Received: 20 December 2018 | Revised: 22 July 2019 | Accepted: 27 July 2019 DOI: 10.1002/fsn3.1284 ORIGINAL RESEARCH Multi-response optimization of extrusion conditions of grain amaranth flour by response surface methodology Julian Atukuri | Brian B. Odong | John H. Muyonga School of Food Technology, Nutrition and Bio-Engineering, Makerere University, Kampala, Uganda Abstract Correspondence John Muyonga, School of Food Technology, Nutrition & Bioengineering, Makerere University, P.O. Box 7062, Kampala, Uganda & Peak Value Industries Ltd, P.O. Box 704, Mukono, Uganda. Email: hmuyonga@yahoo.com of instant grain amaranth flour for complementary feeding. Multi-response criteria Funding information Competeive Grant Scheme, National Agricultural Research Organization The study was designed to optimize extrusion processing conditions for production using response surface methodology and desirability function analysis were employed during the study. The central composite rotatable design (CCRD) was used to determine the level of processing variables and to generate the experimental runs. The process parameters tested included extrusion temperature (110–158°C), screw speed (40–52 Hz), and feed moisture content (11%–16%), while response variable was protein digestibility, sensory acceptability, water absorption index, water solubility index, bulk density, and viscosity. Data obtained from extrusion were analyzed using response surface methodology. Data were fitted to a second-order polynomial model, and the dependent variables expressed as a function of the independent variables. Analysis of variance (ANOVA) revealed that extrusion parameters had significant linear, quadratic, and interactive effects on the responses. Numerical optimization indicated that the optimum extrusion parameters were extrusion temperature of 150°C, extrusion speed (screw speed) of 50 Hz, and feed moisture content of 14.41%. The responses predicted for optimization resulted in protein digestibility 81.87%, water absorption index 1.92, water solubility index 0.55, bulk density 0.59 gm/L, viscosity 174.56 cP (14.55 RVU), and sensory acceptability score of 6.69, with 71% desirability. KEYWORDS complementary feeding, extrusion, gelatinization, grain amaranth, optimization 1 | I NTRO D U C TI O N replace breast milk (Adepeju, Gbadamosi, Omobuwajo, & Abiodun, 2014; Omueti, Otegbayo, Jaiyeola, & Afolabi, 2009). Most regions Malnutrition is problematic during the period of complementary feed- facing malnutrition majorly depend on inadequately processed ing (6–24 months), making this age period crucial in the growth of an traditional foods mainly comprising cereal gruels from maize, sor- infant (Okoth, Ochola, Gikonyo, & Makokha, 2016). Complementary ghum, and millet (Tou, 2007). Gruels from these cereals form very feeding is the delivery of foods or fluids to infants to supplement or viscous pastes during cooking and need excessive dilution to suit This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2019 The Authors. Food Science & Nutrition published by Wiley Periodicals, Inc. Food Sci Nutr. 2019;7:4147–4162. www.foodscience-nutrition.com | 4147 4148 | ATUKURI eT Al. infant feeding (Ikujenlola & Fashakin, 2005; Kikafunda, Abenakyo, Extrusion technology is a continuous high temperature–short & Lukwago, 2006). However, dilution to reduce thickness results time (HTST) food processing technique that combines mechanical in energy and nutrient thinning which reduces energy and nutrient energy with heat energy to gelatinize starch, denature proteins and densities (Amagloh et al., 2013). Furthermore, although cereal crops reorganize food material to form new textured products (Danbaba, are widely available, they are inadequate with respect to nutrients Iro, & Mamudu, 2016). Extrusion has high versatility and efficiency, sufficiency for infants as cereals mainly consist of carbohydrates and low cost, high output per unit time and leads to very little waste have low levels of protein and minerals (Ijarotimi & Oluwalana, 2013). (Nabeshima & Grossmann, 2001). Flours precooked by extrusion can Malnutrition in developing regions is associated with low nutrient quickly increase their viscosity with low tendency to form clumps in density and bioavailability, time constraint for preparation of food both cold and hot water thus extrusion is recommended for prepa- for infants, and inadequate dietary diversity (FANTA-2010, 2005). ration of instant food products (Milán-Carrillo, Montoya-Rodríguez, Grain amaranth is desirable because of its nutritional and Gutiérrez-Dorado, Perales-Sánchez, & Reyes-Moreno, 2012). functional potential, brief growth cycle, and its capability to with- Extrusion can also be used to produce low bulk density instant flours stand unfavorable climate and soil conditions (Capriles, Coelho, which have higher nutrient density than raw flours (Milán-Carrillo et Guerra-Matias, & Arêas, 2008a). The entire plant can be used as al., 2012). Extruded products are therefore more suitable for feed- food (Capriles, Coelho, et al., 2008a). Amaranth seeds contain ing vulnerable persons such as weanlings who are only able to con- more protein than other grains and also have high levels of min- sume limited volumes of