A review of the contribution of cowpea leaves to food and nutrition security in East Africa

Abstract Cowpea leaf is among the African indigenous vegetables that have been recommended for possible alleviation of food and nutrition insecurity in sub‐Saharan Africa (SSA). The vegetable is rich in micronutrients including iron and vitamin A whose deficiencies are prevalent in SSA. Considering the limitation of seasonal availability, preservation techniques have been adopted to enhance availability with little success. This review aims at highlighting the contribution of cowpeas leaves to food and nutrition security as well as research gaps that must be addressed to promote the utilization of value‐added forms that would have extended effect of improving its production and consumption. It was found that preserved and fresh cowpea leaves were rich in beta‐carotene and iron in the ranges of 0.25–36.55 and 0.17–75.00 mg/100 g dry weight, respectively. The proportion of rural households incorporating the vegetable in its various forms in the region can be as high as 30%. With adequate utilization, the vegetable provided up to ≥ 75% and 25% of RDAs for vitamin A and iron, respectively, of children aged 4–8. However, the utilization of preserved forms faced a limitation for a deviation of up to 30% in their sensory scores and decreased nutrient content as compared to the fresh ones hugely hindered their market penetration. Utilization of novel processing techniques incorporating concept of hurdle technology can help address these quality losses. In conclusion, preservation of cowpea leaves should seek not only to enhance the shelf‐life, but also to enhance acceptability of the products with a view of increased utilization.

Africa leading in terms of production area at 10.5 and 0.9 million hectares, respectively (FAOSTAT, 2019).
Cowpea leaves have been exploited for food and feed. They are rich in micronutrients, nutraceuticals, and antioxidants. Some of the antioxidants that have been found in the leaves are alpha tocopherols, flavonoids, lycopene, and anticancer agents (Shetty, Magadum, & Managanvi, 2013). Cowpea vegetables contain important nutrients including vitamins and minerals that can improve the nutritional status of individuals and households with proper utilization (Okonya & Maass, 2014). The rich nutritional property of cowpea leaves makes them ideal for efforts aimed at reducing food and nutrition insecurity.
The burden of malnutrition in SSA is pronounced especially among the under five-year-old children and pregnant mothers (Stevens et al., 2015). Dietary diversification has been recommended as one of the effective strategies in combating vitamin A deficiency (VAD) and other micronutrient deficiencies in SSA. Cowpea leaves as one of the African indigenous vegetables have been incorporated into diets to improve food and nutrition security of the communities (Shiundu & Oniang'o, 2007). However, there is still limited utilization of the cowpea vegetables as it has a short shelf-life thus limiting its availability.
Drying and other preservation methods have been recommended as possible ways of extending the shelf-life and increasing availability of cowpea vegetables (Kiremire, Musinguzi, Kikafunda, & Lukwago, 2010). Over the ages, traditional African communities have subjected cowpea leaves to customized techniques for its preparation and storage. However, some of these processing techniques have deleterious effects on the nutritional composition and sensory quality of the cowpea leaves. Through research, modern techniques of preparation and preservation of cowpea leaves have been developed. The success in terms of increased utilization of cowpea leaves has still not been satisfactory. Questions have been raised with regard to the affordability and feasibility of some of these techniques considering that the production of cowpea leaves is mainly in the small scale. The current review focuses on the trend of utilization of cowpea leaves, the constraints in value addition and prospects to expand its utilization among communities. The review seeks to point out research gaps that can be exploited to improve the quality of preserved cowpea leaves and overcome the constraints that have greatly limited the utilization of cowpea leaves and its contribution to food and nutrition security.

| ME THODOLOGY
A comprehensive literature search was done in the Google Scholar database in order to capture relevant grey literature. A search syntax was (cowpea or Vigna AND unguiculata) "AND leaves AND (food OR nutrition) AND security AND production AND availability AND accessibility AND utilization AND stability)" without a restriction on the publication year. The first 300 articles from the Google Scholar as recommended by Haddaway, Collins, Coughlin, and Kirk (2015) were downloaded, and the abstracts were scrutinized for relevant literature. From these criteria, 86 articles were selected for the review. Six additional reports were extracted from the web pages by Food and Agriculture Organization and Government agencies including Horticultural Crops Directorate of Kenya.

