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Article

Changes in Physicochemical Properties and In Vitro Digestibility of Broken Rice Starch by Ultrasound and Quercetin Dual Treatment

by
Ping Yang
,
Chenhao Qiu
and
Na Zhang
*
College of Food Engineering, Harbin University of Commerce, Harbin 150028, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(8), 4203; https://doi.org/10.3390/app15084203
Submission received: 15 February 2025 / Revised: 30 March 2025 / Accepted: 8 April 2025 / Published: 11 April 2025
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Abstract

:
Applying physical modification methods to raise the resistant starch content is a feasible strategy for developing foods with a low glycemic index (GI) and regulating postprandial hyperglycemia. Here, broken rice starch (C) was modified via ultrasound and quercetin complexation (US-Q). The structure, physicochemical properties, and in vitro digestibility of the US-Q product were subsequently determined. Scanning electron microscopy (SEM) images showed that the modification changed the starch granules’ morphology, forming a more compact and stable structure. Fourier transform infrared (FTIR) spectroscopy images revealed the interaction between the starch and quercetin. An X-ray diffraction (XRD) analysis demonstrated that the crystallinity of the US-Q was lower than that of the C, indicating that the combined modification with ultrasound and quercetin disrupted the long-range ordered structure of the starch and facilitated the formation of a short-range ordered structure from amylose. Size exclusion chromatography (SEC) images showed that both the molecular weight (from 72,080.96 kDa to 85,141.95 kDa) and amylose content (from 15.94% to 26.76%) increased significantly, while the branching degree and average degree of polymerization of amylopectin decreased, suggesting that the ultrasonic treatment processing method had a significant impact on the formation of the quercetin–starch complexes. In terms of in vitro digestion, the resistant starch content of the US-Q was significantly increased from 6.57% to 20.23%, whereas the hydrolysis rate was decreased from 92.6% to 78.35%, indicating that the presence of quercetin reduced the digestibility of the starch complexes by inhibiting the starch-hydrolyzing enzyme activity. Overall, this study improves the understanding of ultrasound and quercetin dual treatment of broken rice starch, providing a theoretical basis for the development of low-GI starch foods for industrial applications.

1. Introduction

The rice industry inevitably produces a large amount of broken rice. The added value of broken rice is low, and its price is less than half that of ordinary rice, which brings huge economic losses to farmers and rice processing enterprises [1]. While the composition, physical, and chemical properties of broken rice are similar to those of ordinary rice, it is a good resource for starch production [2]. Starch serves as a primary energy-supplying nutrient, accounting for a considerable portion of the human dietary structure [3]. Nevertheless, the intake of large amounts of starch leads to an increase in the body’s glycemic index (GI), which is associated with a range of health problems such as diabetes and cardiovascular diseases [4]. Modulating the type of starch consumed to slow the rate and index of postprandial blood glucose elevation is an effective strategy for avoiding these health problems [5]. Studies have shown that the consumption of resistant starch can effectively lower the postprandial blood glucose index and reduce the levels of total cholesterol and low-density lipoprotein cholesterol in serum [6]. Therefore, increasing the proportion of resistant starch in starch using modification techniques has considerable feasibility and potential market value for reducing the incidence of chronic diseases such as diabetes.
Starch modification techniques can be classified into chemical, enzymatic, and physical modalities. Chemical modification methods such as oxidation, esterification, and etherification have been previously explored and applied to functionalize starch [7]. While they can provide a variety of physicochemical benefits, chemical means can affect the molecular structure and reactivity of starch by adding new functional groups, degrading the starch’s polymer structure, and oxidizing or cross-linking starch molecules through free radicals. Moreover, a solvent-free reaction or mild reagent for application in pre-formed particles (e.g., drug delivery) is increasingly required. Enzyme modification can be performed to change the chain length and molecular weight of amylose, as well as amylopectin. Enzyme modification does not improve the swelling capacity of starch granules but can be used for disintegration. In addition, enzyme modification is usually not cost-effective enough for industrial applications [8].
Physical modification alters the stacking structure of starch polymer molecules within the granules and the overall structure of the starch granules without the introduction of foreign substances, having a significant impact on the starch paste, gel, and in vitro digestive properties; thus, this approach is widely employed in the food industry [9]. Such physical treatments include moist heat treatment, aging, high-pressure treatment, radiation heat treatment, annealing, pulsed electric field treatment, and ultrasonic treatment. These methods are chemical-free; environmentally friendly; do not require wastewater treatment; improve the textural properties, stability, and shelf-life; increase the dietary fiber content; and ultimately improve the nutritional value of the product [10]. As a non-thermal technology, ultrasound is a type of physical modification method that has the advantages of being environmentally benign, highly selective, and highly efficient [11]. Through ultrasonic treatment, pores or fissures may appear on the surface of the starch, thereby changing its physicochemical properties [12]. For example, Kaur et al. reported that ultrasonics represents a valuable technique for remodeling the composition, structure, and properties of broken rice starch [13]. Chan et al. probed the effects of ultrasonic treatment on the physicochemical properties and in vitro digestibility of semi-gelatinized high-amylose corn starch and ascertained that the use of high-temperature ultrasonic treatment for a specific time interval could potentiate the short-range ordered molecular structure, whereas the long-range ordered molecular structure waned with low-temperature ultrasonic treatment [14]. Resistant starch formation requires a highly short-range ordered molecular structure to resist enzymatic hydrolysis.
Moreover, it has been shown that the combined application of multiple modification methods can be more effective in reducing the digestibility of starch [15]. In addition to physical modification, the combination of phenolic compounds with starch can increase the content of resistant starch. Starch–polyphenol complexes are generated mainly through hydrogen bonding, hydrophobic interaction, and electrostatic interaction, which contribute to the formation of V-type inclusion and non-inclusion complexes [16]. For inclusion complexes, especially V-type ones, phenolics can directly interact with amylose. The hydrophobic ring of the phenolics partially or completely enters the hydrophobic cavity of the amylose helix, binds to the structure through hydrogen bonding, and forms a helical crystalline region. Phenolic compounds can also form non-inclusion network-like complexes with starch through non-covalent bonds [17]. Quercetin, a flavonoid found mainly in flowers, leaves, and fruits, can directly impede amylase activity by binding to the enzyme’s active site or indirectly affect starch digestion by interacting with starch. Liu et al. grafted quercetin onto Cynanchum bungei starch and found that the thermal stability, resistant starch content, and antioxidant activity of the starch were significantly enhanced. This indicates that quercetin holds potential in the modification of novel resistant starch endowed with antioxidant activity [18]. Inspired by this evidence, broken rice starch was used as raw material in the current study. The modification was achieved first via ultrasonic treatment and then via direct mixing with quercetin. Subsequently, the structure, physicochemical properties, and in vitro digestibility of the produced starch were determined to elucidate the effects of the ultrasound treatment and quercetin addition on the starch’s quality.

