Optimization of extrusion cooking process parameters to develop teff (Eragrostis (Zucc) Trotter)-based products: Physical properties, functional properties, and sensory quality

Abstract

This study was conducted to produce a satisfactory expanded product based on teff, by means of a twin-screw extruder, while optimizing the process parameters of extrusion cooking. The study was performed utilizing the Response Surface Methodology (RSM) using the Box Behnken design. Teff and chickpea composite flour mixes were exposed to extrusion cooking, while considering altered amount of feed moisture content (FMC) (15, 20, 25%), blend ratio (BR) (BR1 (90% teff:10% chickpea), BR2 (85% teff:15% chickpea), and BR3 (80% teff:20% chickpea), barrel-temperature (110, 130, 150°C), and screw-speed (150, 170, 190 rpm) as the independent variables. An analysis was conducted on the physical and functional properties, and sensory quality attributes of the products. The extrudates exhibited notable values for maximum expansion ratio (1.88 mm/mm), maximum water absorption index (WAI) (7.48 g/g), and minimum density (0.22 g/cm³) at the processing conditions of 130°C, 170 rpm, 15% FMC, and BR2. The highest specific length achieved a value of 2.23 cm/g, under the conditions of temperature at 130°C, at 150 rpm, FMC at 15%, and utilization of BR2. The findings of this investigation offer optimistic insights relating to the development of extrudates based on teff and chickpea, along with the associated process optimization studies. The result can be used for product developers in food industries to produce extrudates.

Keywords:

• Blending ratio
• Chickpea
• Extrudates
• Process optimization
• RMS
• Teff
• Twin screw extruder

PUBLIC INTEREST STATEMENT

Ready-to-eat (RTE) extruded products are getting the public and food industry’s attention. Teff-based (teff: chickpea ratio) expanded products using twin-screw extruders are crucial RTE cereal-based gluten-free food products. Optimizing the process parameters (feed moisture content, the blending ratio of teff:chickpea, and barrel temperatures) of this RTE food product’s extrusion cooking is paramount for food industries and processors to improve product quality and save time and money. The extrudates exhibited notable values for maximum expansion ratio (1.88 mm/mm), maximum water absorption index (7.48 g/g), and minimum density (0.22 g/cm³) at the processing conditions of 130°C, 170 rpm, 15% FMC, and 85% teff:15% chickpea. The findings of this investigation offer optimistic insights relating to the development of extrudates based on teff and chickpea, along with the associated process optimization studies. The result can be used by product developers in food industries to produce teff-chickpea-based extrudates.

1. Introduction

Food processing by extrusion is one of the newest technologies for manufacturing various food products (Karwe, 2005). The extrusion process is a unit operation in which a mixture of raw materials is forced through a molded hole (die) at a specific temperature of the barrel, pressure, moisture level of the feed and amount of the feed to produce extruded products of different shapes (Karwe, 2005). This technology has been used to deliver a various of grain-based food products, protein additions, and sausage foodstuffs (Asnakech & Shimelis, 2016). Consumer interest has led food producers to make an extensive variety of high-protein foods and foodstuffs. During the last decade, the industrial fabrication of snack foods, with different plant protein sources as main ingredients, has rapidly grown. Extrusion is now used commercially to produce high-quality grain-based breakfasts and snack foods, such as wheat and corn-based foodstuffs. Nevertheless, this manufacturing approach is not used commercially for legumes-based foods because it is believed that the beans do not expand sufficiently when extruded (Narbutaite et al., 2008). The nutritious demand of high-protein, low-calorie snacks will be a value-added characteristic of extrudates of legume seed products, such as dried beans (Berrios, 2006). Extruded products with high fiber content can improve overall nutritional content and taste by including protein-rich ingredients. The addition of legume flour and other high-protein ingredients has a positive effect on the protein and fiber content of extruded snacks (Asnakech & Shimelis, 2016). In developing countries like Ethiopia, the high cost of high-protein foods makes many people unable to afford high-protein foods. As a result, individuals are in need of cheaper, high-protein foods. Most of the Ethiopian diet consists of mostly carbohydrate-based foods.

