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Ultrason Sonochem
Ultrason Sonochem
Ultrasonics Sonochemistry
1350-4177
1873-2828
Elsevier

S1350-4177(24)00282-7
10.1016/j.ultsonch.2024.107034
107034
Original Research Article
Bactericidal effect of ultrasound on glutinous rice during soaking and its influence on physicochemical properties of starch and quality characteristics of sweet dumplings
Wang Shuli a1
Wang Xiaojie abc1
Liu Yu a
Dong Wenjing a
Fan Huiping abc
Fan Shijia a
Ai Zhilu abc
Yang Yong abc
Suo Biao bsuo@henau.edu.cn
abc⁎
a College of Food Science and Technology, Henan Agricultural University, Zhengzhou, China
b Key Laboratory of Staple Grain Processing, Ministry of Agriculture and Rural Affairs, Zhengzhou, China
c National R&D Center for Frozen Rice & Wheat Products Processing Technology, Henan Province Engineering Research Center of Quick-Frozen Flour-Rice and Prepared Food, Henan Agricultural University, Zhengzhou, China
⁎ Corresponding author at: College of Food Science and Technology, Henan Agricultural University, 63 Nongye Rd., Zhengzhou, Henan 450002, China. bsuo@henau.edu.cn
1 These authors contributed equally to this work and should be considered co-first authors.

16 8 2024
11 2024
16 8 2024
110 10703415 6 2024
11 8 2024
15 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Graphical abstract

Highlights

• Ultrasound effectively reduced viable cell number in glutinous rice after 4 h of soaking.

• Ultrasound destroyed the granule morphology of starch, leading to cracks on the surface.

• Ultrasonic treatment improved the physicochemical properties of glutinous rice starch.

• The quality of sweet dumplings made of ultrasound treated glutinous rice is improved.

• The effects of ultrasound on the physicochemical properties improved with increasing power.

The soaking process of glutinous rice allows the growth and reproduction of microorganisms, which can easily cause food safety problems. In this work, the effects of different ultrasonic powers (150 W, 300 W, 450 W, and 600 W) on the bactericidal effect of glutinous rice, the physicochemical properties of starch and the quality characteristics of sweet dumplings were studied. Compared with soaking for 0 and 2 h, sonication of glutinous rice after soaking for 4 h was more effective at reducing the number of microorganisms in soaked glutinous rice, and the bactericidal effect increased with increasing ultrasound intensity. After 30 min, the total number of bacteria decreased by 2.04 log CFU/g. Moreover, ultrasonic treatment destroys the grain structure of glutinous rice starch, resulting in the formation of dents and cracks on the starch surface, increasing the amylose content, improving its expansion, reducing its short-range order and relative crystallinity, and altering its gelatinization characteristics. In addition, ultrasonic treatment increased the soup transparency of sweet dumplings from 51.8 % to 63.95 %, reducing their hardness, chewiness and adhesiveness. In summary, ultrasonic treatment can not only effectively kill microorganisms in soaked glutinous rice but also improve the quality of glutinous rice dumplings by changing the physicochemical properties of glutinous rice starch. The results of this study provide theoretical support for the application of ultrasonic technology in glutinous rice food production.

Keywords

Ultrasonic treatment
Glutinous rice
Microbial inactivation
Starch physiochemical properties
Sweet dumplings
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pmc1 Introduction

Sticky rice, also known as glutinous rice or sweet rice, is traditionally used as a staple food for Asians because of its soft texture, high viscosity, unique flavor, and high nutritional value [1]. Following the procedure used for sticky rice grains, including washing, soaking, and milling, the resulting glutinous rice flour can be processed into starchy foods such as rice cakes, rice skins, rice noodles, and glutinous rice balls. Among the products, sweet dumplings, made of glutinous rice flour and water in proportion, are a traditional Chinese food symbolizing reunion [2].

With the development of quick-freezing and cold chain technologies, the industrial production of glutinous rice flour products is becoming increasingly popular by integrating the whole production chain from glutinous rice to flour production in one factory because of its advantages of energy savings and quality guarantees [3]. The soaking of glutinous rice is a necessary and critically designed step for flour production. According to previous reports, glutinous rice grains reach the saturation point after 40 min of soaking [4], while 3.5 h is needed to achieve the lowest level of starch destruction, and 4 h of soaking is normally needed when the produced glutinous rice flour is used for making frozen sweet dumplings [5]. However, during the soaking and milling of rice, microorganisms in glutinous rice migrate to water, resulting in a significant increase in the microbial population in the rice slurry [3], [6]. Moreover, microorganisms prone to cross-contamination during pretreatment, manufacturing, processing, and storage might survive even under cold conditions and cause food quality and safety concerns [7], [8], [9] Therefore, inhibiting the growth and reproduction of microorganisms during the soaking process of glutinous rice has become a critical control point for harmful microorganisms during the production of glutinous rice flour products.

