Evaluation of the physicochemical, proximate, and sensory properties of moinmoin from blends of cowpea and water yam flour

Abstract Moinmoin is a steamed cowpea seed‐based pudding native to Nigeria. This study evaluated the physicochemical, proximate, and sensory properties of moinmoin from a blend of cowpea and water yam flours. The pudding was prepared by varying the proportion of cowpea to water yam flour (CWYP). The blends were in the ratio 75:25% (CWYP1), 50:50% (CWYP2), and 100:0% cowpea flour (CPP) which served as the control. Physicochemical evaluation indicated that the products will have good keeping quality and reconstitute easily. Protein content was 26.90%, 20.10%, and 17.60% for CPP, CWYP1 and CWYP2, respectively, and the presence of water yam significantly (p < .05) increased the crude fiber, ash, and carbohydrate contents of the cowpea/water yam products relative to the control. Pasting properties revealed that regardless of the proportion of water yam in the mixture, the moinmoin samples cooked at approximately the same time; and sensory evaluation showed that CWYP1 was best preferred in terms of taste, flavor, color, and overall acceptability. These findings suggest that the cowpea/water yam pudding can meet the nutritional demands of a meal and that the 75%–25% cowpea/water yam mixture was the most acceptable. The study contributes to the knowledge nutritious products with desirable organoleptic qualities from a blend of cowpea and water yam flour, allowing for up to 50% water yam substitution. It also provides an alternative way of utilizing water yam thus preventing wastage during peak production and ensuring food and nutritional security because of the balanced products obtained.

Leguminous species range from the well-known crops such as cowpea (Vignia unguiculata) and soybean (Glycine max) to the lesser known and underutilized wing beans (Psophocarpus tetragonlibas).
Their seeds are highly nutritious with high-protein content, as such; they are used as food for both humans and animals. In addition, cowpea in particular is an important component of cropping systems in the tropics usually used as cover crops, green manure, and as natural fertilizers (Nwokolo, 1996). It is consumed in many forms such as the young leaves, green pods/seeds as vegetables, and dry seeds in various food preparations (Singh, Ehlers, Sharma, & Freire-Filho, 2002). In West Africa, cowpea seeds are boiled with condiments and eaten alone with stew or in combinations with cereals. In Nigeria, they are also used in preparing other food products such as fried cowpea cakes, bean soup, and a steamed popular delicacy called moinmoin. Nutritionally, cowpea seeds contain 61%-66% carbohydrates, 24%-25% proteins, and 1.0%-2.0% lipids (Bressoni, 1985;Madode, Linnemann, Nout, Vosman, & Hounhouigan, 2012).
Dioscorea is a genus with over 600 species of flowering plants in the family Dioscoreaceae. The vast majority of the species are tropical with only a few extending into temperate climates. Several species, known as yams, are important agricultural crops in tropical regions, grown for their large tubers and are particularly important in parts of Africa, Asia, and Oceania. One of the edible species is Dioscorea alata, a tuberous root vegetable, usually bright white in color, also known as water yam. With its origins in the Asian tropics, D. alata has been known to humans since ancient times. The species is characterized by a high-water content and not as sweet as the other edible species.
Nutritionally, water yam has 70%-73% moisture content, 19%-22% carbohydrate, 0.58%-1.0% fat, 6%-8% protein, 4%-5% ash, and a high content of the B vitamins and essential minerals (Wireko-Manu, Ellis, Oduro, Asiedu, & Maziya-Dixon, 2011). It has high yield, high multiplication ratio and better tuber storability than several other edible species. Unfortunately, D. alata has low economic value as it is less used for major yam products such as pounded yam, fufu, boiled or fried yam. This is because its flesh is watery and usually not as firm as those of other edible yams. However, D. alata has an advantage over others. It has higher sustainable cultivation ecology, especially nowadays when the production of other yams seems to be on the decline (Wireko-Manu et al., 2011). Considering the poverty level and inadequate food supply as a result of increase in world population, the ecological adaption for easy and sustainable production of D. alata should be exploited for food security.
According to Singh et al. (2002), the addition of even a small amount of cowpea to cereals, roots, and tubers could enhance the nutritional balance of the diet and protein quality. This is achieved by the synergistic effect of high protein and lysine from cowpea and high methionine and energy from cereals.
Moinmoin is a traditional Nigerian bean dish obtained by steaming homogeneous slurries containing cowpea paste, small amounts of vegetable oil, pepper, and other ingredients. On steaming the slurry in pouches made from leaves, aluminum foil or polyethylene bags, it solidifies into an irreversible gel-pudding (Akusu & Kiin-Kabari, 2012;Okechukwu, Ngoddy, Nnanye-lugo, & Nnanye-lugo, 1992). Galati, Oguntoyinbo, Moschetti, Crescimanno, and Settani (2014) reported that most people from Africa and other developing countries live on diets based mainly on cereals, roots, and tubers. These types of diet provide more than 60% of the total energy supply, with minimal or no protein of animal origin. Although these staples contribute significant amounts of fiber, minerals, and vitamins, they are limiting in some of the essential amino acids especially lysine; thus diets based on cereals, roots, and tubers alone are not sufficient in ensuring balanced diet. Also, because of the low-protein content of cereals, roots, and tubers, protein-energy malnutrition is prevalent in populations where they are used as staples, especially among women and children.
Water yam (D. alata) is rich in cysteine, isoleucine, and lysine (Awoyale, Maziya-Dixon, Sanni, & Shittu, 2016); while cowpeas are good sources of proteins and rich in lysine, though deficient in sulfurcontaining amino acids, such as methionine and cysteine. Blending or combining cereals, roots, and tubers with a protein-rich source will contribute to the improvement of the nutritional quality of such foods (Liao et al., 2004). It follows then that by appropriately combining cowpea with water yam, a well-balanced dish should result.
According to Chandra, Singh, and Kumari (2015), functional properties are the fundamental physicochemical properties that reflect the complex interaction between the composition, structure, molecular conformation, and physicochemical properties of food components together with the nature of environment in which these are associated and measured. Functional characteristics are important in determining product performance and have influence on sensory characteristics of food products. There has been little or no study on the functional and sensory properties of moinmoin made from blends of cowpea and water yam. Therefore, this study was conducted to assess the physicochemical, nutritional, and sensory characteristics of moinmoin products made from cowpea/water yam composite flours toward improving the nutritional quality and diversifying the food uses of water yam to prevent its wastage during peak productivity.