| Production of cowpeas
The global estimates by Food and Agriculture Organization of the United Nations (FAO) on cowpea production only relate to the cowpea grains with Western Africa as the largest producer in the last half a decade as shown in  (Rashid, Hussain, Rahman, Khatun, & Sattar, 2016).
The production of this crop in West Africa is both for domestic consumption and sale (Akpalu, Salaam, Oppong-Sekyere, & Akpalu, 2014). The crop is mainly cultivated in mixed farming with cereals such as sorghum and millet due to its shade tolerance characteristics (Agbogidi, 2010). This has enabled cowpea vegetables and grains not only to aid in dietary diversification but also to serve as a security TA B L E 1 Global production of cowpea grains in tonnes for the period 2012-2016 food in case of failure of the main crop. Cultivation of the crop can be done in soils that are poor as the crop has the ability to fix nitrogen for utilization in growth thereby encouraging its production by farmers in SSA (Edeh & Igberi, 2012;Horn, Ghebrehiwot, & Shimelis, 2016). In most places, the production is mainly in subsistence agriculture and on a small scale (Saidi, Itulya, Aguyoh, & Ngouajio, 2010).
The production of cowpeas is spread across Asia, Europe, Africa, and America (Carvalho et al., 2017;De Souza, Farias, Lima, Ramos, & DeSousa, 2017). However, cowpea still remains a minor crop across Europe with most of the consumed vegetable being imported. Of the developed countries, only USA is a substantial producer and exporter of cowpeas (Directorate Plant Production, 2014). Asia has for a long period of time ranked second to Africa in terms of production of cowpeas (Nedumaran et al., 2015).
Cowpeas are mainly grown for subsistence in SSA with only a small proportion entering the international market (Directorate Plant Production, 2014). Young leaves and immature pods and seeds have been exploited as vegetables, whereas mature seeds have been consumed as pulses. However, harvesting of the leaves has been shown to affect the mean yield of the seeds. Kabululu, Ojiewo, Oluoch, and Maass (2014) reported a mean yield for cowpea leaves of 25 g/ plant/two-week harvest period accompanied by a 59% on farm reduction in grain yield for a local cowpea variety in Tanzania. Similar findings were also reported by Matikiti, Chikwambi, Nyakanda, and Mashingaidze (2009) where termination of leaf harvesting 7 weeks after crop emergence (WACE) resulted in 50.1%-70.4% reduction in grain yield. On the other hand, increasing the frequency of harvesting the leaves from 7-day interval to 14-day intervals was noted to increase the leaf yields by up to 100 percent (Saidi et al., 2010).

| Cultivation of cowpeas in Kenya and East Africa
In East Africa, cultivated cowpeas are a source of vegetables and grains for human consumption. The major cowpea producing countries in East Africa include Kenya, Tanzania, South Sudan, and Uganda (USAID, 2010). The cowpeas that have been utilized across the region include both the landraces and improved varieties (Mamiro, 2011). A study done in Tanzania found that only 11% of the total cowpea leaves harvested ended up in the market for sale (Putter, Koesveld, & Visser, 2007). The average quantity of cowpea leaves sold per salesperson in the market per day in Tanzania was reported as 5.6 kg per day (Lotter, Marshall, Weller, & Mugisha, 2014). This quantity is low when compared to the amount sold per salesperson for other traditional vegetables such as amaranth (7.3 kg), nightshade (8.1 kg), cassava (6.1 kg), and ipomoea leaves (6.8 kg).  (Nderi & Kamau, 2018;Ndiso, Chemining, Olubayo, & Saha, 2016;Oyoo et al., 2017).
The production of cowpeas in East African region is mainly done in the arid and semi-arid lands (ASALS) (Wambugu & Muthamia, 2009). In Kenya, Makueni County ranks the top in terms of cowpea leaves production area and quantity as shown in Table 2 (Kiambi & Mugo, 2016). A study in Machakos and Tharaka Nithi Counties, known for their semi-arid conditions, ranked the crop as the third and second most cultivated, respectively, in the two areas (An, Rm, & Lutta, 2016). The study also found that the farmers are willing to allocate up to 10% of their land for the production of cowpeas. The greatest constraint in the production of cowpea leaves currently being experienced in Kenya is that it is majorly limited to small-scale farming (Ndungu et al., 2018).
In areas where it is grown in East Africa, the availability of cowpea leaves is along the cropping seasons (Okonya & Maass, 2014).
The crop is mostly intercropped with cereals such as sorghum and maize.

