Sorghum–mung bean combination snacks: Effect of extrusion temperature and moisture on chemical, functional, and nutritional characteristics

Nine samples were formulated using whole sorghum–mung bean (70:30) at variable extrusion conditions: barrel temperature (130–170°C) and feed moisture (14–18%), and their effect on properties of snacks was examined. The results revealed that the elevation in temperature and moisture increased the antioxidant activity (ferric‐reducing antioxidant power and 2,2‐DiPhenyl‐2‐Picryl hydrazyl hydrate) and total phenolic content. This could be attributed to that the release of bound phenolics is accelerated by the extrusion treatment. However, higher temperature and lower moisture improved the functional properties such as expansion ratio, in vitro protein digestibility, overall acceptability, and expected glycemic index (50–51) of the snacks. A positive correlation was seen among slowly digestible starch and expected glycemic index of the snacks, water solubility index, and expansion ratio. A strong positive correlation was observed among total phenolic content and antioxidant activity. Higher barrel temperature (170°C) and a lower feed moisture (14%) were found to be the best combination to produce nutritious sorghum–mung bean‐based snacks. Moreover, the snacks were found to be highly likeable among the sensory panel, which proves its commercialization properties.

eat snacks by using the principle of high temperature short time (Tas & Shah, 2021).
Recently, the utilization of sorghum in extruded food products has increased to fulfill the demand for functional, gluten-free, high antioxidant activity (AA) foods. Sorghum is the fifth leading grain (in world production), inexpensive, can be grown in arid as well as semiarid regions of the world (Deepthi et al., 2016), and is a good source of phytochemicals that presents numerous health benefits; however, these compounds are found mainly in the outer parts of the grain (Kaur et al., 2020a). The major negative point is the protein quality and quantity, so incorporation of high protein ingredients can improve the protein quality of sorghum-based food product (Kaur et al., 2020b). Incorporation of legumes in the food products could be a good strategy to promote the production and consumption of legumes (Sahin & Sumnu, 2022). Moreover, there are numerous health-improving properties in legumes such as managing high cholesterol, cancer, and diabetes (Kumar et al., 2021). Legume starch depicts a higher amount of slowly digestible starch (SDS), consequently giving a low glycemic index (GI) to the products (Keskin et al., 2021).
However, an acceptable sorghum-based extruded product can be prepared utilizing sorghum alone, but its nutritional quality is not up to the mark to fulfill consumer satisfaction (Tadesse et al., 2019). One of the main reasons for the same is the poor digestibility of its protein. It turns out that the digestibility value drops down to a significantly lesser value on wet cooking (Belton & Taylor, 2004). Moreover, presence of antinutritional factors such as tannins also lowers down the in vitro protein digestibility (IVPD) (Bekele, Nosworthy, Tyler, Henry, House, & Nosworthy, 2021).
Also, sorghum is deficient in lysine, so a combination with a protein source could fulfill these limitations (Aboubacar et al., 2001).
Being high in protein (22-24%) along with easier digestibility, mung bean is an appropriate candidate to be supplemented into sorghum-based food products (Sharma et al., 2017). It is a staple legume crop in India, having a short growth cycle and a lesser water consumption, even grows in adverse (arid and semiarid) ambiences (Dahiya et al., 2013). It is an excellent protein source-having its protein rich in lysine (in which cereals are mainly deficient), leucine, and threonine . Mung bean consumption does not cause as much flatulence as other legumes due to its easy protein and carbohydrate digestibility (Elobuike et al., 2021).
This investigation aims to (1) determine the effect of barrel temperature (BT) and feed moisture (FM) on the different physicochemical, functional, and nutritional properties of whole sorghum-mung bean-based snacks and (2) determine the best combination of BT-FM to develop snacks with a higher nutritious content (low GI, higher AA, higher total phenolic content [TPC], and higher IVPD) as well as good amount of functional properties.

| Material
Sorghum (2077 B Line) and mung bean (SML 832) were procured from the Directorate of Seed, PAU, Ludhiana, India, and product was formulated at Department of Food Science & Technology, PAU (Lat. 30.899540,Long. 75.804510). Whole grains were milled separately using a lab scale hammer mill (Perten Instruments, Hagersten, Sweden). During preliminary trials, the composition of the blend and improved functional and nutritional properties along with sensory quality were used as criteria for the selection of the proportion of grains. The composition with the best results (the best expansion) came out to be 70:30. The flour was kept in plastic packages till further analysis.