food. Changes in extrusion variables such erals (Kanensi, Ochola, Gikonyo, & Makokha, 2011; Muyonga, as feed compositions, die configuration, feed rate, and processing Nabakabya, Nakimbugwe, & Masinde, 2008). Amaranth protein is temperature among others affect the quality of finished products unique because its amino acid balance is close to the optimum for (Ding, Ainsworth, Tuker, & Marson, 2005). It is therefore crucial to human nutrition (Drzewiecki, 2001). Grain amaranth is a good source appropriately and effectively optimize production variables during of micronutrients especially calcium, iron, potassium, phosphorous, extrusion of products (Danbaba et al., 2016). vitamin A, E, and folic acid (Mnkeni, Masika, & Maphaha, 2007; Response surface methodology (RSM) and central composite de- Valcárcel-Yamani & Lannes, 2012). It has also been shown to con- sign (CCD) provide an ideal tool for process optimization (Danbaba et tain high levels of phenolics and to exhibit high antioxidant activity al., 2016). Therefore, the aim of the study was to determine the op- (Muyonga, Andabati, & Ssepuuya, 2014), properties associated with timal extrusion conditions for production of instant grain amaranth prevention of noninfectious chronic diseases such as cancers (Espín, complementary food with high protein content, protein digestibility, García-Conesa, & Tomás-Barberán, 2007). Because of its high con- sensory acceptability, and low bulk density and viscosity. tent of quality proteins, grain amaranth is highly recommended for complementary feeding (Kanensi et al., 2011). However, grain amaranth is not commonly used for complementary feeding. One of the 2 | M ATE R I A L A N D M E TH O DS challenges is the intrinsic taste, nutty flavor, and sandy mouth feel associated with raw grain amaranth (Capriles, Ameida, et al., 2008b; 2.1 | Processing of extruded grain amaranth flour Macharia-Mutie, Wiel, Moreno-London, Mwangi, & Brouwer, 2011). Another problem with preparation of complementary foods is the Grain amaranth (GA) was purchased from farmers in Uganda through long preparation time, use of more resources in terms of fuel, and Peak Value Ltd. Grain amaranth was washed, dried, and milled using high viscosity. Plant-based complementary foods are often too bulky a commercial mill (30B-C, Changzhou Erbang Drying Equipment for the weanling with a tiny stomach to eat the necessary quanti- Co. Ltd). The obtained GA flour was then extruded using a corotary ties that provide adequate energy and nutrients (Okoth et al., 2016). and intermeshing twin screw extruder (Double screw inflation food According to WHO (2003), a good complementary food should pro- machine DP70-III, Jinan Eagle Machine Co. Ltd), with specs of 7-cm duce a gruel that is neither too thick for the infant to consume nor so screw diameter, 141.7-cm screw length, and 4-mm diameter die thin that energy and nutrient density are reduced. Therefore, food opening. Extrusion was carried out using different extrusion condi- processing technologies that improve nutrient density of comple- tions obtained from design expert® 11.0 (Table 1). Different con- mentary foods are recommended. ditions of extrusion temperature (heating area II), extrusion speed Coded variable level Variable Symbol −α (−1.4142) −1 0 1 +α (1.4142) Extrusion temperature (oC) X1 101.716 110 130 150 158.284 Extrusion speed (Hz) X2 37.9289 40 45 50 52.0711 Feed moisture content (%) X3 11.1716 12 14 16 16.8284 TA B L E 1 Independent variables and levels for the response surface design (rotatable central composite design) | ATUKURI eT Al. 4149 (screw speed), and feed moisture content obtained from the experi- extruded GA for maximum protein content, protein digestibility and mental design were used. Preliminary experiments were conducted sensory acceptability, desirable WAI and WSI, and minimum bulk to determine the range of conditions that gave extrudates with un- density and viscosity. burnt appearance and no clogging of the product in the barrel. Barrel zone I and zone III temperatures were respectively set 20°C lower and 15°C higher than zone II temperatures. Upon extrusion, the ex- 2.4 | Physicochemical analyses trudates were collected in polyethylene bags and allowed to cool. The extrudates were then milled into fine flour to produce instant 2.4.1 | Protein digestibility grain amaranth flour. Feed moisture content was measured as the amount of water added to the grain amaranth flour before extrusion, In vitro protein digestibility (IVPD) was determined according to expressed as a percentage of the weight of the flour. the method by Mertz et al. (1984). In the first stage, crude protein content was determined as described using Kjeldal method (AOAC, 2005). Flour (0.3 g) was weighed into a Kjeldahl digestion flask and 2.2 | Research design for optimization 12 ml of concentrated sulfuric acid added. Selenium was added as a catalyst. The flask was then placed in a digester in a fume hood 2.2.1 | Experimental design cupboard and digested for 45 min until a colorless solution was obtained. Distillation was then carried out using 4% boric acid and 20% Response surface methodology (RSM) using the central composite sodium hydroxide. The distillate was then titrated with 0.6 