Ultrasonication is an emerging green nonthermal sterilization method and a promising alternative to traditional pasteurization for application in food processing [10], [11]. To date, many novel bactericidal technologies have been developed on the basis of ultrasound and have been applied in foods such as apple juice [12], purple kale [13], and salmon [14]. When combined with slightly acidic electrolytic water, ultrasound can effectively reduce the number of Bacillus cereus in brown rice [15]. However, the effectiveness of ultrasonic treatment in inactivating bacterial cells in soaked glutinous rice during flour production has not been evaluated.

In addition to its bactericidal effect, ultrasound is also a promising alternative for improving the quality of flour foods. Ultrasonic waves improved the specific volume and texture properties of steamed bread [16]. The gluten-free dough dumplings prepared by the ultrasound-assisted automatic forming method can homogenize the structure of the dough, improve the rheological properties, increase the volume of the dough and thus increase the yield of dumpling [17]. Ultrasound can significantly improve the viscoelasticity of rice flour dough, soften bread and delay hardening during storage [18]. In addition, proper ultrasonic treatment during the soaking process can improve the nutritional value of white rice and improve its palatability [19]. Compared with those in the control group, parboiled rice grains treated with ultrasound presented lower viscosity, significantly reduced hardness and chewability, and a softer taste [20]. After ultrasonic treatment, the water absorption rate of Shanlan rice increased significantly, which promoted saccharification and alcohol conversion at the initial stage of fermentation and increased the nonvolatile acid content of rice wine [21]. The improvement in quality resulting from ultrasonic treatment can be attributed to the promotion of interactions between different starch molecular chains, mainly through mechanical action, cavitation and thermal effects, destroying the crystal structure, original hydrogen bonds and double helix structure of starch granules, thereby affecting gelatinization performance; these changes affect the morphology, structure, physicochemical indicators and digestive characteristics of starch and food products [22], [23], [24]. However, the influence of ultrasonic treatment on flour texture and food quality has not been reported for soaked glutinous rice, which has a high amylopectin content and high viscosity.

Previous studies have used ultrasonic methods to sterilize rice slurries after milling [25]. However, in factory production, it is difficult to apply ultrasonic treatment to milled glutinous rice slurry. This can be attributed to the following reasons: (1) the milling machine cannot be connected directly to external ultrasonic equipment, and (2) the milled glutinous rice slurry has a high viscosity and is thus difficult to transport to an ultrasound container unless additional energy and resources are consumed. Comparatively, the soaking process of glutinous rice is normally carried out in a standing container, which is easier to connect with external ultrasonic equipment. Thirty minutes of ultrasonic treatment of brown rice during the soaking process produced satisfactory ready-to-eat rice with good appearance integrity and improved brightness and chewiness [26]. However, the bactericidal effect of ultrasonic treatment of glutinous rice during the soaking process and its influence on the quality of the resulting products have not been reported until now.

In this work, the bactericidal effect of ultrasonic treatment on glutinous rice during the soaking process was investigated, and the changes in grain morphology, short-range order, crystal structure, gelatinization characteristics and thermodynamic properties of glutinous rice starch were analyzed to further understand the influence of ultrasonication on product quality. The purpose of this study was to provide a reference for the application of ultrasonic technology in the processing of glutinous rice products.

2 Materials and methods

2.1 Glutinous rice raw material and ultrasonic treatment

Glutinous rice, with a water content of 12.90 %, a protein content of 8.72 % and an amylose content of 1.55 %, was obtained from Yihai Kerry Food Industry Co., Ltd. (Zhengzhou, China). Fifty grams of glutinous rice was soaked in 200 mL of distilled water. The mixture was placed in a constant-frequency ultrasonic cell disruptor (900 W nominal power, 20 kHz, JY92-IIN, Xinzhi Biotechnology Co., Ltd., Ningbo, China). The ultrasonic treatment was performed for 30 min, which was intermittent for 5 s after each 5 s of treatment. The power was set to 150, 300, 450, and 600 W, respectively. The sonicated glutinous rice grains were divided into three groups, which were soaked for 0, 2, or 4 h and then subjected to sonication (30 min). Glutinous rice without ultrasound was used as a control.

