Optimization and characterization of rice–pigeon pea flour blend using extrusion cooking process

This study was carried out to formulate rice and pigeon pea flour blend with the aim of providing nutrient‐enriched and inexpensive food for developing countries where the raw materials are found in abundance. Three factors (screw speed, feed moisture content and feed blend composition) affecting the extrusion cooking process were subjected to face‐centred central composite design (FCCCD), and physical properties were used as the response. Analysis of variance showed that the developed quadratic model was significant with coefficient of determinations (R2) of 0.96 for expansion index, 0.93 for bulk density and 0.88 for water absorption index. Validation experiments were carried out where four rice–pigeon pea flour blends were subjected to physical, mineral and amino acid analyses. Formulation 3 set at screw speed, feed moisture content and feed blend composition of 220 rpm, 30% and 25%, respectively, led to maximum expansion index of 9.98 ± 0.15, bulk density of 0.12 ± 0.01 g/mL and water absorption index of 6.41 ± 0.07. There was significant (p < 0.05) increase in essential amino acids in all the developed rice–pigeon pea flour blends, and Formulation 3 was found to be two‐ and fivefold higher in terms of methionine and lysine contents, respectively, than the control (extruded rice). Similarly, calcium (3.41 ± 0.07 mg/100 g), iron (12.64 ± 0.03 mg/100 g) and zinc (9.33 ± 0.02 g/100 g) contents in Formulation 3 were significantly (p < 0.05) higher than the values of 1.19 ± 0.13, 5.89 ± 0.10 and 2.67 ± 0.05 mg/100 g recorded, respectively, for the extruded rice (control). In conclusion, the extruded rice–pigeon pea flour blend showed better physical properties and nutritional quality than the extruded rice.

. The production of cereal-legume-based products to complement the limiting nutrients has increased significantly over the years; and some of the developed products include porridge, noodles, biscuits, cakes and breads (Muller & Krawinkel, 2005). Extrusion cooking technology has played an important role in enhancing food security for development of a variety of products such as ready-to-eat cereals, chips and flakes (Chaiyakul et al., 2009). This technology requires a high-temperature short-time (HTST) cooking process with considerable prospects, based on its ability to produce shelf-stable foods with minimal microbial loads, reduced anti-nutrient content, improved in vitro protein and starch digestibility and low moisture content (Filli et al., 2011;Nascimento et al., 2017). Products with these characteristics are needed especially in developing countries where food storage infrastructure is limited and inadequate (Filli et al., 2011).
Different factors have been reported to affect the quality of extruded products, and these include temperature, feed-related parameters (moisture, particle size, composition and proportion), screw speed and configuration, nature of the extruder and the geometry of the die (Gbenyi et al., 2015;Pansawat et al., 2008); these factors work synergistically in increasing the physical and chemical properties as well as consumers' acceptability (associated with taste, texture, flavour and desired shapes) of the extruded products.
Rice (Oryza sativa) is a popular staple food for about half of the global population, and there has been a remarkable rise in its production to meet the current demand (FAO, 2016). The wider utilization of rice could be linked to its attractive colour, bland taste and ease of digestion (Kaushal et al., 2012). Thus, in most developing countries, rice accounts for daily provision of 27% energy, 20% protein and 3% fat (Kennedy & Burlingame, 2003).
Pigeon pea (Cajanus cajan) is one of the major leguminous seeds found in many tropical and subtropical countries (Troedson et al., 1990). It was ranked as number four after groundnut, cowpea and Bambara groundnut and highly cherished for its protein content (17%-30%), vitamin B complex, vitamin C and provitamin A (Kaushal et al., 2012). Pigeon pea has been considered to be highly underutilized, and its major limitation is its inability to cook fast; however, it can serve as an ideal supplement to cereal-and tuber-based diets that are known to be generally protein deficient (Eneche, 2009). Therefore, blend of rice and pigeon pea may aid in producing meals with adequate nutrient compositions for utilization by both infants and adults.
Hence, this study is aimed at optimizing the three factors (feed moisture content, feed blend composition and screw speed) affecting the extrusion process of rice-pigeon pea flour blend, and the formulated blends were further characterized based on their nutrient compositions.

