3.1 The contents of main components of mixed rice-wheat flour
The basic components of mixed rice-wheat flour with different rice flour proportions were analyzed, and the results were shown in Table 1. With the addition of rice flour (from 0 to 50%), the water content in the mixed flour decreased significantly from 12.37–11.88%. This phenomenon is due to the fact that protein has stronger hydration capacity than starch, and the water absorption rate of protein is about 5 times greater than that of starch [23]. Because the content of protein in rice flour is lower than that in wheat flour, leading the lower water content after adding rice flour. Significantly, the total starch and amylopectin contents in the selected rice are higher than those in wheat flour, and the fat, protein, amylose and ash contents are lower than those in wheat flour. Therefore, when the addition of rice flour increased from 0 to 50%, the total starch and amylopectin content in the mixed flour significantly increased from 68.45–76.56%, and 52.35–60.53%, respectively. Meanwhile, the fat (from 1.77–1.11%), protein (from 14.72–10.40%), amylose (from 16.10–16.03%) and ash (from 0.79–0.58%) contents were all decreased significantly. Similarly, Grassi et al. studied the partial substitution effects of corn, green banana and rice flour on mixed wheat dough, and found that mixed rice flour dough has low moisture, low protein, low fat and high mineral content compared with wheat flour dough.[11].
Table 1
The contents of main components of mixed rice-wheat flour (on a wet basis)
Addition of rice flour/% | Moisture/% | Fat/ (g·100g− 1) | Protein/ (g·100g− 1) | Total starch /(g·100g− 1) | Amylose/ (g·100g− 1) | Amylopectin/ (g·100g− 1) | Ash/ (g·100g− 1) |
0 | 12.37 ± 0.01f | 1.77 ± 0.01f | 14.72 ± 0.72e | 68.45 ± 0.41a | 16.10 ± 0.01c | 52.35 ± 0.73a | 0.79 ± 0.05d |
10 | 12.27 ± 0.01e | 1.64 ± 0.01e | 13.87 ± 0.57de | 70.07 ± 0.40b | 16.09 ± 0.02c | 53.98 ± 0.68b | 0.75 ± 0.04d |
20 | 12.17 ± 0.01d | 1.51 ± 0.01d | 13.01 ± 0.85cd | 71.69 ± 0.38c | 16.08 ± 0.01bc | 55.61 ± 0.62c | 0.71 ± 0.03cd |
30 | 12.06 ± 0.01c | 1.38 ± 0.02c | 12.16 ± 0.71bc | 73.32 ± 0.37d | 16.06 ± 0.01ab | 57.26 ± 0.47d | 0.66 ± 0.03bc |
40 | 11.96 ± 0.01b | 1.24 ± 0.02b | 11.31 ± 0.44ab | 74.94 ± 0.36e | 16.04 ± 0.02ab | 58.90 ± 0.32e | 0.62 ± 0.02ab |
50 | 11.88 ± 0.02a | 1.11 ± 0.02a | 10.45 ± 0.49a | 76.56 ± 0.35f | 16.03 ± 0.01a | 60.53 ± 0.16f | 0.58 ± 0.01a |
Means with different superscript letters within the same column are significantly different (P < 0.05) |
3.2 Thermo-mechanical properties of the mixed rice-wheat dough
The effect of different amounts of rice flour on thermo-mechanical properties in dough were shown in Table 2. Water absorption is the amount of water required to make the dough reach the target torque. It can be seen from Table 2 that with the addition of rice flour from 0 to 50%, the water absorption of dough decreased from 59.6%±0.1–57.1%±0.2%. This is due to the increase in protein and damaged starch content, which leads to an increase in water absorption of dough. In addition, gluten protein has a greater impact on the water absorption of dough [23]. As the addition of rice flour would reduce the gluten content of dough, the water absorption of dough would decrease as the addition of rice flour.
