Textural Properties of FER
As shown in Table 1, the hardness, adhesiveness, cohesiveness, chewiness and resilience of the SPI added group were lower than the CK group. The decrease of hardness and chewiness was due to the emulsifying and water-holding properties of SPI, and most of them were polar groups. According to the principles of similarity and compatibility, water was a polar molecule that was attracted to the polar SPI and attached to the SPI surface, so the hydrodynamic force of the water was reduced, which provided sufficient conditions for starch swelling. The variability of the resilience of FER was not significant (P > 0.05), and the elasticity was gradually increased. SPI is combined with starch and lipid to form insoluble complexes with gel properties, enhancing the plasticity of FER. Chen and Xi [17] found that polyphenols in coarse cereals could change the structure and properties of protein through covalent and noncovalent interactions, and thus recombine protein to improve the texture of products. Adhesion is a measure of force holding dissimilar particles/surfaces together, the increase of adhesiveness property was closely related to the rheological property. Cohesion decreased gradually, and the increase of moisture leads to cohesive failure. When the content of SPI was 4%, the hardness, cohesiveness and chewiness of FER were the lowest (169.53 g, 89.82 g and 60.25 g, respectively).
Table 1 Effect of SPI on the textural properties of FER
Samples
|
Hardness/g
|
Elasticity/mm
|
Adhesiveness/gs
|
Cohesiveness/g
|
Chewiness/g
|
Resilience/gs
|
CK
|
600.53±27.47a
|
0.48±0.08c
|
0.47±0.01c
|
296.60±9.63a
|
155.56±5.39a
|
0.36±0.19a
|
2%
|
236.35±10.06b
|
0.58±0.02bc
|
0.57±0.01a
|
140.92±2.63b
|
91.23±0.32b
|
0.26±0.01a
|
3%
|
241.25±4.28b
|
0.54±0.01bc
|
0.52±0.01b
|
120.67±4.25c
|
66.39±1.14c
|
0.21±0.01a
|
4%
|
169.53±3.95c
|
0.66±0.01a
|
0.52±0.01b
|
89.82±2.39d
|
60.25±1.16c
|
0.18±0.01a
|
Values are the mean ± standard deviation (SD). Different letters within the same column indicate significantly different at P < 0.05.
Microstructure analysis of FER
The electron microscopic observation micrograph of the flour particles was shown in Fig.1, the microstructure of FER starch grains added with SPI (Fig.1B, 1C, 1D) was agglomerated, which may be related to the emulsification and crosslinking of SPI. The addition of SPI improved the internal structure of FER, making the product more compact and smoother because the spontaneous Maillard reaction coupled protein-polysaccharide to enhance the solubility, emulsification and gel properties of FER [18]. The starch grains of FER without the addition of SPI (Fig.1A) were fragmented and angular. Extrusion was a process that leads to starch gelatinization, protein denaturation and the formation of starch-lipid, and protein-lipid complexes [19]. However, part of the original structure of the extrusion process was not damaged. Under high shear and low moisture extrusion conditions, these primary structures tend to split and form small fragments that affect the microstructure of the FER and eventually disperse during cooking.
Thermal properties of FER
Thermogravimetric analysis (TGA) is a technique used to determine the physical decomposition and chemical kinetics [20]. As shown in Fig.2, the TG curves have a similar trend: the main weight loss occurred in three phases in consecutive reactions (25 °C ~ 250 °C, 250 °C ~ 350 °C and 350 °C ~ 600 °C in Fig.2A). Simultaneously, the characteristic decomposition temperature (250 °C ~ 350 °C) of FER was shown in Fig.2B. The weight loss in the first stage was mainly caused by water loss, and the weight loss ratio was about 15%. The second stage of weight loss was mainly C-C-H, C-O, and C-C bond fracture, which was caused by biodegradation of cellulose, lignin and starch, and the weight loss ratio was about 45%. The weight loss in the third stage was mainly caused by the carbonization of materials, and the weight loss ratio was about 15%.
