A Comparative Study on Extraction and Physicochemical Properties of Soluble Dietary Fiber from Glutinous Rice Bran Using Different Methods

: The methods of hot water extraction and ultrasound-assisted enzymatic treatment were applied for extracting the soluble dietary ﬁber from the glutinous rice bran in the study. Based on the single factor experiment for the hot water method, the optimum parameters of the extraction time of 120 min, solid-liquid ratio 1:20 ( w / v ), and pH 8.0, as well as the extraction temperature 80 ◦ C, were obtained, while the yield and purity of SDF reached 31.83 ± 0.06% and 93.28 ± 0.27%, respectively. Furthermore, the SDF yield was improved to 34.87 ± 0.55% by using the ultrasound-assisted enzymatic treatment under the optimum conditions of cellulase dosage 9 × 10 3 U/g and ultrasonic temperature of 50 ◦ C. Similar polysaccharide compositions were detected based on the infrared spectroscopic analysis. Compared with the SDF obtained from hot water extraction, the whiteness, solubility, water holding capacity, and swelling properties of SDF extracted by ultrasound-assisted enzymatic method improved signiﬁcantly. These results demonstrated that both two strategies could be applied to SDF extraction in practical production, and the ultrasound-assisted enzymatic method might be an effective tool to improve the functional properties of SDF.


Introduction
As a kind of polysaccharide, dietary fiber (DF) was once considered to have little nutrition because it could not be digested or absorbed by the gastrointestinal tract.However, DF has been reported to promote the abundance of beneficial intestinal microbiota and short-chain fatty acids and improve intestinal physiology [1].Although DF could be classified as soluble dietary fiber (SDF) and insoluble dietary fiber (IDF) based on the solubility, SDF would have broader processing properties, such as greater viscosity, gelforming, and emulsifying capacities, and could provide better texture and taste in the food manufacture [2].Furthermore, according to previous research, SDF would not only play a role in increasing digestion, reducing gastric emptying, and slowing nutrient absorption [3], but could also prevent heart disease and obesity [4], reduce blood pressure, and inhibit cardiovascular diseases [5].In addition, the comprehensive utilization of grains and cereal foods, fruits and vegetables, legumes, and nuts has been paid close attention to as they are rich sources of SDF.
Glutinous rice (Oryza sativa var.Glutinosa Matsum) is a typical food in Asia, South America, North America, Europe, and Africa [6].As the important by-product of glutinous rice processing, the bran has not been well utilized yet due to the incomplete processing technology [7].According to the latest report, the production of glutinous rice bran was approximately 40-45 million tons every year [8], and only about 10% of glutinous rice bran has been used to extract oil, natural antioxidants, and bioactive substances, including tocopherol, squalene, and lipopolysaccharide [9].The application of glutinous rice bran in the food industry is greatly limited due to the rancidity and oxidation of oil caused by lipase and lipoxygenase during milling [10].Recent studies have confirmed that glutinous rice bran is rich in fiber of 6-14% (w/w) [11], and the supplement of glutinous rice bran with high content of SDF would be of great significance to prevent modern civilized diseases, such as indigestion, high cholesterol, cancer, urinary calculi, etc. [12].At the same time, rice bran dietary fiber is widely used as a food ingredient and nutritional supplement due to its antioxidant components [13].Therefore, extraction of SDF from the glutinous rice bran would provide a new source for dietary fiber applications in the food industry.
Nowadays, hot water extraction, enzyme extraction, and ultrasound-assisted extraction methods have been applied to extract the SDF and tried to overcome the complex structure of fibers and the physical blocking of plant cell walls [14].Although hot water extraction has the characteristics of simplicity, convenience, and rapidity in practical application, the long and costly process should not be neglected, and the high temperature and prolonged extraction would promote the decomposition of polysaccharides [15].Meanwhile, the enzymatic method also had the problems of a long reaction time and high cost.Cheng et al. [16] have shown that the yields of SDF from potato pulp pretreated by cellulase, xylanase, and a cellulase/xylanase mixture could reach 31.9%(w/w), 25.7% (w/w) and 39.7% (w/w), respectively.Although the complex enzyme could convert insoluble dietary fiber into soluble dietary fiber, part of dietary fiber would be hydrolyzed into oligosaccharides that would not be precipitated by ethanol when the enzyme concentration was higher.Ultrasound-assisted extraction (UAE) would increase extraction efficiency by reducing processing time, energy, and solvent consumption.Zhang et al. [17] recovered soluble dietary fiber (SDF) from the peel of papaya through ultrasound-assisted alkaline extraction with a maximum yield of 36.99%.Cheng et al. [16] used ultrasound-assisted methodology to extract SDF from potato pulp, and a 39.7% yield was obtained.Yan et al. [18] extracted SDF from millet bran by ultrasonic-microwave synergetic enzyme method, and the yield and purity of SDF were 6.35% and 91.27%, respectively, under the optimal conditions.Due to economic reasons, the extraction processes including the hot water extraction and ultrasound-assisted enzymatic method were first investigated in the current study to establish the SDF extraction procedure from the glutinous rice bran.Moreover, the physicochemical properties of soluble dietary fiber obtained by different methods were compared.This study would provide insights into the ideal extraction method and potential application of soluble dietary fiber from glutinous rice bran.