M HCl design (CCD) was used to predict the responses. The independent until formation of a violet color as the end point. A blank was run variables were extrusion temperature (heating area II) (X1), extrusion under the same conditions. Protein content was measured in tripli- speed (X 2), and feed moisture content (X3). Twenty-six (26) experi- cate and calculated using the equation below: mental runs were generated using the rotatable central composite design consisting of 8 factorial runs, 12 axial runs, and 6 center runs. Protein content(%) = The experimental design used to generate the runs was optimized [titer value of sample − blank] × 0.06 × 14 × 6.25 × 100 1000 × weight of sample (2) by considering the design with good G-efficiency and Fraction of Design Space (FDS) score. The independent variables and their levels The second stage involved digesting the sample using pepsin en- are shown in Table 1. The response variables (Y) considered were zyme. Approximately 0.2 g of sample was weighed and placed in a protein digestibility, water absorption index, water solubility index, clean sterile labeled centrifuge tube and 20 ml of buffered pepsin bulk density, viscosity, and sensory acceptability. The dependent solution added, and the mixture incubated in a water bath at 37°C variables were individually expressed as a function of the independ- for 2 hr. The tubes were then centrifuged at 4,500 g for 15 min using ent variable. A second-order degree polynomial equation was used a FisherScientific centrifuge (ThermoFisher Scientific, 297) result- for prediction of the response variables (Equation 1). ing into separation of the digestion mixture in two layers. The upper layer was carefully removed using a dropper and discarded. To the k Y = B0 + ∑ i−1 k Bi X i + ∑ i−1 remaining solution, 10 ml of 0.1 M potassium dihydrogen phosphate k Bii X2i + ∑ Bij Xi Xj + 𝜀 (1) i−1 i<j was added, the tube shaken well and centrifuged as before. The mixture was filtered using Whatman No. 3 until all the liquid drained into the test tubes. The filter paper was rolled up and inserted into where Y is the response function, B0 the center point of the system, ε is digestion flasks. The flasks were then dried in the oven at 100°C for the random error, Bi, Bii, and Bij represent the coefficients of the linear, 15 min, and later, crude protein content determined following AOAC quadratic, and interactive effects, respectively. Xi, Xi2, and XiXj repre- (2005). Protein digestibility was calculated using the equation below. sent the linear, quadratic, and interactive effects of the independent variables (extrusion temperature, extrusion speed, and feed moisture Protein digestibility(%) = content), respectively. A−B A (3) where, A = % protein in the original sample; B = % protein after pepsin digestion. 2.3 | Optimization of extrusion conditions The extruded grain amaranth flours were analyzed for protein digest- 2.5 | Water absorption and solubility index ibility, water absorption index (WAI), water solubility index (WSI), bulk density, viscosity, and sensory acceptability. The experiments Water absorption index (WAI) and water solubility index (WSI) were were randomized to remove bias and reduce systematic errors. The determined using the method described by Kaur and Singh (2005). desirability function approach (DFA) was used for optimizing the A portion (3 g) of the flour was dissolved in 30 ml of distilled water 4150 | ATUKURI eT Al. and heated in a water bath for 15 min at 90°C. The cooked paste 2.9 | Statistical analysis was cooled to room temperature, transferred into preweighed centrifuge dishes, and then centrifuged at 1,500 g for 20 min. The su- Data were analyzed by response surface methodology using design pernatant was thereafter decanted into a preweighed evaporating expert statistical software (DX 11.0; Stat-Ease Inc) to optimize the dish to determine the solid content and the sediment weighed. The extrusion conditions of grain amaranth. To estimate the effect of weight of dry solids was obtained by evaporating the supernatant extrusion temperature, extrusion speed and feed moisture content overnight at 105°C. The WAI and WSI were calculated using the protein digestibility (Y1), water absorption index (Y2), water solubility equations below. index (Y3), bulk density (Y4), viscosity (Y5), and sensory acceptability (Y6), the standardized scores were fitted to a quadratic polynomial WAI = Weight of sediment Weight of flour sample (4) regression model (Equation 1) (Durgadevi & Nazni, 2012). The statistical parameters used to relate the input variables to the responses were p-value and R 2. Significance of the models was determined using model analysis and lack of fit. Weight of dissolved solids in supernatant WSI = Weight of flour sample (5) 2.10 | Validation of RSM results 2.6 | Viscosity To validate the mathematical model, the optimal conditions were Viscosity was expressed as the final viscosity from the rapid visco- used for production of extruded flour and the products were ana- analysis using a Rapid Visco Analyzer (RVA-4, Newport Scientific). lyzed. Predicted were then compared with experimental values. Grain amaranth flour (5 g) was added to 25 ml of distilled water in a Percentage prediction error was calculated using Equation (7) to canister and mixed, loaded onto the RVA and run using the extrusion validate the model (Scheuer et al., 2016). 