2. Materials and Methods

2.1. Materials

The broken rice flour (~76% starch and ~6.8% protein) was obtained from a local supplier in Harbin, China. The quercetin (purity 97%,) was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). The α-amylase from porcine pancreases (A3176, 16 U/mg, Solid) was from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) and the amyloglucosidase from Aspergillus niger (100,000 U/g, Solid) was obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). All other chemicals used in this study were of analytical grade.

2.2. Ultrasound-Assisted Preparation of Modified Starch

The preparation process for the ultrasonic modification of starch (US) was as follows. An appropriate amount of broken rice flour (24%, w/v) was weighed and added to distilled water to prepare a flour slurry. The mixture was magnetically stirred at 25 °C and 200 rpm for 10 min. Subsequently, an ultrasonic treatment (40 kHz) was carried out at a power of 250 W for 30 min [19,20,21]. After the ultrasonic treatment, the slurry was filtered and dried in an oven at 40 °C for 24 h. The dried product was then sieved through a 100-mesh sieve and stored in a desiccator for further use.

2.3. Preparation of Ultrasound-Modified Starch Composites

The modified starch composite was prepared via a combination of ultrasound and quercetin addition (US-Q). Briefly, 3 g of the US sample was added into 30 mL of distilled water to form a 10% (w/v) suspension. The suspension was placed on a magnetic stirrer and stirred for 10 min for pre-gelatinization. Subsequently, the quercetin was dissolved in an ethanol solution (10.0%, w/v), and was then slowly added into the starch dispersion over 30 min under continuous stirring. The final amount of quercetin was 8% of the dry weight of the ultrasonically modified starch. The mixture was then continuously stirred at 85 °C for 90 min. The obtained sample was cooled to room temperature, and the resulting suspension was centrifuged at 4000 rpm (1789× g) for 10 min. The sample was pre-frozen in a −18 °C refrigerator and then freeze-dried at −40 °C. Finally, the dried product was filtered through a 100-mesh sieve and reserved for the subsequent analysis. Broken rice starch (C) and ultrasonic-modified starch (US) were used as the experimental control groups.

2.4. Observation of Particle Morphology

A small quantity of each starch sample was uniformly adhered to the adhesive tape surface. A 10 min treatment of carbon and gold sputtering was carried out using an ion sputtering coater. Afterward, the samples were examined using a scanning electron microscope (SEM) (JSM-6360LV, JEOL, Tokyo, Japan). They were observed at different magnifications of 1000, 3000, 5000, and 10,000 times, respectively. Multiple magnifications and imaging parameters were used, including a voltage of 5 KV, 3.5 spot size, 30 µm objective aperture, and 8 mm working distance [22].

2.5. X-Ray Diffraction (XRD)

The crystal structure of the sample was analyzed using an X-ray diffraction analyzer (Bruker, Berlin, Germany). The characteristic radiation was Cu, with a graphite monochromator. The tube voltage was set at 40 kV, and the current was 30 mA. The measurement angle range was 2θ = 5–60°. The emission and anti-reflection slits were both 1°, the receiving slit was 0.3 mm, the scanning speed was 4°/min, and the step width was 0.02°. The relative crystallinities of the starch samples were calculated using the TOPAS V.5.0 software [23].

2.6. Fourier Transform Infrared Spectroscopy (FTIR)

An FTIR spectrometer (Bruker, Berlin, Germany) was used to qualitatively analyze the changes in the functional groups of the three prepared samples. In brief, 1.0 mg of the starch sample was mixed thoroughly with 100.0 mg of potassium bromide, and the mixture was then pressed into a pellet. The scanning range of the sample was set from 400 to 4000 cm−1, with an accumulation of 64 scans, a resolution of 4 cm−1, and a scanning time of 16 s. All spectra were automatically baseline-corrected and processed using OMNIC 8.0.

2.7. Determination of Molecular Weight and Chain Length Distribution

The molecular weight and chain length distribution were determined according to a reported method with slight modifications [24]. Briefly, the starch samples (5 mg) were mixed with a DMSO solution (5 mL) containing lithium bromide (0.5% w/w) (DMSO/LiBr) and heated at 80 °C for 3 h using a thermostatic mixer. The molecular weight of the starch was measured using SEC-MALLS-RI. The weight- and number-averaged molecular weights (Mw and Mn) and polydispersity index (Mw/Mn) of the starch in the DMSO/LiBr (0.5% w/w) solution were measured using a DAWN HELEOS-II laser photometer (Santa Barbara, CA, USA). The gel filtration columns including OHpak SB-804 HQ (300 × 8 mm) and OHpak SB-803 HQ (300 × 8 mm) were selected based on the properties of the samples. The column temperature was maintained at 60 °C. The injection volume was 200 μL. Mobile phase A was 0.5% LiBr in dimethyl sulfoxide (DMSO). The flow rate was 0.3 mL/min and an isocratic elution was performed for 120 min. A differential refractive index detector (Optilab T-rEX, Santa Barbara, CA, USA) was also connected to give the concentration and dn/dc values of the grades. The starch in the DMSO solution had a dn/dc value of 0.07 mL/g [25,26].