Sample Preparation
Prior to the experiment, the glutinous rice bran was dispersed and stirred in ethanol to remove the lipids as previously described [19] and then dried until constant weight.The dry samples were stored in a sealed plastic bag in a freezer at −18 • C throughout the experiment.SDF was prepared from defatted glutinous rice bran using hot water and ultrasound-assisted enzymatic extraction methods, respectively.

Hot Water Extraction
The method of extracting SDF by hot water extraction was based on a previous method of Daiva et al. [14] with slight modifications.The sample (2.00 g, accurate to 0.01 g) was mixed with hot water (pH of 5.0, 6.0, 7.0, 8.0, 9.0) at specific solid-liquid ratios (1:5, 1:8, 1:10, 1:15, 1:20, w/v) and was incubated in a water bath (40,50,60,70,80, 90 • C) with continuous stirring for 30, 60, 100, 120, 180, 240 min, respectively.The supernatant was obtained after the centrifugation at 5000 r/min for 15 min and concentered by a rotary evaporator; then, 4 times the volume of 95% ethanol was used to precipitate the SDF for 12 h at 25 • C. Then the precipitation was dissolved with distilled water, and the SDF was obtained by the deproteinized procedure with Sevag reagent three times.

Ultrasound-Assisted Enzymatic Extraction
Ultrasound-assisted enzymatic extraction was performed according to the report of Dong et al. [2] with slight modifications.The glutinous rice bran was mixed with deionized water at a solid-liquid ratio of 1:15 (w/v), and cellulase was added at 3 × 10 3 , 5 × 10 3 , 7.0 × 10 3 , 9 × 10 3 , and 11 × 10 3 U/g, respectively.After the solution pH was adjusted to 4.8, the samples were treated for 5, 15, 25, 35, and 45 min, respectively, at 50 • C and 40 kHz under the ultrasonic power of 50 W.Then, the SDF was obtained using the same procedure described above for alcohol precipitation and deproteinization.

Characterization of Purity and Yield of Soluble Dietary Fiber
The fiber obtained by the above extraction method was accurately weighed to 0.2-1.0g (accurate to 0.1 mg, m 0 ) and then digested with the α-amylase, protease, and amyl-glucosidase in sequence.Afterward, enzymolysis products were transferred into the crucible and washed twice with 15 mL of 78% ethanol, 15 mL of 95% ethanol, and 15 mL of acetone.The washing solution was removed by suction filtration and dried overnight at 105 • C. The crucible and product were placed in a DZF-6020-T vacuum drying oven (Shanghai Binglin Electronic Technology Co., Ltd., Shanghai, China) for 1 h at 550 • C and weighed (accurate to 0.1 mg, m 2 ).The yield of soluble dietary fiber was calculated as following: where, m 0 is the weight of the sample and dry crucible; m 1 is the weight of the dry crucible; and m 2 is the weight of coarse fiber and dry crucible.The crude protein content (accurate to 0.1 mg, m p ) was determined by the Kjeldahl method (AOAC, 920.152).Ash was determined by the combustion of the sample in a muffle oven at 550 • C for constant weight (accurate to 0.1 mg, m A ).The purity of soluble dietary fiber was calculated using the following equation: where, m p is the weight of protein in the sample residue and m A is the weight of ash in the sample residue.