Table 2
Effect of different amounts of rice flour on dough thermo-mechanical properties
Addition of rice flour/% | Water absorption/% | Dough formation time/min | Dough stability time /min | C2/Nm | C1-C2/Nm | C3-C4/Nm | C5-C4/Nm | C4/C3 | β |
0 | 59.6 ± 0.1c | 5.38 ± 0.02d | 6.50 ± 0.02b | 0.49 ± 0.01c | 0.60 ± 0.01a | 0.15 ± 0.03a | 1.15 ± 0.04e | 0.92 ± 0.02e | 0.526 ± 0.007a |
10 | 58.3 ± 0.2b | 5.78 ± 0.02e | 7.13 ± 0.04d | 0.48 ± 0.01c | 0.61 ± 0.01a | 0.22 ± 0.00b | 1.03 ± 0.01d | 0.89 ± 0.00d | 0.534 ± 0.018a |
20 | 58.1 ± 0.3b | 5.93 ± 0.04f | 7.50 ± 0.02f | 0.48 ± 0.02c | 0.61 ± 0.01a | 0.34 ± 0.04c | 0.97 ± 0.02c | 0.84 ± 0.02c | 0.554 ± 0.020a |
30 | 58.0 ± 0.1b | 3.82 ± 0.01c | 7.32 ± 0.01e | 0.47 ± 0.01c | 0.62 ± 0.01a | 0.49 ± 0.00d | 0.86 ± 0.03b | 0.77 ± 0.00b | 0.648 ± 0.014b |
40 | 57.9 ± 0.1b | 3.70 ± 0.03b | 6.84 ± 0.03c | 0.45 ± 0.01b | 0.67 ± 0.02b | 0.57 ± 0.02e | 0.70 ± 0.02a | 0.73 ± 0.01a | 0.718 ± 0.011c |
50 | 57.1 ± 0.2a | 3.65 ± 0.06a | 6.13 ± 0.02a | 0.43 ± 0.01a | 0.69 ± 0.02b | 0.59 ± 0.05e | 0.69 ± 0.05a | 0.73 ± 0.02a | 0.716 ± 0.042c |
Means with different superscript letters within the same column are significantly different (P < 0.05) |
Dough formation time refers to the time taken when dough consistency reaches the maximum value, which reflects the formation speed of dough gluten network and hydration capacity of powder particles; dough stability time represents the ability of the dough internal structure resist external mechanical forces, and can characterize the mixing resistance of dough [24]. With the increase of rice flour (from 0 to 50%), dough formation and stability time tend to increase first and then decrease (Table 2). When the amount of rice flour reaches 20%, dough formation time reached the maximum value of 5.93 min, and the dough stability time reached the maximum value of 7.50 min. This may be because when the substitution of rice flour was less than 20%, gluten in rice flour can form disulfide bond [25], which enhances the internal structure of dough, thus masking the dilution effect of rice flour on gluten and increasing the dough formation and stability time. However, when the amount was more than 20%, the gluten strength decreased with the decreased gluten protein content in the dough, resulting in the shortening of dough formation and stability time. Therefore, when the amount of rice flour was 20%, the dough strength was the strongest.
Weakening degree (C1-C2) refers to the difference between the maximum and minimum consistency of dough, which can characterize the protein weakening degree of dough during mixing; C2 refers to the minimum consistency produced by the reaction of dough under mechanical and thermal stress. The smaller C2 indicated that the smaller torque generated by dough under the action of mechanical and thermal stress, leading the weaker dough strength [26]. It can be seen from Table 2 that when the substitution amount of rice flour increased from 0 to 30%, the weakening degree increased from 0.60 to 0.62, and C2 decreased from 0.49 to 0.47, which are not significant. However, when its substitution increased from 30–50%, the weakening degree increased significantly from 0.62 to 0.68, and C2 decreased significantly from 0.47 to 0.43. This indicated that excessive addition of rice flour would reduce the gluten strength of the dough, resulting in a gradual increase of protein weakening and a poor processing property of the dough [27].
Breakdown value (C3-C4) refers to the difference between peak viscosity and trough viscosity, which characterized the stability of the starch paste in the dough. The smaller breakdown value reflects the better stability [28]. The addition of rice flour led to the breakdown value increased significantly from 0.15 ± 0.03 Nm to 0.59 ± 0.05 Nm, which reflected that the shear resistance and stability of the hot paste of the mixed flour dough became worse after adding rice flour.