2.8. Measurement of Pasting Characteristics

The gelatinization characteristics of the samples were analyzed using a rapid viscosity analyzer (RVA) (RVA 4800, PerkinElmer, Shelton, CT, USA). The starch samples (2.5 g each, dry basis) were mixed with 25 mL of distilled water. The test conditions were as follows. The samples were equilibrated at 50 °C for 1 min, heated to 95 °C at a rate of 12 °C/min and maintained for 2.5 min, then cooled to 50 °C at a rate of 12 °C/min and maintained for 2 min. The prepared samples were transferred into the measuring cylinder, and a test program was established to record the gelatinization temperature, complexing time, peak viscosity, minimum viscosity, breakdown value, setback value, and final viscosity [27].

2.9. Thermal Characterization

The thermal characteristics of starch specimens were examined using a thermal gravimetric analyzer (TGA 8000, PerkinElmer, USA). Then, 5.00 mg of the dried sample was precisely weighed within a crucible. An empty crucible without a sample was utilized as a control. The test conditions of the thermogravimetric analyzer were shown as follows. The nitrogen flow rate was maintained at 20 mL/min, and the heating rate was set at a constant value of 10 °C/min. The test temperature range was from 30.0 °C to 500.0 °C. Through this process, the thermogravimetric (TG) curve and the differential thermogravimetry (DTG) curve were acquired [28].

2.10. Dynamic Rheology

The rheological properties were measured using a rheometer (AR-2000, TA Instruments Inc., New Castle, DE, USA). A 5% starch solution was prepared and gelatinized in a 95 °C water bath for 20 min. It was then cooled down to room temperature and quickly placed on a parallel plate with a diameter of 40 mm and a gap of 1 mm. Subsequently, the starch solution was equilibrated at 25 °C for 120 s. The sample was subjected to frequency-sweep measurements to determine the changes in the storage modulus (G′) and loss modulus (G″) with the oscillation frequency. The test conditions were as follows: a temperature of 25 °C, a shear stress of 2%, and a frequency-sweep range between 0.1 and 20 Hz [29].

2.11. Determination of Solubility and Swelling Power

Approximately 150 mg (W0) of starch was weighed and carefully introduced into a centrifuge tube. Subsequently, 10 mL of deionized water was added into the tube and the mixture was subjected to vortex agitation for precisely 10 s. Then, the centrifuge tube was placed in a water bath at 85 °C for 30 min. After the heating process was complete, the centrifuge tubes were quickly transferred to an ice water bath for rapid cooling. Subsequently, these tubes were subjected to centrifugation at 2000 rpm (447× g) for 30 min. After centrifugation, the supernatant was carefully transferred to a small aluminum box. A highly accurate weighing instrument was used to measure the weight of these deposits (Ws). The small aluminum box containing the supernatant was placed in an oven at 105 °C, which was dried to constant weight (W1). The water solubility index (WSI) and the swelling power (SP) calculation formula were as follows [30]:
W S I ( % ) = w 1 w 0 × 100 %
S P % = W s W 0 × 1 − W S I / 100 × 100 %

2.12. Determination of Particle Size of Starch

The starch samples were dispersed in deionized water and then subjected to high-speed dispersion. Subsequently, the samples were injected into a laser particle size analyzer (SALD-7000, SHIMADZU, Kyoto, Japan). The refractive index of the water was set to 1.33, the refractive index of the starch to 1.59, and the absorbance to 0.01 to determine the particle size of the samples. The following conditions were used: temperature (25 °C), refractive index (1.33), signal intensity (~3700 counts per second), and viscosity (0.8878 mPa−s) [31].

2.13. In Vitro Digestibility Measurement

The rapidly digestible starch content (RDS), slowly digestible starch content (SDS), and resistant starch content (RS) values of the samples were determined using a method described elsewhere with some modifications [32]. Then, 100 mg (±5 mg) of the sample was weighed and added to 28 mL of the preheated (37 °C) enzyme solution. These were mixed thoroughly using a vortex oscillator and incubated at 37 °C with shaking for 1 h. After a specific time interval, 1 mL of the reaction mixture was taken for the next steps. A certain amount of 85% ethanol was added and subjected to a boiling water bath for 6 min. Then, the above test solution was centrifuged at 7000 rpm (7104× g) for 10 min. Subsequently, 0.10 mL of the supernatant was pipetted into a test tube, with the addition of 3.0 mL of the GOPOD-p-amino phenazone mixture, which was incubated in a water bath at 50 °C for 20 min, then cooled down to room temperature. The absorbance values of the test solution and the D-glucose standard colorimetric solution were measured at 510 nm. Subsequently, a curve fitting for the sample hydrolysis rate was performed to obtain the hydrolysis rate C∞ and the hydrolysis rate constant k.
G l u c o s e   c o n t e n t = 90 × A 2 × V × D A 1 × m
In the formula, A2 is the absorbance value, A1 is the absorbance value of glucose at a concentration of 1 mg/mL, V is the volume, D is the dilution factor, and m is the sample mass.

2.14. Statistical Analysis

All experimental measurements were performed at least three times, and the data were expressed as the mean ± standard deviation. The data were analyzed with Duncan’s test. An analysis of variance for significant differences was calculated using SPSS 25 with a confidence interval of 95% (p < 0.05).

