3.1. Particle size and potential
The functional characteristics of dietary fiber, such as WHC and OHC, are inextricably linked to its particle size; the effects of DHPM on the size of PDF and PDF-CA particles are shown in Fig. 1. The particle size of all samples was significantly reduced after DHPM treatment and there was a tendency for it to decrease with increasing processing pressure. When the treatment pressure reached 140 MPa, both PDF and PDF-CA had the minimum particle size of 61.29 and 59.9 µm, a significant reduction of 48.92% and 49.14%, respectively, compared to the untreated samples, as reported by Liu et al. (Liu et al. 2022). The results mentioned above show that structural damage to macromolecular materials by shear, cavitation, and high-frequency impact forces from DHPM can result in significant changes in particle size (You et al. 2021). The significantly reduced particle size of the PDF-CA compared to the PDF was due to CA and PDF being bound to each other through non-covalent bonds, resulting in a tightly aggregated complex system.
A higher absolute value of zeta potential is associated with a stronger electrostatic repulsion between molecules, which makes it more difficult for aggregation to occur and results in a more stable complex system. In comparison with the untreated samples, the absolute zeta potential values of the PDF treatment groups increased significantly, with the maximum value reaching 26.00(Fig. 2). This might be due to the structural disruption of the PDF under high pressure, exposing the internal charges and hydrophilic groups and thus making the PDF highly negatively charged (Zeng et al. 2019). The absolute zeta potential values of the PDF-CA treated with DHPM showed a trend consistent with that of the PDF, increasing with the treatment pressure. The overall absolute value of the zeta potential was higher than that of the PDF group, probably caused by the non-covalent bonding between CA and PDF by hydrogen bonding and other non-covalent bonds, thus resulting in changes in the charged condition of PDF, etc.
3.2. Micromorphology
It can be seen in Fig. 3 that the micromorphology of PDF and complexes changed under DHPM treatment at different pressures: the natural particles were larger, the structure was tightly wrapped, the surface wass rough, and there were more folds and furrows. The original microstructure of PDF changed remarkably after DHPM treatment; the particle size was reduced and gradually changed into a lamellar structure, increasing the specific surface area of PDF (Lu et al. 2019). In addition, a few starch molecules (small oval-shaped particles in the figure) were released inside the PDF; these structural changes improved the adsorption characteristics of the PDF (Tao et al. 2012). The addition of CA also changed the PDF structure, so that the intermolecular hydrogen bonding, hydrophobic interactions, and other non-covalent bonding played a role in making the PDF-CA show a particular state of dispersion, and the intermolecular aggregation weakened. With DHPM treatment, the particles had a more uneven surface and the internal void fraction increased, which is reported to be due to the rapid release of pressure during DHPM processing, thus causing swelling of the samples. This also confirmed the changes in particle size and potential of CA-PDF after DHPM treatment. DHPM treatment can affect the microstructure of PDF and PDF-CA, thus impacting the functional and physicochemical properties of PDF.
3.3. FT-IR spectroscopy
The broader absorption peak around 3364.73 cm− 1, as shown in Fig. 4A, was caused by the stretching vibration of O-H, which is the characteristic band of cellulose and hemicellulose (Chu et al. 2019). The peak at 2931.16 cm− 1 is closely related to the stretching motion of the methylene hydrocarbon group; the absorption peak 1738.60 cm− 1 is the result of the stretching vibration of C = O; and that at 1038.81 cm− 1 arise from the bending vibration of C-OH (Li et al. 2010). With an increase of pressure, the absorption peaks of PDF and PDF-CA became narrower, indicating that intermolecular hydrogen bonds and other parts were damaged by the high pressure from DHPM. Following DHPM treatment, the infrared spectrum of each treatment group did not change significantly compared to the blank: only the width and intensity of the absorption peaks changed. In addition, it also had specific effects on the functional properties of PDF (Jia et al. 2019). It can be seen that the O-H absorption peak of PDF changed from 3364.73 to 3384.49 cm− 1, which might be due to a new hydrogen bond in the PDF-CA, indicating that CA interacted with PDF through hydrogen bonding.
3.4. Crystal structure
The X-ray patterns demonstrated the internal crystal structure changes of the fiber. As shown in Fig. 5A and B, the natural PDF had sharp peaks near 2θ of 3.96° and 23.16°, and the diffraction peaks of PDF and complexes were broadened by DHPM treatment, indicating that the crystallinity (peak area/total area) of modified PDF was decreased. The intermolecular hydrogen bonds were broken by the strong force provided by DHPM, as a result, the number of hydrogen bonds was reduced, the orientation of molecular chains deteriorated, and the initially ordered part of the crystalline region was gradually changed into a loose non-crystalline region. The peak positions of PDF and PDF-CA were not significantly changed after DHPM treatment, nor were any new peaks generated, indicating that DHPM did not exert an influence on the overall crystal structure of PDF, which was only reflected in the change of crystal order, similar to the findings of Tang et al. ( Tang et al. 2021).
