Cowpeas: Nutritional profile, processing methods and products—A review

Cowpea (Vigna unguiculata L. Walp) is an important pulse crop grown in sub‐Saharan Africa and in parts of Asia and the Americas. It is a major starchy legume consumed widely in sub‐Saharan Africa as an affordable source of nutrients including protein. The global production of cowpeas has increased 2.7‐folds since 2000. Cowpea is a nutritious food source, rich in protein, digestible and nondigestible carbohydrates, and potassium with very low lipids and sodium content. Cowpeas also contain a number of essential amino acids, and polyphenols with antioxidant activity. The main objectives of this review are to provide information on the nutritional composition of cowpeas, processing techniques used, and consequent effect on nutritional and sensory quality. It focuses on specific processing techniques including traditional processes and the production of cowpea‐based ingredients for potential industrial applications. Additionally, an extensive review of typical foods made from cowpeas is included. Recent developments in cowpea research, notably the use of novel processes and product applications, have also been reviewed.


| COMPOSITION OF COWPEAS
The composition and nutritional profile of raw and cooked cowpeas is summarized in Table 1 (USDA, 2021). The composition can vary due to varietal differences, climatic conditions, and agronomic practices.

| Proteins, amino acids, and protein classification
Cowpea protein is a rich source of essential amino acids except cysteine and methionine (Table 2), which is typical of other legumes. Table 2 also shows the current amino acid scoring patterns for infants, children, and adults. Although cowpea protein is deficient in sulfurcontaining amino acids (cysteine and methionine) for infants, it satisfies the requirements suggested for young children and adults (USDA, 2005). The recommended reference intake of methionine + cysteine is 3.8 g/100 g protein for infants, but cowpeas provide only 2.5 g/100 g protein. However, the recommended reference intake of cysteine + methionine for young children and adults is 2.5 g/100 g protein (USDA, 2005). Chan and Phillips (1994) extracted defatted California Blackeye flour with 0.1-M Na 3 PO 4 , 0.01-N NaOH, and 70% ethanol. The major protein fractions were globulin (66.6% of total) and albumins (24.9%) whiles alkali-extractable glutelins comprised 4.7% and alcohol-soluble prolamins, 0.7%. As shown by SDS-PAGE, globulins had major bands at 65, 60, 56, and 50 kDa and 28-42 kDa minor bands. The albumin fraction contained 99, 91, 32, and 30 kDa subfractions; the glutelin fraction, 101, 68, 31, and 29 kDa; and the prolamin fraction, 105, 62, 50, and 54 kDa subunits. All fractions were rich in aspartic and glutamic acids (9.1-11.8%), with comparable amounts of serine, proline, isoleucine, methionine, tyrosine, and histidine. Albumins contained the highest and glutelins the lowest lysine (9.2 and 7.6 g/100 g protein, respectively). Freitas et al. (2004) reported cowpea protein fractions of 51% globulins, 45% albumins, 3% glutelins, and 1% prolamins. Teka et al. (2020) confirmed the predominant protein fractions were globulin (38.4-49.1%) and albumin (19.6-22.5%), followed by glutelins (6.4-10.4%) and prolamins (1.0-1.14%). In vitro protein digestibility of cowpea flour ranged between 68.7% and 72.0% and had significant but negative correlation with phytic acid (r = À0.673) and globulins (r = À0.846) and no correlation with tannins. Except for isoleucine and histidine, the amino acid score of the cowpea was below the FAO/WHO requirement for essential amino acids for infants and preschool children. Tryptophan was the first limiting amino acid, followed by the sulfur-containing amino acids.