Setback value (C5-C4) refers to the difference between peak viscosity and holding viscosity, which characterized retrogradation degree of the starch cold paste. The smaller setback value, the harder retrogradation of starch. It can be seen from Table 2 that the addition of rice flour (from 0 to 50%) caused a decreased setback value from 1.15 ± 0.04 Nm to 0.69 ± 0.05 Nm, which reflected that starch of the product is not easy to regenerate after adding rice flour. Aging is due to the directional migration of amylose molecules after starch gelatinization to form a three-dimensional network structure, so the aging degree is closely related to the amylose content [29]. The selected rice flour has lower amylose content than wheat flour, so the addition of rice flour reduced the content of amylose in the mixed flour, thus delayed the aging of starch. Therefore, the addition of rice flour can improve the storage characteristics of the mixed dough.
Cooking stability (C4/C3) refers to the ratio of trough viscosity to peak viscosity of dough, which can be used to characterize the stability of starch gelatinized at high temperature. The cooking stability is closer to 1, the more stable the starch paste at high temperature, and the more resistant the product is to cooking [30]. It can be seen from Table 2 that with the addition of rice flour increasing from 0 to 50%, the cooking stability is significantly reduced from 0.92 ± 0.02 to 0.73 ± 0.02, indicating that the stability of mixed flour dough became worse at high temperature by adding rice flour, which is due to the increased degree of protein weakening and the weakening of gluten strength [31].
β value indicates the gelatinization rate of starch. β value increased significantly from 0.526 ± 0.007 to 0.716 ± 0.042 when 0%-50% rice flour was added, indicating that the gelatinization rate of starch in dough increased with the addition of rice flour (Table 2). Gelatinization is a process in which water molecules enter into the crystalline regions of starch particles to make starch swell and break, and finally form a viscous solution [32]. The addition of rice flour significantly increased the content of amylopectin in the mixed flour. The amylopectin has many branches and is easy to form a crystalline regions, which is conducive to the entry of water molecules, thus promoting the gelatinization of starch [33].
3.3 Pasting characteristics of the mixed rice-wheat dough
RVA gelatinization curve of mixed rice-wheat flour is shown in Fig. 1. It can be seen that the shapes of RVA curves are similar, but the curves are offset due to different gelatinization values. The gelatinization parameter values of the mixed flour dough was shown in Table 3. It can be seen that with the addition of rice flour from 0 to 50%, the peak viscosity, trough viscosity and the final viscosity increased significantly from 1434 ± 9 cP to 1853 ± 11 cP, 1026 ± 8 cP to 1254 ± 6 cP, and 2073 ± 18 cP to 2222 ± 9 cP, respectively. And the breakdown value increased significantly from 408 ± 8 cP to 599 ± 5 cP, and the setback value of starch decreased significantly from 1047 ± 14 cP to 968 ± 2 cP. Peak viscosity, trough viscosity and final viscosity refer to the maximum viscosity after heating, the minimum viscosity after cooling and the viscosity after testing respectively. They can all characterize the expansion degree of starch particles in the sample, that is, the gelatinization characteristics of the sample. Because only amylopectin can swell when heated in water, the peak viscosity, trough viscosity and final viscosity of starch all depend on the content of amylopectin. Due to the increase of amylopectin content and the lack of gluten protein in the rice flour after the addition of rice flour, the gluten network formed after gelatinization became worse, so that a large number of starch particles could not be wrapped by the gluten network, resulting in the increase of free starch content and the relative increase of amylopectin concentration. Therefore, the gelatinization value of starch increased after the addition of rice flour [33]. This phenomenon was consistent with the change results of β value in thermo-mechanical properties.