Consequently, there is a need to strategically use inexpensive, protein-rich ingredients to improve nutritional value and supplement the overall amino acid profile needed to overcome nutritional insecurity. As a result, to increase teff consumption in rural and urban areas of our country, we can increase its nutritional value by incorporating other non-cereal ingredients such as chickpeas and peat. Such foods need to be processed with domestic grains, replacing imported ones with cheaper ones, and saving foreign currency, thereby improving food security.

Teff (Eragrostis (Zucc) Trotter) is one of the long-lived crops grown under various environmental conditions in Ethiopia, accounting for the largest percentage of cereal under production (Bultosa, 2007). Teff is considered to be highly nutritious. Nevertheless, there is limited information from the literature on snack food formulation and manufacturing using extrusion techniques. In Ethiopia, teff flour is commonly used as a staple for making flatbreads, a fermented product called injera (Neela & Workneh Fanta, 2020). It contains fiber, Fe, Ca, K and other important minerals (Bultosa, 2007). Adding teff flour to extruded teff-based foods could fortify products with iron and other essential minerals (Bultosa, 2007). Factors, such as agroecology, livelihoods and income, may determine Ethiopia’s cereal consumption (Seyfu, 1997). Teff is a relatively expensive crop in Ethiopia, justifying its importance in urban and semi-urban areas with relatively high incomes (Baye, 2014). Usage information is limited. Efforts in this area have been insignificant, although they will likely develop teff-based products. Teff, on the other hand, is highly nutritious and free of gluten protein. Developing teff-focused food items is anticipated to fascinate the consideration of food product-focused research and development institutes and food manufacturing firms (Fufa et al., 2011). Legumes are used in various food preparations either as such or in combination with cereals due to the limitations of some essential amino acids by cereal proteins (Annor et al., 2014). Therefore, utilization of legumes is crucial as a cheap and concentrated source of proteins since the cost of animal-origin proteins is high and their inaccessibility by the poor population part of the country (Kohajdová et al., 2011). Legumes such as beans and chickpeas (Cicer arietinum L.) are among the furthermost vital crops globally due to their nutritional superiority. Grain legumes are valuable sources of protein. Legume components in the daily diet have been reported to improve physiological processes and controller and inhibit numerous metabolic effects, including diabetics, heart diseases and cancer of the colon (Annor et al., 2014).

Chickpea (Cicer arietinum L.) is full of unsaturated fatty acids, such as linoleic and oleic acid. Furthermore, chickpea is very important because of their importance as a functional constituent in the food processing firm for value-added product processing (Sadik, 2015). So, product development from the blends of teff and chickpea might enhance the nutrient and energy density. In addition, nutrition qualities, the extrusion of composite foods might be taken as a value-addition approach intended for the mixture food crops that are typically conventionally manufactured foods in Ethiopia (Sadik, 2015). Therefore, this study focused on developing a teff-based puffed/extruded product blended with chickpea flour. Moreover, the research was focused on studying the consequences of the extrusion process parameters (moisture content of the, temperature of the barrel, speed of the screw and blending ratio) on the quality of the extruded food produced through an extrusion optimization process.

2. Materials and Methods

2.1. Raw material and blend formulations

The DZ-C1-387 Kuncho Teff (RIL-355) and Ararite chickpea (FLIP-8984C) varieties sampel grains were used for this study. They were obtained from the Adet Agricultural Research Center, Ethiopia, which were harvested in the year 2016/17. All samples were manually cleaned, and the chickpea was decorated using an (AB, Alvan Blanch Decorticator, England, 2007). The samples were milled and sieved through a 710 μm sieve aperture. Then, the blended flours were prepared by mixing as of the proportions obtained from the design optimization models with that of teff flour-based and mixed by the laboratory-scale mixing machine (Model AB, Alvan blanch Type ribbon blender, England, 2007) (Table 1). The blended flours were extruded through a twin-screw extruder (Clextral EV Twin Screw Extruder, France, 2006) to produce extrudate products based on different parameter sets.