| Materials and processing of samples
Rice was obtained from the National Cereals Research Institute, Badeggi, Niger State, Nigeria, and pigeon pea was also purchased from Samaru Market, Zaria, Kaduna State, Nigeria. Processing of rice consists of winnowing and dry cleaning. The kernels were removed using a rice dehuller (Tokyo M3, Rice Dehuller, Japan).
The dried grains were milled into flour and sieved using a 2-mm standard sieve and then stored tightly in a polythene bag for extrusion. Pigeon pea seeds (2 kg) were cleaned manually from all the foreign materials and soaked in water for 8 h (28 ± 2 C), where the seed coats were completely removed. Thereafter, the seeds were dried in a hot air oven maintained at 60 C until constant weight was obtained. The dried seeds were ground into powder using attrition mill (Galenkamp, England) and sieved using a 2-mm standard sieve as described by Anuonye (2012).

| Composite flour formulation
Pigeon pea flour was mixed at different proportions (10%-30%, w/w) with rice flour. The initial moisture content of the rice-pigeon pea flour blends was determined using hot air oven method, which was designated as M a . Thus, the amount of water to be added in the formulation was calculated according to Wilmot (1998).
where W d is the weight of dried flour blend, W w is the amount of water to be added, M a is the initial moisture content and M b is the desired moisture content.

| Bulk density
Bulk density (BD) relates the weight of the extruded sample to its volume as described by Jafari et al. (2017). Ten g of the extruded sample was measured into a clean measuring cylinder (100 mL), and the bottom of the cylinder was tapped repeatedly until no further reduction in volume was observed. All measurements were done in triplicates, and the average volume was calculated as the packed volume. The weight of the extruded sample per unit volume was considered as the BD using the general formula Bulk density g=mL ð Þ = weight of extruded sample g ð Þ volume occupied by the extruded sample mL ð Þ

| Expansion index
Expansion index (EI) shows the relationship between the diameter of the extruded sample and that of the die nozzle of the extruder. Different lengths of the extruded samples were selected at random during collection, and vernier caliper was used to measure the diameter at different positions as described by Alvarez-Marnitez et al. (1988). EI was calculated using the formula Expansion index = diameter of the extruded sample diameter of the die nozzle of the extruder

| Water absorption index
The water absorption index (WAI) was estimated as determined by Jafari et al. (2017), where 1 g of the extruded sample was dissolved in water at a temperature of 25 C for 20 min with gentle stirring at 5-min interval. Thereafter, the mixture was centrifuged at 3000 × g for 15 min, and the WAI was calculated as the weight of hydrated sample obtained after removal of the supernatant per unit weight of the original sample.  Note: (+) = high level; (−) = low level; (0) = centre.

| Validation of the model
Design-Expert 6.0.8 (Stat-Ease, Inc., Minneapolis, USA) so as to validate the model as shown in Table 3. The physical properties (BD, EI and WAI) were used as the response. Also, mineral and amino acid analyses were used to come up with the best rice-pigeon pea flour blend formulation. Extruded rice was used as a control, which was prepared using the laboratory single screw extruder (Duisburg DCE-330 Model, Germany) at screw speed of 220 rpm and moisture content of 30%.

| Mineral compositions and amino acid analysis
Five mineral compositions (magnesium, calcium, zinc, iron and phosphorus) were determined using atomic absorption spectrophotometry

| Statistical analysis
Measurements made in this study were in triplicates and expressed as mean ± standard deviation. Analysis of variance (ANOVA) and post hoc test (Duncan) were carried out to determine the differences among the samples, and significant differences were accepted at p < 0.05. The responses obtained from the FCCCD were used to determine the effects of the selected factors (feed moisture content, feed blend composition and screw speed) on the responses. The coefficient of determination (R 2 ), lack of fit and F-values obtained following the regression analysis were used to evaluate the model fitness.