2.2 Microbial enumeration

The soaked glutinous rice samples were milled for 4 min to make a rice slurry via a pulper (PB9706, Foshan Haixun Electric Co., Ltd., Foshan, China). The rice slurry was serially diluted with a 10-fold gradient of 0.85 % sterile saline. Then, 1 mL of an appropriate dilution gradient solution was spread on plate count agar (PCA, Beijing Auboxing Biotechnology Co., Ltd., China), De-Man Rogosa Sharpe agar (MRS, Land Bridge Technology Co., Ltd., Beijing, China), Baird–Parker agar (BP, Land Bridge Technology Co., Ltd., Beijing, China), and mannitol–yolk polymyxin agar (MYP, Land Bridge Science and Technology Co., Ltd., Beijing, China) to enumerate the numbers of total plates (TPCs), lactic acid bacteria (LABs), Staphylococcus aureus, and Bacillus cereus, respectively. All plates were incubated at 37 °C for 24 h, except for 48 h for MRS.

2.3 Extraction of glutinous rice starch

Glutinous rice starch was extracted from the rice slurry according to the method of Cardoso et al. [27]. One hundred grams of glutinous rice pulp and 1500 mL of NaOH (0.3 %) solution were mixed evenly, stirred with a magnetic stirrer for 4 h, and centrifuged for 5 min at 5000 r/min. The supernatant was discarded, and the sediment was washed with distilled water four times. After that, two volumes of petroleum ether were added to the sediment, which was subsequently filtered twice, dried at 40 °C, ground, and passed through a 100 mesh sieve for use.

2.4 Effects of different ultrasonic powers on the physicochemical properties of glutinous rice starch in rice pulp

2.4.1 Scanning electron microscopy (SEM) observation

The sample was fixed on an aluminum stub using double-sided tape and subjected to gold spraying under vacuum. The particle morphology of the samples was observed using an SEM (TESCAN MIRA LMS, Czech Republic) at an acceleration voltage of 3 kV. The samples were observed at magnifications of × 2,000.

2.4.2 Fourier transform infrared spectroscopy (FT-IR)

The FT-IR spectrum of the glutinous rice starch sample was measured via FTIR (TEN-SOR II, Bruker Technology Co., Ltd., Germany) according to Liu et al. [28]. The determination parameters were as follows: air was used as the background, the scan band was in the range of 400–4,000 cm−1, and the resolution was 4 cm−1. Fourier self-deconvolution was performed on the spectrum in the 800–1,200 cm−1 band range via OMNIC 8.0 software (Thermo, USA), with the half width and enhancement factor set to 38 cm−1 and 1.9, respectively, to obtain 1047 and 1022 cm−1 absorption peak amplitudes. The R value was calculated from the amplitude ratio of the absorbance peaks at 1047 and 1022 cm−1.

2.4.3 X-ray diffraction (XRD) analysis

The X-ray diffraction (XRD) patterns of glutinous rice starch samples were determined with a slightly modified X-ray diffractometer (Bruker D8 Advance, Bruker Technology Co., Ltd., Germany) according to the method of Yang et al.[29]. The operating current and voltage were set to 30  mA and 40  kV, respectively, the scanning range was 5–50°, and the scanning speed was 5°/min. Jade 6.0 software (Materials Data Co., Ltd., USA) was subsequently used to calculate the relative crystallinity of the samples.

2.4.4 Determination of apparent amylose content (AAC)

The apparent amylose content was evaluated via a colorimetric assay [30]. One hundred micrograms of ground rice starch was mixed thoroughly with 1 mL of 95 % ethanol and 9 mL of 1 mol/L NaOH. The mixture was heated in a 100 °C water bath for 10 min. After cooling to room temperature, the solution was diluted to 100 mL with distilled water. Then, 5 mL of the starch solution was mixed with 1 mL of 1 N CH3COOH and 2 mL of iodine solution containing 0.2 % I2 and 2.0 % KI. The mixture was diluted to 100 mL and then allowed to stand for 20 min. The AAC content was calculated according to the absorbance of the mixed solutions at 620 nm measured by a UV Mini 1240 spectrophotometer (Shimadzu Corp., Tokyo, Japan).