| Carbohydrates
Cowpeas contain significant content of both digestible and nondigestible carbohydrate (Table 1). Tuan and Phillips (1991) employed gelatinization, amylase/amylopectinase hydrolysis, and glucose analysis for measuring starch concentration in California F I G U R E 1 Common seed coat colors in cowpea. Source: Weng et al. (2018) Blackeye #5 seeds. Starch content of 48% (control) and 45-54% (stored seeds, at variable temperature and relative humidity) was observed following different pretreatments. Oluwatosin (1998) reported that starch content was 43-64% in 15 Nigerian cultivars grown in three locations. Mallillin et al. (2008) found 34% dietary fiber (TDF) in cowpea, which was comparable with other legumes. Approximately 80% of TDF was reported as insoluble.
Raffinose and stachyose content of 0.45 and 3.30 g/100 g, respectively, were reported in cowpea by Nnanna and Phillips (1988).
These galacto-oligosaccharides (GOS), also present in other legumes, have negative reputation as flatulence-causing compounds.
Various soaking and processing treatments have been suggested to eliminate or significantly reduce their content. However, galactooligosaccharides are now widely recognized as having prebiotic potential, as growth promoters of beneficial intestinal bacteria (Macfarlane et al., 2008).

| Resistant starch and starch digestibility
Starch is the major component (22%-45%) of legumes and has received recent recommendations as an alternative to cereal or tuber starches due to low digestibility and hence low glycemic index (Ma et al., 2017). Cowpea starch granules exhibit the characteristic C-type crystalline structure of legume starches with oval or ellipse shapes. Hamid et al. (2015) reported granule diameters between 20.9 and 48.6 μm. Apparent amylose content ranged from 15.5% to 39.4% with average amylopectin branch chain length between 21.1 to 23.0 (Hamid et al., 2015;Ratnaningsih et al., 2020).
T A B L E 1 Proximate, minerals, and vitamins composition of raw and cooked cowpeas (per 100 g) Cowpea starch is rich in resistant starch (RS) fraction, which is the portion of dietary starch that is not rapidly digested and absorbed; instead, RS enters the large intestine where it is fermented partially or wholly (Rengadu et al., 2020). RS was isolated from five cowpea cultivars and assessed for its potential prebiotic effect. The RS content was 9.3-12.1%, and it significantly stimulated the growth of beneficial bacteria. Further, the starch was sufficiently fermentable by the gut microbes under in vitro conditions. Thus, it was concluded that RS could find potential applications as a prebiotic to maintain the digestive system and improve gastrointestinal health (Rengadu et al., 2020). Ratnaningsih et al. (2020) assessed the effect of 1, 3, or 5 autoclaving-cooling cycles on the physicochemical properties, in vitro starch digestibility, and estimated glycemic index (GI) of cowpea starch. They observed an increase in amylose content, particle size, and thermal properties and decreased pasting temperature and final and setback viscosities due to the autoclave-cooling cycles. The crystalline structure was also modified from C-type into a mixture of B and V-types possibly due to the loss of the amylopectin crystalline region during heating and reassociation of the starch chains within the granules. Further, a decrease in GI was observed, confirming the categorization as a low GI food. The single autoclave-cooling cycle was proposed as a potential processing technique to produce RS with improved thermal stability and low GI for use in functional foods.

| Lipids, minerals, and vitamins
The lipid component of cowpea consists largely of cell membranebound constituents. Ukhun (1984)  Similarly, cowpeas are rich sources of selected of minerals, for example, 100 g provides 41-76% of phosphorous, magnesium, iron, copper, and manganese.
Further, it may potentially bind protein and starch; however, it can act as an anticancer agent and may help against heart disease and diabetes (Campos-Vega et al., 2010). Phytate content of 0.5-3.0% has been reported in cowpeas (Avanza et al., 2013;Oboh, 2006).
3 | COWPEA PROCESSING Figure 2 summarizes approaches for converting dry cowpea seeds to human foods. Typically, cowpeas are consumed as whole cooked or in soups and stews. A major processing challenge is the hard-to-cook (HTC) phenomenon, which develops during improper storage (Bassett et al., 2021;Jombo et al., 2021;Liu et al., 1993;Sefa-Dedeh et al., 1979). In West Africa, besides traditional cooking practices, alkali salts, trona, karwa, and potash are used to overcome HTC effect and soften cowpeas (Uzogara et al., 1988). Similarly, phosphates are added in the soak water during commercial processing of cowpeas.

