Effect of modified okara dietary fiber on the physicochemical properties, intermolecular interactions, and digestion characteristics of wheat starch

Abstract

This study modified okara dietary fiber (ODF) using high-speed homogenization combined with enzymatic treatment (HEDF) and ultrasonication combined with enzymatic treatment (UEDF), and investigated the effects of modified ODF on the physicochemical properties, interaction forces, and digestibility of wheat starch (WS). The results showed that the modified ODF reduced the pasting time and viscosity of WS, while enhancing hydrogen bonding, thereby improving the gel's storage modulus (G′), freeze-thaw stability, and water immobilization capacity. Furthermore, compared with the ODF group, the 6 % UEDF group exhibited a 9.29 % reduction in relative crystallinity and a 15.41 % decrease in enthalpy value (ΔHr), along with the formation of a more compact gel structure, which increased the resistant starch content by 7.32 %. These findings highlight the crucial role of hydrogen bonding in the interaction between modified ODF and WS, underscoring the potential application of modified ODF in low-glycemic index (GI) foods and functional starch-based products.

Highlights

• •Modified ODF optimizes gelatinization, enhancing processing efficiency.
• •The addition of 6 % UEDF formed a denser structure, enhancing structural support.
• •The addition of 6 % UEDF slows starch digestion, aiding control.
• •Hydrogen bonds predominate in the interaction between ODF and WS.

1. Introduction

Wheat (Triticum aestivum L.) is one of the most important food crops globally, serving as a significant source of dietary carbohydrates, the main component of which is starch, constituting approximately 75 % of the grain's dry weight (Toutounji et al., 2019). However, wheat starch (WS) has low solubility in cold water, which limits its effectiveness in applications that require rapid dissolution or high water absorption. Furthermore, WS exhibits susceptibility to disintegration during high-temperature processing and demonstrates weak shear resistance. The occurrence of the starch ageing phenomenon, along with its faster rate of digestion, leads to a decrease in processing characteristics and nutritional quality. Consequently, there is an urgent need to develop effective methodologies to address these inherent limitations in WS.

Dietary fiber (DF), as an essential component of non-starch polysaccharides (NPS), is a functional component with important physiological activity. It primarily originates from the plant cell wall structure and plays a crucial role in regulating blood glucose metabolism, improving intestinal microbiology, and enhancing gastrointestinal motility. It also helps to reduce the risk of chronic metabolic diseases such as obesity, cardiovascular disease, and type 2 diabetes. Studies have shown that the modification effect of NPS on WS structural properties is multi-scale. It mainly forms complexes with starch molecules through hydrogen bonds, hydrophobic interactions, and van der Waals forces, thus changing the configuration of starch, enhancing the stability of starch, effectively alleviating the inherent defects of starch, such as insolubility in cold water and easy disintegration, and can also expand its development and application in the food industry regarding low glycemic index products. Therefore, NPS has long been considered a vital substance to enhance the processing, digestion, and sensory quality of starch (J. Zhuang, Zhu, Cheung, & Li, 2024). For example, He et al. (2023) found in their study on the inhibitory effect of wheat bran insoluble dietary fiber (IDF) on corn starch digestion that the digestible starch content after digesting corn starch without IDF was 61.53 %. However, after adding IDF, due to the different ratios of added corn starch to wheat bran IDF, the degree of decrease in digestible starch content varied. Accordingly, the degree of increase in resistant starch content also differed. When the ratio of corn starch to wheat bran IDF was 1:4, the digestible starch content decreased to 50.48 %, and the resistant starch content increased by 11.05 %. This indicates that IDF can significantly slow down the digestion rate of starch and reduce the digestible starch content, which is influenced by the ratio of added starch to IDF.

Soybean okara is a good source of DF. In the past, it was usually treated as waste or used for animal feed. Due to its economic value being unable to exceed processing costs, it has not been further industrially utilized. Studies have shown that the total dietary fiber (TDF) content in soybean okara is about 60 %. In comparison, the soluble dietary fiber (SDF) content is only 2–3 % (Jing & Chi, 2013), which makes the product's texture relatively rough, thereby affecting its taste and consumer acceptance. Additionally, because IDF is not readily soluble in the food matrix, it may lead to uneven dispersion, which can affect the stability and consistency of the product. Therefore, it becomes imperative to adopt physical, chemical, biological, or synergistic modification methods to improve the processing characteristics of ODF. Chemical modification has several drawbacks, including the potential for chemical residues and environmental concerns, which can negatively impact the product's taste and nutritional value. Therefore, it is not recommended. Physical and biological methods are more widely used because they are simple to operate, can effectively retain most nutrients, and improve fiber texture and dispersibility. They are ideal choices for enhancing the functionality of ODF. At present, most of the research results are limited to adding DF modified by physical-biological synergy technology to starch, which reduces the average diameter and degree of polymerization of starch particles, increases its specific surface area, significantly improves the solubility and fluidity of starch in water, promotes the effective cross-linking of starch during the gel formation process, and finally forms a stronger and more stable gel structure. However, these studies typically focus only on the comparison of apparent properties or the observation of microstructures and involve a discussion of the mechanism of intermolecular interactions to a lesser extent. It is precisely because there is currently no systematic research that deeply explores the molecular structure characterization and intermolecular interaction analysis that it is impossible to determine which intermolecular interaction plays a decisive role in forming the gel network structure.

Therefore, this study primarily investigated the effects of modifying soybean okara DF using high-speed homogenization combined with enzymatic method (HEDF) and ultrasonic combined with enzymatic method (UEDF), followed by adding different proportions of soybean okara DF (4 %, 6 %, and 8 %) on the morphological characteristics, molecular structure, rheological properties, moisture distribution, digestive properties, and intermolecular interaction forces of wheat starch gel. The research findings provide a theoretical basis for designing ideal starch-based food products, thereby enhancing their nutritional value, physical properties, and market application potential.

2. Materials and methods

2.1. Materials

Okara was obtained from Yuxiang Soybean Products Co., Ltd. (Changchun, Jilin Province, China). ODF (Okara has 10.09 % moisture content, 4.06 % ash content, 16.21 % protein content, 7.10 % fat content, 62.14 % TDF content) was obtained from the College of Food Science and Engineering (Changchun University). Wheat starch was obtained from Shanghai Yuanye Biotechnology Co., Ltd. (Shanghai, China). (87.4 % starch content, 31.1 % amylose content, 11.3 % moisture content, 0.6 % fat content, 0.3 % protein content, and 0.2 % ash content (w/w)). Other reagents were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China) and Macklin Reagent Co., Ltd. (Shanghai, China). In addition, all other chemicals and reagents used in this study were of analytical grade.

2.2. Modification and extraction of the ODF

2.2.1. Modification methods

Weigh 5 g of ODF, add deionized water at a liquid-to-material ratio of 30:1 (mL/g), and homogenize using a high-speed homogenizer at 10,000 r/min for 15 min. Simultaneously, weigh another 5 g of ODF, add distilled water at a liquid-to-material ratio of 30:1 (mL/g), and subject the mixture to 500 W ultrasonic treatment for 30 min (3 s:2 s work/rest cycle). Adjust the pH of the dietary fiber samples from both modification methods to 5.0 using 0.1 M HCl. Add a complex enzyme (cellulose: xylanase = 1:1), accounting for 6 % of the total mass, and hydrolyze at 50 °C for 2 h. Terminate the reaction by boiling the mixture in a water bath for 5 min to inactivate the enzymes.

