The effects of extruded corn flour on rheological properties of wheat‐based composite dough and the bread quality

Abstract The effects of extruded corn flour (ECF) on the rheological properties of the wheat‐based composite dough and quality of the bread were investigated. The RVA results of the composite flour with ECF showed weak thermal viscosity and resistance to starch retrogradation. Mixolab tests revealed that the water absorption capacity increased with the increasing amount of ECF, while dough development time (DT) and dough stability (ST) showed a downward trend, and the composite dough became more resistant to retrogradation. The microstructure of the composite dough showed that the presence of both ECF and unextruded corn flour (UECF) resulted in a more broken gluten matrix. The breads made from the composite flour with ECF had significantly softer texture, lower hardening percentage with storage time, darker crust color, larger specific volume, and higher sensory scores than the UECF ones. It is concluded that the extrusion of corn flour is an effective way to improve the quality of the composite bread and retard staling during storage.

Thermal treatment of material is another way to increase the dough cohesion by gelatinization of starch. Extrusion is a crucial method of hydrothermal treatment. The material undergoes the joint action of temperature, humidity, pressure, and mechanical shear during the extrusion process, which causes the starch to gelatinize and damage, the dietary fiber to dissolve, and the protein to aggregate, thus improves the consistency and cohesiveness of the dough, and modifies the functionality of cereal flour (Gómez, Jiménez, Ruiz, & Oliete, 2011;Patil, Rudra, Varghese, & Kaur, 2016;Sun et al., 2018). It has been demonstrated that extruded wheat bran and wheat germ improved dough stability and bread qualities (Gómez, González, & Oliete, 2012;Gómez et al., 2011). Ma et al. (2018) found that the dough with extruded black rice showed more viscous behavior, higher resistance, and extensibility, and the bread with extruded black rice had higher specific volume, lower bake loss, and firmness. Extrusion cooking could also improve the flavor and the overall acceptability of extruded sorghum-wheat composite bread (Jafari, Koocheki, & Milani, 2018b).
The effects of extruded wheat flours (Martínez, Oliete, & Gómez, 2013), extruded finger millet (Patil et al., 2016), and extruded sorghum flour (Jafari, Koocheki, & Milani, 2017;Jafari et al., 2018a) on composite dough rheology and textural and organoleptic properties of breads also had been studied. However, the influences of extruded corn flour on the dough rheology and bread quality have not been fully understood, and more detailed information is needed to clarify the scientific facts and support its potential application.
Extrusion of corn grit was conducted with a laboratory-scale single-screw extruder (YJP100, Shandong University of Technology).
The moisture was adjusted to reach final moisture content of 21% prior to extrusion. The extruded temperature and the screw speed were set at 100°C and 180 rpm, respectively. The circular matrix with 8 mm × 3 diameter was used. Both extruded and unextruded corn grits were ground with a universal grinder (FW-400A, Beijing Zhongxing Weiye Instrument Co., Ltd.) for 2 min and screened through a sieve (80 mesh) to make extruded corn flour (ECF) and unextruded corn flour (UECF).
The premixed flour of WF and ECF or UECF with substitution of 10%, 20%, 30%, and 40% was thoroughly blended by rotating and shaking in a resealable plastic bag.

| Pasting properties of flour
Pasting properties of WF, ECF, UECF, and mixed flour were determined according to the method of LS/T 6101-2002 (Yan, Zhang, & He, 2001) with a Rapid Viscosity Analyzer (RVA-4500, Perten Instruments). Flour samples were weighed 3.5 g on the basis of 14% moisture and then dispersed in 25 ml of distilled water for the RVA measurement. Suspensions were held at 50°C for 1 min and then raised to 95°C at a rate of 12°C/min, held at 95°C for 2.5 min and cooled to 50°C at the same rate, and finally held at 50°C for 2 min.
The stirring rate was 960 rpm for the first 10 s followed by 160 rpm for the remaining test. Peak time (PT1), pasting temperature (PT2), peak viscosity (PV), final viscosity (FV), breakdown (BD), and setback (SB) were obtained from the pasting curve.

| Mixolab measurements
A Mixolab 2 Mixing Tester (Chopin, Tripetteet Renaud) was used to study the agitation and batter characteristics of WF, ECF, UECF, and mixed flour using chopin + protocol according to ICC No. 173 (ICC Standard Methods, 2008). The following parameters were derived from the experimental curves (Rosell, Collar, & Haros, 2007): Water absorption (%) (WA) is the amount of water required to produce a torque of 1.1 ± 0.05 Nm for the dough; development time (min) (DT) is the time to reach the maximum torque at 30°C; stability (min) (ST) is the time that the torque produced could be kept at 1.1 Nm; minimum torque (Nm) (C2) is the torque generated by dough submitted to mechanical and thermal constraints; peak torque (Nm) (C3) is referred to the maximum torque produced during the heating stage; and setback (Nm) (C5-C4) is the difference between the torque produced after cooling to 50°C (C5) and the torque after the heating period (C4).

