tubersum (5.13 to 8.36%), raw Manihotesculenta (5.03%) cassava (3.37%) raw taro (5.11%) calculated on dry matter basis (USDA Food Composition Databases) standard references release.
Coccinicaabyssinica is a good source of carbohydrates, proteins, minerals and fibers. Coccinicaabyssinica could be a healthy food crop with potential source of equal or better elements as compared to other root and tuber crops [3]. Its content is by far greater than other root crops [11]. It is also rich source of calcium which is an important constituent of our bones and teeth [3,48]. It was reported that the nutrient concentrations in Coccinicaabyssinica tuber are higher than that found in equal weight of Solanum tubersum, Dioscoreaabyssinica and Manihotesculentacrantztubers [34,3].
Different scholars reported that nutritional content of Dioscoreaabyssinica varies with species [40]. Atnafua and Endashaw [10] reported that the protein, fat, fiber, moisture and ash content ranges from 3.13 to 5.37%, 0.31 to 1.22%, 1.94 to 4.91%, 9.83% to 12.71% and 32.31% to 3.58% respectvely. Similar finding were also reported that presence of higher protein contents in Dioscoreaabyssinic a tuber compared to other tropical root and tuber crops like cassava, sweet potato and taro which highlights its nutritional superiority as a staple food [38,40,32). Dioscoreaabyssinicaproteins have a better amino acid balance compared to other tropical root crops [12]. The mineral contents of Dioscoreaabyssinica were ranged from 20.3 to 69.7mg/100g for iron, 0.48 to 0.77 mg/100g for zinc, 31.02 to 118.8 mg/100g for calcium, and 15.1 to 56.2 mg/100g for phosphorrous according to Atinafua and Endashaw [10].

Anti-Nutritional Factors of Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica
Anti-nutritional factors have been defined as substances, which by themselves, or through their metabolic products a rising in living systems, interfere with food utilization and affect the health and production of animals [49]. Root crops, in common with most plants, contain small amounts of potential toxins and antinutritional factors [41]. Phytate, tannin, oxalate and cyanide are common ant nutritional factors, which mostly occur in various root crops.
Plectranthus edulis contains terpenoids, flavonoids, phenolics, and cardiac glycoside while, alkaloids, tannins, and antraquinone glycosides were absent [45]. The presence of phenols, terpenoids, cardiac glycosides and flavonoids are recorded and the percent inhibition of the methanol extract of the tuber of P. edulis was 84% at 100 µg/mL which is comparable with ascorbic acid (90% at 100 µg/mL), indicating its strong activity as radical inhibitor [45]. The presence of phenols, terpenoids, cardiac glycosides and flavonoids were found in Plectranthus edulis and known to have some positive effects on health Yadessa and Tolessa [52]. For instances, the presence of phenols and flavonoids in Plectranthus edulis is important as it is reported to decrease the incidence of some cancers and cardiovascular diseases [53]. The presence of phenols recieved much attenction as potential natural antioxidant [51]. Their antioxidant activity is due to their ability to act as radical scavengers and metal chelators.
Ant-Nutritional factors may reduce the nutrient utilization of plants or plant products used as foods. They can limit the digestibility and solubility of certain nutrients such as protiens, minerals and vitamins [6]. Some anti-nutritional factors such as phytate, oxalate, tannin and cyanide in Coccinicaabyssinica were investigated by different scholars [22,11]. Even the raw Coccinicaabyssinica contains low level of anti-nutrients except phytate, when compared to other roots and tubers such as potato, cocayam and taro [22]). The cyanide content of Coccinicaabyssinica tuber falls far below the lethal dose of cyanide [11). Fekadu [22] reported that the tannin content of raw Coccinicaabyssinica tuber is very low compared to its critical toxicity effect and further reduced during processing. Despite this, since further processing of the crop is required; the anti-nutrients could be reduced more to the safest level for consumption. Even traditionally Coccinicaabyssinica is not eaten raw where it is consumed.
The anti-nutritional factors found in Dioscoreaabyssinicalike free phenolics, tannin, hydrogen cyanide, total oxalate, amylase inhibitor and trypsin inhibitor activities were reported by Shajeclaet al., 2011. They reported 0.26 to 2.20 g/100g for total free phenolics, 0.04 to 1.48 g/100g for tannin, 0.16 to 0.34 mg/100g for hydrogen cyanide, and 0.26 to 0.78 g/100g for total oxalate respectively. The presence of phenols has been suggested to indicate that Dioscorea species could act as anti-inflammatory, antioxidants, ant clotting, immune enhancers and hormone modulators [33].
Afiukwaet al. [50] reported that protein digestibility and palatability are reduced when tannin forms complexes with protien. However, their contents in foods are known to reduce through cooking [56]. The trace quantities of tannin available which were reported by Udensilet al. [59] (20 to 255mg/100g) tubers acts as a repellent againt rot in Dioscoreaabyssinica [31]. Phytates and oxalates have been reported to adversely affect the bioavailability of mineral in humans [60]. Abdul-Rasaqet al. (2018) reported that the value of phytic acid (58.6 to 198 mg/100g) and oxalate were ranges from (9.02 to 49.3 mg/100g) which is low compared to the report of Wanasundera and Ravindran [38].