2.2.2. Extraction methods

Adjust the pH of the sample suspension to 6.0, add α-amylase (100 U/g), and stir at 95 °C for 30 min. Adjust the pH to 7.0, add papain (100 U/g), and incubate at 55 °C for 30 min. Adjust the pH to 4.2, then add amyloglucosidase (200 U/g) and react at 60 °C for 30 min. Terminate enzymatic reactions by boiling for 5 min, followed by cooling to room temperature. Centrifuge the digest at 6000 rpm for 15 min. Collect the pellet for IDF preparation through freeze-drying. To the supernatant, add ethanol (95 %, v/v) at a 4:1 ratio (v/v). Allow precipitation at 4 °C for 12 h, then centrifuge at 4000 rpm for 10 min. Collect the precipitate, freeze-dry it, and then sieve it through a 100-mesh sieve. The samples were named unmodified okara dietary fiber (ODF), high-speed homogenization combined with the enzymatic method (HEDF), and ultrasonic combined with the enzymatic method (UEDF), respectively.

2.3. Experimental methods

2.3.1. Preparation of modified ODF-WS composites and determination of their pasting properties

3 g of wheat starch was blended with 4 %, 6 %, and 8 % (w/w, on a dry starch basis) of modified ODF. The resulting mixtures were designated as follows based on the type of ODF modification and the incorporation ratio: native wheat starch (Starch), wheat starch–unmodified okara dietary fiber complex (ODF-4 %, ODF-6 %, ODF-8 %), wheat starch–high-speed homogenization combined with enzymatic treatment okara dietary fiber complex (HEDF-4 %, HEDF-6 %, HEDF-8 %), and wheat starch–ultrasound combined with enzymatic treatment okara dietary fiber complex (UEDF-4 %, UEDF-6 %, UEDF-8 %).

A 25 mL aliquot of distilled water was added to the complex and mixed homogeneously before being transferred into an aluminum canister for a rapid viscosity analyzer (RVA). The pasting properties of each sample were analyzed using a rapid viscosity analyzer (RVATECMASTER, Perten Instruments Ltd., Sweden) according to a standardized temperature profile. Parameters, including peak viscosity (PV), trough viscosity (TV), breakdown (BD), final viscosity (FV), setback (SB), pasting temperature (PT), and pasting time, were recorded. The resulting gel formed after cooling was lyophilized, passed through a 100-mesh sieve, and stored for subsequent measurements.

2.3.2. Determination of gel microstructure

A scanning electron microscope (JSM-6510LA, Shimadzu Corporation, Japan) was used to analyze the freeze-dried gel samples. Imaging was performed at magnifications of 500× and 1000× using an acceleration voltage of 5 kV.

2.3.3. Determination of gel fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy (VERTEX 70, Bruker, Germany) was used to measure the IR spectra of each sample. The samples were mixed with dry KBr powder (1:100, w/w) and pressed into tablets. Spectra were recorded in the range of 400-4000 cm⁻¹ with 64 scans per measurement. Each sample was measured three times, and the average spectrum of the triplicate scans was used for evaluation. Data were processed using Peak Fit software.

2.3.4. Determination of hydrogen bond energy and distance in gels

The data were processed using the aforementioned Peak Fit software. The energy and distance of hydrogen bonds were calculated using eqs. (1), (2), respectively.

$$E=\frac{1}{K}\times \frac{V_0-V}{V_0}$$ (1)

$V_0$ is the standard frequency corresponding to free OH groups (3650 cm⁻¹), $V$ is the wavenumber of the OH groups, and $K$ is a constant ($\frac{1}{K}$=262.5 kJ), $\mathrm{E}$ is the hydrogen bond energy (kJ/mol).

$$V_0-V=4.43\times {10}^3\left(2.84-D\right)$$ (2)

$V_0$is the wavenumber of monomeric OH (3600 cm⁻¹),$V$ is the wavenumber of starch, and $D$is the distance of the hydrogen bond.

2.3.5. Determination of rheological properties in gels

Static and dynamic rheological properties were determined using a rheometer (MCR302, Anton Paar, Austria) with stainless steel parallel plates (25 mm diameter). The sample was transferred onto the rheometer plate, and excess material was trimmed off with a spatula. A fixed gap of 1.0 mm was set, and all measurements were conducted at 25 °C. For steady-state flow analysis, samples demonstrating stable flow behavior were subjected to shear rates ranging from 0.1 to 200 s⁻¹. Shear stress, viscosity, and shear rate were recorded simultaneously. Dynamic rheological measurements were performed within the linear viscoelastic region (1 % strain) over a frequency range of 0.1–10 Hz. The apparent viscosity, storage modulus (G'), and loss modulus (G") were quantified.

2.3.6. Determination of intermolecular forces in gels

Dissolve 0.18 g of ODF and 2.82 g of WS in 25 mL of distilled water. Separately add sodium dodecyl sulfate with concentrations of 0 M, 0.1 M, 0.2 M, 0.3 M, and 0.4 M, urea with concentrations of 0 M, 0.3 M, 0.9 M, 1.5 M, and 3 M, and sodium chloride (NaCl) with concentrations of 0 M, 0.3 M, 0.9 M, 1.5 M, and 3 M. Heat-treated the samples with RVA and cool them to 25 °C. Subsequently, measure the change of G' under the conditions of 0.1–10 Hz and 1 % strain.

2.3.7. Determination of X-ray diffraction (XRD) in gels

The samples were stored at 4 °C for 7 days and then analyzed using an X-ray diffractometer under the following conditions: an accelerating voltage of 40 kV, a current of 30 mA, a step size of 4°/min, and a 2θ range of 5°-60°. The diffraction peaks and relative crystallinity were analyzed using Jade 6 software.

2.3.8. Determination of thermal properties in gels

The starting quantity (To), peak (Tp), conclusion (Tc) temperature, and melting enthalpy (ΔHr) of each sample were measured using a Differential Scanning Calorimeter (DSC, Q2000, TA Instruments, USA). Briefly, 3 mg of the sample was weighed into an aluminum crucible and mixed with 10 μL of distilled water. The crucible was equilibrated at 4 °C overnight to ensure homogeneous moisture. The sample was then loaded into the DSC chamber and heated from room temperature to 100 °C at a constant rate of 10 °C/min. After storage at 4 °C for 7 days, the sample was reanalyzed under identical conditions (temperature range and heating rate).

2.3.9. Determination of water migration and distribution in gels

Transverse relaxation time (T₂) measurements were performed using a low-field nuclear magnetic resonance analyzer (MesoMR23, Shanghai Neumay Electronic Technology Co., Ltd., China) to assess the water distribution and migration properties of the gels. Gel samples were placed in conical vials, loaded into 15 mm diameter NMR tubes, and positioned at the centre of the instrument's detection coil. Measurements were conducted using the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence with an echo time of 0.35 ms, 12000 echoes, and 16 scans. Additionally, sample imaging was performed using the Spin Echo (SE) sequence with a Field of View (FOV) of 200 mm × 200 mm, an echo time (TE) of 50 ms, a recovery time (TR) of 500 ms, and 8 averages.

2.3.10. Determination of gel texture characteristics

A texture analyzer (CTX, Brookfield, USA) was used to assess texture properties, including hardness, springiness, cohesiveness, and chewiness. Composite gel samples were cut into cubes (1.8 cm × 1.8 cm × 1.5 cm) for measurement. A P/36R probe was used to evaluate the texture characteristics under the following instrument settings: a test speed of 1 mm/s, a 50 % compression level, a 5 s dwell time, and a 5 g trigger force. Each measurement was performed in triplicate.