| Preparation of flour blends
Water yam tubers, cowpea seeds, and other ingredients were purchased from a local market in Ilorin, Nigeria. All reagents used in this study were of analytical grade.
Water yam flour was produced using the method described by Udensi, Oselebe, and Iweala (2008) with slight modifications. The tubers were washed, peeled, sliced, and blanched at 80°C for 4 min.
They were dried, milled, and sieved through 100μm mesh.
Cowpea seeds were cleaned, sorted, and steeped in water for 2 hr, dehulled manually by rubbing between the palms and dried at 60°C for 3 hr in a hot air oven. This was then milled and sieved through 100μm mesh.
Flour blends of cowpea and water yam were prepared in ratios of 100:0, 75:25, and 50:50; and coded as CPP, CWYP 1 , and CWYP 2 ), respectively. The flours were separately and thoroughly mixed at these ratios using a Kenwood food mixer to obtain homogeneous blends.

| Formulation and preparation of moinmoin
Three blends of moinmoin from cowpea and water yam were prepared using the formulations shown in Table 1. Each blend was separately mixed with a wooden spoon until all the ingredients were well blended. The resultant blend was dispensed into small aluminum bowls used for pudding preparation and steamed for 45 min.

| Physicochemical properties of flour blends
The pH of the samples was determined using a pH meter (Model 049016, Crison Instruments, S.A., Alella-Barcelona, EU). The titratable acidity (TTA) was determined as previously described by Adeyemi and Oluwamukomi (2002), and the result expressed as percentage lactic acid. Water absorption capacity was measured as described by Chandra et al. (2015), while the reconstitution index was determined as previously described (Oluwatooyin, Osundahunsi, & Aworh, 2002;Onwuka & Ihuma, 2007). All determinations were in triplicates, and the mean value was recorded in each case.