| Extrusion
Extrusion was performed on a corotating and intermeshing twinscrew extruder Model BC 21 (Clextral,Firminy,France). The diameter of the barrel and the ratio of its length to diameter were 25 mm and 16:1, respectively. Feed hopper was maintained at 35-40 C. The BT of the second and third zones was maintained at 70 C and 100 C, respectively, whereas the BT at the last zone (compression and die section) was varied (130-170 C). An 8.5-KW power motor (speed variable = 0 to 682 rpm) was used. A single screw-type volumetric feeder (D.S. and M, Modena, Italy) was used to manage the entry of raw material. The dried extrudates were packaged in polyethylene bags and were stored in plastic airtight containers.

| Specific mechanical energy (SME)
The torque displayed in the extruder which controls the ratio of the actual and maximum torque value of extruder was used to calculate SME, which was presented using Equation (1)

| Bulk density
Bulk density (kilogram per cubic meter) (BD) of the extruded snacks was measured by the volumetric displacement method (Oliveira et al., 2017) and expressed as a mean of five random samples from each experimental run calculated according to Equation (3): 2.6 | Water absorption index and water solubility index Extrusion properties such as water solubility index (WSI) and water absorption index were evaluated using methods described by Anderson et al. (1970).

| Hardness
A texture analyzer (TA-XT2i, Stable Micro System, Surrey, UK) with a 35-mm stainless steel cylinder probe was used to measure the hardness of snacks.

| Color characteristics
The color parameters (L*, a*, and b*) of the snacks were carried out using Hunter color calorimeter (Konia Minolta CR-410 T), and the chroma (C) (Equation 4) and hue (h) (Equation 5) values were calculated using formulas provided below.
Hue angle h 2.9 | AA and TPC

| Extraction
The samples were extracted using method given by Sharma et al. (2019). This supernatant was used for determination of AA and TPC.
Ferric-reducing antioxidant power (FRAP) was evaluated as per method given by Benzie and Strain (1996).

| In vitro protein digestibility (IVPD)
IVPD was determined as per procedure given by that of Hsu et al. (1977).
Exp-GI of the product was determined by using Equation (7).

| Statistical analysis
All tests were performed in triplicate. Analysis of variance using Duncan's multiple test was used to compare means at p < 0.05 using SPSS software (SPSS Inc., USA) version 15. Also, correlation was evaluated in SPSS.

| Specific mechanical energy
The measurement of mechanical energy going to the extruder is important, to get acquainted with the amount of energy consumption and its effects on product characteristics (Sharma et al., 2017). The data show a decrease in SME with increasing BT (130-170 C) ( which reduce the transfer of mechanical energy (Meng et al., 2010).
The results are in accordance to that of Parthi et al. (2019), where functional properties of extruded brown rice grits were evaluated and the rising BT had a negative effect on SME. The highest SME is observed for snacks prepared at 16% MC and 130 C (74.4 Wh/kg), whereas at 18% MC and 130 C, the SME is 69 Wh/kg. It has been indicated that SME decreases with an increase in FM (Table 1). Elevation in FM affects the dissipation of energy by decreasing the viable sites and inculcating the plasticizing effect. Higher MC behaves as a lubricant that diminishes the friction between the particles at movement from barrel to screw, thus reducing SME. Higher MC along with higher BT also reduces the motor torque (Sharma et al., 2017), which is, likely, as high moisture leads to lower melt viscosity (Wang, 2018).

| Water absorption index and WSI
WAI increased from 4.70 to 5.07 g/g with increase in BT (130-170 C) at 16% FM (Table 1). A significantly (p < 0.05) positive effect of BT was noticed, which is possibly due to increased dextrinization at high BTs (Kumar et al., 2010). The results were in accordance to that of Seth et al. (2015) for yam-corn-rice-based snack food, which associated higher BT to an increased disruption of starch granules, thus suggesting enhanced WAI. Similar results were also presented by Singh et al. (2015) and Mugabi et al. (2022) for potato-based snacks and maize-soyabean flour blends, respectively. WAI increments from 4.21 to 4.65 g/g at 14% FM and 4.88 to 5.18 g/g at 18% FM. Pathania et al. (2013) have explained that a higher FM results in a decrease in starch viscosity, thus allowing uniform heating and enhanced gelatinization. A comparable effect of FM on WAI was shown by Ali et al. (2016), owing to the reduced degradation of starch which hikes the water absorption.

| Expansion ratio
Expansion is the most important physical consideration in extrusionbased snacks (Kokini et al., 1992). The findings have shown that ER T A B L E 1 Physical characteristics of sorghum-mung bean-based snacks. giving out an expanded product (Koksel et al., 2004). An elevated BT leads to an increase in the superheating of water, in turn assisting the formation of bubbles and thus depressing the melt viscosity giving a product with higher expansion (Ali et al., 2016).