| Microstructure of dough
The morphology of the dough was observed using a scanning electron microscope (Nova NanoSEM-450, EI) according to the method of Ma et al. (2018). Small portions of sample were cut with a razor blade and immersed in 4% glutaraldehyde at 4°C for 12 hr or more; then dipped four times with phosphate buffer pH 6.8 at a concentration of 0.1 mol/L for 10 min; after rinsing, embedded in a graded ethanol series (30%, 50%, 70%, and 90%) for 15 min at each gradation; and then embedded in 100% ethanol for 30 min three times.
Following that, the samples were dried by supercritical CO 2 and then coated with gold particles for 4 min. The micrographs were taken at 1,000 magnification.

| Preparation of dough and bread
A straight dough method (10-10B, AACC International, 2000) with minor modification was used for preparation of bread. The dough was made using the following formula: WF or premixed flours (300 g), sugar (30 g), salt (2.4 g), instant dried yeasts (5.4 g), lard (9 g), and skimmed milk powder (12 g). The amount of water was calculated from water absorption obtained by Mixolab measurements. All materials except lard were added in a five-speed dough mixer (SM-1688, Shepherd Wang Electrical Hardware Co., Ltd.) and mixed following a schemed sequence as follows: speed of level 2 for 2 min, level 3 for 1 min, level 2 for 1 min after adding lard, level 3 for 1 min, and level 4 for 1 min. The dough was fermented at 30°C and relative humidity of 85% (RH) for 90 min. During the fermentation, the punching was done at 55 min. The dough was divided into two pieces and rolled at the distance of 7.5 and 5.5 mm using a electric roller (300/100 type), respectively. The sheets of dough were rolled up by hand and placed in stainless baking molds (15 × 7 cm 2 in top, 15 × 6 cm 2 in bottom, and 6.5 cm in depth). After proofing for 40 min at 30°C and RH of 85%, the dough was baked in an oven for 20 min (upper temperature 190°C and bottom temperature 200°C). The breads were removed from molds and cooled for 2 hr at 20°C before further analyses. The specific volume of bread was determined by the rapeseed displacement method and calculated in ml/g.

| Hardness of bread
Textural properties of the breads were evaluated using Texture Analyzer (TA-XA Plus, Stable Micro Systems, Ltd.) according to the method of 74-09 (AACC, 2000). The bread was sliced mechanically for 12.5 mm thick from the central loaf. Two slices were stacked together for test sample. A 36-mm-diameter cylindrical aluminum probe was used in a double compression test with 40% penetration depth. The test speed was 1mm/s and with a 5-s delay between first and second compressions. The bread was stored at 20°C in plastic bags for 0, 1, 3, 5, and 7 days, respectively, to measure the change of hardness.

| Color measurement
The color of bread was measured using colorimeter (Ci7x00; X · rite, Glanville) with measurement area ø = 10 mm and Illuminant D65. The color values of the central point on each quarter of crust or crumb were measured and recorded according to color space of CIE L*a* b*.
Every measurement was duplicated for seven times.

| Sensory evaluation
Sensory evaluation of breads was performed with a group of twenty trained panelists using nine-point hedonic scale according to the method of Patil et al. (2016). The bread samples were coded with random three-digit numbers. The panelists were introduced to rinse and swallow water between samples. The following sensory indicators were evaluated: appearance, flavor, texture, taste, and overall acceptability. The score ranged from 1 (dislike extremely) to 9 (like extremely).

| Statistical analyses
Measurements were performed at least in duplicate and analyzed using analysis of variance (ANOVA); then, the results were expressed as mean value ± standard deviation. A Tukey's multiple range test was conducted to establish the significant differences among experimental mean values (p < .05). All statistical analyses were performed using DPS version 7.05 for windows.

| Pasting properties of flour
The pasting parameters of WF, ECF, UECF, and composite flours are shown in Table 1 evaluating the edible quality of cereals (Shi et al., 2016). ECF had the lowest PT1, PT2, FV, and SB, while UECF had the highest PV, FV, and SB. Compared with WF and substituted UECF, the composite flour with ECF had higher PT2, but lower PT1, PV, FV, BV, and SB. With increasing level of ECF substitution, there was a significant (p < .05) decrease in PV, FV, BD, and SB, indicating a severe breakage of the amylose chain, a loss of retrograde ability (Martínez, Rosell, & Gómez, 2014), and a potential ability of antistaling. This may be attributed to molecular degradation (Zeng, Gao, Li, & Liang, 2011) and pregelatinization of starch (Martínez et al., 2013) during extrusion.
The substitution of UECF resulted in a slight increase in PV, FV, and SB, showing higher viscous behavior. Similar results were obtained by Brites et al. (2010) and Martínez and El-Dahs (1993).
The dough from composite flour with ECF had higher WA than UECF and wheat dough. This may be due to the structure disruption and the swelling of highly degraded starch (Zeng et al., 2011). Higher amount of water for dough forming is beneficial to improve the bread yield. Khoshgozaran-Abras, Azizi, Bagheripoor-Fallah, and Khodamoradi (2014) found that the lower DT was more suitable to get appropriate dough. Therefore, dough made with substituted ECF and UECF was desired. ST was observed to decrease with the addition of ECF.

