Effect of ultrasonic treatment on the physicochemical properties of buckwheat starch: Based on the ultrasonic power and moisture content

Graphical abstract

Abstract

The physicochemical property of native buckwheat starch (BWS) limits the application, which attracts more attention in the food industry. The objective of this study was to investigate the effects of different ultrasonic powers combined with moisture contents on the structure and physicochemical properties of BWS. The results showed that ultrasonic treatment significantly reduced the gel hardness and loss modulus of BWS. The increase in water content during ultrasound effectively enhanced the swelling power of BWS and reduced the peak viscosity. Besides, with the increase of water content and ultrasonic power, the crystallinity of BWS decreased significantly, and the formation of ordered structures was suppressed. In addition, after ultrasonic treatment, the particle size of BWS was decreased, and the surface became rough and concave. In short, ultrasonic treatment effectively improves the processability of BWS and provides a new theoretical basis for physical treatment in the production of cereal starch.

1. Introduction

Buckwheat is a traditional grain originating from China and India, which is considered a pseudocereal crop because of its similarity to cereal crops in cultivation and application [1]. Higher nutritional components are contained in buckwheat, which is rich in minerals, vitamins, and other bioactive substances such as flavonoids and inositol. Moreover, buckwheat has positive preventive and therapeutic effects on hypertension and diabetes [2]. As a potential nutritional resource, the research on buckwheat has attracted increasing attention in various fields.

Starch is the main component in buckwheat seeds, which accounts for more than 70 % of the dry weight. Buckwheat starch (BWS) is primarily spherical, oval, or polygonal, and its amylose content accounts for about 25 % of the whole grain, which is higher than that of wheat, corn, and other grains [3]. The unique structure and physicochemical properties of buckwheat starch also largely determine the texture and sensory characteristics of buckwheat food [4]. However, the application of native BWS in the food industry is limited due to its less desirable gelatinization and dissolution properties [5]. For example, the high viscosity of buckwheat starch paste usually leads to poor hardness of its products, which reduces the desire of consumers [6].

In order to improve the processing performance of starch and meet the requirements of modern industry, the properties of starch are mainly modified by physical, chemical, and enzymatic methods [7]. Among these treatments, ultrasonic, as a physical modification method, has been increasingly favoured by researchers due to its beneficial effects in food processing and preservation [8]. Ultrasound is a mechanical wave in which energy is transferred to the sample during ultrasonic treatment through a process known as cavitation, that is, the formation, growth, and rapid rupture of bubbles [9]. At present, many studies have confirmed that ultrasonic treatment can affect the gelatinization, rheology, and other properties of starch [10], [11]. Meanwhile, the changes in ultrasonic conditions may have an impact on the internal structure of starch, such as the different ultrasonic powers that can damage the crystallinity of potato starch [12]. In addition, the breakdown of the cavitation bubbles caused damage to the starch particles, thus creating channels for water to diffuse into the starch interior [13]. Thus, it is hypothesized that water also plays a key role in the ultrasound process, that is, the increase of water content and power may enhance the cavitation effect and mechanical effect in the process of ultrasonic treatment, affecting the structure and performance of BWS.

However, there were few studies on the synergistic effect of water change and ultrasonic power in the ultrasonic modification of BWS, and more work was needed to understand better the role of moisture and power in ultrasonic treatment. This study aimed to explore the effects of ultrasonic treatment with different ultrasonic powers (200 W, 400 W) and different moisture content (the ratios of BWS and moisture were 1:1, 1:2, and 1:3) on the structure and properties of BWS. The effects of ultrasonic treatment on the physicochemical properties, gelatinization, and molecular structure of starch were analyzed from macroscopic and microscopic perspectives. This study has accumulated relevant data for the extensive application of ultrasonic technology in the field of coarse cereal starch and promoted the application and development of buckwheat starch.

2. Materials and methods

2.1. Materials

Buckwheat flour was supplied by Sinthi Valley Buckwheat Co., Ltd (Chaoyang, China). NaOH (96 % purity) was purchased from Sinopharm Group Chemical Reagent Co., LTD (Beijing, China). All chemicals and reagents used were analytical grade.