2.4.5 Rapid viscosity analysis (RVA)

Two grams of starch sample was mixed with 25 mL of distilled water in an RVA aluminum vessel, after which the pasting characteristics were measured using a rapid starch viscosity analyzer (RVA4500, Perten Instruments, Sweden). The temperature program was set according to the following program: incubate at 50 °C for 1 min, heat to 95 °C at a rate of 12 °C/min, maintain for 2.5 min, and finally decrease the temperature to 50 °C at a rate of 12 °C/min for 2 min [31].

2.4.6 Differential scanning calorimetry (DSC) analysis

The thermodynamic properties of the glutinous rice starch samples were determined using a differential calorimetry scanner (DSC-214, NETZSCH Instruments Co., Ltd.) according to Ding et al. [32] with slight modifications. Before DSC evaluation, 3 mg of glutinous rice starch was mixed with 6 μL of distilled water in a DSC aluminum pot and then refrigerated in a 4 °C freezer for 24 h. The thermodynamic properties were determined as the temperature program increased from 30 °C to 100 °C at a rate of 10 °C/min.

2.4.7 Determination of the swelling rate

The solubility and swelling degree of the glutinous rice starch samples were determined according to the methods of Kumar et al. [33] with slight modifications. Four hundred micrograms of glutinous rice starch were placed in a 50 mL preweighed centrifuge tube, after which 25 mL of distilled water was added. The starch suspension was shaken thoroughly and incubated at 80 °C. Then, the completely swollen starch suspension was cooled to room temperature and centrifuged at 5,000 r/min for 20 min. The supernatant was transferred to an aluminum box and dried at 105 °C to a constant weight. The weight of the sediment was also recorded. The solubility (S) and swelling degree (SP) of the starch samples were calculated according to Equations (1), (2).(1) S%=m1m0×100%

(2) SP%=m2m0×(1-S)×100%

where m1 represents the mass of the supernatant dried to a constant weight, g; m2 represents the mass of the sediment, g; m0 represents the mass of glutinous rice starch on a dry basis, g; S represents the solubility, %; and SP represents the degree of swelling, %.

2.5 Evaluation of the attributes of sweet dumplings

2.5.1 Preparation of sweet dumplings

Quick-frozen sweet dumpling was selected as the representative product because it is one of the most popular products made from glutinous rice flour and has the advantages of convenient storage, a long cold storage period, and easy sales and transportation after factory production [34], [35], [36]. A dough with 45 % water content was prepared from 2.2 % rice milk and glutinous rice flour and allowed to stand for 3 min until the water was evenly distributed. A piece of dough (8.0 g) was rounded, put into a disposable tray, and put into a −30 °C high- and low-temperature test chamber (BPH-100A, Yanhe Scientific Instrument Co., Ltd., Shanghai, China) for quick freezing treatment. The frozen dumplings were stored in a freezer at −18 °C.

2.5.2 Mushy soup rate

Five quick-frozen sweet dumplings were removed from the freezer and immediately transferred to a pot containing 500 mL of boiled water [37]. After the rice balls were completely floated on the surface of the boiled water, the cooked soup was cooled to room temperature and then diluted to 1000 mL. The transmittance of the soap was measured at 620 nm with a UV–vis spectrophotometer (UV2000, Shanghai Unocal Instrument Co., Ltd., China), and distilled water was used as a blank control.

2.5.3 Textural characteristics

The textural characteristics of the cooked sweet dumplings were evaluated via a texture analyzer in TPA mode (TA-XT Plus, UK) according to Zhang et al. [38]. The texture analyzer was equipped with a P/36 R probe. The pretest, test and posttest speeds were all set at 1 mm/s. The deformation amount was set to 50 %, and the duration between two compressions was 5 s. The compression component of the probe was aimed at the center of the dough. Five measurements were taken for each group of samples, and the average value was obtained. The textural properties of the sweet dumplings, including hardness, viscosity, elasticity, cohesiveness, chewiness, and resilience, were recorded.

2.6 Statistical analysis

All the data are presented as the average of three measurements and were analyzed for significance (p < 0.05) via Duncan’s multiple tests by using SPSS 25.0 (IBM Corp, Armonk, NY). Origin 2019b software (OriginLab Co. Ltd., Northampton, MA, USA) was used to construct diagrams.