Table 1. Raw material blends proportion

Code	Blending proportion
BR1 BR2 BR3	90% Teff 10% Chickpea 85% Teff 15% Chickpea 80% Teff 20% Chickpea

2.2. Experimental design

The influence of extrusion-cooking process factors on the properties of teff-based extruders was assessed. The findings obtained from the preliminary tests were used in selecting the appropriate extrusion curing factors, such as the die size. The mixed dough is extruded using a twin-screw food extruder at a various feed moisture content (15, 20 and 25%), blend ratio BR1 (90% Teff: 10% Chickpea), BR2 (85%Teff: 15% Chickpea) and BR3 (80% Teff: 20% Chickpea), temperature of the barrel (110, 130 and 150°C) and speed of the screw (150, 170, 190 rpm) on the extrudates.

2.3. Extrusion cooking parameters

Four control factors, such as feed moisture content (X1), blending ratio (X2), temperature of the barrel (X3) and speed of the screw (X4), are considered, and others are maintained at a fixed level. To show each factor’s impact and build an empirical model for individual response, variables including water adsorption index (WAI), water solubility index (WSI), rate of expansion, density, and extrusion firmness were performed using the Box Behnken design. Box-Behnken contains an embedded factorial design with center points so that it can help in finding the finest set of values for a set of parameters, generating an optimal response. Thermocouple connected to the extruder was used to measure the barrel temperature. In this study, the temperature at zone-3 that situated just in front of the die was considered as the independent variable. The temperature was varied at 110, 130 and 150°C and the screw speed was set at 150, 170, 190 rpm.

The mixture ratio of the blending materials was adjusted to be on the extrudates ratio 90%Teff: 10% Chickpea (BR1), 85% Teff: 15% Chickpea (BR2) and 80% Teff: 20% Chickpea (BR3), respectively (Table 1). First, the feed moisture content of the material was modified using the changing the water pump injection set. To improve the feed moisture content, water was poured to extruder near the material inlet. The feeder and pump have been calibrated before extrusion to minimize fluctuations throughout operation. Using the hydration equation, the pump was adjusted to provide moisture content of 15, 20 and 25% in the mixes at a fixed feed rate the material (Molla & Zegeye, 2014). Then, the extruded product was manually cut into 5 cm lengths and cool using air, and the produce was packed using a polyethylene bag aimed at further analysis.

2.4. Extrudates physical and functional properties

Degree of expansion: The extruded product was dried out at 105°C for 3 min period to further analysis. The produce was then cut into about a 5 cm length. According to Divate et al. (2015), the diametric expansion ratio (ER) is well defined as the ratio of the diameter of the extrudates to the diameter of the die hole. ER was computed using Equation 1(1) $\mathrm{E}\mathrm{x}\mathrm{p}\mathrm{a}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}=\frac{\mathrm{D}\mathrm{i}\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{u}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{e}}{Diameterofdie}$(1) :

(1) $\mathrm{E}\mathrm{x}\mathrm{p}\mathrm{a}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}=\frac{\mathrm{D}\mathrm{i}\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{u}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{e}}{Diameterofdie}$(1)

Specific length (cm/g): The specific length (Lsp) (cm/g) of the extrudates was computed as the ratio of the length of extrudates (cm) to weight (g) of the extrudate (Divate et al., 2015) (Equation 2(2) $\mathrm{S}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{L}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}=\frac{\mathrm{L}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{f}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{u}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{e}}{Massofextrudate}$(2) ).

(2) $\mathrm{S}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{L}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}=\frac{\mathrm{L}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{f}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{u}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{e}}{Massofextrudate}$(2)

Density (g/cm³): The extruded product density was computed as the ratio of the extrudate mass to the volume of extrudates (Alam & Kumar, 2014), assuming that the product has a cylindrical shape by means of Equation 3(3) $\mathrm{D}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}=\frac{Massofextrudate}{\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{u}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{e}}$(3) .