| Scanning electron microscopy (SEM) of composite dough
The morphology of composite dough observed by SEM is presented in Figure 1. better structure than the UECF ones, which had little gaps and more sticky substances, and this might because the starch was gelatinized during extrusion and acted as hydrocolloid that could improve the dough structure. These results indicated that the microstructure of composite dough with ECF substitution could be greatly improved.

| Hardness of bread
The effects of substitution of corn flour on the hardness of crumb are shown in Table 3. With respect to the fresh bread, both the addition of ECF and UECF increased the hardness of bread. The hardness of composite bread with unextruded corn flour (UECB) was significantly higher than wheat bread (WB) and composite bread with extruded corn flour (ECB). Similar results were found for extruded flour of maize (Ozola, Straumite, & Klava, 2011), wheat (Martínez et al., 2013), bran (Gómez et al., 2011), and finger millet (Patil et al., 2016). The softer crumb of ECB may be due to the high water retention capacity, gelatinization, and damaged starch (Patil et al., 2016).

| Specific volume and color measurement
The effects of ECF and UECF on the specific volume and bread color are presented in Table 4 and Figure 2. With the addition of ECF and UECF, the specific volume of bread decreased. The specific volume of UECB was lower than ECF at all levels of substitution. Patil et al. (2016) and Martínez et al. (2013) also observed that the higher proportion of damaged starch in extruded finger millet flour and wheat flour was beneficial for better yeast activity and production of fermentable sugars, and the increased gas production and retention made ECB have higher specific volume. The partial gelatinization of starch could increase the consistency of dough and may capture the gas during mixing and baking (Bourekoua, Benatallah, Zidoune, & Rosell, 2016), which thus also improved the specific volume and quality of ECB.
With increase in substitutive level, the value L* and b* of both ECB and UECB crust increased, respectively, while the value a* of UECB crust decreased. Both the values L* and b* of ECB crust were lower than those of UECB at all levels of substitution. The value L* of WB crust was higher than that of 10% ECB, but lower than those of the 40% ECB and all samples of the UECB group, and had no difference to the 20% ECB and 30% ECB. This was mainly due to the higher loaf height (Figure 2) narrowed its distance to the upper heating element. Another reason might be attributed to the sufficient reducing sugars in ECB for Maillard reaction (Martínez et al., 2013).

| Sensory evaluation of bread
The sensory scores of composite breads were shown in stacked column graph (Figure 3). Texture and taste scores of ECB were higher than that of UECB, which accounted for the overall high acceptability score of the extruded corn bread. The addition of ECF and UECF had little effect on the appearance of the bread. It can be observed that higher substitution than 10% of ECF led to a slight increase in flavor score. The bread crumb of ECB was softer than WB and UECB, which had the higher score than UECB. Soft bread crumb usually got high score in sensory evaluation (Torbica et al., 2010). Taste scores of ECB were also higher than UECB, which could be attributed to slight sweetish taste of extruded flour (Patil et al., 2016). All the samples were considered acceptable with the overall scores higher than 5, while the ECB got the highest. This result could be contributed to the opinion that hydrothermal process and extrusion treatment could have a positive impact on the quality of composite bread (Patil et al., 2016;Stokić et al., 2015).

| CON CLUS ION
The composite flour with ECF showed higher pasting temperature; but lower values of peak time, peak viscosity, final viscosity, and breakdown from the results of RVA tests; higher water absorption capacity; shorter dough development time; and lower dough stability from the results of Mixolab tests. The microstructure morphology of the composite dough structure showed that the presence of both ECF and UECF resulted in a more broken gluten matrix, and more entire starch granules were retained for the UECF one. The above facts indicated that the composite flour with ECF would result in softer dough with lower proofing resistance but weak ability of gas retaining during fermentation. The interactive balance of the above factors may be the reason why ECB had larger specific volume than the UECB ones but lower than WB.
F I G U R E 2 Images of WB, ECB, and UECB. 10%, 20%, 30%, and 40%, levels of substitution by ECB or UECB; ECB, composite bread with extruded corn flour; UECB, composite bread with unextruded corn flour; WB, wheat bread F I G U R E 3 Sensory evaluation of WB, ECB, and UECB. 10%, 20%, 30%, and 40%, levels of substitution by ECB or UECB; ECB, composite bread with extruded corn flour; UECB, composite bread with unextruded corn flour; WB, wheat bread Both the results from the RVA and Mixolab tests showed the resistance to starch retrogradation of ECF. The ECB with 20%, 30%, and 40% addition had significantly softer texture than that of UECB, and the breads with 20% and 30% substitution of ECF were significantly softer than both WB and UECB during storage.
The above facts sufficiently approved the antistaling ability of ECF.
The bread from composite flour with ECF had lower hardening percentage with storage time, darker crust color, larger specific volume, and higher sensory scores than the UECB. Consequently, it can be concluded that the extrusion of corn flour is an effective way to improve the quality of composite bread. The authors appreciate Professor Li Hongjun, Shandong University of Technology, for kindly providing the material of the extruded corn flour.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

E TH I C A L A PPROVA L
This study has nothing to do with human and animal testing.