| Utilization of cowpea leaves
The Harvesting of cowpea leaves usually begins as early as two weeks after emergence (WEA) and continues until flowering (Saidi et al., 2010). There is massive utilization of cowpea leaves during glut but less utilization during acute shortages in times of drought (Okello, Hutchinson, Mwang'ombe, Ambuko, Olubayo, 2015). This has been due to the limited shelf-life of the cowpea vegetables. The high moisture content of cowpea leaves renders them highly perishable, while their seasonality in supply has resulted in limited utilization all year round (Njoroge, Matofari, Mulwa, & Anyango, 2016).
Moreover, most consumers utilize the vegetable in its fresh form and make less use of the value-added products. Among West African communities, these vegetables are consumed as accompaniment with cereals or as vegetable sauces with other foods such meat and fish (Madodé et al., 2011). In Kenya, cowpea leaves are consumed as potherbs just as other AIVs . The utilization of cowpea leaves in its different forms in the rural areas in Tanzania is to the tune of 30% of the households (Ochieng et al., 2016). In Asia, the leaves are boiled then sun-dried and stored for later use (Zia-Ul-Haq, Ahmad, Amarowicz, & Feo, 2013). This aims at beating the constraints of seasonality in terms of availability of cowpea leaves. The utilization of cowpea leaf concentrates, which are known to be rich in micronutrients, has been one of the modern techniques of its utilization (Jethwani, Dutta, & Singh, 2015). Other value-added products of cowpea leaves that have been used in high-end markets such as supermarkets include the vacuum-packed, solar-dried, powder, canned, and frozen forms (Jethwani et al., 2015;Okello et al., 2015;Onyeoziri, Kinnear, & Kock, 2018).
However, the incorporation of cowpea leaves into other renowned and widely acceptable products as it has been done with other indigenous and underutilized crops, for example, orange-fleshed sweet potato roots, remains less explored (Owade et al., 2018b).
Cowpea leaves have also been utilized as fodder for livestock (Mahama, 2012). They are recommended as protein-rich animal feeds to be incorporated in feeds during formulation (Martens, Tiemann, Bindelle, Peters, & Lascano, 2012). The utilization of cowpea leaves as livestock fodder is not a common practice in SSA as it is mainly exploited for food.

| Postharvest losses of cowpea leafy vegetables
The AIVs are usually in scarce supply during dry seasons but plenty during rainy seasons that occasion high postharvest losses (Seidu et al., 2012). Postharvest losses for cowpea leaves and other AIVs in East Africa have been estimated to be as high as 30%-40% of the production quantity with some countries recording even higher figures (Babatola, Ojo, & Lawal, 2008 (2016) 2015), and k Imungi and Potter (1983).
TA B L E 3 Nutritional composition of cowpea vegetables (mg/100g dry weight) in which the phytochemicals strongly declined at 4 days (Kirigia et al., 2018). Most of the farmers of cowpea leaves still rely on traditional techniques for handling the vegetables along the value chain resulting in greater losses (Gogo et al., 2018). The smallholder farmers lack necessary facilities such as low temperature storage equipment and reliable transportation means for the postharvest handling of cowpea leaves, and thus, significant proportion of their produce lose the saleable value (Onyango & Imungi, 2007). These produce attract less returns in the market or are rejected at the high-end markets such as supermarkets.