2.3.11. Determination of freeze-thaw stability of gels

The freeze-thaw stability of the gels was evaluated by subjecting the samples to five cycles of freezing at −20 °C for 22 h, thawing at room temperature for 2 h, and centrifuging at 1000 r/min for 20 min to remove released water. The syneresis rate after each cycle was measured to assess the stability.

2.3.12. Determination of in vitro digestibility of gels

A 300 mg sample was mixed with 10 mL of acetate buffer (0.2 M, pH 5.2) and three glass beads to simulate gastrointestinal peristalsis. The mixture was shaken and incubated in a water bath at 37 °C for 10 min. Subsequently, 2.5 mL of an enzyme solution containing 60 U/mL porcine pancreatic α-amylase and 290 U/mL glucosidase was added, followed by digestion with continuous shaking in a thermostatic water bath at 37 °C. At 0, 20, 40, 60, 90, and 120 min, 0.5 mL aliquots of the reaction mixture were collected and mixed with 20 mL of 66 % ethanol to terminate enzyme activity. The reaction solution was then centrifuged at 7300 r/min for 5 min, and 100 μL of the supernatant was added to 3 mL of glucose assay reagent. After reacting at 50 °C for 20 min, the absorbance of each sample was recorded at 510 nm using a UV–visible spectrophotometer. The contents of rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) were calculated according to Eqs. (3), (4), (5), respectively.

$$\mathit{RDS}\left(\%\right)=\frac{\left(G_{20}-G_0\right)\times 0.9}{\mathit{TS}}\times 100\%$$ (3)

$$\mathit{SDS}\left(\%\right)=\frac{\left(G_{120}-G_{20}\right)\times 0.9}{\mathit{TS}}\times 100\%$$ (4)

$$\mathit{RS}\left(\%\right)=100-\mathit{RDS}\left(\%\right)-\mathit{SDS}\left(\%\right)$$ (5)

G₀, G₂₀, and G₁₂₀ (mg) in the formula represent the glucose content released after 0 min, 20 min, and 120 min, respectively, and TS (mg) represents the total starch content.

2.3.13. Statistical data analysis

All experiments were conducted in three independent replications, and the results are expressed as the mean ± standard deviation. Statistical analysis of the experimental data was performed using IBM SPSS Statistics 26 software (IBM Corporation, USA), and graphs were plotted using Origin 2024 (Origin Corporation, USA). Different letters labelled in the same column of data in the table indicate significant differences between group means at the p < 0.05 level.

3. Results and discussion

3.1. Analysis of pasting properties

The RVA was employed to evaluate the pasting parameters of ODF and WS composite gels. The pasting properties of all samples are presented in Fig. 1 and Table 1. Modification of ODF by different methods significantly altered the pasting properties of WS by inhibiting starch granule swelling, enhancing shear stability, and reducing retrogradation. The results indicated that the pasting time and temperature for the 6 % ODF group were 6.13 min and 76.15 °C, respectively. In contrast, these values decreased to 5.69 min and 5.53 min, and 71.11 °C and 70.74 °C for the 6 % HEDF and UEDF groups, respectively. The HEDF and UEDF groups primarily employed a combined physical–biological technology to generate small-molecule ODF and fragmented particles more efficiently. This process significantly reduced the ordered structure between starch molecules, enabling WS to absorb water, swell, and initiate pasting at lower temperatures. As a result, the pasting time for the ODF and WS composite gels was shortened, and less thermal energy was required (Rong et al., 2022).

Fig. 1.

Pasting properties curve of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios. ODF (A), HEDF (B), UEDF (C).

Table 1.

Pasting properties of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios.

Samples	Peak Viscosity (cP)	Trough Viscosity (cP)	Break Down (cP)	Final Viscosity (cP)	Setback (cP)	Pasting Time (min)	Pasting Temp (°C)
Starch	2822 ± 45a	2049 ± 31a	773 ± 12c	3279 ± 34a	1230 ± 9a	6.26 ± 0.03a	83.14 ± 0.12a
ODF-4 %	2725 ± 33b	1904 ± 43b	821 ± 16a	3087 ± 48b	1183 ± 6b	6.21 ± 0.06a	78.27 ± 0.08b
ODF-6 %	2341 ± 41c	1588 ± 25d	753 ± 13d	2625 ± 23c	1037 ± 2c	6.13 ± 0.01b	76.15 ± 0.11c
ODF-8 %	2237 ± 59d	1491 ± 17e	746 ± 16d	2525 ± 26d	1034 ± 1c	6.06 ± 0.02c	75.79 ± 0.03d
HEDF-4 %	2371 ± 61c	1653 ± 36c	714 ± 14e	2657 ± 19c	1004 ± 5d	5.86 ± 0.04c	75.34 ± 0.02d
HEDF-6 %	2046 ± 70e	1263 ± 45f	692 ± 21f	2189 ± 13e	926 ± 4e	5.69 ± 0.07d	71.11 ± 0.03f
HEDF-8 %	1688 ± 52f	1023 ± 54f	665 ± 23g	1759 ± 16f	736 ± 2g	5.54 ± 0.08e	67.05 ± 0.09g
UEDF-4 %	2295 ± 49d	1540 ± 30d	755 ± 14d	2599 ± 20c	1059 ± 4c	5.71 ± 0.04d	74.45 ± 0.05e
UEDF-6 %	1977 ± 35h	1306 ± 23h	783 ± 18g	2179 ± 35f	873 ± 2f	5.53 ± 0.02e	70.74 ± 0.03h
UEDF-8 %	1672 ± 47f	890 ± 33g	782 ± 11b	1522 ± 13g	632 ± 1.2h	5.46 ± 0.01f	66.30 ± 0.02g

Results are expressed as mean ± standard deviation (n = 3), and different letters in the same column indicate significant differences (p < 0.05).

PV reflects the water-holding capacity of WS granules before breakdown, while BD viscosity indicates the extent of granule disintegration, stability, and integrity after swelling and pasting. The FV generally represents the gel-forming ability of WS after cooling and is commonly used for assessing starch quality. As shown in Table 1, compared to the 6 % ODF group, the PV values of the 6 % HEDF and UEDF groups decreased by 295 cP and 364 cP, respectively, while their SB values decreased by 111 cP and 164 cP, respectively. The decrease in PV indicates that the incorporation of modified ODF directly increased the solid content in WS. At the same time, the ODF coated the starch granules, thereby preventing water absorption and swelling—a finding consistent with the results reported by Luo, Niu, Li, and Xiao (2020). Regarding the mechanism underlying the differential reduction in SB, Gan et al. (2023) reported that quinoa and highland barley dietary fibers could effectively suppress the retrogradation of rice starch, leading to a significant decrease in SB. This inhibitory effect was attributed to variations in monosaccharide composition and polysaccharide structure, which further influenced intermolecular interactions among starch chains, thereby altering retrogradation behavior at a microscopic level. Moreover, the FV of the 6 % ODF group was 2625 cP, whereas the 6 % HEDF and UEDF groups exhibited reductions of 436 cP and 446 cP, respectively. In the ODF group, the addition of DF at increasing proportions resulted in a reduction of the BD viscosity from 821 cP to 746 cP. Compared to the ODF group, the BD of the 6 % UEDF group increased by 30 cP, while that of the 6 % HEDF group decreased by 61 cP, indicating that UEDF significantly enhanced the stability of WS.