| Effect of extrusion parameters on the physical properties of rice-pigeon pea blend
Physical properties associated with EI, BD and WAI are among the important quality attributes required in food formulation (Filli et al., 2013). Thus, extrusion cooking process is generally dependent on some parameters, and the effects of the three independent variables associated with moisture content, feed blend composition and screw speed were determined. The combined effects of these factors in terms of physical properties and nutrient compositions of the extruded samples contribute to the overall assessment of the final products.

| EI
The EI of the extruded samples was reported by Ding et al. (2005) to be dependent on feed moisture content; and in this study, EI was found to reach its maximum in Run 12 (9.56), and the lowest value was obtained in Run 5 (2.13) as indicated in Table 1. The higher value of EI observed in Run 12 could be as a result of increase in moisture content and screw speed at decreased feed blend composition. Seker (2005) stated that screw speed and feed composition were the most significant factors that contributed to the increase in EI during extrusion of soybean-corn flour. Similarly, high starch content may lead to increase in EI, especially where significant gelatinization occurs due to dough viscosity (Seker, 2005). High shear force aids in disrupting intermolecular hydrogen bonds, thereby promoting gelatinization and elasticity of the dough, and this could be achieved by changing the screw speed and feed composition during extrusion cooking process (Filli et al., 2013). In case of extruded samples obtained from rice-based expanded snacks, moisture content and barrel temperature affected the EI, which was dependent on dough viscosity and elastic force (Ding et al., 2005).
Also, increase in protein content of rice-pigeon pea blend (based on feed blend composition) could cause puffiness, which breaks down because of its viscoelastic characteristics. Thus, Chaiyakul et al. (2009) found that extrudate of low moisture and low protein contents tend to have higher EI as observed in high- The ANOVA and p-values obtained in this study were used to determine the fitness of the developed model as indicated in Table 2.
The overall model had a p-value of <0.0001, which suggested that the model was significant.
The ANOVA ( indicated the appropriateness of the model because any value greater than 4 signifies its desirability (Salihu et al., 2011). Also, the lower the value of the coefficient of variation (CV = 8.41), the better is the precision and reliability of the experiment; Nath and Chattopadhyay (2007) reported that no reliable models should have CV greater than 10%.
The response surface plots showing the synergistic effects of the selected parameters on the EI of the developed pigeon-pea flour blend were presented in Figure 1. The significance of the interactions is determined by the shape of the response surface, based on the circular or elliptical nature as well as the overall ANOVA values (Manimekalai & Swaminathan, 1999). In Figure 1a, the effect of feed blend composition and feed moisture content on EI at fixed screw speed of 200 rpm (centre point) revealed that EI is dependent on all the factors and their synergistic effect resulted in optimum response, as indicated in Table 2, with a p-value of <0.0001.

| Bulk density
The BD of rice-pigeon pea blend was found to be highest (0.11 g/mL) in Runs 5 and 6, and the lowest value was observed in Run 12 (0.05 g/mL) as presented in Table 1. BD serves as an important physical property that is used in assessing the level of expansion during the extrusion process (Filli et al., 2013). Filli et al. (2011) found that feed blend composition and moisture content were the major factors responsible for the observed increment in BD of soybean-millet extrudate. However, no clear trend was observed in this study as both feed blend composition and feed moisture contents showed different trends on the BD. Also, increase in feed moisture content favours increment in BD by reducing the elasticity, gelatinization and expansion (Ding et al., 2006). The higher BD observed in this study could be related to homogeneous protein matrix of the developed rice-pigeon pea flour blend with compact or no air cavity layers, making it nonspongy upon hydration (Filli, 2009). Thus, the BD values could be linked to the amount of flour particles that are held together and the energy content derivable from the developed blend. Based on this, the developed rice-pigeon pea flour blend may give a desired nutrient density and consistency, which could meet the feeding requirements of different types of people.   Table 2, where all the three factors were found to be significant with p-values of less than 0.05. WAI was reported to be undesirable in the formulation of complementary foods as it affects the nutrient density (Filli, 2009) Table 2. Two of the linear (B and C) and the interaction terms (AC and BC) were found to be significant at p < 0.05. About 88% of the variation in the rice-pigeon pea flour blend could be linked to the independent variables based on its R 2 value of 0.88.
Although this value appears to be low, it is still acceptable due to diverse characteristics of biological systems. In fact, Gassara et al. (2011) suggested that a model can be adequately accepted once the R 2 value is greater than 0.75. A signal-to-noise ratio greater than 4 indicates adequate precision (Salihu et al., 2011), and in this case, an adequate signal was obtained with a value of 12.12. Also, lack of fit that serves as a tool for measuring the failure of a model is expected not to be significant, and a p-value greater than 0.05 (0.544) was obtained.
The response surface plot (Figure 1c) showed the effects of feed blend composition and feed moisture content on WAI. The interaction was statistically significant with a p-value of 0.0005 (Table 2), which suggested that the interaction was significant even at 99.99% confidence level. Thus, increase in feed moisture content resulted in increased WAI (Figure 1c).   Table 3, Formulation 3 resulted in maximum EI of 9.98 ± 0.15, BD of 0.12 ± 0.01 g/mL and WAI of 6.41 ± 0.07. Also, the formulations were further subjected to characterization involving determination of mineral and amino acid compositions to be able to come up with the best formulation of rice-pigeon pea flour blend that meets some nutritional specifications.