| Nutrient contribution of cowpea leaves to human diet
Dietary diversification has been employed over time as a strategy in improving nutrition status of the population. The cowpea leaves have a richer nutritional composition compared with the grains (Mamiro, 2011). The utilization of cowpea leaves for food has mainly been done in various dishes, soups, and sauces (Imungi & Potter, 1983). Cowpea leaves even in their preserved forms are known to be rich in nutrients that are essential for life (Table 3). Cowpea leaves have also been recommended as possible sources of leaf protein to enrich the diet (Ghaly & Alkoaik, 2010). Cowpea leaves are known to be rich in proteins, vitamins such as provitamin A, folate, thiamin, riboflavin, and vitamin C, and minerals, such as calcium, phosphorus, and iron (Kirakou, 2014;Xiong et al., 2016). Okonya and Maass (2014) reported the crude protein and iron contents of fresh cowpea leaves on a dry mass basis to be as high as 33 g/100g and 379 µg/g, respectively. A study by Van Jaarsveld et al. (2014) reported that a 90g portion of cowpea leaves could meet ≥ 75% and 25%-50% RDAs for vitamin A and iron, respectively, for children 4-8 years.
However, it is worth noting that the nutrient composition of cowpea leaves varies depending on the cultivar (Carvalho et al., 2012), and thus, cultivar selection should be carefully done.

| Antinutritional factors in cowpea leaves
Cowpea leaves have antinutritional factors such as oxalates, phytates, and nitrates which are known to have negative impact on the nutrient intake of individuals (Muchoki et al., 2010;Oulai, Zoue, & Niamke, 2015). Optimal processing and preservation techniques should seek to reduce or minimize the accumulation of these antinutritional factors as a way of ameliorating the nutritional quality.
Some of the processing techniques that have been used successfully in reducing antinutrients in foods include fermentation, soaking, germination, debranning, and autoclaving (Ertop & Bektaş 2018).
Removal or reduction in the antinutrients serves to improve the nutritional quality of the food by increasing the bioavailability of nutrients such as protein, calcium, iron, and zinc. Muchoki (2007) reported a reduction of 38.4% and 8.3%, respectively, in the nitrate and oxalate contents of dried cowpea leaves that were subjected to fermentation. Another study by Chikwendu, Igbatim, and Obizoba (2014) similarly reported a reduction of 33.3%, 73.9%, 85.9%, and 70.7% in the tannin, saponins, flavonoid, and polyphenols contents, respectively, for cowpea leaves that were parboiled, sun-dried, and drained. The practice in the latter study may not be encouraged as it would also result in greater losses in the important nutrients especially the micronutrients. Any techniques aimed at reducing these antinutritional factors must have minimal deleterious effects on essential nutrients.

| Traditional processing and preservation of cowpea leaves among East African Communities
There are different customized recipes among the communi-

| Fermentation
Fermentation of cowpeas has been in practice among traditional African communities for a long time. The overall aim of the fermentation process has been to improve on the shelf-life of the cowpea leaves, while at the same time the nutritional quality has also been improved. Muchoki et al. (2010) reported a reduction of 71.9% in the nitrate content of cowpea vegetables subjected to fermentation. However, the fermentation techniques employed in these traditional practices are spontaneous and the product quality in terms of the nutritional and sensory is still varied and not optimal. Attempt has been made to try and standardize the fermentation processes of these vegetables. Kasangi et al. (2010) used fermentable sugars at the rate of 1%-3% to enhance the cowpea leaves fermentation process and achieved a crude fiber and ash contents of 16.29%-17.61% and 22.37%-22.61%, respectively. These values were higher than those he reported for cowpea leaves that were solar-dried (12.76% and 13.08% crude fiber and ash contents, respectively) or blanched solar-dried (11.76% and 9.49% crude fiber and ash contents, respectively). However, It is also recommended that in fermentation of cowpea vegetables and fermentable sugar such as glucose or fructose should be added at the level of 2%-3%, followed by a starter culture; this gives a better product in terms of its quality compared with the cowpea leaves subjected to spontaneous fermentation (Wafula et al., 2016). Sun drying is one of the preservation techniques that have been recommended to improve the availability of cowpea vegetables to aid in the promotion of food and nutrition security in SSA.