| Bulk density
Lower BD is an ideal characteristic of an extruded snack: Puffed and airy product is favored. BD of the snacks diminished with amplifying BT: BD at 16% and 130 C is 105 g/L, and at 16% and 170 C, it decreased to 58.05 g/L. Higher BT raises the dough BT above its boiling point, leading to sudden pressure change at the exit, causing the internal moisture to flash off. The process results in formation of bubbles in the matrix producing low-density extrudates (Altan et al., 2008). The effects of rising BT on BD were in agreement with the studies reported by other authors as well: for rice-and mung bean-based extrudates (Sharma et al., 2017) and for maize-chickpea (Ali et al., 2016). The value of FM was found to be ranging from 52 to 80 g/L (at 14% FM and 170-130 C), similarly 58.05-105 g/L at 16% MC and 85-108 g/L at 18% MC (Table 1). These relationships are certain, as a higher FM facilitates the lubrication, which reduces the SME as well as degradation of starch (Singh et al., 2015). Similar effects of MC on BD were presented by Wang et al. (2019) and Sharma et al. (2017) for chickpea:sorghum:maize extrudates and rice:mung bean extrudates, respectively. A higher BT when combined with a lower FM assists the gradient of vapor pressure and pressure increases within the barrel leading to a puffed product (Sharma et al., 2017). As per correlations are concerned, BD has shown a positive correlation with hardness (r = 0.81, ρ = 0.008) as well as RS (r = 0.87, ρ = 0.002). A significantly negative correlation has been recorded among BD and IVPD (r = À0.90, ρ = 0.001) ( Table 6).

| Hardness
The results have depicted that the hardness of sorghum-mung bean snacks has an inverse relationship with BT. The decrease in hardness with elevating BT was also reported by Mugabi et al. (2022) for maize-soyabean flour blends, which was found to be due to escalat-  (Table 6).

| Color characteristics
Because color is an important physical quality characteristic, it is evaluated for sorghum-mung bean-based snacks as well ( forth. Maillard reaction generally takes place at high BT as well as higher screw speed at an optimum FM (Geetha et al., 2014).
However, the L*, a*, and b* values did show a positive correlation with each other. However, a highly positive correlation is observed among b* value and chrome (r = 0.99, ρ = 0.00) and a* value and RS (r = 0.78, ρ = 0.013), whereas a negative correlation of a* value was seen with texture (r = À0.80, ρ = 0.009). Overall, a strong significantly positive correlation was found among color and OA (r = 0.92, ρ = 0.001) ( Table 6).

| Antioxidant activity
AA evaluates the ability of a material to scavenge the free radicals.
DPPH and FRAP were applied to determine the AA of extruded snacks. AA values are expressed as trolox equivalent, where trolox is a water-soluble antioxidant and used as a standard in antioxidant assays. The AA values for snacks are given in Table 3. Using both evaluation methods has confirmed that the AA elevates by increasing FM as well as BT. As per evaluation using DPPH, at 14% MC, the AA was found to increase from 12.52 μ mol TE/g dw (130 C) to 27.11 μ mol TE/g dw (170 C) at 18% MC. Using FRAP, the values elevated from 6.43 μmol/g (dw) (130 C BT, 14% MC) to 16.13 μmol/g (dw) (170 C BT, 18% MC) ( Table 3) As whole grains were utilized in this product, the presence of bran portion has also incremented the AA, as it has been reported that the TPC of the bran portion is supposed to be 15-18 times more than the endosperm portion (Adom & Liu, 2002). Parallel to this study,  have also shown an elevation in DPPH levels for a different proportion of chickpea-sorghum snacks with increase in BT. It was found that the antioxidant levels at 160 C were higher as compared with 120 C or 140 C. In the present study, the DPPH and FRAP were observed to also enhance with increase in  Table 6).
The correlation results ensured the effect of TPC on AA, as reported by above mentioned studies.

| Total phenolic content (TPC)
TPC is expressed as gallic acid equivalent, where gallic acid is a natural compound present in many plants and is used as standard in TPC assays. As per TPC is concerned, the amount incremented from 4578.01 mg/100 g (dw) (130 C) to 5117.23 mg/100 g (dw) (170 C) at 14% MC (Table 3). It has been mentioned that the short-time processing by extrusion gives a potential advantage for maintaining phenolics as compared with other heating treatments (Sarka et al., 2020).