| Traditional processing-Whole seed
Traditional processing utilized whole cowpea seeds, flours or meals, and pastes. Over 20 home-cooked foods are made by the Hausa, Yoruba, and Fulani peoples across West Africa, and other ethnic groups in Ghana (Dovlo et al., 1976). The traditional recipes utilize many processes to prepare soups, stews, steamed or fried cakes, and sauces. Other staples or ingredients are also added, for example, maize, gari, rice, sorghum, meats, fish, eggs, and selected vegetables and herbs.
In West Africa, fried cowpea paste, akara, is a popular food. In India, cowpeas are used in papad, a traditional snack food (Bhagirathi et al., 1992), extruded and fried dough-sev (Annapure et al., 1998), idhli, and dhosal/dosa (Enwere & Ngoddy, 1986). In Brazil, acaraje (fried fritter like akara) is sold by street vendors. In the United States (mainly in the southeast), cowpea is sold in raw/dry, canned, or frozen F I G U R E 2 Conversion of cowpea seeds to foods forms and consumed primarily as cooked whole seeds. Typically, cowpeas consumption forms part of meals with different vegetables and/or fried corn cakes or baked corn bread.

| Soaking and boiling
Soaking in water or salt solution (to facilitate softening) is a common first step in cowpea processing. Tuan and Phillips (1991)  between the two varieties. This was attributed to conformational changes and reduction of antinutritional factors. A reduction in the degree of starch hydrolysis was also reported in unsoaked, boiled pink and white hull cowpeas whereas an increase was recorded in red hull cowpeas irrespective of treatment (boiling, soaked or unsoaked) (Torres, Muñoz, et al., 2019). Onwuka (2006) showed significant reduction in trypsin inhibitors (TIA) and phytohemagglutinin (PHA) activity in cowpeas after 13-h soaking, boiling alone, or soaking followed by 40-min boiling. The effect of different processing methods on TIA in various cowpea products is shown in Figure 3. Processing methods also significantly affect raffinose, stachyose, phytic acid, and tannins ( from seeds soaked (30 C-95 C) were treated to observe starch, proteins, cellulose, and pectin. Water uptake and dry matter loss were also monitored. Parenchymatous cells of cotyledons changed significantly with soaking temperature and water uptake occurred either through the micropyle (30 C, 60 C) or testa (95 C).

| Germination and fermentation
Germination or fermentation of the hydrated cowpeas can enhance nutritional quality and remove/reduce antinutrients. Phillips (1988, 1990) reported that germination resulted in an increase in the activity of alpha-galactosidase, alpha-amylase, and protease (within 12-24 h). Germination (25 C and 30 C, for 24 h) decreased flatulence by 77% and improved digestibility of protein and starch.
Niacin, thiamin, and riboflavin content increased but total protein and carbohydrates were unaffected. Wang et al. (1997)