| Mineral compositions of the formulated rice-pigeon pea flour blend
Minerals are indispensable components of foods that are required for normal metabolic activities of the body (Kadan et al., 2003). Sodium is one of the minerals whose low intake is always encouraged because its high concentration in form of salt has been implicated in hypertension and other coronary complications (Kadan et al., 2003). The concentration of sodium observed in the control (extruded rice) was found to be 1.53 ± 0.11 mg/100 g, and there was significant (p < 0.05) increase in sodium content (Table 4) (Table 4). Thus, sodium is an essential cation required for acid-base balance, muscle contraction and regulation of osmotic pressure (Kanu et al., 2009). Potassium is primarily an intracellular cation, whose functions include maintenance of electrolyte balance, transmission of impulses and muscle contraction. It also functions in synergy with sodium in maintaining the normal pH equilibrium (Adeyeye & Agesin, 2007). The highest potassium content was found in Formulation 3 (5.01 ± 0.01 mg/100 g), and the lowest (3.73 ± 0.09 mg/100 g) was found in the control (Table 4) Magnesium as an activator of many enzyme systems is required in energy metabolic processes. It works in tandem with calcium during muscle contraction, blood clotting and the regulation of blood pressure (Adeyeye & Agesin, 2007). Thus, the findings from this work as presented in Table 4 showed that Formulation 1 had significantly (p < 0.05) higher magnesium (2.99 ± 0.05 mg/100 g) and phosphorus (1.52 ± 0.10 mg/100 g) contents when compared with the other formulations and the control. Similarly, there was no significant (p > 0.05) difference between the control and Formulation 2 in terms of phosphorus contents. Also, there were significantly (p < 0.05) higher iron and zinc contents in Formulation 3, with values of 12.64 ± 0.03 and 9.33 ± 0.02 mg/100 g, respectively (Table 4). Similarly, iron and zinc act as cofactors for enzyme-catalysed reactions during the normal metabolic processes (Agunbiade & Ojezele, 2010). In addition, iron forms an important component of two essential proteins (myoglobin and haemoglobin), whereas zinc is a component of living cells required for membrane stabilization and immune functions (Agunbiade & Ojezele, 2010). Generally, researchers (Dalbhagat et al., 2019;Singh, Chauhan, et al., 2000) reported significant increase in mineral compositions (especially iron, calcium, phosphorus and copper) of extruded snacks developed from rice flour blends.