Table 3
Effect of different amounts of rice flour on gelatinization properties
Addition of rice flour/% | Peak viscosity/cP | Trough viscosity/cP | Breakdown value/cP | Final viscosity/cP | Setback value/cP |
0 | 1434 ± 9a | 1026 ± 8a | 408 ± 8a | 2073 ± 18a | 1047 ± 14d |
10 | 1472 ± 10b | 1046 ± 9b | 426 ± 2b | 2077 ± 33b | 1031 ± 4c |
20 | 1499 ± 7c | 1051 ± 3b | 448 ± 2c | 2079 ± 6c | 1028 ± 13c |
30 | 1642 ± 5d | 1165 ± 2c | 477 ± 3d | 2147 ± 6d | 982 ± 3b |
40 | 1695 ± 2e | 1182 ± 1d | 513 ± 1e | 2158 ± 2d | 976 ± 3b |
50 | 1853 ± 11f | 1254 ± 6e | 599 ± 5f | 2222 ± 9e | 968 ± 2a |
Means with different superscript letters within the same column are significantly different (P < 0.05) |
3.4 Dynamic rheological properties of mixed rice-wheat dough
Dynamic rheological frequency measurement can be used to analyze the viscoelasticity of dough samples, and has great application value in dough processing [34]. The changes of G', G" and tan δ with frequency in the mixed flour system with different amounts of rice flour were shown in Fig. 2. G' is the storage modulus, also known as the elastic modulus, which refers to the energy stored by dough due to elastic deformation, reflecting the elastic size of dough; G′′, namely loss modulus, also known as viscosity modulus, refers to the energy loss of dough due to viscous deformation, reflecting the viscosity of dough [35]. With the increase of oscillation frequency from 0.1 Hz to 10 Hz, both G' and G'' of those dough increased (Fig. 2A&B). This is because the increase of oscillation frequency made the starch particles in the dough act as fillers, enhanced the strength of gluten, and produced a strong binding force, thus increasing G' and G'' of dough [7]. In addition, when the addition of rice flour was increase from 0 to 50%, G' and G'' increased first and then decreased. Notably, G' and G'' reached the maximum when the amount of rice flour was 20%. This may be because when the amount was less than 20%, the gluten in the rice flour could form disulfide bonds, which enhanced the internal structure of the dough gel system, strengthened its strength and increased its viscoelasticity. However, when rice flour is continuously added, both G' and G'' are reduced. This is because when too much rice flour was added, the content of gluten protein in the dough decreased and the formation of gluten network was difficult, resulting in the reduction of air chamber, muscle strength and viscoelasticity [36].
tan δ is loss tangent value, which refers to the ratio of G'' to G'. When tan δ<1, G' is larger than G'' and dough samples are similar to solid; when tan δ>1, G'' is larger than G', and dough samples are similar to fluid [35]. As shown in Fig. 2, the tan δ of these dough were always less than 1, indicating that the dough mainly underwent elastic deformation, its properties were similar to solid, and its internal starch and colloid showed typical weak gel viscoelastic characteristics [36]. In addition, with the increase of oscillation frequency from 0.1 Hz to 10 Hz, tan δ of those dough decreased first and then increased. When oscillation frequency was between 0.1–0.4 Hz, tan δ decreased with the increase of frequency. Because the gluten protein could form a good network shape at low frequency, and the dough system was relatively stable, thus increasing the elastic ratio; When the frequency was greater than 0.4 Hz, tan δ gradually increased with the increase of frequency, which indicated that the cross-linking between molecules were weakened. Therefore, the fluidity of the dough gel system was enhanced, the viscosity ratio was increased, and the phenomenon of "shear thinning" occurs [37], which caused the dough system to be unstable and easy to be destroyed.
tan δ is related to the degree of molecular crosslinking in the dough system. The smaller the tan δ, the higher the degree of molecular crosslinking, and the greater the degree of component polymerization in the dough system. With the addition of rice flour increasing from 0 to 50%, the tan δ decreased gradually. The addition of rice flour can compete with wheat flour for water absorption, so that the degree of molecular cross-linking polymerization dough gradually increased, and the elastic ratio increased [38]. Xu et al. reported that the tan δ of dough gradually decreased with the increase of potato powder from 0 to 30%, which is consistent with the results of this study [39].
3.5 Microstructural characteristics of mixed rice-wheat dough
SEM analysis of rice-wheat flour mixed dough with different amounts of rice flour were shown in Fig. 3. It can be seen that the pores of wheat flour dough were continuous and complete, the gluten in the dough could form a good network structure, and starch granule integrity; when the amount of rice flour was 20%, the stomatal walls inside the dough began to break and connect each other, and the irregular voids and small damaged starch increased; when the amount was more than 30%, the gluten structure of these dough was seriously damaged, the pore wall was seriously broken, the pores collapsed, and the damaged starch increased significantly.
These results revealed that adding rice flour will destroy organizational structure of dough, and make it impossible to form a continuous three-dimensional protein network structure. These not only makes the dough lose its swelling ability during fermentation, but also fails to maintain the pore structure produced by yeast fermentation, resulting in the lack of elasticity and softness of dough and products [40]. Sun et al. also showed that with the increase of wheat germ powder, discontinuous and irregular matrix formed around starch particles, and the gluten network structure was damaged [41].