3 Results and discussion

3.1 Effects of ultrasound on microorganisms in glutinous rice during soaking

The changes in microbial colonies after ultrasonic treatment are shown in Fig. 1. After soaking for 4 h, the ultrasonic sterilization effect was better than that after soaking for 0 h and 2 h. This difference can be attributed to the softer and more uniform system because full soaking is more conducive to the function of ultrasonic treatment [39]. Previous studies have shown that ultrasonic treatment can shorten the soaking time and promote the soaking effect [26]. A duration of more than 4 h of soaking was not evaluated for possible improvement in the bactericidal effect because this period is enough to produce glutinous rice flour [5]. Moreover, prolonged soaking consumes processing time and increases the risk of microbial proliferation [3]. Although the softer system obtained by longer soaking is more conducive to improving the bactericidal effect, the milled rice slurry was not selected for ultrasonic treatment. The reason is that during actual factory production, milled glutinous rice slurry is difficult to treat with external ultrasound equipment because it is very difficult to clean and is more susceptible to microbial cross-contamination. Therefore, the ultrasonic treatment step was designed for the soaking process of glutinous rice.Fig. 1 The change in the number of glutinous rice colonies soaked after ultrasonication at different powers. (A: Total number of viable cells in glutinous rice that were soaked for 0 h, 2 h and 4 h before ultrasonic treatment; B: The samples were soaked for 4 h prior to 30 min of ultrasonic treatment. The plate count (PCA), De-Man Rogosa Sharpe (MRS), mannitol–yolk polymyxin (MYP), and Baird–Parker (BP) agars were used to enumerate the viable cell numbers of the total plate (TPC), lactic acid bacteria (LAB), Bacillus cereus, and Staphylococcus aureus, respectively.

During the soaking process, compared with that in the control group, the total number of viable cells decreased by 1.09--2.04 log CFU/g under different ultrasound conditions. The bactericidal effect continuously enhanced as the ultrasound power increased. This is because the mechanical action and cavitation of ultrasound in solution generate high temperatures and high pressure, which directly destroy the cell wall or cell membrane of microorganisms, promote the decomposition of water molecules, lead to the generation of free radicals with strong oxidizing effects, and inactivate key enzymes in microorganisms, causing microbial cell rupture and cell death [40]. With increasing ultrasonic power, the amount of E. coli decreased gradually. When the power increased from 100 W to 200 W, the number of E. coli decreased by 41 % [41]. After ultrasonic treatment, the number of E. coli in vegetable juice can be reduced by 3 log CFU/mL, and the subsequent growth of E. coli in vegetable juice can be inhibited [10]. Ultrasonic (60 W/L) treatment reduced the total number of barley grass microbial colonies by 33 % [42]. Ultrasonic treatment alone for 6 min could reduce Salmonella typhimurium in onion by 0.98 log CFU/g, and the bactericidal effect increased with extending time [43]. Ultrasound combined with a bacteriostatic agent reduced 2 log CFU/g mold and yeast in green asparagus [44]. The reason for the improved bactericidal effect may be that the increase in ultrasonic power can expand the range of ultrasonic cavitation in the liquid, promote the high-intensity cavitation effect, and increase the formation and rupture of bubbles, resulting in the destruction and death of cells.

3.2 Effects of ultrasound on the structural characteristics of glutinous rice starch

3.2.1 Morphological characteristics

The particles of all the starch samples presented a polygonal structure, and the surface of the untreated glutinous rice starch samples was smoother, with fewer cracks and dents (Fig. 2). After ultrasonic treatment, the surface of the glutinous rice starch particles was rough, with more cracks and depressions, and the degree of damage increased with increasing ultrasonic power. When the ultrasonic power reached a maximum of 600 W, more debris appeared on the surface of the starch particles. This result is consistent with the effects of different ultrasonication times on the appearance of kiwifruit starch particles [45]. This is mainly because the strong shearing force generated by the rupture of cavitation bubbles caused by ultrasonic vibration acts on large starch granules, destroying the surface structure and causing cracks, depressions, holes, etc.[46]. Similar results were also obtained in corn starch, where the surface of starch particles became rough and bubbles burst on the surface of starch particles caused by ultrasonic cavitation, resulting in pits and pores [47].Fig. 2 Scanning electron microscopy (SEM) micrographs of starch from glutinous rice flour milled from glutinous rice after ultrasonic treatment at different powers (A: CK; B: 150 W; C: 300 W, D: 450 W; E: 600 W).