3.2. Microstructure analysis

The SEM results reveal the structural changes within the composite gels of ODF and WS under different modification methods and addition ratios. As shown in Fig. 2A, the native WS samples exhibited a continuous and uniform porous structure, indicating molecular-level pore formation during starch gelatinization, which resulted in a typical gel network. As shown in Fig. 2B, with the addition of ODF, the pores within the gel network gradually increased in size, accompanied by the emergence of a small number of non-uniform cellular and lamellar structures, along with a noticeable aggregation of fibers. Furthermore, a comparison between Fig. 2C and Fig. 2D indicated that although the gel structure of the HEDF group exhibited increased porosity, the UEDF group demonstrated the most prominent pores, characterised by a discontinuous and more sparsely distributed network. This observation suggested that while HEDF could moderately enhance the interfacial binding between ODF and WS, the effect was limited in magnitude. In contrast, the incorporation of UEDF more effectively improved the compatibility between ODF and WS, thereby strengthening the gel network structure. This was consistent with the research results of Maleki, Aarabi, Far, and Dizaji (2024).

Fig. 2.

SEM microstructure of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios. Starch (A), ODF (B), HEDF (C), UEDF (D).

Based on the structural changes observed in ODF and WS composite gels under different modification methods and addition ratios, as shown in Fig. 2, we inferred that the phenomena above primarily originated from interactions between DF and amylose leached during gelatinization, leading to significant alterations in the gel network structure. Ye et al. (2018) also found in related studies that the addition of inulin DF increased the micropore size of rice starch, making the structure more transparent. Additionally, the introduction of DF promoted the formation of a gel-like structure. However, when excessive DF was added, it inhibited this promoting effect. The potential reason for the analysis was that a competitive relationship existed between DF and WS during the water uptake process, thereby interfering with the formation of a gel network structure (X. Zhuang et al., 2020).

3.3. FT-IR analysis

Fig. 3 presents the FT-IR spectra of composite gels formed by ODF and WS with different modification methods and addition ratios. No new functional groups were observed in the spectra of any samples. A broad and intense absorption peak in the 3550–3200 cm⁻¹ region indicates O—H stretching vibrations associated with various types of intra- and intermolecular hydrogen bonds in the formed complexes, suggesting the presence of hydroxyl groups in both DF and WS. The absorption bands observed at 2800–3000 cm⁻¹ correspond to C—H stretching vibrations, demonstrating that hydrogen bonding is the primary interaction between DF and WS, followed by electrostatic and hydrophobic interactions. (J. Li, Liu, Wu, & Tan, 2024). In the region of 800–1300 cm⁻¹, the starch samples exhibited C—C and C—O stretching vibrations. The hydrogen bonds in the starch chains contributed to the formation of an absorption peak at 995 cm⁻¹, and the absorption peaks at 1022 and 1047 cm⁻¹ were related to the amorphous and crystalline regions, respectively.

Fig. 3.

FTIR spectra of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios. (A), Second Derivative Spectroscopy of FTIR in the 800–1200 cm⁻¹ Range (B), Second Derivative Spectroscopy of FTIR in the 3240–3360 cm⁻¹ Range (C), Second Derivative Spectroscopy of FTIR in the 3400–3600 cm⁻¹ Range (D).

Moreover, by calculating the ratios of 1047/1022 and 995/1022, the short-range order degree (DO) and double helix degree (DD) of starch molecules can be obtained (W. Ma et al., 2025). As shown in Table 2, the higher the DO value, the better the short-range order of starch molecules. Compared with the ODF group, the DO ratios of the 6 % HEDF and 6 % UEDF treatment groups decreased by 0.18 and 0.08, respectively, indicating that the introduction of modified ODF caused the ordered structure of starch molecules to be disordered. With the increase in the amount of DF added, this interference effect became more significant (N. Wang, Huang, Zhang, Zhang, & Zheng, 2020). The DD value can reflect the change in the double helix structure of starch granules. The higher the DD value, the stronger the double helix nature in starch molecules. Compared with the ODF group, the DD ratios of the 6 % HEDF and 6 % UEDF groups decreased by 0.21 and 0.16, respectively, indicating a reduction in the number of hydrogen bonds in the starch system, which suggests that ODF inhibited the formation of a tighter double helix structure. Wen et al. (2024) also found that adding Lentinus edodes dietary fiber hindered the recrystallization of amylose in extruded starch. This hindrance can be attributed to spatial effects, which impede the formation of a hydrogen bond network. These results suggest that DF may interact with starch molecules through hydrogen bonding, thereby altering their structure and properties. To quantitatively analyze the strength of hydrogen bonds, the corresponding hydrogen bond energy can be calculated using V₀ (the standard frequency of free hydroxyl groups, 3650 cm⁻¹), V (the wavenumber of the OH group), and the constant K (1/K = 2.625 × 10² kJ).

Table 2.

Short-range ordered structure and crystal structure of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios.

Samples	DO (1047/1022)	DD (995/1022)	RC (%)
Starch	0.732 ± 0.002a	1.452 ± 0.004a	28.55 ± 0.21a
ODF-4 %	0.297 ± 0.004d	1.312 ± 0.002c	27.29 ± 0.17b
ODF-6 %	0.392 ± 0.006c	1.296 ± 0.004d	26.78 ± 0.19c
ODF-8 %	0.501 ± 0.003b	1.057 ± 0.003h	25.63 ± 0.16d
HEDF-4 %	0.172 ± 0.004g	1.177 ± 0.005e	25.57 ± 0.12d
HEDF-6 %	0.208 ± 0.006f	1.084 ± 0.008g	24.78 ± 0.11e
HEDF-8 %	0.381 ± 0.001c	1.338 ± 0.002b	24.24 ± 0.10f
UEDF-4 %	0.266 ± 0.004e	1.142 ± 0.007f	25.35 ± 0.14d
UEDF-6 %	0.379 ± 0.003e	1.131 ± 0.002f	24.29 ± 0.17f
UEDF-8 %	0.313 ± 0.002d	1.065 ± 0.009h	23.37 ± 0.16g

DO, degree of order; DD, degree of double helix. Results are expressed as mean ± standard deviation (n = 3), and different letters in the same column indicate significant differences (p < 0.05).

3.4. Analysis of hydrogen bond energy and distance

As shown in Fig. 3C and Fig. 3D, the broad absorption peak in the range of 3600–3000 cm⁻¹ is related to the stretching vibration of the -OH group. By deconvoluting the OH band (3600–3000 cm⁻¹) in the FTIR spectrum, the overlapping absorption peaks were separated, and the hydrogen bond energy (E) and hydrogen bond distance (D) were further calculated. According to Hu et al. (2024), the hydrogen bonds between double helices and chains are located at approximately 3290 cm⁻¹ and 3511 cm⁻¹, respectively, whereas free hydrogen bonds typically appear above 3500 cm⁻¹. Notably, only the hydrogen bonds between double helices and chains participate in intermolecular interactions, whereas free hydrogen bonds do not (Garcia-Valle, Bello-Pérez, Agama-Acevedo, & Alvarez-Ramirez, 2021). As shown in Table 3, the hydrogen bonds between molecular chains and those between double helices in the 6 % ODF group appeared at 3522 cm⁻¹ and 3291 cm⁻¹, respectively. In the 6 % HEDF group, the corresponding absorption peaks were observed at 3516 cm⁻¹ and 3508 cm⁻¹, while in the 6 % UEDF group, these peaks were located at 3289 cm⁻¹ and 3287 cm⁻¹. These results suggest that modified ODF may alter the interaction patterns between molecular chains, leading to redistribution or enhancement of hydrogen bonds.