3. Results and Discussion

3.1. SEM Analysis

Figure 1a shows that the broken rice starch (C) granules present a regular polyhedral appearance. Their surfaces are smooth, flat, and integral, with a relatively complete structure and well-defined edges. Some starch granules of different sizes agglomerate together to form larger clusters. The starch granules in broken rice are non-uniform and their sizes also vary, with diameters approximately in the range of 3–8 μm.
In contrast, the ultrasonic treatment makes the starch surfaces become rough (Figure 1b). Depressions, pores, and channels can be observed on the starch granules, and distinct depressions of varying degrees appear at the edges, which is consistent with the previous research results [13,33]. This may be due to the formation of cavitation bubbles during the ultrasonic process leading to the rupture of starch granules and mechanical damage. High-pressure gradients and high local velocities are generated in the nearby liquid layer, thereby creating shear forces that break the starch polymer chains and damage the granules [34].
As shown in Figure 1c, the US-Q surface becomes smoother and more compact when starch is complexed with quercetin, which is in agreement with a previous study [35]. The composite material obtained via the combined modification methods of ultrasound and quercetin is in a lamellar form and relatively loose, and the connections are no longer close (Figure 1c). This is also because of the gelatinization process, resulting in the loss of granular integrity compared to the morphology of the starch granules in Figure 1a,b. This has been reported to disrupt the molecular ordering within the starch granules, leading to granule swelling, crystal melting, loss of birefringence, increased viscosity, and dissolution [36]. The partially solubilized quercetin acts on the starch and water molecules in the system through hydrogen bonding, and the undissolved portion may serve to dilute the starch matrix and reduce the formation and cross-linking of amylose double-helical structures. As a result, this hinders the re-polymerization process among starch molecules, forming a relatively loose structure.

3.2. X-Ray Diffraction Analysis

Figure 2a suggests that the sharp peak diffraction feature and the diffuse diffraction feature correspond to the crystalline region and the amorphous region of the starch granules, respectively. The C exhibited relatively strong diffraction characteristic peaks at diffraction angles of 15.0°, 17.0°, 17.9°, and 23°, which belonged to the typical A-type crystalline structure and possessed relatively high stability [35]. No new crystallization peaks emerged after the ultrasonic treatment, and the difference between the C and the US was not significant. Given that the fillers in the crystalline and amorphous parts of starch granules are different, their sensitivity levels to ultrasound vary. Consequently, it can be inferred that the ultrasonic treatment mainly takes place in the amorphous region of the starch, representing a physical modification process [37]. Yang et al. also found that the amorphous region of starch was destroyed by ultrasound instead of the crystalline region based on the XRD analysis [38]. These data indicate that ultrasonic disruption tends to occur preferentially in the amorphous regions of the starch granule structure [39].
As shown in Figure 2b, the diffraction peaks of quercetin in the US-Q are present at 10.7°, 12.4°, and 27.1°, and distinct diffraction peaks appear at 7.0°, 13.4°, and 20.1°. This indicates that the combined modification with ultrasound and quercetin has an impact on both the crystalline and amorphous regions of starch granules. Diffraction peaks of the V-type crystalline structure emerge, further suggesting that there is an interaction between quercetin and starch; that is, V-type starch is formed, indicating the formation of an inclusion complex. In another study, the amylose–proanthocyanidin complex showed a prominent peak at 19.8°, indicating that this complex was a V-type crystal inclusion complex [40]. Based on a similar principle, this demonstrates that there may be an interaction between quercetin and starch molecules, resulting in the formation of a V-type crystal inclusion complex. The relative crystallinity rates of C, US, and US-Q are 20.83%, 20.00%, and 19.28%, respectively. The crystallinity of the US-Q is lower than that of the C, indicating that the combined modification with ultrasound and quercetin disrupts the long-range ordered structure of the starch, and a short-range ordered structure is formed from amylose. This may be because the addition of quercetin introduces hydroxyl groups that are prone to binding with starch molecules through hydrogen bonds, thereby affecting the aggregation among the starch molecules. Similarly, Su et al. showed that a combination of moist heat treatment and ultrasonic treatment could destroy amylopectin crystals, resulting in an unstable lamellar arrangement of starch granules, which is manifested by a decrease in short-range ordering and long-range ordering [41].

3.3. FTIR Analysis

To explore the impacts of ultrasound and the combination of ultrasound and quercetin on the short-range crystal structure of broken rice starch (C), FTIR spectroscopy was adopted for the determination of the infrared spectra of the three above samples, as illustrated in Figure 3a. All three starch samples exhibited a broad and strong absorption peak around 3300 cm−1, primarily due to the absorption caused by the stretching vibration of the -OH bond [42]. The absorption peak at 2930 cm−1 resulted from the stretching vibration of -CH and -CH2 in the starch. In comparison with the infrared spectrum of C, after the ultrasonic treatment, there was no significant alteration in the shape or position of any characteristic group within the C. Meanwhile, no characteristic absorption peak disappeared, nor did any new one emerge, indicating that the short-range crystal structure remains unchanged during the ultrasonic process, thereby making it a physical modification method [38]. When compared with the unmodified broken rice starch, its relative absorbance decreased, suggesting that ultrasonic treatment affects the absorption intensity of the hydroxyl groups of broken rice starch molecules. This may be because the ultrasonic treatment induces mechanical action and cavitation effects that change the ordered structure of the starch crystalline structure, thereby causing the alteration of the intensity of the starch absorption peak. The absorption peak of quercetin (Figure 3b) at 3414 cm−1 is the stretching vibration of the hydroxyl group. After compounding with quercetin, the absorption peaks appearing at 1610 cm−1, 1560 cm−1, and 1515 cm−1 could be attributed to the stretching vibration of the benzene ring bond. In the wavelength range of 1665–1022 cm−1, several absorption peaks of different intensities emerges, and these absorption peaks were consistent with the positions of the absorption peaks in the FTIR spectrum of quercetin, indicating that quercetin is bound to the starch and suggesting the existence of an interaction between starch and quercetin [43].