1 profile. Viscosity was expressed as the final viscosity at a temperature of 49.95°C in centipoise (cP) (1RVU = 12 cP). Predicted error(%) = (experimental − predicted) × 100 predicted 2.7 | Bulk density 3 | R E S U LT S A N D D I S CU S S I O N Bulk density was determined using a method by Maninder, Kawaljit, 3.1 | Protein digestibility (7) and Narpinder (2007). Grain amaranth flour was weighed into a cylinder of known volume in triplicates. The cylinder was gently tapped Protein digestibility is an essential determinant of protein quality. on a laboratory bench until no further diminution and flour filled to Plant proteins generally have lower digestibility compared with ani- the volume mark. This was done to eliminate air spaces. Weight was mal proteins due to antinutritional factors such as trypsin inhibitors measured using a laboratory balance, and the results expressed as (Nyakuni et al., 2008; Singh, Gamlath, & Wakeling, 2007). In vitro weight to volume ratio (Equation 6). protein digestibility (IVPD) was determined and the effect of extrusion conditions on protein digestibility is represented in Equation (8), Weight of sample(g) Bulk density(g∕ml) = Volume of sample(ml) (6) which shows the model in terms of coded levels of the variables. A negative coefficient denotes decrease in the response with increase in the level of the parameter whereas a positive coefficient indicates 2.8 | Sensory acceptability increase in the response as the level of parameter increases (Filli, Nkama, Jideani, & Abubakar, 2012). Extrusion temperature had a Sensory evaluation was carried out to determine the accept- negative linear effect while extrusion speed and feed moisture had ability of porridges obtained from the extruded grain amaranth positive linear effects on protein digestibility of GA flour. flour. Porridges were prepared by adding 33 g of extruded grain amaranth flour to 100 ml of hot water and stirred to uniform and Protein digestibility = 80.29 − 1.39X1 + 1.71X2 + 1.86X3 (8) drinkable consistency (2,500–3,000 cP) (Akande, Nakimbugwe, & Mukisa, 2017). A semitrained panel of 25 members consisting Increase in extrusion temperature resulted in decrease in protein 44% males and 56% females (approximately 75% of whom were digestibility while speed and feed moisture content increased with mothers) was used. Panelists were presented with coded sam- protein digestibility (Figure 1). The model was insignificant (p = .63) ples and asked to evaluate them according to their preference and had a nonsignificant lack of fit (p = .99). Observed decrease in using a 9-point Hedonic scale, from 1 (dislike extremely) to 9 (like IVPD with increased extrusion temperature could be due to modifi- extremely). cations during extrusion. During extrusion, heating leads to Maillard ATUKURI eT Al. | 4151 F I G U R E 1 Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture on protein digestibility of grain amaranth flour (Constant moisture (14%), (Constant screw speed, 45 Hz) reactions between the free amino groups of protein and carbonyl (Njoki, Silva, & Onyango, 2014; Singh et al., 2007). According to groups of reducing sugars (reducing protein digestibility) and also re- Okpala and Chinyelu (2011), decrease in IVPD results from nonen- duces antinutrients which inhibit digestion of proteins by enzymes zymatic browning reactions which cause nonreversible formation 4152 | ATUKURI eT Al. of compounds thus a decrease in protein available for digestion. extrusion typically causes gelatinization of starch, which increases Maillard reactions particularly occur at high temperatures (Peluola- water-holding properties. The unfolding and loosening of biopolymer Adeyemi, Idowu, Sanni, & Bodunde, 2014), and this could explain chains enhance availability and easier accessibility of structures by the decrease in protein digestibility with increase in extrusion water molecules thus, the increase in WAI may be attributed to un- temperature. Similar effect of temperature on IVPD was reported covering of hydrophilic groups (Marzec & Lewicki, 2006). However, by Akande et al. (2017). Screw speed and feed moisture increased further increase in extrusion temperature lowers the WAI due to protein digestibility because higher shear enhances denaturation of formation of complexes between the soluble proteins and sugars proteins more easily, which facilitates enzyme hydrolysis (Singh et through Maillard reactions (Mburu, Gikonyo, Kenji, & Mwasaru, al., 2007). Their disruption aids protein unfolding and thus digest- 2011). On the other hand, increasing extrusion speed and feed mois- ibility (Brennan & Grandison, 2012). Furthermore, denaturation of ture resulted in decrease of WAI of flour (Figure 2). The increase proteins also improves nutritional quality by increasing accessibil- in WAI with lower feed moisture could be due to increased depo- ity of the molecules to proteases thus making them more digestible lymerization of polysaccharides at low moisture content (Obradović, (Brennan, 2006; Peluola-Adeyemi et al., 2014). Feed moisture prob- Babić, Šubarić, Aćkar, & Jozinović, 