| Pasting properties of flour blends
Pasting viscosity of the flour blends was determined as described by AACC (2001) using a Rapid Viscosity Analyzer (RVA-1998, Model RVA-SUPER3; Newport Scientific, Australia). Pasting temperature, peak viscosity temperature, peak viscosity during heating and viscosity on cooling to 50°C were recorded.
Briefly, 3 g of flour samples was weighed into a dry empty canister; 30 ml of water was added and thoroughly mixed before fitting into the visco analyzer. Each suspension was kept at 50°C, then heated up to 95°C at 12.2°C per min and held for 2.5 min at this temperature. The suspension was then cooled to 50°C at 12.0°C and held for 2 min at 50°C. All determinations were in triplicates, and the mean value was recorded in each case.

| Proximate analysis
The moisture, crude protein, ash, crude fat, and crude fiber contents of the blends were determined using the AOAC (2002)

| Sensory evaluation
Sensory properties of the moinmoin were determined by panels of 20 untrained people familiar with the product. The coded samples were presented in white plastic plates, and panelists were instructed to rinse their mouth with water between samples. The assessors were asked to appreciate how much they liked the taste, appearance, flavor, texture, and the overall acceptability of the moinmoin on a hedonic scale varying from 1 (dislike extremely) to 7 (like extremely).

| Statistical analysis
Results were analyzed using Minitab ® , version 12, software. Analysis of variance was used for data relating to sensory evaluation, and Duncan's Multiple Range Test was used to separate and compare the group means. Significance was accepted at p < .05.

| Physicochemical properties of flour blends
Results for TTA, pH, water absorption capacity, and reconstitution index are presented in Table 2. The pH values of all the samples were in the acidic range but decreased slightly from control (CPP) to CWYP 2, while titratable acidity increased from 0.28 into 0.45. There were also significant (p < .05) differences in water absorption capacity and reconstitution index of CPP relative to CWYP 1 and CWYP 2 with the lowest reconstitution index value.

| Pasting properties of flour blends
The pasting properties of the flour blends are presented in Figure 1.
There was a significant (p < .05) difference in the peak viscosity (PV) of the control when compared to the cowpea: water yam blends.
The control (CPP) had the highest viscosity, followed by CWYP 1 and T A B L E 1 Blends of cowpea/water yam flours and ingredients for moinmoin production

| Proximate and Energy content of moinmoin from flour blends
The data obtained for the proximate analysis of the samples are presented in Table 3

| Sensory evaluation of moinmoin prepared from cowpea/water yam blends
Results of the organoleptic tests of the pudding using the flour blends are as presented in Figure 2. CWYP 1 was rated highest in terms of taste, flavor, color, and overall acceptability. It was closely followed by CPP, while CWYP 2 was the least accepted.