3.2.2 Short-range ordered structure of starch

FT-IR spectroscopy is an effective method for analyzing molecular structure changes in starch. The amplitude of the absorbance peak at 1047 cm−1 was related to the number of ordered starch domains, the amplitude of the absorbance peak at 1022 cm−1 was related to the number of amorphous starch domains, and the short-range ordered structure of starch granules was represented by R1047/1022 [48]. As shown in Fig. 3, the FT-IR spectra of all the glutinous rice starch samples exhibited no differences in terms of the absorption or characteristic peaks. This finding indicates that sonication cannot produce new functional groups. The R value of the samples in the control group was 1.1606, and those in the 150 W, 300 W, 450 W and 600 W treatment groups were 1.136, 1.109, 1.0526 and 0.9987, respectively. Compared with the control, ultrasonic treatment of soaked glutinous rice obviously decreased the R1047/1022 ratio of milled starch (p < 0.05), and this decrease improved with increasing ultrasonic power. The results indicated that ultrasonic treatment reduced the short-range order of glutinous rice starch and that the magnitude of the reduction increased with increasing ultrasonic power. These results agreed with a previous report that ultrasound had an attenuation effect on R1047/1022 of sweet potato starch [49], likely because the cavitation effect of ultrasound destroys the hydrogen bonds between or inside the starch molecular chains, resulting in the irregular arrangement and twisting of the starch molecules [50].Fig. 3 FT-IR spectra and R1047/1022 values of flour milled from glutinous rice after ultrasonic treatment at different powers.

3.2.3 Crystal structure

XRD is an effective indicator reflecting changes in starch crystal structure. As shown in Fig. 4, all glutinous rice starch samples exhibited strong diffraction peaks at 15°, 17°, 18°, and 23°, with no difference in diffraction peak positions, indicating that sonication of glutinous rice did not change the crystal structure of the starch. The relative crystallinity of the untreated glutinous rice starch was 23.16 %. After ultrasonic treatment at 150 W, 300 W, 450 W and 600 W, the relative crystallinity of the glutinous rice starch samples was 21.57 %, 20.95 %, 20.63 % and 19.8 %, respectively, and there were significant differences among all the samples (p < 0.05). Compared with that of the untreated control, the relative crystallinity of the starch milled from the ultrasound-treated glutinous rice clearly decreased with increasing ultrasonic power. A previous report on oat starch revealed that ultrasonic treatment could reduce the relative crystallinity without changing the crystalline form [51]. These results can be attributed to the fact that the free radicals and cavitation generated by ultrasound destroy the surface and internal molecular structure of starch granules, resulting in the dissolution of amylose and a decrease in crystallinity [20].Fig. 4 XRD patterns and relative crystallinity of glutinous rice flour treated with different ultrasonic powers.

3.3 Effects of ultrasound on the physicochemical properties of glutinous rice starch

3.3.1 Apparent amylose content

The amylose content affects many physical, chemical, and functional properties of starch, such as gelatinization and swelling characteristics. As shown in Table 1, the amylose content of glutinous rice starch without ultrasonic treatment was 1.55 %. After ultrasonic treatment at different powers (150 W, 300 W, 450 W and 600 W), the relative amylose contents of glutinous rice starch were 2.0 %, 2.03 %, 2.13 % and 2.19 %, respectively. The results revealed that ultrasonic treatment significantly increased the amylose content of soaked glutinous rice starch. This effect should be due to the significant cavitation and mechanical impact of the ultrasonic treatment, which forces cracking of the linear chains of amylose and promotes the rapid diffusion of starch molecules. and dissolution [52]. Similarly, ultrasound significantly increased the amylose content of kiwi starch by destroying the starch molecular chains to increase the linear fragmentation of amylose [45]. Ultrasound-assisted alkaline extraction increased the amylose content of pea starch [53]. Ultrasonic treatment improved the amylose content, swelling power, solubility and transmittance of oat starch [51].Table 1 Changes in the physicochemical properties of glutinous rice flour after ultrasonic treatment at different power levels.