2014). Depolymerization of poly- ably increases digestibility because it facilitates swelling and soften- saccharides increases with increase in shear stress and lower mois- ing, which enhances disintegration and hydrolysis at high speed. This ture content of raw material (Esposito et al., 2005; Moscicki, 2011). is supported by Njoki et al. (2014) who reported that wet cooking Earlier studies have also reported lower WAI for starchy materials improves protein digestibility better than dry cooking. During heat extruded at high moisture content (Alam, Kumar, & Khaira, 2015; treatment, changes in protein digestibility are influenced by the de- Charunuch, Limsangouan, Praset, & Butsuwan, 2011). Charunuch et gree of formation of complexes between proteins and other grain al. (2011) attributed the low WAI for materials extruded under high components, and matrix disintegration which affects access of pro- feed moisture to low extent of gelatinization due to loss of thermal teolytic enzymes (Muyonga et al., 2014). During extrusion, antinutri- energy during extrusion reduction. Water absorption index is an in- tional factors which inhibit digestibility are reduced which increases dication of the ability of flour to absorb and retain water (Narbutaite, digestibility. According to Muyonga et al. (2014), grain amaranth pro- Makaravicius, Juodeikiene, & Basinskiene, 2008). In complementary teins have higher digestibility compared with maize and sorghum. foods, high water absorption index is undesirable because it contributes to dietary bulk (Afam-Anene & Ahiarakwem, 2014). During cooking, foods with high WAI absorb large amount of water to form 3.2 | Water absorption index voluminous low energy and nutrient food (Omueti et al., 2009). Therefore, the low water absorption values observed in this study Water absorption index is a measure of volume the starch occupies are desirable as it is appropriate for making thinner gruels which after swelling in excess water and is an indication of the integrity have high caloric density per unit volume (Ijarotimi & Oluwalana, of starch (Leonel, Freitas, & Mischan, 2009; Mesquita, Leonel, & 2013). This is particularly crucial during processing of flours for com- Mischan, 2013). It measures the water absorbed by starch and can plementary feeding. be used as an index for gelatinization, an important effect of extrusion (Saini, 2015; Yang, Peng, Lui, & Lin, 2008). Quadratic Equation 9 showed that extrusion temperature (X1), speed (X 2), and feed mois- 3.3 | Water solubility index ture content (X3) had significant effect on water absorption index of grain amaranth flour. Interaction and quadratic effects of the pro- Water solubility index (WSI) measures soluble components from cess variables were also observed. starch after extrusion and is an indication of molecular degradation (Milán-Carrillo et al., 2012; Saini, 2015). There was a quadratic re- Water absorption index = 1.97 + 0.0456X1 − 0.2217X2 − 0.0654X3 + 0.0533X1 X2 + 0.2783X1 X3 − 0.0363X2 X3 − 0.2867X21 + 0.3790X22 (9) lation between extrusion conditions and water solubility index of grain amaranth, and the relation is shown by Equation 10. The model was significant (p = .036) and had a nonsignificant lack of fit (p = .81). Positive coefficients of X 2 and X3 indicate that extrusion speed and feed moisture content had positive linear effects on water solubility The model was significant (p < .01) and had a nonsignificant index whereas extrusion temperature had negative linear but posi- lack of fit (p = .88). Extrusion temperature had a positive linear but tive quadratic effect on grain amaranth flour WSI. Negative inter- negative quadratic effect on WAI while extrusion speed and feed action existed between extrusion temperature and feed moisture moisture content both had negative linear effects. Increase in ex- content while positive interaction existed between extrusion speed trusion temperature resulted in increase in WAI of grain amaranth and feed moisture. flour in the low temperature range (Figure 2). However, further increase to higher temperatures results in decreased WAI (Equation 9). According to Simons (2013) and Hernández-Nava, Bello-Pérez, San Martín-Martínez, Hernández-Sánchez, and Mora-Escobedo (2011), Water solubility index = 0.5536 − 0.0051X1 + 0.0186X2 + 0.0033X3 + 0.0019X1 X2 − 0.0254X1 X3 + 0.0040X2 X3 + 0.0257X12 − 0.0346X22 (10) ATUKURI eT Al. | 4153 F I G U R E 2 (I) Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture content on water absorption index of grain amaranth flour (Constant moisture, 14%). (II) Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture content on water absorption index of grain amaranth flour (Constant screw speed, 45 Hz) Increase in extrusion temperature was associated with decrease into release of amylose and amylopectin, dextrinization and other in WSI; however, higher temperatures increased WSI of GA flour reactions also occur during extrusion leading to formation of low (Figure 3). This could be attributed to dextrinization of starch which molecular weight compounds which influence WSI (Mesquita et al., could have increased with extrusion temperature and feed moisture 2013). Although Gui, Gil, and Ryu (2012) reported