3.4. Analysis of Molecular Weight and Chain Length Distribution

Compared to the control starch group with a weight-averaged molecular weight (Mw) of 72,080.96 kDa, the Mw of the ultrasound-treated starch decreased (US: 70,744.22 kDa), indicating that the ultrasonic treatment led to the degradation of molecular chains in the starch, further confirming that both amylose and amylopectin molecules were disrupted. This finding has also been observed in other studies [32]. However, it is likely to cause starch degradation by shear force in the SEC separation system. This is a common phenomenon when using SEC to separate giant molecules, such as amylopectin. To avoid this, a low flow rate may not result in a full or partial change in the apparent size distribution, or cause artifacts via shear fracture [44]. Herein, the flow rate of 0.3 mL/min was used to separate the broken rice starch samples before and after ultrasonic treatment, which is suitable for the SEC or GPC of native starch in an eluent system [45].
After complexation with quercetin, the starch exhibited a higher molecular weight (US-Q: 85,141.95 kDa), which could be attributed to the added quercetin forming complexes with the starch. Additionally, the ultrasonic treatment processing method had a significant impact on the formation of the complexes [46]. Furthermore, the chain length distributions of C, US, and US-Q are shown in Figure 4. The two peaks in the chain length distribution represent amylopectin (branched polymer) and amylose (linear polymer). Peak 1 represents A chains and short B chains in amylopectin, peak 2 represents long B and C chains in amylopectin, and peak 3 represents amylose. As a result, compared to the broken rice starch serving as the control (15.94%), both the ultrasonically modified starch (31.76%) and the modified starch composite (26.76%) exhibited a significant increase in relative amylose content. The degree of branching of amylopectin in the ultrasonically modified starch was noticeably decreased, accompanied by a significant increase in the average degree of polymerization. The degree of branching of amylopectin in the modified starch composite also demonstrated a significant reduction. However, in contrast to the ultrasonically modified starch, the degree of branching of amylopectin in the modified starch composite exhibited a slight recovery, while the average degree of polymerization underwent a sharp and abrupt decline.

3.5. Analysis of Starch Gelatinization Characteristics

Starch gelatinization represents a complex transformation of starch granules from an ordered state to a disordered state [47]. A comparison of the gelatinization characteristics of C, US, and US-Q is presented in Table 1. Compared to the US, the peak viscosity and breakdown value of the C following ultrasonic treatment exhibited a significant increase, whereas the increase in the setback value was not pronounced. Notably, the peak viscosity, trough viscosity, and final viscosity of the modified starch composite all decreased significantly. After the addition of quercetin, the breakdown value decreased from 1116 to 685, indicating that the composite modification of broken rice starch by the combination of ultrasound and quercetin led to a marked enhancement in the starch’s thermal stability and a reduced likelihood of starch granule fragmentation. The setback value of the C, which was 1021, was significantly higher than that of the US-Q, which was 407. A lower setback value implies better anti-aging properties of the starch. After ultrasonic modification, the setback value was relatively higher than that of the unmodified broken rice starch. However, it decreased significantly upon the addition of quercetin, demonstrating that the addition of quercetin led to a reduction in the setback value, thereby significantly delaying the starch’s retrogradation and substantially improving the anti-aging properties. The decline in gelatinization performance might be attributed to the fact that the hydroxyl groups on the surfaces of polyphenols exist in the form of hydrogen bonds and form a complex with the side chains of starch molecules under the action of van der Waals forces [48]. This, in turn, hindered the interactions between starch molecules and inhibited the recrystallization of the starch after cooling, as well as the leaching of amylose, thereby achieving the effect of delaying the starch’s retrogradation.

3.6. Thermal Performance Analysis

Thermogravimetric analyses (TGs) of the C, US, and US-Q were performed (Figure 5a,b). Mass losses in all three types of starch were detected in two stages, mainly resulting from the thermal decomposition of starch and the complexes. The water evaporation caused the samples’ weight loss between 40 and 150 °C. The mass of C decreased from 92% to 40%, the mass of the starch after ultrasonic treatment (US) dropped from 96% to 30%, and the mass of the complex (US-Q) decreased from 93% to 45%. This indicates that the thermal stability of the starch is reduced after ultrasonic treatment. However, when combined with quercetin, the mass loss of the starch is decreased, and the thermal stability is enhanced. Similarly, the DTG analyses of C, US, and US-Q are shown in Figure 5c. The maximum weight loss of the C control sample was greater than that of the US and US-Q samples at around 300 °C. Moreover, when the temperature reached between 400 and 500 °C, the weight loss of US-Q happened at the highest temperature, followed by the US and C samples. This indicated that the interaction in the starch–quercetin complex resulted in better thermal stability, which was also supported by the compact structure of the starch granules (Figure 1). These results were consistent with the studies of maize starch–quercetin complexes [49] and Tartary buckwheat starch–quercetin complexes [50].
In addition, Amoako et al. studied the formation of resistant starch in the V-type complex within the helix between polymeric proanthocyanidins and amylose. Through differential scanning calorimetry (DSC), they found that the amylose–proanthocyanidin complex has a characteristic melting peak at approximately 120 °C, also indicating the formation of an inclusion complex. Through TG analyses, it was proven that broken rice starch forms an inclusion complex after ultrasonic treatment and combination with quercetin [40]. Meanwhile, compared with the unmodified broken rice starch, the thermal stability of the starch is enhanced by the combined modification with ultrasound and quercetin. Similarly, Raza et al. demonstrated that the combined treatment of ultrasound and high-pressure homogenization was able to enhance the thermal stability of the starch–phenolic acid complexes, a synergistic effect on the complexes’ thermal stability [51]. This suggests that the synergistically modified complexes of starch and polyphenol have a more complex molecular structure and a larger molecular weight.