Parameters	Ultrasonic Power (W)	
CK	150	300	450	600	
AAC (%)	1.55 ± 0.05d	2.00 ± 0.02c	2.03 ± 0.01c	2.13 ± 0.01b	2.19 ± 0.03a	
S (%)	0.24 ± 0.02c	0.26 ± 0.01c	0.37 ± 0.03c	0.69 ± 0.02b	1.06 ± 0.02a	
SP (%)	17.40 ± 0.57d	19.00 ± 0.71cd	20.23 ± 0.71bc	21.75 ± 0.56ab	22.83 ± 0.60a	
PV (cP)	1312.00 ± 4.24a	1303.00 ± 15.56a	1222.50 ± 13.44b	1180.00 ± 5.66c	580.00 ± 1.41d	
TV (cP)	816.00 ± 9.90a	801.50 ± 7.78a	738.00 ± 9.90b	700.00 ± 7.07c	290.50 ± 2.12d	
BD (cP)	501.50 ± 7.78a	496.00 ± 5.66ab	484.50 ± 3.54ab	480.00 ± 12.73b	289.50 ± 0.71c	
FV (cP)	961.50 ± 12.02a	929.00 ± 8.49b	866.00 ± 9.90c	808.50 ± 3.54d	357.50 ± 4.95e	
SB (cP)	145.50 ± 2.12a	127.50 ± 0.71b	128.00 ± 0.00b	108.50 ± 3.54c	67.00 ± 2.83d	
PT (°C)	70.68 ± 0.60a	71.07 ± 0.04a	71.05 ± 0.07a	70.55 ± 0.57a	70.65 ± 0.57a	
TO (°C)	64.75 ± 0.35a	63.95 ± 0.35a	62.15 ± 0.49b	61.50 ± 0.28bc	61.05 ± 0.07c	
TP (°C)	70.55 ± 0.49a	70.30 ± 0.14a	68.60 ± 0.00b	68.03 ± 0.67b	68.45 ± 0.07b	
TC (°C)	77.00 ± 0.42a	75.90 ± 0.28ab	75.35 ± 0.07ab	74.45 ± 1.48ab	75.80 ± 0.57b	
ΔH (J/g)	6.84 ± 0.05a	6.22 ± 0.24b	5.67 ± 0.26c	5.37 ± 0.03cd	5.15 ± 0.02d	
Note: AAC, apparent amylose content; RC, relative crystallinity; PV, peak viscosity; TV, trough viscosity; BD, breakdown; FV, final viscosity; SB, setback viscosity; PT, pasting temperature; To, onset temperature; Tp, peak temperature; Tc, end temperature; ΔH, enthalpy value. Different letters represent significant differences (p < 0.05).

3.3.2 Pasting characteristics

As shown in Fig. 5 and Table 1, compared with those of the control group, ultrasonic treatment of glutinous rice significantly reduced the peak viscosity, trough viscosity, ultimate viscosity, disintegration value, and retrogradation value of starch (p < 0.05). The magnitude of the reduction increased with increasing ultrasonic power, but it had no significant effect on the pasting temperature. The decrease in starch viscosity might be because the cavitation generated by ultrasonic treatment destroyed the structure of the starch granules, causing breakage of the starch macromolecular chains and reducing the flow resistance [54]. Moreover, the destruction of the starch structure also exposes more hydroxyl groups on the starch, which strengthens the hydrogen bonding between the starch molecules and the water molecules, increasing the resistance to aging [55]. The results was agreed with a previous report that the viscosity of sprouted and husked rice decreased significantly after ultrasonic treatment [54]. Similar results were reported for the ultrasonic treatment of starch from Agriophyllum squarrosum seeds [56].Fig. 5 Paste viscosity curves of the samples ultrasonically treated at different powers.

3.3.3 Thermodynamic properties

In the DSC thermodynamic characteristics, To represents the temperature at the beginning of starch gelatinization, which is called the onset temperature; Tp represents the intermediate temperature during gelatinization, which is called the peak temperature; Tc represents the temperature at the end of gelatinization and is called the end temperature; and ΔH represents the energy required for the depolymerization and melting of the double helix ordered structure during the gelatinization of starch granules [57]. As shown in Table 1, compared with those in the control group, ultrasonic treatment of glutinous rice significantly reduced the To, Tp, Tc and ΔH of starch (p < 0.05), and the magnitude of the reduction increased with increasing ultrasonic power. The results indicated that glutinous rice starch obtained using high-power ultrasonic treatment requires less energy for gelatinization and is easier to gelatinize. This may be because ultrasonic treatment destroyed the double helix and crystallization region of glutinous rice starch, resulting in a lower gelatinization temperature and a lower enthalpy of gelatinization. Similarly, ultrasonic treatment reduced the To, Tp and Tc of sweet potato starch [49] and decreased the ΔH values of corn, potato and pea starches [58].

3.3.4 Solubility and swelling

The solubility and swelling degree are important indicators reflecting the interaction between starch and water molecules. Ultrasonic treatment of glutinous rice improved the solubility and degree of swelling of starch, and the improvement was enhanced with increasing ultrasonic power (Table 1). This could be attributed to the fact that ultrasonic waves destroyed the structure of glutinous rice starch and promoted its interaction with water molecules in the amorphous regions, thereby increasing its solubility and degree of swelling. Similar results have been reported for potato, wheat, corn, and rice starches [59]. In addition, ultrasound can effectively remove proteins from the surface of starch granules, which also contributes to increased swelling [51].