| In vitro protein digestibility (IVPD)
In the present study, the IVPD values have elevated with the rise in BT (Figure 1). The highest IVPD (71.44%) was shown in the extrudates formulated at 14% FM and 170 C BT. Similar results have been revealed in a recent study by Bekele, Nosworthy, Tyler, Henry, House, and Nosworthy (2021), where chickpea:sorghum extrudates were formulated at 50:50 and 70:30 ratios. Diverse reasons might be associated for this phenomenon, that is, opening of the protein structure owing to the alteration in noncovalent interaction as well as the inactivation of protease inhibitors and other antinutritional factors; an increment in the protein digestibility with increase in BT could also be linked to the decline in anti-nutrients (Ghumman et al., 2016) or processing-induced fluctuations in the actions of endogenous hydrolytic enzymes or structural changes in the storage proteins (Bishnoi et al., 1994). However, a contrary relationship has been seen in case of FM, where decrease in IVPD was observed with rise in FM (at 14% MC, 130 C: 54.15%). The same might be proved due to the low shear rate with increase in FM. The results in the present study were in line with that mentioned by Wang et al. (2019) for extruded kabuli chickpea. In another study, the rise in IVPD values was related to the inactivation of polyphenols, enzyme-inhibiting factors (trypsin and chymotrypsin inhibitors), and exposure of protein aiding in enzyme digestion (Bai et al., 2018). IVPD did show a significantly strong positive correlation with WSI (r = 0.79, ρ = 0.010) and ER (r = 0.72, ρ = 0.028) and a strong negative correlation with BD (r = À0.90, ρ = 0.001) and hardness (r = À0.89, ρ = 0.001) ( Table 6).

| In vitro starch digestibility (IVSD)
A digestible starch is classified into RDS, SDS, and RS. The RDS is found to have been elevated with the increasing BT (18.01-20.33%) from 130 C to 170 C at 14% MC (Table 4) The SDS fraction is related to the double helix structure of starch (Gonzalez et al., 2020). In contrast to RDS, SDS levels have shown a reduction (30.84% to 26.93%) with augmenting BT (130-170 C) at 14% FM. It has been suggested that higher BT would elevate the thermal processes of the melt which aids in increased starch gelatinization (Koa et al., 2017) F I G U R E 2 Effect of feed moisture and barrel temperature at expected glycemic index (GI) of the sorghum-mung bean-based snacks.
T A B L E 5 The sensory characteristics of the sorghum-mung bean snacks. decreasing the RS content. Furthermore, the higher the BT is, the more significant is the damage to the starch structure. The highest RS content (higher amylose content) was achieved at higher FM and a lower screw speed for extruded green banana flour (Sarawong et al., 2014). Similar to the present study, RS content increased with high FM, probably because extrusion at high FM elevated the percentage of starch retrogradation and thus RS content (Koa et al., 2017).

| Expected GI (Exp-GI)
There are multiple factors that influence the GI of foods such as amylose amylopectin ratio, food processing methods, food composition, degree of ripening, particle size, RS, and so forth, and there are various methods to formulate low GI foods (Kaur et al., 2021). The lowest GI was seen for the samples prepared at 18% MC (50), which might be due to the higher amount of RS (38.32-41.41%), which actually reduces the GI by slow release of glucose as well as its slower metabolization (Kaur et al., 2021) (Figure 2). It has been observed that GI of the product did not show a significant decrease at all BTs; however, at 16% FM, the Exp-GI has shown quite a trend where GI is 51 at 130 C, which decreased to 50 at 145 C and 170 C. It has been observed that the heat moisture treatment leads to an increased stability at the structural level by rearrangement of amylase chains to an ordered domain, giving a higher RS content and thus a lower Exp-GI (Klein et al., 2013). Exp-GI was found to have a significant positive correlation with SDS (r = 0.83, ρ = 0.005) ( Table 6).

| Sensory characteristics
Sensory analysis is important to know the probable response of the target population regarding the developed product. A randomly selected group of people analyzes the product (using sensory organs) and give their reviews on a 9-point hedonic scale. The OA of the snacks is given in Table 5. The snacks developed at higher BT (170 C) were highly acceptable (8.2 at 14% MC, 7.5 at 16% MC, 7.3 at 18% MC), which show that FM was negatively related to OA (Table 5). A significant difference in OA was observed for the samples prepared at different BTs. Texture of an extruded product is considered as a vital factor for liking it: Snacks prepared at higher BT and the lowest moisture had the maximum expansion and a crunchy mouthfeel that turns to be the most favorable characteristic for an extruded product.
Moreover, the combination of sorghum and mung bean (70:30) made a product with a good OA scores. A strong significant positive correlation has been observed for taste with texture (r = 0.97, ρ = 0.00) and OA (r = 0.91, ρ = 0.001). Also, texture is found to have a very strong positive correlation with OA (r = 0.95, ρ = 0.001) ( Table 6).

| CONCLUSIONS
In the present study, nine samples were formulated at different moisture and BT (keeping the screw speed constant) from sorghum-mung bean (70:30), and their various properties were analyzed. It was achieved that a higher BT along with lower FM is required to produce snacks with a good functional and nutritional properties along with best functional and sensory characteristics. These products could make a healthy option for every age group of population. However, additional research is required to cover the storage studies as well as the one with variable screw speed.

CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest.

FUNDING INFORMATION
The research received no specific grant from any funding agency.

DATA AVAILABILITY STATEMENT
No shared data.