(3) $\mathrm{D}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}=\frac{Massofextrudate}{\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{u}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{e}}$(3)

Extrudate Hardness: The force needed to breakdown extruded items was measured using a universal texture analyzer (Model-TA Plus, England, 2005). The sharp testing blade (3 mm thick, 6.94 mm wide) was used to squeeze the extruded sample (5 cm), which was positioned on the platform transversally over a metal sheet support (1 cm thick). The probe three-point bending rig (HDP/3PB) was lowered by the texturometer head at a rate of 3.0 mm/s till the extrudates were broken.

Water Absorption Index(WAI) (g/g): Water absorption index of the flour, composite flour, and extrudate was calculated using Equation 4(4) $\mathrm{W}\mathrm{A}\mathrm{I}=\frac{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}}$(4) (Peluola-Adeyemi & Idowu, 2014).

(4) $\mathrm{W}\mathrm{A}\mathrm{I}=\frac{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}}$(4)

Water Solubility Index (WSI) (%): The supernatant preserved from WAI determination was evaporated at 105°C the whole night. The WSI was calculated using Equation 5(5) $\mathrm{W}\mathrm{S}\mathrm{I}=\left[\frac{\begin{array}{c}\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{v}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{s}\\ \mathrm{i}\mathrm{n}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{s}\mathrm{u}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}\end{array}}{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}}\right]$(5) .

(5) $\mathrm{W}\mathrm{S}\mathrm{I}=\left[\frac{\begin{array}{c}\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{v}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{s}\\ \mathrm{i}\mathrm{n}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{s}\mathrm{u}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}\end{array}}{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}}\right]$(5)

2.5. Sensory evaluation

Sensory evaluation was performed using 30 judges (20 males and 10 females) who were nominated from the staff members, postgraduate and undergraduate students of the School of Chemical and Food Process Engineering of Bahir Dar University. The judges had technical knowledge of sensory evaluation parameters and familiarity with food sensory evaluation terms.

2.6. Statistical analysis

Response surface methodology was used to analyze the influence of independent variables (extrusion parameters and material ratios). Design Expert Stat-Ease software version 12.0 was also used in determining the optimal extrusion factors. To achieve this optimal goal, maximum and minimum factor levels were determined. For this reason, more complex models should be proposed for consideration. Therefore, we need a Box-Behnken design to find the “best fit” for the selected factors.

3. Results and Discussion

3.1. Expansion of the extrudates

This study obtained the highest expansion ratio (1.88 mm/mm) at 20% feed moisture content, 80% teff flour: 20% chickpea flour blend mix ratio, 150°C temperature of the barrel and 170 rpm speed of the screw. The rate of expansion was directly proportional to the rise in barrel temperature until it stretched to its maximum value and decreased slightly as the temperature continued to increase beyond the optimal value. The puffing/expansion property of the extruded product is a complicated phenomenon that happens at elevated temperatures and low moisture content extrusion process (Camacho-Hernándeza & b, 2014). The puffing process might be due to several reasons, such as structural change of the starch-protein biopolymer, phase change, nucleation, swelling, bubbling and bubble collapse dynamic factors (Camacho-Hernándeza & b, 2014). Suitable extrudate expansion has been observed to depend not only on the extrusion parameters but also on the composition of the feed, which can affect the water-binding capacity (Mesquita et al., 2013). The Chickpea level significantly affected the expansion of extrudates (p ≤ 0.05). It might be seen from Figure 1. That increasing chickpea levels caused in a decrease of puffing for all the chickpea levels; this agrees with the study by Sadik (2015) reported that the addition of 0–20% soy protein resulted in a small expansion for maize-based extruded product. It is probably related to the starch and protein proportion of the soy interaction that might affect the puffing property indirectly through the application of mechanical energy and disrupt the starch matrix and, hence, reduce the extensibility of the cell structure.

Figure 1. Contour plots for an expansion ratio of extrudates as a function of (a) moisture content (MC), blending ratio (BR), and (b) moisture content (MC) barrel temperature (BT).

