Plant foods still maintain the greatest possible fountainhead of basic nutrients for major population globally. They are major sources of food for mankind that cannot be replaced. Plant foods are more advantageous over other sources of food due to the fact that they are readily available, cheap, affordable and generally acceptable. Ignored and underutilized plant foods [1] such as the African yam beans is a tropical leguminous plant cultivated mainly in West African countries. Above the ground, AYB produces a good yield of edible seeds. It could be found in forests, open wooded grasslands, rocky fields and marshy grounds as weed and cultivated crops. It grows on wide range of soils including acid and highly leached sandy soils at altitudes from sea level to 1950 m. AYB is cultivated mainly for home consumption and only about 30% of the dry grain produced is sold [2]. 

Cassava is predominantly carbohydrate, with a deficiency in protein of 1 - 2%, fresh weight [3]. A rapid postharvest deterioration limit its utilization as a vital food and feed in Africa [4-8]. African yam bean (Sphenostylis stenocarpa) is a valuable but neglected source of plant protein in the diet of Nigerians cultivated as a pulse for human consumption. According to Eromosele et al. [9], African yam bean (AYB) was said to contain about 21-29 % protein with 50 % carbohydrates mainly as starch. Oshodi et al. [10], also reported AYB to contain calcium and amino acids: lysine, cysteine, methionine, phenylalanine and pyrone. AYB, non-conventional pulse has been brought into focus by some previous researchers as it is known to have a nutritive and culinary value [11]. African Yam Bean (AYB) is also known to be rich in crude fiber [12,13]. Although it contains anti-nutritional factors such as haemagglutinins, tannins and oligosaccharides, processing destroys most of these components making them of more nutritional value when consumed.

Oseni and Akindahunsi, [14], stated that fermentation could increase protein content of food through secretion of microbial proteins, accumulation of other nitrogenous microbial components such as chitin and hydrolysis of peptides. Crude protein content of a food product could also increase when protein in the substrate is concentrated as carbohydrates are consumed by microorganisms [15]. Supplementation or blending of African yam bean equally has resulted in products with high nutritional values. Consequently, the effect of incorporation of African Yam Bean (AYB) on the pasting, proximate and sensory attributes of cassava was studied by Nwokeke et al. [16]. As a rider to this, limited information is available on the in-vitro protein digestibility, starch digestibility and sensory property of fermented and extruded blends of cassava and African yam bean to determine the nutritional effect of their fortification, which cause has necessitated this study.

Source of Raw Materials

Matured cassava (Manihot esculenta) tubers were gotten from Federal University of Technology, Akure, farm Nigeria while African yam beans (Sphaerostylis sternocarpa) seeds were purchased from a local market in Akure, Nigeria.

Sample Preparation

The tubers of cassava were cleaned by washing them in clean water, after which they were peeled and cut into small cubes and kept in a clean bucket with cover. The seeds of African Yam Beans (AYB) were sorted out for bad seeds, dirt and stones. They were washed with clean water and thereafter dried.

Formation of Cassava: African Yam Beans Blends

Cassava and African yam beans were mixed in varying proportions as shown in Table 1.

The blends were kept in sterile air- tight plastic containers and properly labelled as A, B, C, D, E and F correspondingly. This was done following Ojokoh and Fagbemi, [17], method with little modification.

Processing of the Cassava: African Yam Beans Blends

The blends of cassava and African yam beans were grouped into four categories. The first group was fermented only, the second group was extruded only and the third group was subjected to both fermentation and extrusion while the last group was neither fermented nor extruded (control).

Fermentation of Cassava: African Yam Bean Blends

The samples were fermented by adding 2 litres of sterile water to 1 kg of each blend. Each sample was covered and allowed to ferment for 120 hours. The fermentation was stopped by drying the samples in an oven at temperature of 550C for 24 hours.

Table 1: Formulation of Cassava: African Yam Bean Blends

Extrusion of Cassava: African Yam Bean Blends

The blends of cassava and African yam beans were extruded with the use of a Brabender 20DN. The process was done as described by [18].

In-Vitro Protein Digestibility Determination

The in-vitro protein digestibility of each sample was evaluated using a sequential pepsin and pancreatin digestion model according to the method of Chavan et al. and Nunes et al.

In-Vitro Starch Digestibility Determination

In vitro stach digestibility was determined by using 1% carboxymethyl cellulose (CMC) in sodium acetate buffer (pH 5.5) as substrate. This was done using the method of Englyst et al., 1992.

