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As illustrated in Table 1, the hardness, adhesiveness, cohesiveness, chewiness and resilience of The SPI added group exhibited a lower value in comparison to 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 & Xi[15] 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 SPI content was 4%, FER had the least hardness, cohesiveness, and chewiness (169.53, 89.82 and 60.25 g, respectively).
Table 1. Effect of soybean protein isolate on the textural properties of FER.
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 The values represent the average value plus or minus the standard deviation (SD). The presence of distinct letters within the identical column signifies a significant disparity at a significance level of p < 0.05. Microstructure analysis of FER
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The extrusion process caused starch to become gelatinous, proteins to become desiccated, and complexes to form between starch and lipids[16]. Despite this, some of the initial components of the extrusion process remained intact. When subjected to intense shear and minimal moisture extrusion, these primary structures have a tendency to fracture and create tiny pieces that can influence the microstructure of the FER and eventually spread out during the cooking process. The electron microscopic observation micrograph of the flour particles is shown in Fig. 1, the incorporation of SPI enhanced the internal structure of FER, resulting in a more compact and smoother product due to the Maillard reaction combining protein and polysaccharide, which improved the solubility, emulsification, and gel characteristics of FER. The starch grains of FER without the addition of SPI (Fig. 1a) were fragmented and angular. the microstructure of FER starch grains added with SPI (Fig. 1b−d) was agglomerated, which may be related to the emulsification and crosslinking of SPI. Through the molecular docking of SPI and starch (Fig. 2b−d), we verified this point, and found that the binding force between SPI and starch was very strong (the maximum binding energy was −3.16), which suggests that SPI has an impact on the thermal characteristics and rheological characteristics of the FER.
Figure 1.
Electron microscopic observation: (a) represents CK, (b) represents SPI 2%, (c) represents SPI 3%, and (d) represents SPI 4%.
Thermal properties of FER
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As shown in Fig. 3, the TG curves have a similar trend: the main weight loss occurred in three phases in consecutive reactions (25~250 °C, 250~350 °C and 350~600 °C in Fig. 3a). Simultaneously, the characteristic decomposition temperature (250~350 °C) of FER was shown in Fig. 3b. At the outset, the majority of the weight loss was attributed to water loss, with a 15% reduction in weight. The breakdown of the C-C-H, C-O, and C-C bonds during the second phase of weight loss was mainly attributed to the decomposition of cellulose, lignin, and starch, resulting in a weight loss ratio of around 45%. The weight loss in the third stage was mainly caused by the carbonization of materials, and the weight loss ratio was about 15%.
Figure 2.
Thermal properties and rheological properties of soybean protein isolate on FER: (a) TG curve; (b) DTG curve; (c) Storage modulus (G') curve; (d) Loss modulus (G") curve.
Figure 3.
(a) Food quality analysis radar chart. (b) The schematic diagram of the surface static electricity in the docking of starch and soy protein isolate molecules. (c) Diagram of the binding energy in all docking times, where the binding energy is less than 0, demonstrated that docking can be achieved, and the higher the value, the more powerful the binding. (d) Indicated that they were combined by hydrogen bond and hydrophobic force, and the bond position and length were shown in the figure.
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 decrease in Tm leads to a decrease in the thermal stability of the sample. The greater the Rm and TML, the poorer the thermal stability of the raw material[17]. The SPI experimental group exhibited higher Tm, Rm, and TML values compared to the CK group, suggesting that SPI had the ability to elevate Tm levels to 270.79 °C, yet failed to decrease Rm and TML. The Tm exhibited an initial increase followed by a subsequent decrease as the SPI rose. 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 soybean protein isolate 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
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The storage modulus (G') was a measure of the energy held back during each cycle of dynamic oscillation and could be indicative of the elasticity of the FER[18]. The FER's viscous properties can be inferred from the loss modulus (G). The FER gels formation temperature was observed to be between 55 °C and 60 °C, as demonstrated in Fig. 3c & d, with G' and G" increasing as the temperature rose to 55 °C and 60 °C. Nevertheless, the experimental groups (SPI 2%, SPI 3%, SPI 4%) displayed significant variation ranging from 70 to 75 °C, as compared with CK (55~60 °C). The presence of hysteresis in the SPI-induced gel's gel point demonstrated its thermal stability and its ability to amplify the heat-sensitive active components, which was in agreement with the experimental findings of Zhang et al.[19]. The G' and G" declined at the temperature of 75~80 °C. Podlena et al.[20] 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
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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. Although there was no significant difference in taste and appearance (p > 0.05), the score of FER was high (4.63, 4.89, respectively). The artificial sensory test could be prone to mistakes due to its reliance on the assessment of a variety of sensory traits, such as age, taste sensitivity, taste preference, and other elements[21]. The rice taste analyser was used to determine the quality of rice consumption.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. 2a, 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 FER 4.46 ± 0.27a 4.03 ± 0.19b 4.48 ± 0.25a 4.85 ± 0.08a 86.50 ± 1.08b Paddy rice 4.63 ± 0.18a 4.65 ± 0.33a 3.67 ± 0.42b 4.89 ± 0.05a 92.00 ± 1.31a The values represent the average value plus or minus the standard deviation (standard deviation). The presence of distinct letters within the identical column signifies a significant disparity at a significance level of p < 0.05. -
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
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About this article
Cite this article
Li L, Li D, Li X. 2024. Characterisation of fresh extruded rice with added soybean protein isolate. Food Materials Research 4: e009 doi: 10.48130/fmr-0023-0044
Characterisation of fresh extruded rice with added soybean protein isolate
- Received: 02 November 2023
- Accepted: 27 December 2023
- Published online: 01 March 2024
Abstract: Incorporating proteins into gluten-free foods can often improve their nutritional value. Plant-based proteins are often used as a good source of protein due to their easy absorption in the body and low environmental impact. The utilization of Soy Protein Isolate (SPI) in an extruded food product aimed to examine the impact of SPI on the physicochemical characteristics of Fresh Extruded Rice (FER) in this study. We used rheological techniques and thermal analysis to determine the suitability of the extrusion process and the loss of heating mass. The microstructure, textural properties, sensory evaluation and rice taste analyser scores of FER were determined. A new gluten-free food product was produced and its quality was improved by the addition of SPI. When the content of SPI was 3%, the microstructure and texture properties showed that the FER had medium hardness, good elasticity and cohesion, which was better than paddy rice in food quality analysis. In the extrusion process, SPI has the potential to enhance not only the rheological, thermogravimetric, microstructure, and texture properties of FER, but also serve as a dietary supplement to elevate the sensory experience of FER.
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Key words:
- Gluten-free /
- Soy protein isolate /
- Extruded food /
- Artificial rice /
- Quality