3.7. Analysis of Dynamic Rheology of Starch

Studies commonly use the storage modulus G′, loss modulus G″, and loss factor tanδ to evaluate the dynamic rheological properties of starch samples. Figure 6 reflects the influence of ultrasound on broken rice starch and the combined modification of ultrasound and quercetin on the dynamic rheological properties of broken rice starch. Compared with C, the storage modulus and loss modulus of the US decreased to a certain extent, which follows the research of others [13]. This might be because ultrasonic treatment leads to the appearance of holes and depressions on the internal surface of the broken rice starch, resulting in a loosening of the internal structure and a subsequent decrease in viscoelasticity [52]. Ultrasonic treatment reduces the loss modulus of broken rice starch to a large degree. However, after composite modification, the loss modulus of broken rice starch increases substantially. When the frequency is 0.26 Hz, the loss modulus of the US-Q is equal to that of the C, and after 0.26 Hz, the loss modulus of the US-Q is always higher than that of the C. The tanδ value of the US-Q increases and is always higher than that of the broken rice starch. During the entire process of dynamic rheological measurement, both the G′ and G″ values of the three types of starch increase with the increase in frequency, and the storage modulus is always greater than the loss modulus. When quercetin is added, the tan δ value of the starch increases, indicating that quercetin increases the proportion of viscosity in the system and improves the fluidity. This may be related to the large number of hydroxyl groups in quercetin molecules. Quercetin can act on the side chains of starch molecules through hydrophobic bonds, hydrogen bonds, and van der Waals forces, increasing the number of entanglement points among molecular chains within the gel system. This enables the system to form a relatively stable network structure, generating a weaker gel with the property of being inclined toward a viscous fluid [53]. According to a previous study, starch molecules were able to interact with carbonyl- and hydroxyl-rich flavonoids through hydrogen bonding, inhibit the contact between the chains of straight-chain starch molecules through van der Waals forces, impede the molecular ordered rearrangement, and limit the formation of hydrogen bonds within the microcrystalline bundles of starch, thereby delaying starch retrogradation [54].

3.8. Analysis of Starch Solubility and Swelling Power

The solubility and swelling degree of C, US, and US-Q are shown in Table 2. The order of the starch solubility and swelling power is as follows: US > C > US-Q. The broken rice starch had relatively high levels of solubility and swelling power, and both the solubility and swelling power of the US increased. The ultrasonic effect causes amylose to be released into the aqueous medium, thereby increasing the solubility. Ultrasonic treatment can cause changes in the physical geometry of holes and channels on the surfaces of starch granules, which is consistent with the results of SEM. This makes it easier for water molecules to penetrate large-volume granules, thereby increasing the solubility of the granules and enhancing the binding ability of starch and water molecules. Some studies suggested that the increase in swelling power caused by ultrasonic treatment might be due to the decomposition of intermolecular bonds and the destruction of the internal crystalline molecular structure of the starch [55]. When water molecules are connected to the free hydroxyl groups of amylose and amylopectin through hydrogen bonds, starch granule clusters are broken and the structure is changed, resulting in an increase in water absorption by starch granules. After compounding with quercetin, the solubility decreased from 23.27% to 12.73%, and the swelling power decreased from 10.26 to 9.85, significantly lower than the results for C and US. This may be because the addition of quercetin can limit the leaching of amylose during the heating and gelatinization process. Meanwhile, the hydroxyl groups of quercetin can interact with the hydrogen bonds between water molecules, thereby slowing down the movement speed of water molecules and inhibiting the water absorption and swelling of the starch during heating. It is also possible that because quercetin has relatively low solubility in water, water-dispersed quercetin is adsorbed by broken rice starch, which to some extent hinders the interaction between starch and water molecules, leading to a decrease in starch solubility [56].

3.9. Starch Particle Size Analysis

As shown in Table 2, following the ultrasonic treatment, the average particle size of the broken rice starch expanded from 131.5 nm to 314.53 nm. The increase in the average particle size of the starch granules after the ultrasonic treatment might be attributed to the cavitation and mechanical effects of the ultrasonic process. Such cavitation could lead to the breakage of intramolecular hydrogen bonds within the starch structure, generating more pores and fissures on the surface. In turn, this may have enlarged the structure of the starch granules by inducing changes in their physical geometry, thereby resulting in a significant increase in particle size [57]. Upon compounding with quercetin after the ultrasonic treatment, the average particle size decreased from 314.53 nm to 242.53 nm. This could potentially be because quercetin penetrated through the pores into the depths of the starch granules and attached to them, forming a complex and undergoing re-association, consequently leading to a reduction in particle size.