| Sun drying
Consumption of sundried cowpea leaves together with other traditional indigenous vegetables was shown to improve the mean serum retinol content by 25.9% in a 13-week feeding trial study (Nawiri, Nyambaka, & Murungi, 2013). Thus, the preserved vegetables are recommended for amelioration of nutritional status of vulnerable populations.

| Modernized cowpea leaves processing techniques
Processing of cowpea vegetables has been used to improve the nutritional and keeping qualities of the vegetables using modernized technologies with the aim of increasing their utilization. Some of the processing techniques that have been utilized include solar drying, freezing, freeze-drying, blanching, and vacuum packaging (Okello et al., 2015). The degree of deterioration in the nutritional and sensory quality of these preserved products vary depending on the cooking methods and preservation techniques in use (Okonya & Maass, 2014). Kirakou et al. (2017) reported low retention of beta-carotene and ascorbic acid at 52.78% and 20.24%, respectively, for cowpea vegetables that were blanched in salty water for 2 min at 94°C followed by solar dying. Conventional cooking of cowpea vegetables that entailed boiling for 10 min resulted into total loss of vitamin C (Rashid et al., 2016). Thus, the processing technique used should be carefully selected with the aim of maximum retention of the nutritional and sensory quality.

| Blanching
Blanching has been used to preserve cowpea vegetables and extend The other non-nutritional benefit of blanching is that it improves the rehydrability, reduces microbial load, and increases the ease of packaging as it shrinks and softens the vegetables (Njoroge et al., 2016).

| Solar drying
Solar drying has also been used in the preservation of cowpea leaves as a modern technique. The greatest concern with this preservation technique still remains the retention of micronutrients such as ascorbic acid and β-carotene as it exposes these nutrients to oxidation in the presence of oxygen. Visqueen-covered solardried cowpea vegetables showed great losses for β-carotene and vitamin C at 34% and 71%, respectively (Ndawula et al., 2004).
Blanching of the vegetables before solar drying improved the retention of β-carotene and vitamin C by 15% and 7.5%, respectively. This gives greater credence to the use of hurdle technology in the preservation of cowpea leaves.

| The concept of hurdle technology
A combination of at least two preservation techniques has been reported to produce the best results in terms of nutritional and keeping quality. Njoroge et al. (2016) reported that blanching at 80°C for 10 min and hot-air drying of cowpea vegetables had a retention of 53.7% and 53.8% for vitamin C and β-carotene, respectively, whereas blanching at 80°C for 10 min and solar drying recorded a retention of 58.2% and 49.2% for vitamin C and β-carotene, respectively.
These values were higher when compared with that of conventional method of boiling at 100°C for 30 min. In the traditional preparation of cowpea leaves, blanching was not included in the preparatory processes prior to sun drying, and lower retention of β-carotene was noted (Mulokozi & Svanberg, 2003). Without blanching, sundried vegetables are exposed to continued enzyme activity that would easily expose the carotenoids to oxidation reactions.

| Consumer acceptance of value-added cowpea products
The greatest incentive for promotion of value addition practices is successful adoption of the products (Owade, Abong, Okoth 2018a).
Increased utilization of cowpea leaves can only be achieved when the preserved cowpea leaves are marketable. A study by Okello et al. (2015) reported that Kenyan consumers were willing to pay as high as KES: 5 for both the sun-dried and frozen cowpea leaves. The same study notes that some consumers attach no great significance on value addition practices and would not be willing to pay extra for such. The current low consumption can be attributed to wrong consumer perception about the cowpea leaves. Lekunze (2014) in his market analysis study of cowpea leaves reported that households who were regular consumers of the vegetables would more likely substitute these vegetables with other vegetables in the likely event that their income levels increased.
Another study that evaluated the acceptability of different cowpea leaves established that nutritional composition such as ascorbic acid, moisture and phosphorus content, and the leaf size was correlated with the acceptability of cowpea leaves (Ahenkora, Adu Dapaah, & Agyemang, 1998). Dehydration of cowpea leaves as a way of preservation results in alteration of the nutritional and thus the sensory quality too. Descriptive sensory analysis of dehydrated cowpea leaves revealed that the solar-dried leaves had similar appearance and texture to the fresh cowpea leaves, whereas greater alterations existed in the sundried cowpea leaves (Nyambaka & Ryley, 2004). The greatest gap that has to be filled is to have the consumers adopt value-added cowpea leaves.