dextrinization (Peluola-Adeyemi et al., 2014). Besides gelatinization which results as dominant during the extrusion process, Chang and Ng (2011) 4154 | ATUKURI eT Al. F I G U R E 3 (I) Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture content on water solubility index of grain amaranth flour (Constant moisture, 14%). (II) Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture content on water solubility index of grain amaranth flour (Constant screw speed, 45 Hz) reported that at higher temperatures, the combination of thermal the quadratic effect of higher temperature on WSI. According to and mechanical energies fully cooks the starch leading to greater Yang et al. (2008), WSI indicates the degree of molecular degrada- degree of starch gelatinization and degradation. This could explain tion and measures extent of starch conversion during extrusion, that | ATUKURI eT Al. 4155 is, the quantity of soluble polysaccharide released from the starch. In speed; however, further increase in speed causes increase in BD. In contrast, increasing extrusion speed and feed moisture content also a study on extrusion of a mixture of rice, lentils, and carrot pomace, increased WSI. At low feed moisture content, dextrinization could Alam et al. (2015) also observed a negative relationship between have occurred easily thus increasing soluble starch and hence WSI bulk density and extrusion temperature. Bulk density is influenced (Gui et al., 2012). The increase in WSI with extrusion speed could by flour particle size and determines the packaging requirements be due to mechanical degradation of starch molecules resulting in and material handling of the flour (Alawode et al., 2017; Ijarotimi & increased solubility. According to Mezreb, Goullieux, Ralainirina, Oluwalana, 2013). It measures flour heaviness and indicates that the and Queneudec (2003), increasing screw speed induces an increase volume of the flour in a package will not excessively reduce during in specific mechanical energy which mechanical shear degrades storage (Alawode et al., 2017). During packaging, a large free space macromolecules, thus decreases molecular weight of starch gran- is undesirable because it creates a large reservoir for oxygen how- ules hence increasing WSI. Compaoré, Nikièma, Bassole, Savadogo, ever lower bulk density result in greater oxygen transmission in the and Hounhouigan (2011) asserted that WSI increases because high packed food (Adepeju et al., 2014; Omueti et al., 2009). Decrease in molecular weight carbohydrates and proteins are hydrolyzed into bulk density will reduce packaging and transportation costs (Bolaji, simpler components. Gui et al. (2012) also reported similar effect Oyewo, & Adepoju, 2014; Inyang & Effiong, 2016). of screw speed and feed moisture on WSI. Njoki and Faller (2001) reported that high WSI can be used to predict the ease of digestion of complementary foods by infants. 3.5 | Viscosity Viscosity is an important attribute which indicates increase or 3.4 | Bulk density decrease of bulk of a cooked product and affects taste intensity (Mburu et al., 2011). Viscosity was determined as the final viscosity Functional properties are important because they determine the of the RVA, since the RVA can be used to determine the viscosity of a application of food materials for numerous food products (Awolu, sample exposed to different temperatures. Final viscosity measures Omoba, Olawoye, & Dairo, 2016). Bulk density (BD) is an indica- reassociation of starch and for extruded products, it depends on tion of the load that the sample can carry when resting directly on the structural modifications of granules and molecules (Hernández- one another (Ijarotimi & Oluwalana, 2013). The effect of extrusion Nava et al., 2011). Quadratic Equation 12 shows positive coefficients conditions on bulk density of grain amaranth flour is represented by of X1 and X3 but negative coefficients of X 2, with interactions. Equation (11). Viscosity = 162.60 + 6.86X1 − 30.13X2 + 0.7584X3 Bulk density = 0.6544 − 0.0036X1 − 0.0360X2 − 0.0054X3 − 6.31X1 X2 + 19.52X1 X3 − 8.57X2 X3 + 39.02X22 (12) − 0.0108X1 X2 + 0.0050X1 X3 − 0.0361X21 + 0.0238X22 (11) Extrusion temperature and feed moisture had positive linear Extrusion temperature, speed, and feed moisture had negative effect on viscosity of grain amaranth porridges. The model was linear effect on bulk density of grain amaranth flour with interac- significant (p < .001) and had a nonsignificant lack of fit (p = .30). tions between process variables (Figure 4). The model was significant However, extrusion speed had negative linear effect but positive (p < .001) and had a nonsignificant lack of fit (p = .73). Temperature quadratic effect on viscosity. Increase in viscosity with temperature had a negative, while speed had a positive quadratic effect on BD could be due to gelatinization of starch. Increase in viscosity results (Equation 11). The negative effect of temperature on BD could be from the swelling and recrystallization of native starch (Adegunwa, attributed to effect of heating. Increasing extrusion temperature Adebowale, Bakare, & Kalejaiye, 2014); therefore, gelatinization increases the degree of superheated water in the extruder which which occurs