3.10. Effect of Modification on the In Vitro Digestibility of Broken Rice Starch

The starch hydrolysis rates of the broken rice starch and modified starch composite were determined by measuring the glucose content. The amylase digestion curves are shown in Figure 7. The RDS, SDS, and RS contents, as well as the kinetic parameters of the starch digestion models, were calculated for the broken rice starch and modified starch composite, with the fitted data presented. The correlation coefficients were all above 0.98, indicating a good fit [58]. As shown in Table 3, after ultrasonic treatment, the RDS content of the broken rice starch increased from 70.94% to 76.63%, while the SDS content decreased from 22.49% to 18.65% and the RS content decreased from 6.57% to 4.72%, indicating a general decrease in resistant starch content. This could be due to the increased porosity of starch granules after ultrasonic treatment, making them more susceptible to enzyme actions [59]. The destruction of the double-helical structure of the starch after the ultrasonic treatment facilitated enzyme access to the enzyme-sensitive sites, leading to the degradation of amylopectin and a decrease in RS content [32]. After complexation with quercetin, the RDS content was lower than that of unmodified broken rice starch, with a decrease of 21.43%, indicating a significant reduction in rapidly digestible starch. The SDS content of the US-Q was 30.26% and the RS content was 20.23%, representing an increase of 7.77% in SDS content and 13.66% in RS content compared with C.
As shown in Figure 7a, the hydrolysis curve of the modified starch composite was significantly lower compared to the unmodified broken rice starch. The hydrolysis rate of unmodified C at 20 min was 70.94%, while the hydrolysis rate of US-Q decreased by 21.43%. Model fitting of the starch hydrolysis curve was performed to further understand the effect of ultrasonic synergistic quercetin composite modification on in vitro digestibility, and the fitting results are shown in Figure 7b. The hydrolysis parameter C∞ and k values for US-Q were significantly lower than those for unmodified C. The fitting results indicated that when the digestion time approached infinity, the hydrolysis rate of the broken rice starch could reach 92.6%, while that of the composite-modified starch was 78.35%, representing a decrease of 14.25%. Previous studies have shown that quercetin is an effective inhibitor of pancreatic amylase, which plays a major role in starch digestion. Starch must first be correctly oriented within the active site of the enzyme, where the α-1,4 glycosidic bonds of starch are cleaved [16]. Therefore, when quercetin–starch complexes form, quercetin can bind to the hydrophobic helical regions of starch, which inhibits the binding of starch to α-amylase and suppresses the α-amylase activity [60]. Additionally, the strong interaction between quercetin and starch, as indicated by the XRD analysis, resulted in the formation of V-type crystals. V-type crystals are highly ordered, which also contributes to reducing the starch digestibility. Thus, the ultrasonic synergistic quercetin-modified starch composite exhibited greater resistance to digestion compared to the broken rice starch, significantly delaying the starch hydrolysis. Günal-Köroğlu et al. also confirmed that quercetin played an important role in the interaction with starches and reduced their digestibility, increased the formation of resistant starch, inhibited digestive enzymes, and lowered the glycemic index based on in vitro or in vivo data [61]. Zhou et al. showed that quercetin exhibited very strong inhibitory activity on α-glucosidase in in vitro and in vivo assays [62]. Furthermore, experiments involving the oral administration of quercetin–starch complexes in rats suggested that the maximum level of postprandial blood glucose was delayed and reduced. Despite quercetin being able to efficiently reduce the rate of starch digestion, its bioavailability is often relatively low (<10%) in the human digestive system [63]. This is because of its poor water solubility, chemical stability, absorption properties, and complex food matrix effects. Many efforts have been made to improve its bioavailability and efficacy, such as in emulsions, liposomes, hydrogel-based systems, protein nanoparticles, cyclodextrin complexation, and crystal engineering to control its size and physical form.

4. Conclusions

In this study, ultrasonically modified starch and starch modified with ultrasound combined with quercetin were assessed, and their structure and physicochemical properties were investigated. The results showed that the ultrasonic modification led to some dents and pores on the broken rice starch granules, while combining quercetin with ultrasonic modification resulted in surfaces with smoother and denser granules. The modified starch composite exhibited a V-type crystalline structure, with interactions occurring between the starch and quercetin. The molecular weight and amylose content significantly increased, while the degree of branching and average degree of polymerization of the amylopectin decreased, resulting in smaller particle sizes. The modified starch composite demonstrated enhanced thermal stability and network structure, with the addition of quercetin restricting the water absorption and swelling of the broken rice starch during heating. Both the ultrasonic and ultrasonic–quercetin modifications effectively increased the RS content and decreased the starch’s in vitro digestibility. Therefore, the use of ultrasonic treatment combined with quercetin modification is an effective approach for reducing the glycemic index of starch after consumption, and it holds great promise for the development of foods that can effectively lower postprandial blood glucose levels. In future studies, the interaction between rice starch and quercetin in the presence of proteins will be investigated, since the proteins can also form complexes with polyphenols through non-covalent interactions, potentially decreasing the formation of starch–polyphenol V-type complexes. Additionally, the mechanism of changes in starch properties under specific microwave and phenolic conditions and the suitability for application in different food products need further studies.

Author Contributions

Conceptualization, P.Y. and N.Z.; methodology, P.Y.; software, C.Q.; formal analysis, P.Y.; investigation, P.Y. and C.Q.; resources, P.Y.; data curation, P.Y.; writing—original draft preparation, P.Y.; writing—review and editing, P.Y. and N.Z.; visualization, N.Z.; supervision, N.Z.; project administration, N.Z.; funding acquisition, N.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Program Project (2023YFD2100803), National Natural Science Foundation of China (32372387), National Natural Science Foundation of China (32402015), Heilongjiang Province Outstanding Youth Fund Project, Heilongjiang Province “Double First-Class” Discipline Collaborative Innovation Achievement Project, and Heilongjiang Province Key Research and Development Program Project of the Revealing the List and Taking the Lead Model (2023ZXJ08B03).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

Broken rice starch (C), ultrasonic modification of starch (US), ultrasound and starch–quercetin complex (US-Q), scanning electron microscope (SEM), Fourier transform infrared (FTIR), X-ray diffraction (XRD), size exclusion chromatography (SEC), rapid viscosity analyzer (RVA), thermal gravimetric analyzer (TGA), rapidly digestible starch (RDS), slowly digestible starch (SDS), resistant starch (RS)