Similarly, a common effect might attribute to the expansion reduction in the increased chickpea amount. Furthermore, the increase in lipid content of the blend at higher chickpea levels could also attribute to a decrease in puffing ratio. The interaction effects of different process parameters are shown in Figure 1. The highest expansion ratio (ER) was observed in 15% fed moisture content, 80:20 blending ratio, 150°C temperature of the barrel and 170 rpm speed of the screw.

3.2. The density of the extrudates

The density of the extruded products ranges from 0.22 to 0.67 g/cm³. In this study, the maximum density was 0.67 g/cm^3, and it was attained at 15% moisture content the feed, 85% teff: 15% chickpea blending ratio, 130°C temperature of the barrel, and 150 rpm speed of the screw. Moisture content of the feed has also been found to influence the density of the produce. Amplified feed moisture content throughout extrusion reduced the expansion ratio and enhanced density. The high dependence of density and expansion on feed moisture might show its effect on the elasticity of the starch-based foods (Chulaluck et al., 2011). Density is also enhanced as the feed composition enlarged. Gbenyi et al. (2016) likewise perceived that reducing the moisture content of the feed throughout extrusion caused in amplified expansion and reduced density. The density of extrudates reduced with a rise in barrel temperature. It might be because of starch gelatinization. Therefore, the density and gelatinization temperature are inversely correlated, which keeps the volume increment of the extrudate volume and decreases the density. At elevation temperatures, the vapor pressure of the free moisture is also advanced, which might make an enlarged rate of moisture flashing and puffing upon leaving from the die. A rise in protein level is one cause for the rise in density in extruded products, and the study confirms that a rise in density occurs in extruded products with higher protein content. This could be related to their crude fiber content, as it was advanced in these foodstuffs (Asnakech & Shimelis, 2016), which was reported that crude fiber affects density. The density is amplified through increasing moisture content of the extrudates (Figure 2). Regression analyzes show that density decreases with decreasing humidity.

Figure 2. Contour plots for density as a function of (a) moisture content (MC), blending ratio (BR), (b) moisture content (MC), and barrel temperature (BT).

3.3. The hardness of the extrudates

The hardness of the extrudate was in the range of 8.54 N − 15.82 N. The minimum hardness was attained from the 20% moisture content of the feed, 85% Teff: 15% Chickpea blend ratio, 110°C temperature of the barrel, and 190 rpm screw speed. The maximum was gained at 15% feed moisture content, 85% teff: 15% chickpea blend ratio, 110°C barrel temperature, and 170 rpm screw speed. The results displayed a tendency to rise expansion, decrease density and decrease the breaking force with the temperature of the barrel from 110°C to 130°C. The influence of extrusion environments on extrudate hardness is shown by the contour plot (Figure 3). Amplified protein content in the feed material formed a less expanded product and a more inflexible complex, resulting in higher resistance to shear. Nevertheless, feed moisture and barrel temperature likewise significantly affect the hardness of the extrudate.

Figure 3. Contour plots for the hardness of extrudate as a function of (A) moisture content (MC), blending ratio (BR), (B) moisture content (MC), barrel temperature (BT), and (C) blending ratio (BR), and barrel temperature (BT).

Asnakech and Shimelis (2016) stated that the firmness of extrudate rises as the moisture content of the feed rises, which is similar to this work. Increasing moisture content will cause reduced expansion (Planini et al., 2012). Additionally, the rise in moisture content of the feed raises the product’s density, and the product becomes denser. As a result, denser products show better fracture resistance. The extrusion temperature appears to have a negative influence on the samples’ hardness, compared to the moisture of the feed. The further puffed extrudate foods become more crispy, which produces a softer firmness.

Additionally, the firmness of the extrudates rises with increasing feed rate. Since extruded dough in high feed rates is exposed to lesser shear forces due to minor residence time, which causes fewer deprivation. Hence, it makes an inferior degree of cooking that produces a more rigid texture extrudate food product. Brnčić et al. (2006) reported that an increase in fiber content in the mixture decreased the puffed diameter. The authors also said that the longitudinal puffing property was affected little or none. The more puffed product resulted in a crisper and hence a softer mouthfeel. Low firmness, which is also a superior characteristic of extrudates, was detected at low moisture content of the feed material, high speed of the screw and temperature of the barrel, which is similar to the study by Meng et al. (2010).