Fourier Transform Infrared Spectroscopy (FTIR)
Two milligrams of SDF samples were mixed with 200 mg KBr powder and pressed into tablets.The FTIR spectra were obtained by using an FTIR-1500 infrared spectrometer (Zhongshi Walker Tianjin Technology Development Co., Ltd., Tianjin, China).The range of wavenumbers was 4000-400 cm −1 by accumulating 5 scans with a resolution of 4 cm −1 .

Physicochemical Properties of Soluble Dietary Fiber from Glutinous Rice Bran
The SDF obtained by hot water extraction after optimizing extraction conditions (extraction solution pH 8.0, extraction temperature 80 • C, time 120 min, and the solid-liquid ratio of 1:15 (w/v)) was recorded as B-SDF, and the SDF extracted by the ultrasound-assisted enzymatic method after optimizing extraction conditions (cellulase dosage 9 × 10 3 U/g, ultrasonic temperature 50 • C, power 50 W, and ultrasonic treatment time 35 min) was named as A-SDF.

Color Difference
The L, a, and b values of SDF were measured by the NR200 color difference instrument (Shenzhen 3NH Technology CO., LTD, Shenzhen, Guangdong, China).L value represents the brightness degree from 0 to 100, where the 0 value means black, while −a to +a values indicate the green-red degrees, and −b to + b values suggest the blue-yellow degrees.In order to identify the total color differences among samples, ∆E was calculated between two samples measurements as follows: Twenty-five milligrams of SDF was mixed with 20 mL distilled water in a 50 mL centrifuge tube to be stirred continuously for 30 min; then, the mixture was centrifuged at 2500 r/min using the TG 16G high-speed centrifuge (Kate Industrial Technology Co., Ltd., Yancheng, China) for 5 min.The supernatant was transferred into a weighing dish and dried to constant weight by heating at 105 • C. The solubility was calculated as follows: where, m 3 is the mass of the sample; m 4 is the mass of weighing dish; and m 5 is the mass of the tared dish and sample.

Water-Holding Capacity (WHC)
The WHC was performed according to the method described by Wang et al.
[20] with minor modifications.Two hundred milligrams of SDF was mixed with 4 mL deionized water in a 10 mL centrifuge tube and mixed evenly with a vortex oscillator.After the centrifugation at 5000 r/min using the TG 16G high-speed centrifuge (Kate Industrial Technology Co., Ltd.) for 10 min, the supernatant was discarded, and the water-holding capacity was calculated as follows: where m 6 is the dry mass of the sample and m 7 is the wet mass of the sample.

Swelling Capacity (SC)
The swelling capacity of SDF was determined according to the report of Liu et al. [21].First, 0.1 g of SDF was mixed with 5 mL deionized water.After a 24 h standing at room temperature, the swelling capacity was calculated as follows: where m 8 is the mass of the sample; V 1 is the volume before expansion; and V 2 is the volume after expansion.

Statistical Analysis
All the tests were performed in triplicate and results were expressed as mean ± standard deviation (SD).The data were analyzed by the Microsoft Excel software (Mathworks, Natick, MA, USA) and the value of p < 0.05 was considered a statistically significant difference.The charts were drawn with Origin software (Origin 9.8, Northampton, MA, USA).

Hot Water Extraction
The yield and purity of SDF were used as evaluation indicators for SDF extraction by hot water, and the effects of pH, temperature, time, and solid-liquid ratio were studied.