2.2. Starch preparation

Regarding the previous study, BWS was extracted using the alkali method with slight modification [5]. BWS was soaked in 0.3 % (mass fraction) NaOH solution and mixed with a magnetic stirrer at room temperature (25 °C) overnight. Subsequently, the slurry was filtered through a 100-mesh sieve, and the filtrate was centrifuged at 4000 g for 15 min (TGL-20bR, Shanghai Anting scientific instrument Co., LTD., Shanghai, China). The supernatant was removed, and the precipitate was washed with distilled water until a tight white starch precipitate was obtained at the bottom of the centrifuge bottle. Finally, the precipitate was dried in an oven (DGG-9070A, Shanghai Senxin instrument, Shanghai, China) at 40 °C for 12 h, and the BWS sample was obtained after crushing through a 100-mesh screen.

2.3. Ultrasonic treatment

BWS and distilled water were evenly mixed according to the 1:1, 1:2, and 1:3 different ratios to obtain the BWS suspensions. Subsequently, the suspension samples were placed in sealed bags, and the sealed bag was secured 6 cm away from the bottom of the tank. The samples were then treated for 30 min by an ultrasonic bath device (SB25-12 DTD, Ningbo Ultrasonic Instrument Co., LTD., Ningbo, China) with a frequency of 40 kHz at 200 W and 400 W power, respectively. The temperature of the water was maintained at 25 ± 1 °C through a constant temperature water bath system [14]. Subsequently, both native BWS and ultrasonically treated BWS samples were subjected to drying in a 40 °C oven until reaching an identical moisture content of 9.8 %. The native and the ultrasonic BWS were named NB (native BWS), UP200-1 (ultrasonic power 200 W with the BWS-water ratio of 1:1), UP200-2 (ultrasonic power 200 W with the BWS-water ratio of 1:2), UP200-3 (ultrasonic power 200 W with the BWS-water ratio of 1:3), UP400-1 (ultrasonic power 400 W with the BWS-water ratio of 1:1), UP400-2 (ultrasonic power 400 W with the BWS-water ratio of 1:2), UP400-3 (ultrasonic power 400 W with the BWS-water ratio of 1:3), respectively.

2.4. Texture analysis

The textural properties of BWS gels were determined by a texture analyzer (TA.XTC-18, Shanghai Bosin, China), and the hardness and strength of the gels were obtained [15]. A 10 % BWS suspension was placed in a water bath and heated at 95°C for 30 min. After heating, the sample was stored at 4°C overnight to obtain the gel. The test was performed using TA/0.5 probe with a test distance of 15 mm, induction force of 5 g, and compression variable of 50 %. The experiment was repeated three times.

2.5. Rheological properties

The dynamic rheological properties of BWS were measured using a DHR-2 rheometer (RH-20, Shanghai Bosin, China) based on the prior description [16]. The cooled pastes prepared by RVA were transferred onto the parallel and filled suffused the gap (1 mm). Parallel plates with a diameter of 40 mm were selected for frequency scanning. The frequency range was set at 0.1–10 Hz, the strain amplitude was set at 1 %, and the test temperature was set at 25 °C. The storage modulus (G’) and loss modulus (G”) of the gel were determined.

2.6. Particle size distribution

The BWS was uniformly dispersed in deionized water at a ratio of 1:100 (w/v). Particle size was tested with a laser particle size analyzer (MS3000, Malvern, Worcestershire, UK) with 780 nm laser at a scattering intensity of 3 mW, a scanning area of 0.02 to 163°, and a measuring range of 0.02 to 2000 μm [17].

2.7. Pasting properties

The pasting properties of BWS were studied using a rapid viscosity analyzer (Rapid-20, Shanghai Bosin, Shanghai). The parameters were set as follows: 50 °C for 1 min, heating at a constant rate of 12 °C/min to 95 °C for 2.5 min, and then cooling at the same rate from 95 °C to 50 °C for 2 min [18]. The peak viscosity, though viscosity, breakdown value, final viscosity, setback value, and pasting temperature were recorded.