during extrusion causes a decrease in native starch enhances formation of bubbles and reduces melt viscosity hence which hinders increase in viscosity. According to Martínez, Calviño, leading to reduced density (Gui et al., 2012). Additionally, increase Rosell, and Gomez (2014), extrusion causes swelling and rupture of in BD with temperature could also be due to starch gelatinization. starch granules, destroying the organized granule structure com- Increase in gelatinization increases volume of extruded products pletely or partially. Increase in viscosity with temperature could be which results in a decrease in BD (Alam et al., 2015). This is sup- attributed to disintegration of starch granules which increases sus- ported by the suggestion that the structure of starch polymers influ- ceptibility to hydration (Muyonga et al., 2014). Screw speed had a ences bulk density and loose polymer structure results in low bulk more significant effect on viscosity than temperature or feed mois- density (Alawode, Idowu, Adeola, Oke, & Omoniyi, 2017). Increasing ture as evidenced by the higher coefficient of X 2 and the contours on screw speed and feed moisture decreases BD because high screw the response surface plot (Figure 5). Increasing screw speed reduces speed lowers the melting viscosity and increases elasticity of the gelatinization because of decrease of residence time in the extruder mixture which reduces BD (Ding et al., 2005). The quadratic effect which results in low swelling and volume (Obradović et al., 2014; of screw speed indicates that BD decreased with increase in screw Yu, Ramaswamy, & Boye, 2013). Leonel et al. (2009) reported low 4156 | ATUKURI eT Al. F I G U R E 4 (I) Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture content on bulk density of grain amaranth flour (Constant moisture, 14%). (II) Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture content on bulk density of grain amaranth flour (Constant screw speed, 45 Hz) viscosity for cassava extruded with low feed moisture and attributed indicates that on cooking and cooling, the flours would form low vis- this to increased frictional damage especially at high screw speed. cosity gruels (Omueti et al., 2009). This implies that it is impossible Generally, low viscosities were observed during the study which to prepare gruels with relatively high solids content, and therefore | ATUKURI eT Al. 4157 F I G U R E 5 (I) Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture content on viscosity of grain amaranth porridge made with mass ratio of 1:5, flour:water (Constant moisture, 14%). (II). Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture content on viscosity of grain amaranth porridge (made with mass ratio of 1:5, flour:water) high caloric density per unit volume while maintaining drinkable 3.6 | Sensory acceptability consistency (Ikujenlola & Fashakin, 2005; Otegbayo, Aina, Asiedu, & Bokanga, 2006). Low viscosity is required to make the food easy to The quadratic model for sensory acceptability in terms of coded lev- consume by infants (Mburu et al., 2011). els of the variables is shown in Equation 13. 4158 | ATUKURI eT Al. F I G U R E 6 (I) Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture content on response surface plot showing effect of extrusion temperature, extrusion speed, and feed moisture content on sensory acceptability of grain amaranth porridge made with flour to water mass ratio of 1:3 (Constant moisture, 14%). (II) Response surface plot showing effect of extrusion temperature, screw speed, and feed moisture content on response surface plot showing effect of extrusion temperature, extrusion speed, and feed moisture content on sensory acceptability of grain amaranth porridge made with flour to water mass ratio of 1:3 (Constant screw speed, 45 Hz) Sensory acceptability = 6.42 + 0.0997X1 + 0.1824X2 Extrusion temperature and speed had positive linear effect − 0.1048X3 + 0.0170X1 X2 + 0.2784X1 X3 on sensory acceptability while feed moisture content had nega- + 0.0170X2 X3 + 0.1469X21 − 0.1700X23 tive linear and quadratic effect on sensory acceptability (Figure 6). (13) Interactions between the process variables were observed. The | ATUKURI eT Al. TA B L E 2 Predicted and experimental values of the model Response Protein digestibility (PD) Predicted value 81.87 Experimental value t Test 77.85 0.052 Deviation −4.02 Relative deviation (%) Prediction error (%) −5.11 −4.91 Bulk density 0.59 0.51 ± 0.01 0.016 −0.08 −15.69 −13.56 Water absorption index (WAI) 1.92 1.60 ± 0.07 0.056 −0.32 −20.00 −16.67 Water solubility index (WSI) 0.55 0.67 ± 0.01 0.02a 0.12 17.91 21.81 174.56 237 ± 4.51 0.0068a 62.44 26.35 35.77 6.69 7.36 ± 0.81 0.11 0.67 9.10 10.01 Viscosity Sensory acceptability a 4159 a Significantly different. model was significant (p = .012) with nonsignificant (p = .4) lack of fit, were those that resulted in the highest desirability (0.705) (within ac- and the adequate prediction ratio showed a sufficient signal implying ceptable range for desirability). The optimal extrusion conditions were that the model could be used to navigate the design space. Gbenyi, as follows: extrusion temperature of 150°C, extrusion speed of 50 Hz, Nkama, Badau, and Idakwo (2016) demonstrated the