3.6 Textural properties of mixed rice-wheat dough
The textural properties for the dough samples were shown in Table 4. The hardness is the force for the sample to reach certain deformation, and the springiness represents the rate at which a sample recovers to its pre-deformation after removing deformation forces. Within a certain range, the smaller hardness, the greater elasticity, and is softer for the sample [42]. It can be seen that with the addition of rice flour (from 0 to 50%), the hardness of the dough first decreased and then increased, while the elasticity change trend was opposite. When the amount was 20%, the hardness reached the minimum value of 86.50 ± 3.70 gf, and the elasticity reached the maximum value of 0.92 ± 0.03. These results were consistent with the determination results of G′ and G′′ in dynamic rheological properties of dough.
Table 4
Effect of different amounts of rice flour on dough texture characteristics and tensile characteristics
Addition of rice flour/% | Hardness/gf | Springness | Resilience | Tensile resistance/gf | Tensile distance/mm |
0 | 96.49 ± 2.12b | 0.84 ± 0.02bc | 0.08 ± 0.01c | 30.67 ± 5.20bc | 49.54 ± 0.01d |
10 | 91.73 ± 0.67a | 0.88 ± 0.01c | 0.08 ± 0.00c | 38.48 ± 4.22c | 34.65 ± 8.61c |
20 | 86.50 ± 3.70a | 0.92 ± 0.03c | 0.07 ± 0.00c | 40.13 ± 3.47c | 33.22 ± 3.21c |
30 | 116.68 ± 3.54c | 0.88 ± 0.01c | 0.06 ± 0.00b | 23.71 ± 3.13b | 21.81 ± 1.97b |
40 | 130.30 ± 14.36cd | 0.80 ± 0.02ab | 0.06 ± 0.00b | 21.60 ± 1.04b | 17.77 ± 1.56a |
50 | 151.87 ± 11.05d | 0.78 ± 0.01a | 0.05 ± 0.00a | 14.42 ± 3.01a | 16.28 ± 1.45a |
Means with different superscript letters within the same column are significantly different (P < 0.05) |
Resilience refers to the ability of dough to recover from deformation, which characterized the toughness of dough [43]. When the amount of rice flour is 20–50%, the resilience decreased significantly from 0.07 to 0.05. This result was consistent with the determination results of weakening degree and C2. The weakening degree is obviously increased, the dough toughness is reduced, the gluten strength is weakened, and the processability is deteriorated.
3.7 Tensile properties of mixed rice-wheat dough
Effect of different amounts of rice flour on the tensile properties of dough were shown in Table 4. Tensile resistance can used to evaluate the strength of dough. The greater tensile resistance of dough reflects better elasticity, the stronger the tensile force, and the greater air holding capacity [44]. When the rice flour was added to 20%, the tensile resistance of the dough did not change significantly; when the amount increased from 20–50%, the tensile resistance was significantly reduced from 40.13 ± 3.47 gf to 14.42 ± 3.01 gf, indicating that excessive addition of rice flour would weaken the dough strength and deteriorate its processing characteristics (Table 4). These results supported the determination results of weakening degree. Weakening degree is obviously increased, the dough toughness is reduced, the gluten strength is weakened, and then the process ability is deteriorated. These were also consistent with the resilience results that adding too much rice flour will reduce the toughness of dough and make it difficult to process. .
Tensile distance can evaluate the plasticity of dough. The longer the tensile distance, the better the formation state of the gluten network for dough, and the less likely it is to break [45]. The tensile distance of the dough decreased significantly from 49.54 ± 0.01 mm to 17.77 ± 1.56 mm as the addition of rice flour from 0 to 50%, and the decrease was more obvious when the addition was more than 20% (Table 4). These indicated that the formation of the gluten network of the dough became worse and easier to break with the addition of rice flour. The results were consistent with the microstructural characteristics results that adding a large amount of rice flour will reduce the gluten content of mixed flour dough, destroy the organizational structure of dough, and make it unable to form a continuous three-dimensional gluten network structure.