2.8. Swelling power and solubility

The swelling power (SP) and solubility (WSI) were measured with a slight modification of the previous study [19]. 0.3 g BWS (W₀) and distilled water were mixed in the centrifuge tube to prepare a 3 % (w/v) suspension. The suspension was then heated at 95 °C for 30 min while continuously stirring. After heating, the sample was cooled to room temperature and centrifuged at 5000 g for 15 min. Then, the supernatant was removed, and the precipitate was weighed (Ws). The supernatant was then dried at 105 °C to a constant weight (W₁).

WSI = W₁/W₀ × 100 % (1)

SP = W_s/(W₀×(100 % − WSI)) (g/g) (2)

2.9. X-ray diffraction analysis (XRD)

According to the previous research, the XRD method has been adjusted appropriately [16]. The analysis was performed using an X-ray diffractometer (Empyrean, PANalytical, Netherlands). The test was determined at a scan rate of 4°/min over a diffraction angle range of 4° to 40°. The target voltage was 40 kV, and the current was 40 mA. The peaks and relative crystallinity were analyzed with Jade 6.5 software.

2.10. Fourier transform infrared spectroscopy (FTIR) and Raman spectrometry

FTIR spectral analysis was performed using the previous method [20]. The BWS sample was mixed with potassium bromide in a ratio of 1:100. The spectrum of the mixture was determined using a Fourier transform infrared spectrometer (NEXUS 670, Nicolli Instruments, USA) with a wavelength of 400 cm⁻¹ to 4000 cm⁻¹ and a resolution of 4 cm⁻¹. Each spectrum was obtained by averaging 32 scans and analyzed using OMNIC software.

Referring to previous measurement, Raman spectra of BWS were determined using a Raman spectrometer (LabRam HR Evolution, HORIBA, Japan) with a 785 nm laser source [21]. The laser power was set to 17 mv, the spectrum of each sample was collected over a scan range of 3200–400 cm⁻¹, and the half-peak width at 480 cm⁻¹ was recorded.

2.11. Scanning electron microscopy (SEM)

The BWS samples were sprayed with gold using the previous method [22]. Afterwards, the BWS sample was transferred into a scanning electron microscope (Sigma300, Zeiss), and all samples were observed at a magnification of ×1000.

2.12. Statistical analysis

The statistical significance of the obtained data was tested by one-way Analysis of Variance (ANOVA), and Duncan's multiple range test was used for mean comparison (p < 0.05). Statistical analysis was carried out using SPSS 19.0 statistical software.

3. Results and discussion

3.1. Texture property

The texture property was an important index of the gel property of starch, and the result can reflect the crosslinking degree of starch structure [16]. Hence, the gel hardness and gel strength characteristics of BWS under varied moisture content and different ultrasonic power treatments were detected.

As shown in Fig. 1A, the outcomes demonstrated that ultrasonic treatment significantly decreased the hardness of BWS gel. This may be due to the destruction of BWS particles by ultrasound, which then caused the gel hardness of starch to decrease. Based on recent research, the structure of starch granules could have been damaged by the combination of the cavitation effect and mechanical action during the ultrasonic treatment process, which led to the degradation of the branches of amylose and amylopectin, thus weakening the gel network structure [23]. Besides, the increase in moisture content caused a significant decrease in the gel hardness of starch from 26.91 g (control) to 11.35 g (UP200-3). This could be because the rise of moisture content enhanced the dielectric property of starch, which was associated with the ability of starch to absorb ultrasonic energy [24]. Therefore, under the same ultrasonic power condition, the gel hardness prepared by BWS with higher moisture content showed a decreasing trend. Besides, compared with 200 W ultrasound, the gel hardness of the starch treated with 400 W ultrasound was lower (except UP400-3). This may be because when a more powerful ultrasound treatment was applied, the BWS starch particles were further destroyed, causing less potential for gel organization [11]. In addition, the reason why the hardness of UP400-3 was higher than that of UP200-3 may be due to the fact that the long molecular chain of starch in UP400-3 sample was destroyed and turned into short chains, and then recombination led to a higher hardness.