| Essential amino acid composition of the formulated rice-pigeon pea flour blend
Essential amino acids are needed to be supplied in the diet for growth and development. Table 5 shows the compositions of essential amino acids; significant (p ≤ 0.05) differences in amino acid compositions were observed in the four formulations when compared with the control (extruded rice), and these could be linked to variations in feed blend composition. The limiting essential amino acids in rice and pigeon pea are lysine and methionine, respectively. Lysine is thermolabile, and any attempt to retain its composition during extrusion cooking process is of primary interest. The lysine content of extruded rice (control) was found to be 0.66 ± 0.13 g/100 g, and the ricepigeon pea blends showed up to five-fold increment in lysine content with maximum value of 3.44 ± 0.04 g/100 g recorded in Formulation 3. Similarly, the methionine content of Formulation 3 was found to be 1.48 ± 0.02 g/100 g, which was two times higher than what was obtained in the control (0.72 ± 0.03 g/100 g). Masatcioglu et al. (2014) found that decrease in lysine contents in some extruded products could be attributed to low moisture contents, longer period of extrusion and the nature of the extruder. Leucine was found to be the most predominant amino acid where the control had a value of 2.93 ± 0.14 g/100 g, and higher values were obtained in Formulation 3 (5.53 ± 0.10 g/100 g) and Formulation 4 (5.60 ± 0.07 g/100 g) as shown in Table 5. This agrees with the findings of Aremu et al. (2010) that plant products of Nigerian origin have leucine as the most abundant essential amino acid.
In this study, there was significant (p < 0.05) increase in the compositions of eight essential amino acids (leucine, isoleucine, lysine, phenylalanine, valine, methionine, arginine and threonine) in the developed formulations when compared with the control (extruded rice), which could be linked to the addition of pigeon pea. However, no significant (p > 0.05) difference exists between Formulation 2 and the control in terms of tryptophan and histidine contents (Table 5).
Several studies including that of Filli et al. (2011) reported that addition of soybean flour during the production of extruded fura resulted in higher essential amino acid content. Singh et al. (2007) also showed that mild extrusion conditions enhance the nutrient contents including the amino acids and some physical characteristics, whereas high extrusion conditions associated with high screw speed (>250 rpm), low moisture (<20%) and/or improper feed blend compositions affect the nutrient contents of extruded products. In contrast to this study, Anuonye et al. (2010) found a significant reduction in essential amino T A B L E 5 Essential amino acid compositions (g/100 g) of formulated rice-pigeon pea blend  acid content in extruded acha-soybean blends. Thus, based on the essential amino acids, the formulated rice-pigeon pea flour blends show a great potential for utilization in developing different products made from conventional flour sources that could meet the nutrient requirements of vulnerable population.

| CONCLUSION
The effect of different factors affecting extrusion of rice-pigeon pea flour blends was studied using FCCCD. The optimum independent variables were established at screw speed of 220 rpm, feed moisture content of 30% and feed blend composition of 25%, which resulted in maximum EI of 9.98 ± 0.15, BD of 0.12 ± 0.01 g/mL and WAI of 6.41 ± 0.07. Out of the four formulations prepared for validation experiments, Formulation 3 was found to be better in terms of physical properties, mineral and essential amino acid contents. Based on this, different complementary foods can be produced from ricepigeon pea flour blend using extrusion method. The use of these locally available raw materials could help in management of some malnutrition-related cases in developing countries where these raw materials are found in abundance.

ACKNOWLEDGEMENTS
The assistance of technical staff of the Department of Biochemistry, Ahmadu Bello University, Zaria, and that of National Cereals Research Institute, Badeggi, Niger State, is greatly appreciated by the authors throughout the period of the research.

CONFLICT OF INTEREST
No conflict of Interest during the research and preparation of the manuscript.

FUNDING
No specific grant was received for this research from any funding agencies in the government or private sector.

AUTHOR CONTRIBUTION
Ndaliman, Salihu, Muhammad and Bala contributed in the conceptualization and design of the study. Ndaliman and Salihu carried out the experimentation and analysis of data. All the four authors contributed equally in the interpretation and revision of the contents of the manuscript, and their approval was sought before submission.

ETHICAL APPROVAL
Ethical principles for research were adhered to, and for food characterization-based study like this, institutional ethical committee of Ahmadu Bello University, Zaria, ensures that hygienic/safety measures have been taken into consideration. I wish to declare that human subjects/animal models have not been used in this study and the two samples (rice and pigeon pea) are commonly used as staple foods in Nigeria and no reported toxicity on any one of them has been documented.

DATA AVAILABILITY STATEMENT
The authors wish to state that the data obtained that supported the findings reported in this study are available upon reasonable request from the corresponding author.