Table 3.

Hydrogen bond energies (E) and distances (D) of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios.

Samples	Free hydrogen bonds			Inter-strand hydrogen bond			Inter-double helices hydrogen bond
	Band position (cm−1)	E (kJ/mol)	D (Å)	Band position (cm−1)	E (kJ/mol)	D (Å)	Band position (cm−1)	E (kJ/mol)	D (Å)
Starch	3543 ± 0.23a	7.695 ± 0.02a	2.827 ± 0.01a	3525 ± 0.19a	8.990 ± 0.03a	2.823 ± 0.05a	3295 ± 0.36a	25.531 ± 0.04a	2.771 ± 0.03a
ODF-4 %	3542 ± 0.17b	7.767 ± 0.03b	2.827 ± 0.03c	3524 ± 0.24e	9.062 ± 0.05c	2.823 ± 0.02a	3294 ± 0.83d	25.603 ± 0.02d	2.771 ± 0.05c
ODF-6 %	3541 ± 0.38c	7.839 ± 0.05b	2.827 ± 0.04e	3522 ± 0.33b	9.205 ± 0.07d	2.822 ± 0.04c	3291 ± 0.69e	25.818 ± 0.06e	2.770 ± 0.04e
ODF-8 %	3544 ± 0.43d	7.623 ± 0.06e	2.827 ± 0.02d	3519 ± 0.48d	9.421 ± 0.04e	2.822 ± 0.03e	3289 ± 0.13c	25.962 ± 0.07e	2.770 ± 0.02f
HEDF-4 %	3546 ± 0.62d	7.479 ± 0.03d	2.828 ± 0.05g	3512 ± 0.15c	9.925 ± 0.06e	2.820 ± 0.04f	3287 ± 0.27b	26.106 ± 0.06b	2.769 ± 0.01d
HEDF-6 %	3545 ± 0.57e	7.551 ± 0.04c	2.828 ± 0.06h	3516 ± 0.46f	9.637 ± 0.02f	2.821 ± 0.07g	3289 ± 0.18g	25.962 ± 0.04f	2.770 ± 0.06f
HEDF-8 %	3544 ± 0.33c	7.623 ± 0.03e	2.827 ± 0.02f	3518 ± 0.74g	9.493 ± 0.04g	2.821 ± 0.02h	3287 ± 0.42e	26.106 ± 0.05h	2.769 ± 0.03g
UEDF-4 %	3546 ± 0.42d	7.479 ± 0.02f	2.828 ± 0.04b	3507 ± 0.42d	10.284 ± 0.05f	2.819 ± 0.05b	3284 ± 0.41f	26.322 ± 0.06c	2.769 ± 0.02d
UEDF-6 %	3545 ± 0.59f	7.551 ± 0.06g	2.828 ± 0.03f	3507 ± 0.39h	10.284 ± 0.03g	2.819 ± 0.02d	3287 ± 0.48h	26.106 ± 0.04e	2.769 ± 0.03g
UEDF-8 %	3546 ± 0.56g	7.479 ± 0.04h	2.828 ± 0.05f	3525 ± 0.41e	8.990 ± 0.06h	2.823 ± 0.03f	3295 ± 0.52g	25.531 ± 0.04f	2.771 ± 0.05h

Results are expressed as mean ± standard deviation (n = 3), and different letters in the same column indicate significant differences (p < 0.05).

Furthermore, based on the hydrogen bond energy and distance data provided in Table 3, the UEDF group exhibited the highest intermolecular hydrogen bond energy of 10.284 kJ/mol, followed by the HEDF group at 9.637 kJ/mol. In comparison, the ODF group showed the lowest value of 9.205 kJ/mol. Ultrasonic treatment may significantly impact the microstructure of ODF, disrupting its internal organisation and facilitating the formation of new hydrogen bonds. In contrast, high-speed homogenization primarily focuses on mechanical disruption and homogenization at the macroscopic level, exhibiting relatively limited efficacy in modifying hydrogen bond structures at the molecular scale. Consequently, it is less effective than ultrasonic treatment in enhancing hydrogen bond energy. According to D. Wang et al. (2023), increasing the explosion puffing pressure resulted in a decrease in the E value and an increase in the D value in expanded surimi-starch samples, indicating a reduction in the quantity of hydrogen bonds and a more loosely organized structure. In summary, it is hypothesized that hydrogen bonding is the primary interaction between WS and DF.

3.5. Rheological properties analysis

The rheological properties, including viscosity, elasticity, and flow behavior, directly influence the applicability of gels in food processing and the quality of the final product. Fig. 4 illustrates the steady-state flow behavior of starch paste. The shear stress of all samples increased with rising shear rate. In contrast, the viscosity exhibited an opposite trend, decreasing with increasing shear rate, demonstrating typical shear-thinning behavior, which aligns with the findings of Rong et al. (2022).

Fig. 4.

Flow behavior (A), viscosity (B), storage modulus (G′) (C), and loss modulus (G″) (D) of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios.

Fig. 4 also presents the dynamic rheological properties of starch paste. The storage modulus (G′) represents the elastic behavior, while the loss modulus (G″) reflects the viscous characteristics. The results indicated that G′ was higher than G″, and both moduli increased with frequency, suggesting a dominant gel-like rheological behavior in all samples (H. Wang et al., 2020). Compared to the ODF group, the HEDF and UEDF groups exhibited an overall increasing trend in G′; however, as the addition level of DF increased, G′ showed a declining trend, with the most pronounced reduction observed in the UEDF group. This phenomenon may be attributed to ultrasonic cavitation, a physical modification that positively influences G′ under optimal conditions, thereby strengthening the elastic network structure of the gel. However, excessive ODF addition may increase system viscosity and reduce fluidity, adversely affecting G′ and leading to a significant decline in overall elasticity. These findings are consistent with the observations of S. ma et al. (2022), who reported that G′ and G″ initially increased but later decreased with rising dietary fiber content. The observed viscosity reduction aligns with the results of the previous RVA analysis.

3.6. Analysis of intermolecular interactions

The interactions between polysaccharides and starch typically involve hydrogen bonding, hydrophobic interactions, and electrostatic forces (Liu et al., 2022). Specifically, the hydroxyl groups in WS molecules form hydrogen bonds with cellulose and hemicellulose in ODF, collectively establishing a three-dimensional gel network. Urea competes with WS or DF for hydrogen bonds, leading to a decline in G' and a looser gel structure. As shown in Fig. 5. A-C, the G' value progressively decreases with increasing urea concentration (0 M–3 M), indicating that urea effectively disrupts the hydrogen bonds between ODF and WS. Compared to the ODF group, the HEDF and UEDF groups exhibit higher G' values, attributed to the mechanochemical treatment-induced reduction in DF particle size and increased surface hydroxyl exposure. This structural modification enhances hydrogen bonding between DF and WS molecules, thereby improving the stability of the gel. Additionally, Pan, Zhou, Liu, and Wang (2021) observed that urea exerted the most pronounced effect on gels prepared with amidated low-methoxyl pectin (AOP), suggesting that hydrogen bonding dominates the gelation process of AOP. In contrast, hydrophobic interactions and disulfide bonds play minor roles.