Fig. 1.

The gel hardness (A) and gel strength (B) of native and ultrasonic modified BWS. Different lowercase letters represent significant differences (p < 0.05).

In addition, as shown in Fig. 1B, it can be found that with the increase of moisture content and ultrasonic power, the gel strength of BWS also decreased significantly. This may be because ultrasound destroyed the internal network structure of the starch, leading to the decomposition of excessive amylopectin side chains, which made it more challenging to crosslink and polymerize between starch molecules, thereby reducing the gel strength [25]. It was noteworthy that compared with the increase of ultrasonic power, the increase of moisture content can promote the further degradation of BWS structure by ultrasonic, weakening the gel network of starch molecules [9]. In conclusion, the increase of moisture content and power caused by ultrasonic waves would cause more severe damage to starch particles, significantly reducing the gel hardness and strength of BWS.

3.2. Dynamic properties

To further investigate the impact of ultrasonic treatment with different moisture and power on the dynamic rheology of starch, the viscoelasticity of different BWS gels was ascertained through frequency scanning. The alterations of G’ and G” of both native and ultrasonic-treated BWS gels concerning frequency were shown in Fig. 2. It was evident that the values of G’ and G” for all samples escalated with the rise in frequency, demonstrating a pronounced frequency dependence. By comparing the G’ value and the G” value, it can be observed that the G’ value exceeded the G” value, signifying that BWS gel exhibited excellent elasticity [26]. Meanwhile, the G” value of BWS gel declined following ultrasonic treatment. This might be attributed to the fact that ultrasonic treatment would disrupt starch molecular chains and covalent bonds via the strong shear force generated by cavitation and mechanical action, resulting in a reduction of viscosity [23]. It can be discerned that the moisture content exerted a considerable influence on the G” value of BWS gel, which could be because the increase in moisture content facilitated the absorption of more ultrasonic energy by starch [24]. The higher moisture content aggravated the damage of starch particles by ultrasonic shear force, leading to the further straightening of amylose molecules and the weakening of the shear action in the fluid layer, thereby causing the viscosity to decrease [27]. Similarly, under the same ultrasonic power, the G’ value of BWS gradually decreased with the increase of moisture content. This result may be because, with the increase in moisture content, the dielectric properties of starch will also be significantly enhanced under the influence of moisture content, making starch more susceptible to ultrasonic cavitation and causing more severe damage to starch particles [28]. When the moisture content of the suspension reached 75 %, the damage to the starch crystallization zone was further aggravated. This leads to amylopectin fragmentation and an increase in amylose molecule content, so the G’ value was the lowest at this time [27].

Fig. 2.

The dynamic rheological properties of native and ultrasonic modified BWS. (A) and (B) represent the G’ and G”, respectively.

3.3. Particle size distribution

The particle size of starch is related to the swelling ability of starch and starch gel properties, so the particle size distribution analysis for BWS was conducted. The cumulative particle size distribution and average particle size results of native and ultrasonic treatment of BWS were presented in Fig. 3 and Table 1. In contrast to native starch, the peak height of ultrasonic-treated starch increased, and the peak shifted to a narrower peak. This indicated that sonication caused the BWS particles to tend to move toward smaller particle sizes, while also improving the uniformity of the BWS particle size [29].

Fig. 3.

The particle size distribution of native and ultrasonic modified BWS.

Table 1.

The particle size distributions of native and ultrasonic modified BWS.

Samples	D10 (μm)	D50 (μm)	D90 (μm)
NB	4.63 ± 0.01a	10.53 ± 0.03a	27.40 ± 0.06a
UP200-1	4.55 ± 0.01b	10.23 ± 0.03b	25.93 ± 0.03c
UP200-2	4.52 ± 0.02c	10.03 ± 0.03d	24.63 ± 0.01e
UP200-3	4.32 ± 0.02f	9.17 ± 0.02f	20.93 ± 0.03f
UP400-1	4.48 ± 0.01d	10.13 ± 0.03c	26.23 ± 0.02b
UP400-2	4.52 ± 0.01c	9.95 ± 0.01d	25.53 ± 0.02d
UP400-3	4.40 ± 0.01e	9.29 ± 0.02e	20.73 ± 0.03g

Values followed by different superscripts in the same column present significantly different (P < 0.05).