potential of and feed moisture content of 14.41%. These extrusion conditions re- using sensory scores in optimization of processes. In this study, sen- sulted in instant grain amaranth flour with IVPD of 81.87%, WAI of sory acceptability of grain amaranth porridge increased with tem- 1.92, WSI of 0.55, bulk density of 0.59 (g/ml), final viscosity of 174.56 perature and speed. However, further increase in temperature and cP (14.55 RVU), and overall sensory acceptability score of 6.69. screw speed resulted in a decrease in acceptability. Low extrusion temperature and screw speed resulted in low sensory acceptability of grain amaranth porridge. During extrusion, low speed resulted 3.8 | Validation of results into porridges with a browner color which panelists described as having a burnt taste. During extrusion, dextrinization and Maillard The suitability of the developed model for prediction was tested reactions occur, which causes formation of melanoidins and in turn by comparing the predicted and experimental values (Table 2). t modifies color of flours (Martínez et al., 2014). These reactions not Test showed that the predicted values for PD, WAI, and sensory only affect color but also the taste and flavor of the product, affect- acceptability were not statistically different from experimental val- ing overall product acceptability. High speed extrusion is associated ues. Predicted value for BD was higher than experimental value with short extrusion times, and this seems to result in products with while the predicted viscosity was significantly lower than experi- desirable sensory attributes. Lower feed moisture content increased mental value. The observed differences between predicted and acceptability probably due to enhanced dextrinization, which leads experimental values may be attributed to difference in physical to formation of dextrins and other complexes that contribute to the variables such as humidity, room temperature, and experimental flavor, taste, and color. However, the interaction between screw variations (Scheuer et al., 2016). The results, however, still show speed and the other variables indicates the importance of exposure that response surface models correctly predicted more than half time. Chemical reactions that occur during extrusion are important the variables, including sensory acceptability, a key attribute with because they impart desirable sensory qualities such as flavor and respect to uptake of product. This technique is therefore still useful color. Filli et al. (2012) reported that color changes in extruded prod- for optimization especially when used in combination with other ucts are due to pigment decomposition and chemical reactions such techniques. as caramelization of carbohydrates. Despite their positive effects, these reactions, if uncontrolled, negatively affect sensory attributes. The interaction effects indicate the need to consider the different 4 | CO N C LU S I O N processing variables together when designing extrusion protocols. The study showed that extrusion parameters of temperature, screw speed, and feed moisture content together with their interactions 3.7 | Optimization of extrusion conditions affected the properties of extruded grain amaranth, with extrusion speed having the most significant effect. Response surface meth- Optimization of the process variables was carried out using the nu- odology was successfully used to optimize extrusion of GA flour for merical method. Desired goals were assigned for all the parameters complementary feeding. The optimal extrusion conditions for grain to obtain optimal values for the responses. During desirability de- amaranth flour were extruding at a temperature of 150°C (heating termination, protein digestibility and sensory acceptability were area II) at a speed of 50 Hz with feed moisture content of 14.41%. maximized, WAI and WSI were kept in range, while viscosity and bulk Porridge from flour made using optimal conditions was found to density were minimized. The extrusion variables selected as optimal be highly acceptable and to exhibit relatively low bulk density. The 4160 | ATUKURI eT Al. above conditions can therefore be used for producing instant grain amaranth flour suitable for use in complementary foods. AC K N OW L E D G E M E N T This study was funded by the National Agriculture Research Organization through the Competitive Grant Scheme C O N FL I C T O F I N T E R E S T No conflict of interest was declared by the authors. E T H I C A L A P P R OVA L This study does not involve any human or animal testing. ORCID John H. Muyonga https://orcid.org/0000-0002-5598-1462 REFERENCES Adegunwa, M., Adebowale, A., Bakare, H., & Kalejaiye, K. (2014). Effects of treatments on the antinutritional factors and functional properties of Bambara groundnut (Voandzeia subterranean) flour. Journal of Food Processing and Preservation, 38, 1875–1881. Adepeju, A. B., Gbadamosi, S. O., Omobuwajo, T. O., & Abiodun, O. A. (2014). Functional and physico-chemical properties of complementary diets produced from breadfruit (Artocarpus altilis). African Journal of Food Science and Technology, 5, 105–113. Afam-Anene, O. C., & Ahiarakwem, J. H. (2014). 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