3.4. Water absorption index (WAI) and water solubility index (WSI)

The water absorption index (WAI) is the quantity of water absorbed by starch, and it might be used as an indicator for the degree of starch gelatinization (Chulaluck et al., 2011). It is influenced by the presence of hydrophilic food components that can hold and fix water molecules and food matrix. The composite blended flour and its extrudate exhibited the WAI values of 1.65 g/g solid and 7.48 g/g solid, respectively. It shows that the WAI value of the extrudate is comparatively higher than that of the formulated flour mixture. It might be due to the starch gelatinization, protein denaturation and fiber swelling during extrusion that might increase the WAI value. In this work, the WAI value was higher in the extruded formulas of the 20% moisture content of the feed, 80% teff: 20% chickpea blending ratio, 130°C temperature of the barrel, and 150 rpm screw speed). According to Mesquita et al. (2013) and Asnakech and Shimelis (2016), increases in moisture content reduce the water absorption index. It is because moisture can act as a plasticizer during extrusion cooking, reducing starch granules degradation, and resulting in an amplified water absorption capacity, as revealed by Figure 4(a,b) .

Figure 4. Contour plots for WAI as a function of (A) moisture content (MC), blending ratio (BR), (B) moisture content (MC), and barrel temperature (BT).

Varsha and Mohan (2016) reported that the water solubility index (WSI) is a marker of the starch molecule and other components degradation of extruded food products. It measures the number of soluble constituents generated or extruded from protein and other molecules. The higher WSI value resulted in the higher the digestibility of the product in an in-vitro analysis (Varsha & Mohan, 2016). At p ≤ 0.05, the blending ratio, screw temperature and speed significantly affected the water solubility index of the extrudate product. The WSI varied from 12.50 to 22.50. The increasing temperature would result in the degradation of molecules, increasing WSI (Alisis, 2011). As the feed moisture content increases, the result shows that the WSI was increased initially (Figure 5), which may be due to proper gelatinization.

Figure 5. Contour plots for WSI as a function of moisture content (MC) and barrel temperature (BT).

3.5. Sensory quality of the extrudate

Sensory perception is a crucial quality aspect of extruded snacks and is vital for customer approval. Sensory assessment is a key quality factor in product development, enhancing product quality, and optimization. Skilled panel members scrutinized the sensory characteristics of the extruded item, including color, look, taste, crunchiness, and overall acceptability (Table 2). Sensory evaluation of extruded product made at 110°C barrel temperature, 20% feed moisture content, 80 Teff: 20 chickpea blending ratios, and screw speed of 170 rpm were significantly (p ≤ 0.05) affected, while better in color (6.30 ± 0.92) and appearance (6.05 ± 0.75) were observed.