D₁₀, D₅₀, and D₉₀ respectively represented the particle size corresponding to the cumulative particle size ratio of 10 %, 50 %, and 90 %, while D₅₀ was usually used to represent the average particle size [30]. It can be perceived intuitively from Table 1 that the particle size of all ultrasonic-treated starches was lower than that of native starches, and the average particle size of BWS decreased from 10.5 μm to 9.29 μm. Compared with 200 W ultrasonic treatment, the average particle size of BWS after 400 W ultrasonic treatment was smaller (except UP400-3). This might be attributed to the fact that the increase in power would intensify the mechanical action and oscillation of the ultrasonic wave, breaking the BWS particles into smaller particles and fragments, thereby reducing the average particle size of starch [31]. A similar trend was also discovered in the ultrasonic treatment of kiwi starch, which further substantiated our finding [20]. The particle size of starch in UP400-3 group was larger than that in UP200-3 group, which may be due to the dominant effect of ultrasonic wave on starch particle crushing in UP200-3 group. However, the energy input during the treatment of the UP400-3 group led to the activation of the starch particle surface, which promoted the agglomeration of the starch particles, thus contributing to this result.

In addition, the particle size of starch under ultrasonic treatment with different moisture content was also measured. As indicated, the average particle size of BWS decreased with the rise of moisture content. This might be attributed to the fact that the cavitation microbubbles would break and cause the pressure of the liquid layer around the starch to rise and then induce the shear forces that could damage the starch granules [32]. Moreover, the increase of moisture could lead to more cavitation microbubbles. That is to say, the increase of moisture would lead to a more substantial effect of shear force, promoting the degradation of polymer by ultrasonic waves and the further destruction of particles [33]. Simultaneously, the fragmented particles, after being damaged by ultrasonic waves, did not aggregate. Therefore, the size of the starch particles was decreased with the increase of the moisture content during the treatment [20].

3.4. Swelling power and solubility

The solubility and swelling power of starch were related to its hydration ability and structural integrity [20]. In order to investigate the effects of diverse ultrasonic powers and moisture contents on the molecular characteristics of BWS, the solubility and swelling power were measured. As shown in Fig. 4A, it can be observed that ultrasonic treatment enhanced the swelling power of BWS, which might be attributed to the disruption of the starch molecular structure [23]. Moreover, with the increase in moisture content and ultrasonic power, more severe damage to starch particles and starch chains was promoted by ultrasonic waves. This damage caused more holes to appear on the surface of the particles, which allowed more water to enter the interior [34]. The water molecules that entered the particle were linked with the free hydroxyl group of amylose and amylopectin via hydrogen bonding, enabling BWS to absorb and retain more water [26]. Therefore, the BWS prepared under 75 % moisture content and 400 W ultrasonic treatment exhibited the maximum swelling power.

Fig. 4.

The swelling power and solubility of native and ultrasonic modified BWS. Different lowercase letters represent significant differences (p < 0.05).

In addition, according to the solubility results (Fig. 4B), it can be identified that it also showed an increasing trend. That is, with the increase of moisture content and power, the solubility of BWS also gradually rose, which might be related to the leaching of amylose [23]. Ultrasonic damage led to the loosening of the structure of starch particles. The augmentation of power and moisture content would facilitate the further degradation of the BWS structure by ultrasonic waves, thereby resulting in more excellent release of amylopectin side chains and the leaching of amylose [35]. Therefore, the solubility of BWS after ultrasonic treatment would increase significantly.

