Chemical Composition and Organoleptic Attributes of Cowpea Leaves and Husks

Introduction

Cowpeas (Vigna unguculata L. Walp) belong to the Family Fabaceae (Ibrahim, Satish, Ajay, & Karunakara, 2017; OECD, 2016). Production of cowpeas is both for its grains and vegetables. It is among the recognized African indigenous nutrient‐rich vegetables with the potential to promote food and nutrition security in sub‐Saharan Africa (SSA). The crop is originated from West and Central Africa, from where its cultivation and production spread to Latin America and South East Asia (Ano & Ubochi, 2008; Edeh & Igberi, 2012). The 2016 global estimates show that 12.3 million hectares of land are utilized in the production of cowpeas with Western and Eastern 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.

Utilization of Cowpea Leaves

The underexploitation and lack of proper utilization of cowpea leaves and other traditional leafy vegetables has been a major undoing in the promotion of food security in sub‐Saharan Africa (SSA). A study by Madodé et al. (2011) cited the limited shelf‐life of cowpea leaves of just 24 hour at ambient temperatures as a major bottleneck to its utilization. For promotion of utilization of cowpea vegetables, extension of the shelf‐life, optimization of production and processing techniques, and reduction in postharvest losses have been recommended as possible strategies (Kirakou, Margaret, Ambuko, & Owino, 2017).

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.

Cowpea leaves have also been consumed as boiled, blanched, dried, or fermented vegetables (Kasangi, Shitandi, Shalo, & Mbugua, 2010; Kirakou et al., 2017; Muchoki, Imungi, & Lamuka, 2010). 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 pot herbs just as other African indigenous vegetables (AIVs) (Kanali et al., 2017). 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 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). Gogo, Opiyo, Ulrichs, and Huyskens‐Keil (2018) reported postharvest losses exceeding half of the production quantity of cowpea leaves and other AIVs among some of the farmers in Kenya. Postharvest losses in these vegetables have largely been attributed to handling, production practices, distribution, and marketing dynamics (Ndirangu et al., 2017). The quality of cowpea leaves deteriorates with storage. Storage conditions based on their temperature affect the physicochemical attributes of cowpea leaves in terms of antioxidant properties, nutritional quality, color, and texture (Kirigia, Winkelmann, Kasili, & Mibus, 2018; Natabirwa, Mukiibi, Zziwa, & Kabirizi, 2016). Value addition of cowpea leaves through adoption of optimal preservation techniques is seen as a possible way of improving the shelf‐life while retaining the nutritional and sensory quality (Zheng et al., 2009).

Poor postharvest handling practices aggravate the losses, which may rise up to 70% (FAO & World Bank, 2010). Poor transport and distribution network are some of the underlying economic factors that may increase these losses (Shiundu & Oniang’o, 2007). Low temperature storage (4°C) improves retention of the phytochemicals in the fresh cowpea leaves, thus enhanced keeping quality compared with those stored at room temperature (50%–55% relative humidity) 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. 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

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There are different customized recipes among the communities growing the cowpea vegetables. In Kenya, cowpea leaves can be consumed stewed as vegetables or even as mixed dishes with other vegetables such as jute mellow (FAO & GoK, 2018). Cowpea leaves are a common delicacy among Kenyan communities with the Mijikenda consuming it boiled mixed with coconut milk (Okello et al., 2015). Other preparation techniques that cowpea leaves have been subjected to include boiling with lye (traditional salt) and milk and frying (Akello, 2014). The boiled vegetables are at times subjected to fermentation for about 48 hour before its consumption in certain communities or even sun‐dried and stored for later use. The per capita consumption of cowpea leaves in the producing areas in Tanzania was reported to be 41–200 g (Mamiro, 2011). The vegetables are consumed in varied forms including the preserved ones.

a.     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, the study fell short of establishing the effect of such treatment on the sensory appeal and color changes of the vegetables. Muchoki et al. (2010) while using the one‐factor method had established the salt and sugar concentrations each at 3% as the optimal for cowpea leaves fermentation. However, this has been shown to ignore the influence of interactions of the factors under investigation on the response. Further work should consider incorporating the influence of the interactions of the factors on the product quality. 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).

b.    Sun Drying

Utilization of sun drying as a technique of food preservation was reported by Nnadi, Chikaire, and Ezudike (2013) as the most practiced technique by people, 94% of households, in traditional communities in Nigeria. The dried vegetables have been utilized in Uganda to overcome the shortage of cowpea leaves during drought (Aleni, 2017). Sun‐dried leaves are at times first steamed before being dried (Directorate Plant Production, 2014). The method is known to have concentration effects on nutrients thus increases the nutrient density (Chikwendu et al., 2014). However, the limitation of this technique has been that it results in decreased micronutrient contents. Ndawula, Kabasa, and Byaruhanga (2004) reported a nutrient loss of 58% and 84% for β‐carotene and vitamin C, respectively, during open drying. In another study, Chikwendu et al. (2014) reported a decrease in the iron, zinc, calcium, iodine, and phosphorus contents by 90.3%, 87.1%, 96.5%, 73.8%, and 64.6%, respectively, for cowpea leaves that had been boiled and sun‐dried. Drying under a shade is recommended to minimize the deleterious effects on micronutrient content (Directorate Plant Production, 2014).

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. 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 minutes 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.

a.     Blanching

Blanching has been used to preserve cowpea vegetables and extend their utilization. Variation in the blanching temperature–time combination has an effect on the nutritional composition and microbial quality of the vegetables (Njoroge et al., 2016). Oulai et al. (2015) reported β‐carotene and vitamin C losses of up to 55.5% and 61.1%, respectively, for cowpea vegetables blanched in a pressure cooker (121°C) for 15 minutes. Increasing the blanching time resulted in more nutrient losses, however, with a positive impact of reduction in antinutritional components. Aathira, Sudarsa, and Siddhuraju (2017) also reported a similar trend in the loss of protein in cowpea leaves blanched at 100°C for 20 minutes (35.4%) compared with those blanched at 100°C for 15 min (31.3%). Even so, blanching also has the positive impact of attenuating the deleterious effects of drying on antioxidant activity such as 2,2‐diphenyl‐2‐picryl hydrazyl (DPPH) scavenging activity; thus, blanched vegetables are highly recommended as alternative sources of antioxidants (Nobosse, Fombang, & Mbofung, 2017).

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).

b.    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 solar‐dried 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.

c.      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.

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Utilization of a combination of different storage technologies and appropriate storage conditions can also improve the micronutrient retention. Anyango (2015) reported the least losses of 13.6%–13.8% in β‐carotene content of sun‐dried cowpea leaf vegetables under inert conditions as compared to losses of 56.3%–57.3% and 29.6%–33.3% under normal conditions and modified atmospheric packages, respectively. However, this too has had a fair share of challenges in achieving faultless results as storage conditions affected acceptability. A study by Natabirwa et al. (2016) that evaluated the acceptability of cowpea leaves that were blanched and then either sun‐dried or solar‐dried reported that the preserved products had lower acceptability scores (5.6–6.5) than the fresh cowpea vegetables (8.0) on a 9‐point hedonic scale. It is therefore important that these studies must also establish the impact of these technologies on the acceptability of the product.

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). A study in Malawi and Mozambique reported that the weak value chain linkages and less value addition practices for traditional vegetables including cowpea leaves as a major bottleneck in their marketing (Chagomoka, Afari‐Sefab, & Pitoroc, 2014). Another study in South Africa found that these farmers lacked technical knowledge and ability of value addition that would be important in improving their revenues (Senyolo, Wale, & Ortmann, 2018).

Conclusion

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 techniques 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.

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