Maximum mass loss rate temperature (Tm), maximum mass loss rate (Rm) and total weight loss (TML) were commonly used parameters in the thermogravimetric analysis. As shown in Table 2, the decomposition rate of FER was the maximum at about 270 °C. A lower Tm results in lower thermal stability of the sample. The higher the Rm and TML, the lower the thermal stability of the raw material [21]. The Tm, Rm and TML in the SPI experimental group were larger than the CK group, which indicated that SPI could increase Tm up to 270.79 °C, but could not reduce Rm and TML. With increasing SPI, the Tm increased first and then decreased. Rm and TML showed an increasing trend, and the mass loss rate increased from 0.7275%/°C to 0.7648%/°C. This was due to the emulsification and dissolution of SPI, and the molecular migration velocity of the bio-based components dissolved in FER was completely accelerated at the high temperature (above 250 °C), which finally led to the increase in mass loss rate.
Table 2 Effect of SPI on the thermogravimetric properties of FER
Samples
|
Tm (°C)
|
Rm (%/°C)
|
TML (%)
|
CK
|
262.54±0.80b
|
0.7252±0.0073b
|
67.70±0.16d
|
2%
|
269.74±1.04a
|
0.7625±0.0022a
|
71.30±0.27c
|
3%
|
270.79±0.66a
|
0.7643±0.0009a
|
73.48±0.42a
|
4%
|
269.25±0.16a
|
0.7648±0.0024a
|
74.60±0.09a
|
Values are the mean ± standard deviation (SD). Different letters within the same column indicate significantly different at P < 0.05.
Rheological properties of FER
Temperature affects the rheological properties of the food material and thus the extrusion of the material [22]. The storage modulus (G') indicated the energy reserved during every cycle of dynamic oscillation and could reflect the elastic properties of the FER. The loss modulus (G") could reflect the viscous properties of the FER [23]. As shown in Fig.3, G' and G" increased with the temperature to the range of 55 °C and 60 °C, which indicated the FER gels formation temperature was approximately between 55 °C and 60 °C. However, the experimental groups (SPI 2%, SPI 3%, SPI 4%) exhibited severe fluctuation between 70 °C and 75 °C, as compared with CK (55 °C ~ 60 °C). The gel point of SPI-induced gel had hysteresis, which fully illustrated that SPI gel had thermal stability and could enhance the heat-sensitive active components in the system, which was consistent with the experimental results of Zhang et al [24]. The G' and G" declined at the temperature of 75 °C ~ 80 °C. Podlena et al [25] found that the thermal analysis transition temperature of unmodified SPI was 73.8 °C. Therefore, it could be preliminarily speculated that the rheological properties of FER may be caused by the denaturation of SPI. The rheological profile of FER was flattest when SPI was added at 3%, indicating that it is more suitable for extrusion processing.
Food quality analysis of FER
From the rheological properties, thermal properties and microstructure, it was concluded that SPI with 3% was more suitable for extrusion food production and therefore a quality analysis of FER was required before entering the consumer market. The sensory evaluation and taste analyser score method was used to judge the quality of FER as shown in Table 3. There was no significant difference in taste and appearance (P > 0.05), although the score of FER was high (4.63, 4.89, respectively). The artificial sensory test method could be error-prone as it depends upon the evaluation of various sensory characteristics, including age, taste sensitivity, taste preference, and other factors [26]. Rice taste analyser was an instrument to determine the quality of rice consumption [27]. The score of the FER taste analyser was higher than paddy rice and that variability was significant (P < 0.05) in Table 3, this indicated that the consumer quality of FER was better. Due to the significant variability (P < 0.05) between FER and paddy rice flavour, E-nose was used to test odour sensitivity to exclude subjective human preference. The sensitivity of each sensor to FER was greater than paddy rice as shown in Fig. 4, indicating that FER had a higher response, which was consistent with the conclusion obtained from the sensory evaluation (Table 3, flavour). S8 and S10 were the most sensitive, that is, the odour components contain more alcohols, aldehydes, ketones and long-chain alkanes.
Table 3 Food quality analysis of FER
samples
|
taste
|
flavour
|
colour
|
appearance
|
taste analyser score
|
Paddy rice
|
4.46±0.27a
|
4.03±0.19b
|
4.48±0.25a
|
4.85±0.08a
|
86.50±1.08b
|
FER
|
4.63±0.18a
|
4.65±0.33a
|
3.67±0.42b
|
4.89±0.05a
|
92.00±1.31a
|
Values are the mean ± standard deviation (SD). Different letters within the same column indicate significantly different at P < 0.05.