3.8 Water distribution of mixed rice-wheat dough
The combination of moisture and other ingredients in flour has an important influence on the rheological properties of dough.[46]. The relaxation time (T2) of dough represents the fluidity and binding degree of water. A shorter T2 relaxation time indicates a lower degree of water freedom [47]. Figure 4 showed the nuclear magnetic resonance (NMR) curves of mixed flour dough. The curves showed four CPMG proton populations: T21 (0.12–0.87 ms), T22 (0.87-4 ms), T23 (4–37 ms), and T24 (37–115 ms), which represented tightly bound water, loosely bound water, adsorbed water, and free water of the moisture in the dough, respectively [47]. T23 is the main peak, and its signal amplitude accounts for the largest proportion, which indicates that water mainly exists in the form of adsorbed water in the dough. Li et al. explored the characteristics of remixed fermented dough and its influence on the quality of steamed bread, and found that water mainly exists in the dough in the form of adsorbed water.[48].
The change of T2 relaxation time and the relative peak area after adding rice flour was shown in Table 5. T21, T23 and T24 did not change significantly after adding rice flour from 0 to 50%. T22 increased from 3.01 ± 0.01 ms to 3.51 ± 0.02 ms and changed significantly when the amount was more than 30%, indicating that the addition of rice flour made some bound water in the dough migrate to free water. It can be seen that with the addition of rice flour (0 to 50%), A21 was significantly reduced from 8.32%±0.07–1.42%±0.28%, A22 was significantly reduced from 28.02%±0.18–19.77%±0.18%, A23 was significantly increased from 63.17%±0.09–73.93%±0.14%, and A24 was significantly increased from 0.49%±0.35–4.88%±0.60%, which indicated that rice flour has a significant impact on the water distribution in the dough. Rice flour has the strong water binding capacity and compete water absorption process with wheat flour, leading the water that can be bound by wheat flour was reduced, and the water mobility was increased. Part of the bound water migrated to free water, resulting in the decrease of the bound water content and the increase of the adsorbed water content.
Table 5
Effect of different amounts of rice flour on the T2 relaxation time and relative peak area of each component peak in dough
Addition of rice flour/% | T2 relaxation time /min | Relative peak area/% |
T21 | T22 | T23 | T24 | A21 | A22 | A23 | A24 |
0 | 0.66 ± 0.01a | 3.01 ± 0.01a | 16.30 ± 0.14a | 100.00 ± 2.02a | 8.32 ± 0.07e | 28.02 ± 0.18e | 63.17 ± 0.09a | 0.49 ± 0.35a |
10 | 0.68 ± 0.01ab | 3.01 ± 0.02a | 16.30 ± 0.23a | 100.00 ± 1.52a | 8.25 ± 1.29e | 27.07 ± 0.37d | 63.27 ± 0.12a | 1.41 ± 0.12b |
20 | 0.70 ± 0.00b | 3.05 ± 0.01a | 16.30 ± 0.15a | 100.00 ± 1.30a | 6.36 ± 0.26d | 25.73 ± 0.39c | 66.34 ± 0.42b | 1.57 ± 0.02b |
30 | 0.71 ± 0.00b | 3.05 ± 0.04a | 16.30 ± 0.32a | 100.00 ± 1.22a | 4.76 ± 0.47c | 23.48 ± 0.71b | 70.26 ± 0.04c | 1.50 ± 0.05b |
40 | 0.71 ± 0.03b | 3.51 ± 0.05b | 16.30 ± 0.14a | 100.00 ± 1.34a | 3.04 ± 0.04b | 20.22 ± 0.05a | 74.48 ± 0.08d | 2.26 ± 0.41b |
50 | 0.73 ± 0.03b | 3.51 ± 0.02b | 16.30 ± 0.22a | 100.00 ± 1.23a | 1.42 ± 0.28a | 19.77 ± 0.18a | 73.93 ± 0.14d | 4.88 ± 0.60c |
Means with different superscript letters within the same column are significantly different (P < 0.05) |
The experimental results showed that rice flour competed with wheat gluten to absorb water, making the continuity and uniformity of gluten network structure worse, interfering with the formation of gluten network structure and reducing the stability of dough [22]. The results were consistent with the micro-structural characteristics results. Similarly, Huang et al. found that adding 9% fenugreek fiber to wheat flour can destroy the gluten network structure of the dough, reduce the bound water content and increase the free water content [49].