3.5. Pasting properties

To further explore the impact of ultrasound on starch swelling, the pasting behavior was measured. As shown in Table 2, ultrasonic treatment significantly reduced the viscosity of BWS compared to native starch. Peak viscosity (PV) referred to the viscosity of starch particles when the volume of starch particles expanded to the maximum level during heating [19]. As can be seen, the PV value of native BWS was 2329 mPa, and after ultrasonic treatment, the PV value of BWS was reduced to 2105 mPa (400 W, BWS-water ratio of 1:3). This may be related to the destruction of starch particles and the increased permeability of water [37]. In addition, the cavitation and oscillation of ultrasonic waves shorten the length of starch chains and weaken the interaction force between the starch particles, reducing starch viscosity [12]. Under the conditions of high power and high moisture content, the viscosity of BWS was observed to decrease more significantly. This may be because more muscular ultrasonic treatment conditions would weaken the tangles between starch molecular chains to a greater extent and simultaneously reduce the flow resistance of starch [22]. Recent study had found that the peak viscosity also depended on the size of starch particles, and the peak viscosity increased with the increase of particles [38]. In the previous analysis of BWS particle size, it can be seen that the increase of moisture and power led to a further reduction of BWS particle size. This was consistent with our pasting results, which further verified that increased moisture and power promoted the decrease of starch PV value.

Table 2.

The pasting properties of native and ultrasonic modified BWS.

Sample	PV (cP)	TV (cP)	BD (cP)	FV (cP)	SB (cP)	PT (°C)
NB	2329 ± 1.42a	2015 ± 38.18a	314 ± 36.76a	2236 ± 24.74a	221 ± 13.43a	93.67 ± 0.03a
UP200-1	2270 ± 35.35b	1929 ± 57.27b	340 ± 21.92a	2101 ± 95.45b	172 ± 38.18b	93.37 ± 0.04a
UP200-2	2225 ± 34.64bc	1834 ± 18.38c	391 ± 16.26b	1982 ± 50.20c	148 ± 31.81b	93.32 ± 0.04a
UP200-3	2129 ± 14.84d	1626 ± 14.14d	503 ± 0.71c	1715 ± 9.19d	89 ± 4.94cd	92.65 ± 0.07b
UP400-1	2203 ± 23.33c	1851 ± 25.45c	352 ± 2.12ab	2017 ± 28.28bc	166 ± 2.82b	93.32 ± 0.03a
UP400-2	2193 ± 0.71c	1802 ± 12.72c	391 ± 13.43b	1937 ± 14.84c	135 ± 2.12bc	92.87 ± 0.38b
UP400-3	2105 ± 27.57d	1610 ± 9.89d	495 ± 17.67c	1693 ± 2.12d	83 ± 7.77d	92.60 ± 0.14b

Data are expressed as means ± SD of duplicate assays. Values followed by different superscripts in the same column present significantly different (P < 0.05). PV: Peak viscosity. TV: Trough viscosity. BD: Breakdown value. FV: Final viscosity. SB: Setback value. PT: Pasting temperature.

The setback (SB) value indicated the short-term retrogradation of starch, which can reflect the ability of starch to resist retrogradation [18]. As can be seen from Table 2, with the increase of moisture and power, the SB value of starch paste gradually decreased, which may be due to the interference of ultrasonic wave on amylose rearrangement, weakening the hydrogen bond formed between leached amylose and amylopectin, and inhibiting the formation of gel network [39]. At the same time, higher moisture content facilitated the dispersion of starch molecules and hindered the formation of crystallization, thereby delaying the retrogradation trend of starch [40]. Pasting temperature (PT) was an index of the structural resistance of starch to heat-induced swelling and cracks in water [16]. It can be found that the PT of BWS decreased slightly after ultrasonic treatment, which indicated that the swelling resistance of BWS was weakened by ultrasonic treatment [41]. This may be due to the breakdown of starch granules, disruption of the crystalline layer, and improved crystal hydration [42].