Fig. 5.

Changes in the storage modulus (G′) of composite gels formed by wheat starch and okara dietary fiber treated with different modification methods upon addition of urea (A–C), NaCl (D—F), or SDS (G–I).

As an electrolyte, NaCl enhances electrostatic interactions between WS and ODF, thereby influencing the formation and stability of the gel network. Its mechanism can be divided into two phases: promotion at low concentrations and inhibition at high concentrations, governed by the dynamic regulation of ionic strength on intermolecular forces. As depicted in Fig. 5. D—F, increasing NaCl concentration (0 M–3 M) elevates ionic strength, reducing the system's free energy and strengthening intermolecular interactions, thereby increasing G'. Thus, low NaCl concentrations enhance the elasticity of the gel. However, at high concentrations, G' declines due to excessive ions competitively binding hydroxyl groups, inducing electrostatic shielding effects that weaken crosslinking and reduce mechanical strength, ultimately destabilising the gel. This aligns with Huang et al. (2023), who reported that excessive NaCl impaired the strength of sweet potato starch-polysaccharide hydrogels.

Sodium dodecyl sulfate modulates hydrophobic crosslinking and competitive binding at varying concentrations, affecting gel stability. As illustrated in Fig. 5. G-I, G' initially rises and then falls with increasing sodium dodecyl sulfate concentration. As an anionic surfactant, sodium dodecyl sulfate exhibits a dual mechanism. At low concentrations, it may enhance electrostatic attraction between DF molecules, elevating G' and promoting a denser gel network with improved elasticity and mechanical strength. Beyond a critical concentration, excessive sodium dodecyl sulfate disperses DF particles, disrupting the gel network and significantly reducing G'.

In the investigation of gel formation mechanisms, analysis of intermolecular interactions revealed that both HEDF and UEDF groups significantly enhance gel properties by strengthening these forces. Hydrogen bonding was identified as the primary interaction between ODF and WS molecules, playing a decisive role in the formation of the gel network structure. Meanwhile, electrostatic and hydrophobic interactions functioned as secondary forces, cooperatively contributing to the construction of the gel architecture. These findings provide a theoretical basis for applying ODF in WS-based foods, such as those with a low glycemic index (GI).

3.7. Crystal structure analysis

XRD analysis revealed the significant influence of different modified ODF on the crystalline properties of WS. As shown in Fig. 6A, all samples exhibited a distinct diffraction peak at 2θ = 20°, confirming the V-type crystalline structure of ODF-WS composite gels, which arises from the helical inclusion complex formed between amylose and natural fatty acids through hydrophobic interactions. The formation of V-type complexes not only effectively inhibits the rearrangement of amylose (Liang et al., 2024) but may also alter the digestive properties of starch at the molecular level. Additionally, characteristic peaks observed at 2θ = 15°, 17.5°, and 24° represented typical B-type crystalline features, formed by double helices consisting of amylose and amylopectin, which serve as a key indicator of starch retrogradation. Notably, the development of B-type crystallinity involves the ordered arrangement of water molecules within the double-helical structure, a phenomenon consistent with the B-type crystalline characteristics observed in β-cyclodextrin-treated corn starch in the study by Ji et al. (2021), suggesting a common molecular rearrangement mechanism during the ageing process of different starch systems.

Fig. 6.

The X-ray diffraction patterns (A) and DSC curve (B) of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios.

The relative crystallinity (RC), reflecting the extent of starch rearrangement, is primarily associated with the double helices of amylopectin. Data in Table 2 demonstrated a dose-dependent decrease in RC with increasing ODF concentrations (4 %–8 %). The ODF group exhibited higher crystallinity and an ordered gel structure, attributed to the intact crystalline domains of cellulose and hemicellulose. In contrast, the HEDF and UEDF groups exhibited significantly reduced and broadened diffraction peaks, with RC decreasing by 9.29 % and 7.46 %, respectively. This indicates that the modification disrupted the crystalline regions of the fibers, causing them to transform into an amorphous state. The UEDF group exhibited the most pronounced effect, as the combined ultrasonic-enzymatic treatment thoroughly disrupted the crystalline regions. Ultrasonic cavitation generated high-frequency vibrations and localised high-temperature/high-pressure conditions, leading to molecular chain depolymerisation, while enzymatic treatment selectively hydrolysed the amorphous areas of the fibers. Their synergistic effect facilitated the transition to an amorphous structure, enhancing water retention and improving product texture. Moreover, the resulting physical barrier delayed starch enzyme accessibility and efficiency, thereby inhibiting starch retrogradation.

From the perspective of molecular interaction mechanisms, the reduced crystallinity of DF exposed more reactive groups, such as free hydroxyl and carboxyl groups, which influenced ODF-WS composite gels properties through three primary pathways: (1) competition between ODF and starch for water molecules, restructuring the hydrogen bond network and modifying starch plasticization behavior; (2) penetration of fiber molecules into the starch continuous phase, forming a physical barrier that restricts starch chain mobility and rearrangement; and (3) complexation between reactive groups on the fiber surface and starch molecules, interfering with starch crystallization.

3.8. Thermal properties analysis

Differential scanning calorimetry (DSC) is a powerful technique for studying the phase transition behavior of gel systems. The migration and evaporation of water, hydrogen bonding interactions between DF and starch molecules, as well as the thermal denaturation of trace protein or polysaccharide components, may collectively influence the temperatures and enthalpy value (ΔHr) of endothermic peaks. These complex multicomponent interactions further highlight the unique thermodynamic behavior of composite gel systems. As shown in Fig. 6B, the DSC thermograms of all samples in the modified ODF-WS composite gel systems exhibited a distinct endothermic peak, primarily attributed to the starch pasting process. This process involves disrupting the crystalline structure of starch granules and unwinding the molecular chains, requiring substantial heat absorption, which results in a characteristic endothermic peak (Liang et al., 2024). Furthermore, the addition of DF induced a shift in the endothermic peak, resulting in a reduction in the pasting temperature. As indicated in Table 4, compared to the ODF group, the ΔHr values decreased by 15.41 % and 7.13 % in the 6 % UEDF and 6 % HEDF groups, respectively. Moreover, the decline in these values became more pronounced with increasing DF content, particularly in the UEDF group. This phenomenon may be attributed to the competition for water between DF and starch in the UEDF group, wherein the DF encapsulates the surface of starch granules, inhibiting their swelling and subsequently forming a barrier that impedes starch hydration. This process disrupts the crystalline structure of starch, leading to a reduction in ΔHr and an extension of the gelatinization process. Additionally, studies by W. Li et al. (2024) demonstrated that macrocyclic dextrins delay the retrogradation of potato starch. Due to enhanced swelling and higher amylose leaching, macrocyclic dextrin-starch mixtures undergo dissociation, thereby altering the structure of the amorphous region. The ordered microcrystals formed during retrogradation require less energy to melt compared to those in native starch granules. Therefore, modified ODF can interact with WS, contributing to the reduction in ΔHr.

Table 4.

Thermal properties of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios.