Table 2. Sensory quality evaluations of the extrudate

	Independent process parameters					Sensory attributes
S/N	x1	x2	x3	x4	Color	Appearance	Flavor	Crispness	OAA
1	−1	−1	0	0	5.35±1.08^cdefg	4.40±1.72^j	5.50±0.82^ab	5.75±1.37^abcdef	5.10±0.91^cdefg
2	1	−1	0	0	5.55±0.68^bcdef	5.05 ±0.88^defghi	4.35±1.08^fg	5.30±1.08^defgh	4.95 ±0.88^defg
3	−1	1	0	0	5.20±1.19^efgh	5.20±1.00^bcdefghi	4.10±1.58^g	5.30±1.55^defgh	4.95±0.99^defg
4	1	1	0	0	4.90±0.91^ghi	4.80±0.76^ghij	4.65±0.67^efg	5.35±1.34^cdefgh	4.80±0.83^fg
5	−1	0	−1	0	6.30±0.92^a	5.90 ±1.58^ab	4.70±1.34^defg	4.95±1.50^fghi	4.85 ±1.56^efg
6	1	0	−1	0	4.70±1.03^hij	4.55±1.19^ij	4.95±0.99^bcdef	3.40±2.01^j	4.80 ±1.10^fg
7	−1	0	1	0	5.75±1.58^abcde	5.55±1.09^abcdef	5.35±1.30^abcd	6.15±0.81^abcd	5.45±0.88^bcd
8	1	0	1	0	4.90±0.71^ghi	5.00 ±0.85^efghij	5.15±0.81^abcde	5.40±0.94^cdefgh	5.15±0.67^bcdefg
9	−1	0	0	−1	5.40±1.04^cdefg	5.05±1.19^defghij	4.80±0.83^bcdef	5.30±1.30^defgh	4.95 ± 0.60^defg
10	1	0	0	−1	5.40±0.59^cdefg	5.60±0.50^abcdef	5.20±0.52^abcde	5.20±1.15^efgh	5.40±0.59^bcde
11	−1	0	0	1	5.85±0.48^abcd	5.70±0.65^abcde	5.50±0.68^ab	5.35±1.13^cdefgh	5.50 ±0.82^bcd
12	1	0	0	1	5.25 ±1.25^defgh	5.30±0.97^bcdefgh	4.65±1.89^efg	5.45±1.60^cdefgh	4.75 ±1.25^g
13	0	−1	−1	0	5.25±0.85^defgh	4.40 ±1.39^j	5.20±1.36^abcde	5.30±1.78^defgh	4.75 ±1.40 ^g
14	0	1	−1	0	6.30±0.86^a	6.05 ±0.75^a	5.40±0.68^abc	4.75±1.74^ghi	5.45 ±0.68^bcd
15	0	−1	1	0	4.55±1.09^ijk	4.90 ±1.11^fghij	5.20±1.05^abcde	4.60±1.90^hi	4.75 ± 1.16^g
16	0	1	1	0	5.55±0.68^bcdef	4.95±1.23^fghij	5.55±0.82ab	5.65±1.53^bcdef	5.35 ±0.87^bdcef
17	0	−1	0	−1	4.05±1.27^k	4.75 ±1.44^hij	5.20±1.05^abcde	4.65±1.66^ghi	4.85 ± 1.03^efg
18	0	1	0	−1	5.25±0.78^defgh	5.05 ±1.05^defghij	5.60±0.94^ab	6.40±0.50^ab	5.35±0.81^bcdef
19	0	−1	0	1	4.10±1.25^jk	4.55±1.57^ij	5.55±0.88^ab	5.50±1.31^cdefg	5.30±0.92^bcdefg
20	0	1	0	1	5.10 ±2.02^fghi	4.70 ±1.49^hij	4.65±1.13^efg	4.15±1.78^ij	4.85±1.18^efg
21	0	0	−1	−1	5.60±0.82^bcdef	4.65±1.22^hij	5.15±1.30^abcde	5.85±1.13^abcde	5.20±0.61^bcdefg
22	0	0	1	−1	6.15±0.48^ab	5.75±0.91^abdc	5.75±0.96a	6.55±0.68^a	6.15±0.67^a
23	0	0	−1	1	5.80±0.76^abcde	5.80 ±0.76^abc	5.75±0.63a	6.10±1.20^abcd	5.60±0.88^abc
24	0	0	1	1	5.90 ±0.71^abc	5.50±0.88^abcdefg	5.25±0.85^abcde	6.15±0.87^abcd	5.70±0.47^ab
25	0	0	0	0	5.50±0.60^cdefg	5.10 ±1.16^cdefghij	5.55±0.82ab	6.20±1.19^abc	5.60±0.88^abc

All figures are averages of triplicate ± standard deviation; figures in the same column with dissimilar alphabets show notable differences (p ≤ 0.05); where OAA-Overall acceptability; Coded levels of parameters (X1, X2, X3, and X4); Factors (FMC, BR, BT, and SS); where X1- FMC (%), X2- BR/Blend ratio, X3- BT (0^C) Barrel temperature and X4-SS (rpm) screw speed.