Sensory Evaluation of Fermented and Extruded Cassava: African Yam Bean Blends

The design and analysis for evaluation of sensory acceptability as detailed by Nwatarali et al. and Ihekoronye and Ngoddy, was used to determine the general acceptability of the food samples. A panel of 10 judges (untrained but familiar with extruded products) was set up. Coded samples of the raw blends, extruded unfermented blends, fermented un-extruded blends and fermented extruded blends were served to the panelists. The panelists were asked to rate the samples based on the colour, aroma, texture and overall acceptability by scoring them on a nine-point hedonic scale with 9 being extremely liked and 1 being extremely disliked.

in-Vitro Protein Digestibility Profile of Cassava-African Yam Bean Blends

The efficacy of fermentation and extrusion on the in-vitro protein digestibility of Cassava-African yam bean blends of sample RF (A through F) ranged from 67.98±0.00 to 71.94±0.00. Sample FU (A through F) ranged from 72.10±0.01 to 78.52±0.00. Sample UE (A through F) ranged from 71.96±0.00 to 76.41±0.00. Sample FE (A through F) ranged from 78.11±0.00 to 82.11±0.00 as illustrated in Table 2.

Table 2: Effect of Fermentation and Extrusion On the In-Vitro Protein Digestibility of Cassava-African Yam Bean Blends

A: 100% Cassava; B: 90% Cassava: 10% African yam beans; C: 80% Cassava: 20% African yam beans; D: 70% Cassava: 30% African yam beans; E: 60% Cassava: 40% African yam beans; F: 50% Cassava: 50% African yam beans RF: Raw flour; FU: Fermented unextruded blends; UE: Unfermented extruded blends; FE: Fermented extruded blends. Data are presented as Mean±S.D (n = 6). Figures with the similar superscript letter(s) along the equivalent row are not significantly diverse (p<0.05).

In-Vitro Starch Digestibility Profile Of Cassava-African Yam Bean Blends

The effect of fermentation and extrusion on the in-vitro starch digestibility of Cassava-African yam bean blends of sample RF (A through F) ranged from 32.33±0.66 to 36.61±0.03. Sample FU (A through F) ranged from 35.51±0.04 to 41.18±1.73. Sample UE (A through F) ranged from 35.00±0.00 to 41.02±0.58. Sample FE (A through F) ranged from 37.42±0.29 to 46.59±0.01 as shown in Table 3.

Table 3: Effect of Fermentation and Extrusion On the In-Vitro Starch Digestibility of Cassava-African Yam Bean Blends

A: 100% Cassava; B: 90% Cassava: 10% African yam beans; C: 80% Cassava: 20% African yam beans; D: 70% Cassava: 30% African yam beans; E: 60% Cassava: 40% African yam beans; F: 50% Cassava: 50% African yam beans RF: Raw flour; FU: Fermented unextruded blends; UE: Unfermented extruded blends; FE: Fermented extruded blends. Data are presented as Mean±S.D (n = 3). Figures with the similar superscript letter(s) along the equivalent row are not significantly diverse (p<0.05).

 Sensory Evaluation Rating of Fermented and Extruded Cassava-African Yam Bean Blends

The aroma rating of sample RA through RF of fermented and extruded Cassava-African yam bean blends after organoleptic evaluation ranged from 4.96±0.47 to 6.98±0.17. The aroma of sample FUA through FUF of fermented and extruded Cassava-African yam bean blends after organoleptic evaluation ranged from 5.89±0.06 to 6.71±0.11. The aroma of sample UEA through UEF of fermented and extruded Cassava-African yam bean blends after organoleptic evaluation ranged from 6.07±0.39 to 6.88±0.56. The aroma of sample FEA through FEF of fermented and extruded Cassava-African yam bean blends after organoleptic evaluation ranged from 6.17±0.19 to 8.33±0.01.

The colour rating of sample RA through RF of fermented and extruded Cassava-African yam bean blends ranged from 7.60±1.00 to 8.56±0.03. The colour of sample FUA through FUF of fermented and extruded Cassava-African yam bean blends ranged from 5.97±0.59 to 6.63±0.16. The colour of sample UEA through UEF of fermented and extruded Cassava-African yam bean blends ranged from 6.27±0.69 to 6.85±0.95. The colour of sample FEA through FEF of fermented and extruded Cassava-African yam bean blends ranged from 6.91±0.19 to 8.01±0.01. 

The texture rating of sample RA through RF of fermented and extruded Cassava-African yam bean blends ranged from 7.10±0.07 to 8.16±0.03. 

The texture of sample FUA through FUF of fermented and extruded Cassava-African yam bean blends ranged from 6.20±0.79 to 8.26±0.08. The texture of sample UEA through UEF of fermented and extruded Cassava-African yam bean blends ranged from 7.06±0.70 to 7.95±0.08. The texture of sample FEA through FEF of fermented and extruded Cassava-African yam bean blends ranged from 6.91±0.19 to 8.01±0.01 (Table 4).