Samples	To (°C)	Tp (°C)	Tc (°C)	ΔHr (J/g)
Starch	61.35 ± 0.03a	66.21 ± 0.06a	73.34 ± 0.07a	10.23 ± 0.09a
ODF-4 %	56.83 ± 0.02b	62.37 ± 0.04b	70.24 ± 0.06c	7.21 ± 0.07d
ODF-6 %	56.36 ± 0.04d	62.12 ± 0.02c	70.53 ± 0.03b	7.85 ± 0.03b
ODF-8 %	56.04 ± 0.03f	61.95 ± 0.01d	69.81 ± 0.04d	7.15 ± 0.04d
HEDF-4 %	56.71 ± 0.01c	62.83 ± 0.03b	70.67 ± 0.03b	8.13 ± 0.06b
HEDF-6 %	56.47 ± 0.05d	62.63 ± 0.02b	70.21 ± 0.04c	7.29 ± 0.03c
HEDF-8 %	56.05 ± 0.02f	61.67 ± 0.06d	69.76 ± 0.03d	6.78 ± 0.06e
UEDF-4 %	56.94 ± 0.05b	62.34 ± 0.03c	70.29 ± 0.05c	7.54 ± 0.05c
UEDF-6 %	56.59 ± 0.02e	62.23 ± 0.05c	70.40 ± 0.01d	6.64 ± 0.02e
UEDF-8 %	56.16 ± 0.01e	61.13 ± 0.04e	69.18 ± 0.02e	5.87 ± 0.03f

Results are expressed as mean ± standard deviation (n = 3), and different letters in the same column indicate significant differences (p < 0.05).

3.9. Analysis of water migration and distribution

As shown in Table 5, the water characteristics of the samples, including moisture content, water distribution, and migration behavior, were analyzed using low-field nuclear magnetic resonance (LF-NMR). Three distinct types of water molecules were observed in the relaxation spectra of all samples, with their relaxation times denoted as T₂₁, T₂₂, and T₂₃. The peak area ratios of each water type, represented by A₂₁, A₂₂, and A₂₃, indicated the relative content of different water states. Specifically, water with T₂₁ < 1 ms was classified as bound water, water with T₂₂ in the range of 1–30 ms as weakly bound water, and water with T₂₃ > 100 ms as free water (Zheng et al., 2020). Compared to the 6 % ODF group, the 6 % HEDF group exhibited reductions in T₂₁, T₂₂, and T₂₃ by 3.28 ms, 1.42 ms, and 34.37 ms, respectively, suggesting that HEDF enhanced the restriction of weakly bound water. In the 6 % UEDF group, T₂₁ and T₂₂ decreased significantly by 0.83 ms and 2.08 ms, respectively, while T₂₃ increased by 6.52 ms. This indicated that the addition of UEDF facilitated the conversion of free water to bound water within the gel matrix. These findings align with the conclusions of Wang et al. (2020), who attributed the increase in bound water content to an enhanced water-holding capacity. It is hypothesized that UEDF may interact with starch molecules, thereby improving water-binding capacity and contributing to the stability of the gel system.

Table 5.

The relaxation times and relative percentages of relaxation peak areas of okara dietary fiber-wheat starch composite gels under different modification methods and addition ratios.

Samples	Relaxation time (ms)			Relaxation peak area(%)
	T21	T22	T23	A21	A22	A23
Starch	7.72 ± 0.26a	–	251.23 ± 8.27a	3.46 ± 0.14c	–	96.52 ± 0.63a
ODF-4 %	7.55 ± 0.35a	32.18 ± 1.98b	254.08 ± 8.41a	2.54 ± 0.34de	0.98 ± 0.41f	96.48 ± 0.76a
ODF-6 %	6.34 ± 0.42b	23.87 ± 1.44cde	242.72 ± 8.52b	1.48 ± 0.23e	2.66 ± 0.15e	95.85 ± 0.71bc
ODF-8 %	4.67 ± 0.19d	20.74 ± 2.54f	237.43 ± 8.39b	1.52 ± 0.19e	2.77 ± 0.20e	95.32 ± 0.51bc
HEDF-4 %	5.71 ± 0.23c	21.62 ± 0.83e	224.34 ± 8.83c	4.38 ± 0.24b	7.83 ± 0.06bc	87.80 ± 0.21d
HEDF-6 %	4.53 ± 0.17d	22.36 ± 1.55de	208.35 ± 4.84d	5.75 ± 0.08a	9.72 ± 0.08a	84.54 ± 0.41e
HEDF-8 %	3.06 ± 0.14f	26.02 ± 1.65cd	195.27 ± 9.27e	5.87 ± 0.12a	8.58 ± 0.07b	85.56 ± 0.35de
UEDF-4 %	6.76 ± 0.32b	39.65 ± 1.80a	213.71 ± 5.68cd	2.37 ± 0.18de	5.98 ± 0.12d	91.64 ± 1.39c
UEDF-6 %	5.51 ± 0.38c	32.79 ± 3.51b	210.24 ± 7.57cd	2.95 ± 0.20d	6.35 ± 0.14c	90.70 ± 0.55c
UEDF-8 %	4.26 ± 0.26d	26.68 ± 1.47c	205.15 ± 4.24d	3.66 ± 0.11c	6.71 ± 0.11c	89.65 ± 0.42c

Results are expressed as mean ± standard deviation (n = 3), and different letters in the same column indicate significant differences (p < 0.05).

As illustrated in Fig. 7B, the comparison of relaxation peak areas revealed that, relative to the 6 % ODF group, the A₂₂ values of the 6 % HEDF and UEDF groups increased by 7.06 % and 3.69 %, respectively. In comparison, A₂₃ decreased by 11.31 % and 5.15 %. This suggested that HEDF and UEDF exhibited stronger water absorption capacity than ODF, leading to a reduced proportion of free water (A₂₃). Furthermore, Fig. 7C displays the moisture distribution images of ODF-WS composite gels prepared with different modification methods and addition ratios. The pseudo-color images reflect the distribution of hydrogen protons in hydrodynamic motion, where red regions indicate higher water content and blue regions represent lower water content (Zhang, Chen, Chen, & Chen, 2019). A shift from red to yellow corresponded to a decrease in water mobility. Darker red areas indicated a higher concentration of mobile hydrogen protons and more heterogeneous water distribution. Compared to the ODF group, the HEDF and UEDF groups displayed smaller red regions, indicating lower free water content and more uniform water distribution, consistent with the peak area analysis. This phenomenon may arise from interactions between modified ODF and starch molecules. Further evidence suggests that modified ODF alter water distribution and migration properties by competing with starch molecules for water (Liao et al., 2022).

Fig. 7.

Relaxation time (A), peak area (B), and water migration and distribution (C) of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios.

3.10. Texture profile analysis

Table 6 presents the TPA (Texture Profile Analysis) parameters of ODF-WS composite gels, including hardness, springiness, chewiness, and cohesiveness. The results indicate that the addition of modified ODF significantly influenced the hardness and chewiness of WS composite gels. As the ODF content increased from 4 % to 8 %, the hardness and chewiness of the ODF group increased by 140.2 and 68.18, respectively. The incorporation of ODF may enhance the hardness and chewiness of the composite gel by partially replacing starch components, promoting greater water absorption, and restricting the full expansion of starch granules. This observation aligns with the findings of Jiang et al. (2024), who reported that heat-induced starch-surimi gels exhibited increased hardness, springiness, and cohesiveness, likely due to improved water absorption and swelling capacity of starch. However, compared to the ODF group, the hardness and chewiness of the 6 % HEDF group decreased by 91.01 % and 47.28 %, respectively, while those of the 6 % UEDF group decreased by 79.01 % and 51.28 %. This reduction may be attributed to the steric hindrance effect induced by modified ODF, which impedes the formation of a cross-linked network structure in the starch composite gels, thereby weakening the hardness and chewiness of the composite gel. Su, Xu, Wang, Zhou, and Xu (2024) demonstrated that the addition of konjac glucomannan reduced the hardness and chewiness of starch gels. Due to its superior water-absorption capacity, konjac glucomannan not only restricts starch granule swelling but also hinders the formation of effective intermolecular connections among starch molecules. Therefore, selecting appropriate modification methods and optimal ODF addition levels is crucial for optimizing the processing characteristics of ODF-WS composite gels.