The crispness of the extrudate product is highly affected by the blending ratios of the components. Chickpea level significantly affected the crispness of extrudates (p ≤ 0.05). Higher chickpea levels resulted in extrudates that had lower crispiness scores. Sadik (2015) also reported that an increased level of chickpeas resulted in a decrease in crispness with higher chickpea levels could be due to reduced product expansion. In this study, moisture content of the feed had a reducing influence on the crispness of the majority of the extrudates. This might be understood as the decrease in expansion of extruded dough, which effects when increased amount of moisture content of the feed is applied (Asnakech & Shimelis, 2016). The protein component reduces starch conversion and compresses the bubble growth (Varsha & Mohan, 2016). Hence, it resulted in the production of very dense, structured extruded products. Consequently, less crisp products are produced (Varsha & Mohan, 2016). Sensory quality assessment of extrudate made at 130°C barrel temperature, 150°C barrel temperature, 20% feed moisture content, 85% teff: 15% chickpea blending ratios, and screw speed of 150 rpm were significantly (p ≤ 0.05) different and good crispiness (6.55 ± 0.68) and better overall acceptability (6.15 ± 0.67) observed.

4. Conclusions

This study was accompanied with the overarching goal of increasing the relevance of teff-based food products and investigating their appropriateness for incorporation into extruded food items through several blend ratio formulations. This might have important implications for Ethiopia and other African countries wherever teff is used as a staple food. Thus, process parameters optimization for the development of teff-based extruded snacks or ready-to-eat-foods revealed that extrusion cooking at barrel temperature of 130°C, screw speed of 170 rpm, feed moisture content of 15% and teff: chickpea flour mixture ratio of 15%: 85% produced the extrudate of improved quality with the highest expansion ratio (1.88 mm/mm), maximum WAI (7.48 g/g) and minimum density values (0.22 g/cm³) of the extrudates. In addition, acceptable extrudates were significantly (p ≤ 0.05) better in color, flavor, crispness, and overall acceptability. It has been found that feed moisture content and barrel temperature are the most important process parameters affecting the expansion ratio, hardness and bulk density of the extrudates. Protein-energy malnutrition is still the biggest nutritional challenge amongst preschool children in developing countries like Ethiopia. Therefore, protein-rich developed extrudates can help enhance the nutritional value of protein, mineral and fiber constituents in particular, which can help reduce protein-energy malnutrition problems in developing countries like Ethiopia. Finally, the result identified the potential use of teff to manufacture extruded value-added food products. Additionally, Ethiopia can obtain by processing teff flour and renovating into different value-added products, which can turn the potential into reality through research and development, technology transfer and establishment of manufacturing industries to produce products.

Conflict of interest and authorship confirmation form

This statement certifies that all authors have seen and approved the submitted manuscript. We warrant that the article is the author’s original work. We warrant that the article has not received prior publication and is not under consideration for publication elsewhere. The corresponding author shall bear full responsibility for the submission on behalf of all co-authors. All authors have participated in the conception and design, analysis and interpretation of the data, drafting the article or revising it critically for important intellectual content, and approving the final version.

The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript.

Acknowledgment

The authors would like to express warm appreciation to Haramaya University, Bahir Dar University and Adet Agricultural Research Center for the support to conduct the extrusion process and providing research samples for experimentation, respectively.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Mikyas Kebede Ali

Mikyas Kebede is a food engineer (researcher and lecturer) at the Department of Food Technology and Process Engineering, Haramaya University, with over 10 years of experience in teaching, research and development. Research interest in the areas of food processing, food engineering, and nutrition intervention research.

Solomon Abera

Solomon Abera (Dr. Eng) is an associate professor of Food Engineering at the Department of Food Technology and Process Engineering, Haramaya University, with over 30 years of experience in teaching, research and development. He specializes in Food Engineering and mainly works and publishes in food engineering, food science and food engineering.

Getachew Neme Tolesa

Getachew Neme Tolesa (PhD) is an assistant professor at the Department of Food Science and Postharvest Technology, Haramaya University, with over 16 years of experience in teaching, research and development. He specialized in Postharvest technology and food science. Research interest in the areas of postharvest food preservation, postharvest handling, food value-addition and food science.