3.4 Effects of ultrasonic treatment of glutinous rice on the cooking quality of prepared quick-frozen sweet dumplings

3.4.1 Mushy soup rate

The mud soup rate is used to represent the amount of flour that falls off when sweet dumplings are boiled and is closely related to the cooking quality of the flour [60]. As shown in Table 2, with increasing ultrasonic power, the light transmittance of the soup gradually increased from 51.8 % to 63.95 %. These results can be attributed to the destruction of the starch structure by ultrasonic treatment, which enhances the binding between starch and water molecules, improves its structural stability, and reduces the dissolution of starch and protein [51].Table 2 Changes in the quality characteristics of sweet dumplings after ultrasonic treatment at different powers.

Ultrasonic Power(W)	CK	150	300	450	600	
Transparency (%)	51.80 ± 0.14e	54.45 ± 0.07d	56.60 ± 0.28c	59.20 ± 0.14b	63.95 ± 0.07a	
Hardness (g)	1263.32 ± 0.91a	1118.42 ± 25.81b	1085.04 ± 3.19bc	1065.23 ± 6.15c	993.00 ± 14.95d	
Adhesiveness	−162.82 ± 19.99a	−198.39 ± 3.25b	−231.69 ± 7.62c	−268.02 ± 0.86d	−293.10 ± 0.49e	
Springiness	0.92 ± 0.00b	0.93 ± 0.00b	0.94 ± 0.01a	0.92 ± 0.01b	0.93 ± 0.00ab	
Chewiness (g)	792.59 ± 12.77a	732.08 ± 4.91b	713.80 ± 4.29c	673.61 ± 0.13d	662.68 ± 5.18d	

3.4.2 Textural characteristics

Texture is an important attribute reflecting the quality of boiled glutinous rice dumplings. With increasing ultrasonic power, the hardness, viscosity and chewability of boiled dumplings gradually decreased, whereas the elasticity did not significantly change (Table 2). This occurred because ultrasonic cavitation destroys the starch structure in glutinous rice starch, strengthens the hydrogen bonding interaction between starch molecules and water molecules, and weakens the gel strength of boiled dumplings, thus decreasing the hardness and chewability. Ultrasound promoted the breakage of starch macromolecular chains and reduced the viscosity of boiled dumplings, which was also consistent with the viscosity change in the RVA structural analysis. Some studies have shown that ultrasonic treatment reduces the gel strength of oat starch, which is consistent with the results of this study [51]. Another report on brown rice indicated that ultrasonic power is negatively correlated with hardness and that ultrasound promotes the destruction of starch structure and the dissolution of amylose, thus leading to starch gelatinization and reduced hardness and viscosity [61].

4 Conclusions

In this study, we evaluated the effects of different ultrasonic powers on the bactericidal effect of glutinous rice, the physicochemical properties of starch and the quality characteristics of sweet dumplings. The number of viable cells in glutinous rice decreased with increasing ultrasonic power. Ultrasound-assisted soaking destroyed the grain structure of glutinous rice starch, increased its amylose content, increased its degree of expansion, reduced its proximity degree and relative crystallinity, and changed its gelatinization characteristics. With increasing ultrasonic power, the soup transparency of sweet dumplings gradually increased, and their hardness, chewiness and adhesiveness gradually decreased. The results of this study provide theoretical support for the application of ultrasonic technology in the production of glutinous rice flour.

CRediT authorship contribution statement

Shuli Wang: Writing – original draft, Investigation. Xiaojie Wang: Investigation. Yu Liu: Investigation, Data curation. Wenjing Dong: Methodology. Huiping Fan: Visualization. Shijia Fan: Validation. Zhilu Ai: Resources, Conceptualization. Yong Yang: Software. Biao Suo: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization.

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.

Acknowledgments

This research was funded by the 10.13039/501100012166 National Key R&D Program of China (2021YFD2100200/2021YFD2100201), the 10.13039/501100001809 National Natural Science Foundation of China (32272441 ), the Program for Science & Technology Innovation Talents in Universities of Henan Province (22HASTIT034 ), and the Innovative Training Program for College Students of 10.13039/501100009010 Henan Agricultural University (2023CY065 ).
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