Limitation in Utilization of Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica
The review revealed that the available information on the indigenous root and tuber crops of Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica are minimal. Thus, Integration of root and tuber crops into the food system of the people may help to minimize the problem of food insecurity [29]. Mekibib and Deressa [28] listed the major broad spectrum factors limiting production and sustainable use of root and tuber crops are displaced by other crops, little research attention, drought, short shelf life, shortage of planting materials, limited knowledge of youth and pests. It was noted that inappropriate agronomic packages, such as lack of recommended optimum fertilizer rates [2] and other agricultural practices may be some of the factors limiting productivity; quality and nutritive value of the crops Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica. Generally, the main challenges for utilization of the crop in Ethiopia may be ignorance to the tuber crops, lack of awareness of the society and lack of support by policy direction.

Effect of Processing on Nutritional Value of Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica
Different processing technology of treatment conditions are reported to eliminate or minimize anti-nutrients in root crops. Reduction or inactivation of anti-nutritional factors through process technology requires knowledge of the type, distribution, chemical reactivity and thermal sensitivity of these factors within the seed matrix and complex knowledge of process technologies. The processing method of tuber Plectranthus edulis has significant effects on anti-nutritional factors [55]. This researcher reported that, cooking process such as boiling, steaming, and baking are known to improve the palatability and digestibility of tubers of Plectranthus edulis. Gifty and Bruno [55] reported that different processing methods reduces the ant-nutritional factors available in tubers and enhance the palatability and digestibility of the Plectranthus edulis.
Cocciniaabyssinica was commonly processed traditionally by boiling after or before peeling [22]. The traditional processing methods were found to be effective methods to reduce anti-nutrient contents of Cocciniaabyssinica tubers [22]. Additionally, Shebabaw [48] reported the effects of three processing methods (boiling, roasting and fermentation) on anti-nutritional contents of Cocciniaabyssinica and reported that fermentation was the most efficient in reducing all the anti-nutritional factors (phytate, oxalate, tannin and cynade) found inCocciniaabyssinica.
CocciniaabyssinicaBoilingwas stated to increase crude fiber content and improve the bioavailability of zinc and on the other hand, processing was reported to decrease the crude protein, total ash, calcium, iron and zinc content of the crop [21]. Cocciniaabyssinica tuber conserved majority of nutrients since this method prevents them from leaching out into the cooking water. Fermentation of Cocciniaabyssinica tuber enhanced protein, fat, ash, potassium (K) and zinc (Zn) content [48].
The Dioscoreaabyssinica tuber treated with species, pretreatment and drying methods had significant effect on most of the anti-nutritional and vitamin contents of the Dioscoreaabyssinica flour (Abdul-Rasaq et al., 2018). The anti-nutritional factors were significantly reduced by blanching compared with the use of potassium metabisulphite as the method of pretreatment. Fermentation is a very interesting process used in plant foods to increase the nutritional quality and remove undesirable compounds. Fermentation enhances the nutrient content of foods through the biosynthesis of vitamins, essential amino acids and proteins, by improving protein quality and fiber digestibility. It also enhances micronutrient bioavailability and aids in degrading anti-nutritional factors [54]. Fermentation enhances the nutritional quality of foods and contributes to food safety particularly under conditions where refrigeration or other food processing facilities are not available [58].
Generally, processing methods could be effective to reduce the amount of anti-nutritional factors available in tubers down to tolerable levels. Among the alternatives that can reduce the toxicity of anti-nutrients to the minimum level and ensure the safety of consumers, the use of effective processing methods including cooking, fermentation, soaking and drying are quite viable.

Conclusions
Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinicahave a great potential for contributing to food security. Especially, they play major role to fill food gaps that might exist until harvest of major crop. The crops are source of carbohydrate, fibers and minerals and also used as medicine for human being and contains some anti-nutritional factors. The toxic substances like anti-nutritional factors found in those crops can be reduced by fermentation, boiling and cooking to tolerable level. The potential uses of indigenous Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinica crops for food security and income generation has not been fully exploited. Taking into account their roles of crops in traditional agriculture system and people's culture, better attention need to be given by all actors to fully utilize in daily diet. Promotion of Plectranthus edulis, Cocciniaabyssinica and Dioscoreaabyssinic a crops in different areas, could play a crucial role in improving the food and nutrition security of the fast growing population of the country.

Conflict of Interest
There is no conflict of interest