3.5. Thermal stability
The thermodynamic properties of PDF and PDF-CA are shown in Fig. 6A and B, respectively; it can be seen that there was a thermal absorption peak at about 90 ℃, essentially an endothermic peak, at which time the cellulose and hemicellulose were decomposed by heat. The absorption peak temperatures of PDF showed a significant decrease after DHPM processing, suggesting that the modified PDF had weaker thermal stability, probably owing to the destruction of the tightly wrapped PDF structure (Wang et al. 2018). The peak temperatures of the PDF-CA (Fig. 6B) were all significantly higher than for PDF (Fig. 6A), indicating that the addition of CA increased the thermal stability of the fiber, and the peak temperature of PDF-CA under DHPM treatment had an overall increasing trend. The addition of CA generated new chemical bonds with PDF, and in cooperation with DHPM treatment, the organic compounds inside PDF-CA were exposed. Thus, PDF-CA would gain more chemical energy so that a higher temperature was needed to destroy the crystal structure of PDF-CA (Xu et al. 2004; Zhang et al. 2011). The above demonstration confirmed that DHPM has different effects on the thermal stability of PDF in different systems.
3.6. WHC and OHC of PDF and complexes
WHC and OHC refer to the capacity of fiber particles to retain water and absorb oil. Good water-holding and oil-holding properties not only improve the emulsification of fiber but also contribute to intestinal health and other effects (Huang et al. 2020). The WHC values of untreated PDF and PDF-CA were 12.33 and 14.92 g/g, respectively (Fig. 7). DHPM treatment of the samples significantly enhanced their WHC, by 85.56% and 52.01%, respectively, indicating that DHPM remarkably improved the WHC of PDF, consistent with the results obtained by Liu (Liu et al. 2016). It can be seen that after DHPM treatment, the OHC values of both PDF and PDF-CA increased, to 20.48 and 26.70 g/g, respectively (Fig. 8). The slight reduction of OHC at 140 MPa might be attributed to the excessive comminution of the particles so that the oil molecules were hardly being trapped. The improvement of WHC and OHC could be explained by the strong shear force from DHPM, which caused the formation of smaller fragments of cellulose and hemicellulose molecules after being truncated, and microstructural changes such as the reduction of particle size, thereby increasing the area in contact with water and oil for all samples. Hydrophilic and lipophilic groups inside the fiber were exposed, facilitating the combination of particles with water and oil, thus leading to an increase in the ability to retain water and oil (Li et al. 2021).
The WHC and OHC values of the PDF-CA were generally higher than PDF. On the one hand, CA and PDF combined through non-covalent interactions, so that the internal structure of the fiber changed and the void rate increased, increasing the capacity of PDF-CA to adsorb oil; on the other hand, the above experiments prove that PDF-CA particles were smaller than PDF particles, which was more conducive to the absorption of water and oil.
3.7. GAC
The unique physical and chemical characteristics of PDF enabled it to preserve glucose molecules in it, so as to weaken the diffusional motion of glucose in the gastrointestinal tract and decrease its effective and robust uptake in the small intestine, which plays a role in lowering blood sugar (Qi et al. 2015). All samples in the DHPM-treated group showed an increasing trend of GAC compared with untreated PDF; DHPM facilitated the GAC of PDF and PDF-CA, which was consistent with the water- and oil-holding properties mentioned above and with the findings reported by Yu et al. (Yu et al. 2022). Not only did the DHPM treatment reduce the particle size and increase the specific surface area, it also promoted the transformation of insoluble dietary fiber into soluble fiber, thus increasing the viscosity of the sample and enhancing the force of the fiber binding to glucose (Guo et al. 2018).
Meanwhile, the GAC of PDF-CA was significantly enhanced compared to that of PDF, reaching a maximum at 120 MPa (p < 0.05). By modifying PDF through non-covalent bonds, CA changed the microstructure of PDF, and with the further action of pressure, the hydrogen bonds and van der Waals forces between molecules were strengthened. In addition, CA had some glucose-lowering function, effectively cutting the diffusion movement of glucose, so as to improve the binding effect and ability to capture glucose molecules (Huang et al. 2015; Ma et al. 2016). Therefore, the ability of PDF to adsorb glucose was able to be significantly enhanced by DHPM treatment with the synergistic addition of CA, thus effectively controlling the rise of human blood glucose after meals.
3.8. CBC
The ability of dietary fiber adsorb cholesterol is regarded as an essential functional property of dietary fiber (Zhao et al. 2022). Converting cholesterol in the blood into bile acids or directly combining with cholesterol and excreting it from the human body, in turn, achieves a reduction of the cholesterol content in the human body (Zhang et al. 2022). As can be seen, the CBC was significantly improved by DHPM treatment compared to untreated fiber. This was attributable to the tendency of gradually decreasing and more uniform distribution of fiber particle size; enhanced capillary action allows for increased cholesterol adsorption and cholesterol-lowering effects. During processing of the sample, more binding sites were exposed that could react with the hydroxyl groups in bile acids, thus the sample obtained had a stronger cholesterol-lowering ability (Shi et al. 2021; Wen et al. 2017). It was also evident that the CBC of PDF and PDF-CA was dependent on the pH of the reaction system: the CBC of samples at pH = 7.0 (simulated intestinal environment) was much lower than that at pH = 2.0 (simulated gastric environment); this coincided with the experimental results of PDF studies presented by Liu et al. (Liu et al. 2021). The binding of CA to PDF facilitated an improvement in the samples’ CBC; the value was also correlated with the OHC: the better the OHC, the stronger the cholesterol adsorption. In general, DHPM treatment remarkably improved the adsorption of cholesterol by CA-PF, which may effectively combat some human diseases.