| Constraints of value addition practices for cowpea vegetables
The primary bottleneck for the utilization and value addition practices for cowpea leaves stems from the production practices. The production practices in the SSA for cowpea leaves are below the optimal levels, and thus, low production quantities have been realized (Oyewale & Bamaiyi, 2013). Some of agricultural practices also impoverish the soil thus affecting the nutrient content of cowpea leaves produced from these soils. All the above highlighted factors thereby result into lower economic returns that greatly discourage the production of cowpea leaves (Mucheru-Muna et al., 2010).
The marketing system for cowpea leaves and other AIVs is mainly through the informal sectors where quality parameters are less observed (Onyango & Imungi, 2007). The seasonal nature of the supply of cowpea leaves makes its handling in the less organized informal market difficult resulting to major losses in form of spoilage (Omulo, 2016). These farmers rely on traditional preservation techniques such as storage under a shade and sprinkling of water on vegetables in stores which has its limitations as the keeping quality is only extended by a few days (Kirigia et al., 2018).
The modernized cold storage facilities such as cold rooms are unaffordable to the smallholder farmers (Onyango & Imungi, 2007).
Lack of time, inadequate knowledge, and additional costs have resulted into less practice of value addition among most handlers in the value chains (Kirui, Kisang, & Kiptum, 2017

| Gaps in knowledge
In spite of the many value-added products of cowpea leaves in the market, the uptake is still very low. The practices are also very limited with a large proportion of the cowpea leaves still sold as fresh produce. In some instances, the less optimal traditional preservation techniques are preferred due to affordability. The focus of research should be to promote affordable techniques of preservation of cowpea leaves that are acceptable among the value chain actors that in turn will promote their utilization.
The quality in terms of nutritional and sensory quality of the value-added cowpea leaves greatly deviates from those of fresh produce pointing to less optimal practices. There is need for research on optimization of some of these traditional techniques such as fermentation to develop highly acceptable products whose uptake in the market would be almost as much as the fresh cowpea leaves.

| CON CLUS ION
Cowpea is a popular crop in the SSA that is rich in nutrients and phytochemicals but is yet to be fully harnessed. Both the leaves and grain of cowpeas are exploited for food. The leaves are of high nutritional value and would be of great importance in strategies aimed at addressing food and nutrition security. However, the concerns of shelf stability of the fresh product and the nutritional and sensory quality of the processed product greatly limit the utilization of this vegetable. Some of the preserved products have poor sensory and nutritional quality as compared to the fresh leaves. Different preservation techniques are utilized to preserve cowpea leaves. The efficiency of these techniques in achieving different quality requirements has great variations.
The use of hurdle technology provides better results as compared to utilization of preservation techniques separately. Some of these tech- niques are yet to be evaluated in the preservation of cowpea leaves.
Acceptability of a product hugely determines its uptake in the market and thus influences its successful adoption. The major focus in the preservation of these vegetables should not just be on the nutritional quality but must also consider the sensory appeal of the product. The preservation should seek to improve the sensory quality of the preserved cowpea leaves that largely influences its consumer acceptability.

ACK N OWLED G M ENT
This work was financially supported by the German Federal Ministry of Food and Agriculture (BMEL) based on the decision of the Parliament of the Federal Republic of Germany.

CO N FLI C T O F I NTE R E S T
The authors declare that they do not have any conflict of interest.

E TH I C A L A PPROVA L
The study did not involve any animal or human testing.

I N FO R M E D CO N S E NT
The study did not involve human subjects.