Table 6.

Texture parameters of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios.

Samples	Hardness	Springiness	Chewiness	Cohesiveness
Starch	67.21 ± 3.64f	0.72 ± 0.02c	32.51 ± 1.26d	0.68 ± 0.02f
ODF-4 %	157.98 ± 10.54c	0.71 ± 0.03ab	67.94 ± 3.96c	0.71 ± 0.02ab
ODF-6 %	226.24 ± 12.51b	0.70 ± 0.02ab	105.97 ± 3.64b	0.65 ± 0.03bc
ODF-8 %	298.18 ± 5.33a	0.68 ± 0.06b	136.12 ± 2.70a	0.74 ± 0.01a
HEDF-4 %	129.01 ± 5.92e	0.74 ± 0.02ab	53.67 ± 7.24d	0.65 ± 0.04bc
HEDF-6 %	135.23 ± 9.48d	0.71 ± 0.02ab	58.69 ± 4.09d	0.60 ± 0.01de
HEDF-8 %	140.13 ± 5.74c	0.70 ± 0.01b	66.97 ± 2.45c	0.57 ± 0.03e
UEDF-4 %	134.01 ± 12.94d	0.76 ± 0.06a	48.67 ± 2.72e	0.58 ± 0.05de
UEDF-6 %	147.23 ± 9.97bc	0.75 ± 0.01a	54.69 ± 3.92d	0.56 ± 0.02e
UEDF-8 %	150.13 ± 4.53d	0.74 ± 0.02ab	68.67 ± 5.41c	0.55 ± 0.04e

Results are expressed as mean ± standard deviation (n = 3), and different letters in the same column indicate significant differences (p < 0.05).

3.11. Freeze-thaw stability analysis

Freeze-thaw stability is a critical indicator for evaluating the ability of frozen foods to retain moisture during freezing and thawing processes (Muadklay & Charoenrein, 2008). The freeze-thaw stability of starch refers to its resistance to physical changes during these cycles, typically measured by syneresis rate. A lower syneresis rate indicates better freeze-thaw stability, reflecting superior water retention capacity. As shown in Fig. 8A, the syneresis rate of all ODF-WS composite gels increased with additional freeze-thaw cycles, suggesting that repeated freezing and thawing accelerated water expulsion from the WS composite gel network. After five freeze-thaw cycles, the water exudation rates of the 6 % HEDF and 6 % UEDF groups decreased by 4.38 % and 10.08 %, respectively, compared with the 6 % ODF group. These results indicate that the incorporation of modified ODF significantly improved the freeze-thaw stability of the gel. Moreover, at the same incorporation level of 4 %–8 %, the UEDF group consistently exhibited significantly lower water exudation than the HEDF group. This suggests that the UEDF group effectively inhibited ice crystal formation by enhancing the water absorption capacity of the WS gel, thereby reducing water release and minimizing ice crystal growth during storage (Xin, Nie, Chen, Li, & Li, 2018). This mechanism reduces water migration and controls ice crystal growth, ultimately improving freeze-thaw stability. These findings align with previous moisture migration analyses, further confirming the beneficial role of UEDF in enhancing the freeze-thaw performance of starch-based gels.

Fig. 8.

Freeze-thaw stability (A) and digestible fragment content (B) of okara dietary fiber and wheat starch composite gel with different modification methods and addition ratios.

3.12. In vitro simulated digestion analysis

The digestibility of starch refers to the extent to which starch is hydrolysed into glucose during digestion, which directly affects the rate of energy release and blood glucose levels, thereby playing a significant role in glycemic management (Qin et al., 2025). Fig. 8B illustrates the variations in the content of RDS, SDS, and RS in composite gels of WS and ODF, as modified by different methods and varying addition ratios. Compared with the ODF group, the 6 % HEDF group exhibited a decrease in RDS content from 41.56 % to 38.9 %, a reduction in SDS content from 11.44 % to 9.43 %, and an increase in RS content from 47.01 % to 51.66 %. Similarly, the 6 % UEDF group showed a decline in RDS content from 41.56 % to 36.72 %, a decrease in SDS content from 11.44 % to 8.96 %, and a rise in RS content from 47.01 % to 54.32 %, with the UEDF group demonstrating the most pronounced increase in RS. These results indicate that the incorporation of modified ODF significantly reduces the final content of digestible starch, primarily due to the inhibitory effect of modified ODF on the interaction between digestive enzymes and starch, which diminishes the number of enzyme binding sites and consequently slows the hydrolysis rate of starch. This finding is consistent with the results reported by Bai et al. (2021) for purified DF components, such as cellulose, pectin from citrus peel, and guar galactomannan, all of which demonstrated significant inhibitory effects on starch digestion. Furthermore, Liao et al. (2022) demonstrated that electrostatic interactions facilitate the adhesion of DF to the starch surface in probiotic-fermented, modified wheat bran IDF, thereby further reducing the digestion rate of starch gels and decreasing the number of enzyme binding sites.

4. Conclusions

This study demonstrated that modified ODF has a significant effect on the physicochemical properties, structural characteristics, and digestibility of WS. The results demonstrated that both HEDF and UEDF, as modified DF, effectively reduced the pasting temperature and time of WS. Specifically, the 6 % UEDF group exhibited a pasting temperature of 70.74 °C, with PV and SB value reduced by 364 cP and 164 cP, respectively, indicating that UEDF significantly inhibited starch retrogradation. SEM and rheological analyses confirmed that UEDF promoted the formation of a more compact WS composite gel network, resulting in a significantly increased G′. FT-IR and hydrogen bond energy analysis revealed that ODF and WS primarily interact through hydrogen bonding, with the 6 % UEDF group exhibiting a hydrogen bonding energy of 10.284 kJ/mol, which was significantly higher than that of the ODF group, accompanied by reduced short-range order and double-helix structures. Correspondingly, XRD and DSC results indicated that the 6 % UEDF group exhibited a decrease in relative crystallinity and ΔHr of 9.29 % and 15.41 %, respectively. Low-field NMR and freeze-thaw stability analysis demonstrated that the UEDF group enhanced water-binding capacity and promoted a more uniform water distribution, thereby significantly improving the freeze-thaw stability of the WS composite gel. Importantly, in vitro digestion revealed that the 6 % UEDF group increased resistant starch RS content by 7.32 % while reducing RDS, highlighting its potential to attenuate starch digestibility and lower the glycemic index (GI). Collectively, these findings confirm that UEDF-modified dietary fiber most effectively enhances the processing performance, structural stability, and nutritional functionality of WS, providing new insights for the valorization of ODF and the development of low-GI functional starch-based foods.

CRediT authorship contribution statement

Sheng Li: Writing – original draft, Funding acquisition, Conceptualization. Xiaoyan Qin: Writing – original draft. Yuqian Zheng: Software. Wenlong Xie: Software. Zhilong Chen: Software. Wenyan Wang: Data curation. Jun Zhao: Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

No data was used for the research described in the article.