| Flours and air-classified fractions
Diverse culinary practices have led to the removal of the seed coat and hilum/"black-eye" (i.e., decortication) to produce traditional foods. The seed coat is reported to reduce digestibility and cause abdominal distress, especially in children (Enwere & Ngoddy, 1986).
The seed coat type is a major consideration for decortication; for example, crowder and blackeye are the two major cowpeas in the United States. Crowder has thick, smooth, loosely adhering seed coats that are easily removed by cracking and aspiration.
In West Africa, decortication of seeds with tightly adhering seed coats is traditionally done using a wet process and rubbing soaked seeds manually or in a mortar. Phillips (1982)  Physical and functional properties of decorticated cowpeas and flour have been reported widely: color (Jarrard et al., 2007), flours/ meals functionality (Akissoé et al., 2021;Kethireddipalli et al., 2002), and particle size, specific gravity, water absorption, pasting viscosity, protein solubility, and thiamin and riboflavin content (McWatters et al., 1988). Table 4 shows selected functional properties of raw, germinated, fermented, and heat-treated cowpea flours. Proximate analysis of raw and preprocessed (whole and defatted) cowpea flours is presented in Table 5. Gunawardena et al. (2011) reported that coarse (starch-rich) and fine (protein-rich) fractions of milled legume/cereal flours can be separated by air-classification, based on density and particle size. Cloutt et al. (1986) air-classified cowpea flour that was observed to contain a high proportion of small starch. This process produced fines with a higher starch content in cowpea flour than other legumes studied.

| Extrusion processing
Extrusion processing has been used to prepare cowpea-based nutritious weaning foods. Cowpea flour/meal, alone or in combination with other ingredients, can be extruded to produce snack products and weaning foods. Phillips et al. (1984)  Improved swelling and water-and oil-holding capacity, and water extractable total proteins was achieved by microfluidization process; however, mean particle size and bulk density decreased significantly.
Microfluidization has potential to produce high-quality functional cowpea flour with improved physicochemical properties for use in diverse food applications with enhanced nutritional properties and health benefits. produce a superior quality akara compared with dry-milled flour (McWatters, 1983). Phillips and Baker (1987) reported that protein quality of akara is comparable with other heat-processed cowpea foods. Cowpeas with HTC defect produce a poor quality product (McWatters et al., 1988). The frying process significantly reduces the phenolic content and radical scavenging capacity of the product (Apea-Bah et al., 2017).

| Moin-moin, cowpea stew, and waakye
Moin-moin (Alele), a steamed cowpea paste, is major cowpea dish, especially in Nigeria. Except for whipping, other initial preparation steps are similar to akara. Fish/crayfish or egg may be added along with pepper, onion, tomato puree, and oil. The paste is portioned into tins, or leaves, and steamed until the product sets/gel. Jarrard et al. (2007) reported that cowpea flour makes a firmer, stickier gel than the traditional coarser meal or whole cowpea.
Cowpea stew or Red-Red, a popular dish in Ghana, is served with fried plantain. Cooked whole red cowpeas are combined with a sauce prepared from fish, shrimp, onion, pepper, tomato, palm oil, and salt.
Waakye is another popular dish, where cowpeas are boiled with rice and eaten with a meat stew, pepper sauce, and optional vegetables.
However, a substantial proportion of consumers liked the biscuits.
The composite biscuits had higher dietary fiber content but similar protein quality as the standard. Campbell et al. (2016) investigated the sensory acceptability and textural properties of leavened wheat bread and sponge cake fortified with CPIs, denatured and glycated by thermal treatment. Addition of CPI improved water absorption of dough resulting in softer texture but significantly increased bread crumb hardness than the control.
Higher sensory acceptability scores were recorded for bread containing glycated CPI.
Beany flavor in legumes, including cowpeas, has been reported to be a negative sensory attribute that can potentially impact consumer acceptance of products made with cowpea composite. One notable exception is Moin-moin, a traditional cowpea product, where beany flavor is preferred by consumers (McWatters, 1990;Okaka & Potter, 1979;Xu et al., 2020). A preliminary steaming treatment of cowpea flour was shown to lessen the beany aroma and flavor of biscuits made with the cowpea flour (McWatters, 1990). Okaka and Potter (1979) reported that acidified water soaking of cowpeas, followed by blanching, reduced the beany flavor of cowpea powders prepared by drum drying. Xu et al. (2020) reported that the use of germination as an extraction pretreatment was effective in reducing the characteristics beany flavor of pulse ingredients.

DATA AVAILABILITY STATEMENT
The manuscript reviewed previously published research; therefore, data sharing/accessibility statement does not apply.