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Figure 1. Microstructures of broken rice starch (a), ultrasound-modified starch (b), and modified starch composites (c). After ultrasound treatment, depressions, pores, and channels can be observed on the starch granules (blue arrow).
Figure 1. Microstructures of broken rice starch (a), ultrasound-modified starch (b), and modified starch composites (c). After ultrasound treatment, depressions, pores, and channels can be observed on the starch granules (blue arrow).
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Figure 2. (a) The X-ray diffraction patterns of broken rice starch (C) (black line), ultrasound-modified starch (US) (red line), and modified starch composite (US-Q) (blue line), as well as quercetin (b).
Figure 2. (a) The X-ray diffraction patterns of broken rice starch (C) (black line), ultrasound-modified starch (US) (red line), and modified starch composite (US-Q) (blue line), as well as quercetin (b).
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Figure 3. (a) Infrared spectroscopy images of broken rice starch (black line), ultrasound-modified starch (red line), and modified starch composite (blue line), as well as quercetin (b).
Figure 3. (a) Infrared spectroscopy images of broken rice starch (black line), ultrasound-modified starch (red line), and modified starch composite (blue line), as well as quercetin (b).
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Figure 4. Chain length distributions of broken rice starch, ultrasonically modified starch, and modified starch composite.
Figure 4. Chain length distributions of broken rice starch, ultrasonically modified starch, and modified starch composite.
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Figure 5. Plots of the thermogravimetric analysis of the broken rice starch, ultrasonically modified starch, and modified starch composite (a,b) and the DTG curve (c). The grey part in (a) is amplified as (b).
Figure 5. Plots of the thermogravimetric analysis of the broken rice starch, ultrasonically modified starch, and modified starch composite (a,b) and the DTG curve (c). The grey part in (a) is amplified as (b).
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Figure 6. Dynamic rheological properties of ground rice starch, ultrasonically modified starch, and modified starch composite: (a) energy storage modulus (G′); (b) loss modulus (G″); (c) depletion coefficient tan δ.
Figure 6. Dynamic rheological properties of ground rice starch, ultrasonically modified starch, and modified starch composite: (a) energy storage modulus (G′); (b) loss modulus (G″); (c) depletion coefficient tan δ.
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Figure 7. (a) Hydrolysis rates of crushed rice starch and composite modified starch. (b) Hydrolysis fitting curves of crushed rice starch and modified starch composite.
Figure 7. (a) Hydrolysis rates of crushed rice starch and composite modified starch. (b) Hydrolysis fitting curves of crushed rice starch and modified starch composite.
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Table 1. Comparison of pasting characteristics of broken rice starch, ultrasonically modified starch, and modified starch composite.
Table 1. Comparison of pasting characteristics of broken rice starch, ultrasonically modified starch, and modified starch composite.
SamplePeak Viscosity
(cp)		Minimum Viscosity
(cp)		Attenuation Value
(cp)		Final Viscosity
(cp)		Return to Life Value
(cp)
C2782 ± 16.34 b2782 ± 16.34 a2782 ± 16.34 b2782 ± 16.34 a2782 ± 16.34 b
US3795 ± 20.32 a3795 ± 20.32 b3795 ± 20.32 a3795 ± 20.32 b3795 ± 20.32 a
US-Q1368 ± 8.95 c1368 ± 8.95 c1368 ± 8.95 c1368 ± 8.95 c1368 ± 8.95 c
Note: Different letters within the same column denote a significant difference (p < 0.05).
Table 2. Comparison of solubility, swelling, and particle size results for ground rice starch, ultrasonically modified starch, and modified starch composite.
Table 2. Comparison of solubility, swelling, and particle size results for ground rice starch, ultrasonically modified starch, and modified starch composite.
SampleSolubility
(%)		Swelling Power
(%)		Average Particle Size (nm)
C20.33 ± 0.02 a10.01 ± 0.05 a131.5 ± 10.65 c
US23.27 ± 0.01 a10.26 ± 0.07 a314.53 ± 15.21 a
US-Q12.73 ± 0.03 b9.85 ± 0.12 b242.53 ± 13.33 b
Note: Different letters within the same column denote a significant difference (p < 0.05).
Table 3. Model parameters of RDS, SDS, and RS contents and starch digestion kinetics in crushed rice starch and modified starch composite.
Table 3. Model parameters of RDS, SDS, and RS contents and starch digestion kinetics in crushed rice starch and modified starch composite.
SampleRDS (%)SDS (%)RS (%)C∞k
C70.94 ± 0.53 a22.49 ± 0.13 a6.57 ± 0.13 a92.6 ± 1.30 a0.07 ± 0.01 a
US76.63 ± 0.58 c18.65 ± 0.16 c4.72 ± 0.14 c94.21 ± 0.82 c0.08 ± 0.01 c
US-Q49.51 ± 0.42 b30.26 ± 0.13 b20.23 ± 0.13 b78.35 ± 1.47 b0.05 ± 0.01 b
Note: RDS: rapidly digestible starch content; SDS: slowly digestible starch content; RS: resistant starch content; C∞: starch hydrolysis rate at the end of the reaction; k: enzymatic hydrolysis rate. Different letters within the same column denote a significant difference (p < 0.05).
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Yang, P.; Qiu, C.; Zhang, N. Changes in Physicochemical Properties and In Vitro Digestibility of Broken Rice Starch by Ultrasound and Quercetin Dual Treatment. Appl. Sci. 2025, 15, 4203. https://doi.org/10.3390/app15084203

AMA Style

Yang P, Qiu C, Zhang N. Changes in Physicochemical Properties and In Vitro Digestibility of Broken Rice Starch by Ultrasound and Quercetin Dual Treatment. Applied Sciences. 2025; 15(8):4203. https://doi.org/10.3390/app15084203

Chicago/Turabian Style

Yang, Ping, Chenhao Qiu, and Na Zhang. 2025. "Changes in Physicochemical Properties and In Vitro Digestibility of Broken Rice Starch by Ultrasound and Quercetin Dual Treatment" Applied Sciences 15, no. 8: 4203. https://doi.org/10.3390/app15084203

APA Style

Yang, P., Qiu, C., & Zhang, N. (2025). Changes in Physicochemical Properties and In Vitro Digestibility of Broken Rice Starch by Ultrasound and Quercetin Dual Treatment. Applied Sciences, 15(8), 4203. https://doi.org/10.3390/app15084203

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