3.6. Long-range ordered structure

In order to further study the effect of ultrasonic treatment on the type of starch crystallization and long-range order, different treated BWS were tested by XRD, and the results were shown in Fig. 5A [43]. The XRD pattern of native starch exhibited prominent diffraction peaks at 15° and 23°, and two continuous diffraction peaks along 17° and 18°, showing that BWS was a typical A-type starch [44]. Compared with native starch, there was no change in the diffraction pattern of ultrasonic-treated starch, and no new diffraction peaks were observed, indicating that ultrasonic treatment did not change the crystallization pattern of BWS [26]. However, as can be seen from the graph, the intensity of the diffraction peaks of the ultrasonically treated starch decreased, indicating lattice distortion and a reduction in crystal size.And there was an obvious correlation among them with the increase of ultrasonic power and moisture content. The relative crystallinity of native BWS was 19.62 %, and with the increase of moisture content from 50 % to 75 %, the relative crystallinity of starch decreased significantly to 13.09 %, suggesting that the crystalline structure of starch was susceptible to the influence of moisture content. This may be because the increased moisture content promoted hydration in the amorphous region of BWS, thus enhancing the effect of ultrasound on starch [45]. Besides, the increase of water assisted the ultrasonic wave to produced more free radicals, which, combined with cavitation, destroyed the surface and internal molecular structure of the starch particles to a greater extent, leading to the decrease in crystallinity [46]. Comparing the crystallinity of BWS treated with different ultrasonic powers, it can be found that compared with 200 W ultrasonic treatment, the relative crystallinity of BWS under 400 W ultrasonic treatment was further reduced (except UP400-3), and a similar trend of relative crystallinity reduction was also observed in the study of waxy corn starch [47]. This may be because ultrasonic destroyed the amorphous region of starch and weakened the crystal region, resulting in loose crystal structure and lattice accumulation [22]. With the increase of ultrasonic power, this damage was enhanced, and eventually, the smaller starch crystal region was formed.

Fig. 5.

The long-range and short-range ordered structure of native and ultrasonic modified BWS. (A): XRD pattern, (B): FTIR spectra, (C): DD value, (D): DO value, (E): Raman spectra, (F): FWHM.

3.7. Short-range ordered structure

FTIR and Raman spectroscopy can be used to study the short-range ordered molecular arrangement of starch. The FTIR spectra of BWS prepared by different ultrasonic treatments were shown in Fig. 5B. It can be seen that the FTIR spectra of all BWS samples did not show differences in terms of characteristic absorption peaks, suggesting that no new functional groups or covalent bonds were formed by sonication [29]. A broad and strong characteristic peak was observed between 3000 and 3600 cm⁻¹, which may be related to the stretching vibration of the –OH group [48]. At the same time, a sharp peak was observed near 2900 cm⁻¹, which represented the stretching vibration of the C-H group [35]. The spectrum of the ultrasonically treated starch and the native starch were similar, and no new absorption peaks emerged, which indicated that the ultrasonic treatment had not changed the types of the original chemical groups [36].

The absorption peaks at 800 to 1200 cm⁻¹ in the infrared spectrum were associated with stretching vibrations of C-O and C-C, the characteristic peaks at 1047 cm⁻¹ and 1022 cm⁻¹ indicated the ordered and amorphous structures of starch, respectively, while the characteristic peaks at 995 cm⁻¹ were associated with the proximal sequence of the double helix [15]. In addition, the absorbance ratio of 1047/1022 cm⁻¹ (R_1047/1022) was commonly used to describe the degree of order (DO), and the conformational change of the double helicity (DD) can be indicated by the ratio of 995/1022 cm⁻¹ (R_995/1022) [49]. As shown in Fig. 5C-D, with the increase of ultrasonic power and moisture content, the DO value showed a decreasing trend, which may be caused by the cavitation effect of ultrasound together with the mechanical action to destroy the crystalline and amorphous regions of starch, resulting in irregular arrangement and distortion of starch molecules [20]. This was similar to the previous study in which DO values were also observed to decrease with the increase of ultrasonic power in the ultrasonic treatment of rice starch [37]. Similarly, in this study, DD values also showed a decreasing trend similar to DO values, which may be related to hydrogen bond breakage. The ultrasonic treatment destroyed the hydrogen bond in the starch molecular chain, which unwound the helix structure, resulting in the formation of a more loose spiral structure of BWS [46].