| DISCUSSION
The acidity of flour is an important indication of its freshness. TTA and pH of the cowpea/water yam pudding flour samples revealed that addition of water yam did not lead to any significant increase in acidity. Low TTA ensures the proper absorption of mineral elements and is also an indication that the product will have better keeping quality because the acidity could prevent or delay the growth of  However, the final viscosity (FV) and the viscosity on cooling (PT) to 50°C showed a reverse trend, increasing with increase in the proportion of water yam flour. These observations suggest that the water absorption capacity of the water yam flour is far greater than that of the CPP as confirmed earlier in Table 3. In all cases, the final viscosities of the flours were greater than the peak viscosities, implying that even on cooling, the starches in the mixtures continued to absorb water and become more swollen, which might result in stiffer moinmoin from the blends.
Setback viscosity is the tendency of starch granules to retrograde on cooling. According to Iwe, Onyeukwu, and Agiriga (2016) the higher the setback viscosity, the lower the retrogradation of the flour paste during cooling and the lower the staling rate of the products made from the flour. In this study, the CWYP 1 blend had the highest setback viscosity which implies that its products are likely to have a very good texture and not stale quickly. Both the pasting temperatures and peak times, which are indicators of the temperature at which gelatinization begins and the time to reach peak viscosity, did not present much variation, though the highest value was recorded for CWYP 2. The implication of this is that regardless of the proportion of water yam, the mixture would cook at approximately the same time and at the same temperature as the control.
The moisture content of any food is an indicator of its water content, stability, and susceptibility to microbial contamination (Uyoh, Ita, & Nwofia, 2013). The highest moisture recorded for CWYP 1 was still relatively low when compared to other studies (Davis et al., 1991;Ibeanu, Onyechi, Ani, & Ohia, 2016). The low moisture content of the moinmoin is an indication of storage stability and longer shelf life. The high-protein content observed in the CPP moinmoin could be attributed to the significant quantity of protein (about 24%) in cowpea seeds (Jimoh & Olatidoye, 2009 F l a v o u r C o l o u r and lipid content compared with the pudding with 50:50% cowpea water yam substitutions. The lower fat content of the moinmoin from cowpea/water yam blends will reduce the rate at which rancidity sets in subsequently increasing the shelf life. This also makes the products suitable for weight management. The presence of 75% and 50% contents of cowpea significantly increased the protein content of the water yam-containing products (CWYP 1 and CWYP 2 ) considering that water yam has only 6%-8% protein content (Jimoh & Olatidoye, 2009). This expected increase was the basis for formulating the blends in order that the final products will have higher protein content and can thus be used to prevent malnutrition. Similar increase in protein content has been reported when cassava flour was supplemented with soy flour (Olatidoye & Sobowale, 2011) and when plantain flour was fortified with soy flour (Abioye et al., 2011). The two composite pudding products CWYP 1 and CWYP 2 also exhibited higher ash, crude fiber, and carbohydrate contents when compared with the CPP products as a result of the presence of water yam in the composite. The ash, crude fiber, and carbohydrate content increased with increasing level of water yam substitution.
This high nutrient content of the blends will be of nutritional importance in most developing countries where many people can hardly afford high-proteinous foods because of the high costs. The lowenergy level observed in the pudding products can be attributed to their low crude fat and high-protein contents. This is an indication that the puddings are low-calorie foods which may be very helpful in weight management.
Overall, proximate values for all the samples showed that the three products irrespective of the water yam content have high nutritional quality and low-energy values.
Sensory evaluation showed that both CPP and CWYP 1 had similar overall acceptability. This is an indication that the addition of 75% cowpea to 25% water yam gave a product that is acceptable both organoleptically and nutritionally as the 100% cowpea pudding. Sample CWYP 2 was least acceptable probably because of the high proportion of the water yam in it which darkened the color, making it unattractive and rendering its taste much less acceptable.
The result of the sensory evaluation revealed that pudding from cowpea paste alone and that produced from the addition of 75:25 cowpea water yam was rated alike in almost all the quality attributes evaluated indicating the acceptability of moinmoin from a blend of cowpea and water yam. These blends are suitable for combating protein-energy malnutrition in a predominantly starch-and cerealbased diet; and their consumption can suppress nutrient deficiencies as a viable long-term, food-based, strategy to control nutrient deficiencies.

| CONCLUSION
This study has revealed that addition of cowpea to water yam flour in the proportion of 75% or 50% gives a product that is nutritionally acceptable and organoleptically better (25% substitution only) than the well-known moimoin from cowpea alone. The 100% moinmoin (control) and that of water yam: cowpea 25:75% (CWYP1) was rated alike in almost all the quality attributes.
While water yam is abundant, it is less utilized for major food products as a result of traditional bias which fails to recognize its unique quality characteristics. It can be inferred therefore that water yam/ cowpea pudding could be an acceptable addition to the menu especially in regions where PEM is prevalent and both crops are produced in abundance. It also provides an alternative way of utilizing water yam thus preventing wastage during peak production and ensuring food security. This is especially important because yam production seems to be on the decline as a result of high cost of production, low yields, and postharvest losses.

ACKNOWLEDGMENT
The authors acknowledge the technical assistance of SO Oladapo.