Table 4: Sensory Evaluation of Fermented and Extruded Cassava-African Yam Bean Blends

RA = Raw 100% Cassava FLOUR;                    RB = Raw 90% Cassava: 10% African yam beans; RC = Raw 90% Cassava: 10% African yam beans; RD = Raw 90% Cassava: 10% African yam beans; RE = Raw 90% Cassava: 10% African yam beans; RF = Raw 90% Cassava: 10% African yam beans; FUA = Fermented Unextruded 100% Cassava; FUB = Fermented Unextruded 90% Cassava: 10% African yam bean; FUC = Fermented Unextruded 80% Cassava: 20% African yam beans; FUD = Fermented Unextruded 70% Cassava: 30% African yam beans; FUE = Fermented Unextruded 60% Cassava: 40% African yam beans; FUF = Fermented Unextruded 50% Cassava: 50% African yam beans; UEA = Unfermented Extruded 100% Cassava; UEB = Unfermented Extruded 90% Cassava: 10% African yam beans; UEC = Unfermented Extruded 80% Cassava: 20% African yam beans; UED = Unfermented Extruded 70% Cassava: 30% African yam beans; UEE = Unfermented Extruded 60% Cassava: 40% African yam beans; UEF = Unfermented Extruded 50% Cassava: 50% African yam beans; FEA = Fermented Extruded 100% Cassava; FEB = Fermented Extruded 90% Cassava: 10% African yam beans; FEC = Fermented Extruded 80% Cassava: 20% African yam beans; FED = Fermented Extruded 70% Cassava: 30% African yam beans; FEE = Fermented Extruded 60% Cassava: 40% African yam beans; FEF = Fermented Extruded 50% Cassava: 50% African yam beans

The general appearance rating of sample RA through RF of fermented and extruded Cassava-African yam bean blends ranged from 6.37±0.10 to 8.00±0.08. The general appearance profile of sample FUA through FUF of fermented and extruded Cassava-African yam bean blends ranged from 6.41±1.04 to 8.01±0.38. The general appearance profile of sample UEA through UEF of fermented and extruded Cassava-African yam bean blends ranged from 6.10±0.26 to 7.58±0.05. The general appearance profile of sample FEA through FEF of fermented and extruded Cassava-African yam bean blends ranged from 6.89±0.63 to 8.25±0.04.

This study set out to investigate the effect of fermentation and extrusion on the in-vitro protein digestibility, in-vitro starch digestibility and sensory properties of cassava and African Yam Bean (AYB) blends. The essence of fermentation of cassava and African yam bean blends in this study could have increase protein content of food blends through secretion of microbial proteins, accumulation of other nitrogenous microbial components such as chitin and hydrolysis of peptides [14]. The increasing protein digestibility profile of cassava and African Yam Bean (AYB) blends observed in this study could be owed to African yam bean (AYB) (Sphenostylis stenocarpa) being a valuable and prominent source of plant proteins and it’s said to contain about 21-29 % protein [9]. The protein repertoire of AYB is made up of over 32 % essential amino acids, comprising the predominant lysine and leucine [16,19]. This augments the rationale for the combination of cassava and African yam bean in our study because the utilization of cassava is restricted by its exceptionally low protein content and so the consumption of its products has been implicated in undernourishment. This low protein intake has been attributed to the mounting skyrocketing cost of conventional sources of animal protein. As a respite to this, AYB, a legume, is a low-priced source of protein rich food that has been important in ameliorating protein malnutrition [16,20]. Additionally, the crude protein content of the blends could also increase when protein in the substrate is concentrated as carbohydrates and are consumed by microorganisms as reported by Day and Morawicki, [15]. This is important because the fortification and combination of inexpensive staples such as cassava and legumes has resulted in end products of high nutritional value [16,21].

The increasing starch digestibility profile of cassava and African yam bean (AYB) blends in this study could be due to cassava being predominantly carbohydrate, with a deficiency in protein of 1 - 2% [3], since cassava is a major source of dietary energy for low income consumers [16]. More importantly, African yam bean has also been revealed like most grain legumes to be rich in carbohydrates (62.6%), [16,22]. Eromosele et al. [9], also reported AYB constitutes 50 % carbohydrates mainly as starch which supports this finding in our study. 

The rating for aroma extensively differs from each other and relatively increased further with concurrent inclusion of AYB flour. This could be accredited to the enhance bean- off flavour associated with African yam bean legume [23]. The considerably high difference in the colour and general appearance ratings of the blended cassava and African yam bean (AYB) might be attributed to the nature and chemical composition of the major raw materials in the food blends [16], ability because of closeness in the blending proportions [16].

Findings from this study have shown that the nutritional benefits of a cassava enhanced through African Yam Beans supplementation and fortification with wide ranging sensory properties. This suggests the enhancement of acceptable, nutritionally-enriched starchy-proteinous staple foods that can be stored at home and can be produced for consumption in areas where protein intake is low.

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