The Raman band located at 480 cm⁻¹ was generally associated with stretching vibrations of the C-O-C bond of starch [21]. Moreover, the full width at half maximum (FWHM) at this point represented the short-range ordered structure of starch [50]. The FWHM values exhibited a significant negative correlation with the ordered structure of starch. Specifically, an increase in FWHM values corresponds to a reduction in the degree of structural ordering. Fig. 5E-F showed the Raman spectral curves and FWHM values of the BWS sample, respectively. It can be intuitively seen that different ultrasonic-treated starches showed similar Raman spectra curves, and no new characteristic peaks were generated. This indicated that sonication did not cause changes in the groups in BWS [51]. At the same time, with the increase of moisture content and power, the FWHM value also gradually increased. This suggested that ultrasound may tend to disrupt the crystallizing region of starch by disturbing the ordered arrangement of the double helix in the starch particles [23]. This result was in agreement with our XRD results. In conclusion, sonication inhibited the ordered arrangement of BWS molecular structures.

3.8. SEM

Our previous study found that the particle size and physicochemical properties of starch had changed after treatment, indicating that the starch structure was changed. In order to investigate the microstructure of BWS, the changes were detected by SEM. As shown in Fig. 6, it could be seen that the surface of native starch particles was smooth, and there were no cracks on the outside, most of them were oval or polygonal [52]. After ultrasonic treatment, the surface of BWS particles became rough, and certain depressions and holes appeared. In addition, the particle size was also reduced. For BWS samples with higher moisture content, a greater degree of particle destruction and dispersion was observed. This may be related to the sonochemical effect of ultrasonic waves, which caused the cracking of water molecules to produce OH free radicals, then OH free radicals would attack starch particles and promote the degradation of starch particles [47]. In this study, the increase of water could provide more water molecules to be cracked by ultrasonic waves, thus producing a more significant number of OH radicals to destroy starch particles. Therefore, after ultrasonic treatment with 75 % moisture content, the BWS particles became coarser, with more cracks and holes appearing on the surface. In addition, the increase of ultrasonic power also seemed to increase the degree of particle damage. This may be because the increase in power will enhance the high-shear and high-frequency jet action of the ultrasonic wave, resulting in a greater degree of damage to the starch particles [20]. The SEM results confirmed that ultrasonic treatment could affect the morphology and size of BWS particles, which was helpful in improving the uniformity of starch particles.

Fig. 6.

The microstructure of native and ultrasonic modified BWS. (A): NB. (B): UP200-1. (C): UP200-2. (D): UP200-3. (E): UP400-1. (F): UP400-2. (G): UP400-3.

4. Conclusion

The results showed that compared with different ultrasonic power levels (200 W, 400 W), the moisture content of the starch suspension had a more notable impact on the experimental results. And the physicochemical properties of starch changed significantly when the ratio of starch to water reached 1:3. The results showed that ultrasonic treatment could improve the texture and rheological properties of BWS gel. With the increase of the moisture content of the suspension during treatment, the swelling power and solubility of BWS gradually increased, and the peak viscosity and setback value decreased, indicating that ultrasonic treatment could delay the staling of starch. With the increase of moisture content and power, the relative crystallinity and short-range order of starch decreased further. This confirmed that ultrasonic treatment could hinder the formation of ordered structures in starch crystalline and amorphous regions. In conclusion, ultrasonic treatment could improve the physicochemical and structural properties of BWS to a certain extent and provide theoretical support for its wide application in the food industry.

CRediT authorship contribution statement

Yue Yan: Writing – original draft, Methodology, Investigation. Meihan Jia: Formal analysis, Conceptualization. Zuohang Zhou: Resources. Shensheng Xiao: Methodology, Formal analysis. Peili Lin: Resources, Formal analysis. Yiying Wang: Validation, Methodology. Yang Fu: Writing – review & editing, Project administration, Funding acquisition. Xuedong Wang: Writing – review & editing, Validation, 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.