Effect of pH on SDF Yield and Purity
In the experiment of exploring the influence of pH values (5.0, 6.0, 7.0, 8.0, 9.0) on the extraction rate of SDF, the hot water extraction parameters were set at a temperature of 70 • C, time of 100 min, and a solid-liquid ratio of 1:10 (w/v).Figure 1A shows the effect of pH on the yield and purity of SDF.The yield and purity of SDF increased significantly (p < 0.05) to the maximum (20.60 ± 0.71% and 90.77 ± 0.65%) with the increase of pH to 8.0, and then decreased with a further increase of pH beyond the optimum, which was consistent with the results of Jujube SDF extraction by hot water extraction [22].These results could be attributed to the increase of polysaccharide solubility due to the increase of ionization constant with the increasing pH value.According to previous reports, the extraction solution would process a high concentration of hydrogen and hydroxide ions with increasing ionization degree and then would enhance the degree of hydrolysis reaction [23].The hydrogen bond and glycosidic bond in the cellulose chain would be partially broken under the mild alkaline condition, leading to the reduction of polymerization and mechanical strength of cellulose [24].However, the abstriction would be active with a further increase of pH to 9.0, then the monosaccharides and small molecular oligosaccharides were produced, which could not be easily precipitated by the ethanol [25].Therefore, the optimum pH for hot water extraction was selected at 8.0, and the yield of 20.60 ± 0.71% and the purity of 90.77 ± 0.65% were obtained under the current condition.

Effect of Extraction Temperature on SDF Yield and Purity
The effect of extraction temperature (40, 50, 60, 70, 80, and 90 • C) on the extraction rate of SDF was studied under the conditions of pH 8.0, extraction time of 100 min, and a solid-liquid ratio of 1:10 (w/v).In Figure 1B, the yield of SDF firstly increased from 40 to 70 • C and then decreased with the increase of temperature to 90 • C.However, the purity of SDF showed a fluctuating upward trend in the range from 60 to 70 • C, and a decreasing trend was observed from 80 to 90 • C. With the increase of extraction temperature, the molecular thermal movement would accelerate the increase of yield; however, the viscosity of the extracted aqueous solution would also increase, which would hinder the solute diffusion [26] and reduce the yield at a higher temperature of more than 80 • C.Moreover, the degradation of polysaccharides in the thermal treatment also reduced the purity [27].As a consequence, 80 • C was the optimal temperature to extract SDF.

Effect of Extraction Time on SDF Yield and Purity
Under the optimum hot water extraction conditions of pH 8.0 and a temperature of 80 • C, the effect of extraction time (30 to 240 min) on the extraction rate of SDF was studied at a solid-liquid ratio of 1:10 (w/v).The extraction time also had a significant (p < 0.05) impact on the yield and purity of SDF (Figure 1C).With the extension of time from 30 to 120 min, the yield and purity of SDF increased continuously (p < 0.05), and then the yield and purity decreased with a further increase of the extraction time to 240 min.According to previous studies, the SDF structure would be destructed by a long extraction in the alkaline environment [28].Consequently, the optimum extraction time was set as 120 min, and the yield of SDF and purity could reach 28.77 ± 0.70% and 92.60 ± 0.26%, respectively.

Effect of Extraction Temperature on SDF Yield and Purity
The effect of extraction temperature (40, 50, 60, 70, 80, and 90 °C) on the extraction rate of SDF was studied under the conditions of pH 8.0, extraction time of 100 min, and a solid-liquid ratio of 1:10 (w/v).In Figure 1B, the yield of SDF firstly increased from 40 to 70 °C and then decreased with the increase of temperature to 90 °C.However, the purity of SDF showed a fluctuating upward trend in the range from 60 to 70 °C, and a decreasing trend was observed from 80 to 90 °C.With the increase of extraction temperature, the molecular thermal movement would accelerate the increase of yield; however, the viscosity of the extracted aqueous solution would also increase, which would hinder the solute diffusion [26] and reduce the yield at a higher temperature of more than 80 °C.Moreover, the degradation of polysaccharides in the thermal treatment also reduced the purity [27].As a consequence, 80 °C was the optimal temperature to extract SDF.

Effect of Extraction Time on SDF Yield and Purity
Under the optimum hot water extraction conditions of pH 8.0 and a temperature of 80 °C, the effect of extraction time (30 to 240 min) on the extraction rate of SDF was studied at a solid-liquid ratio of 1:10 (w/v).The extraction time also had a significant (p < 0.05) impact on the yield and purity of SDF (Figure 1C).With the extension of time from 30 to 120 min, the yield and purity of SDF increased continuously (p < 0.05), and then the yield and purity decreased with a further increase of the extraction time to 240 min.According to previous studies, the SDF structure would be destructed by a long extraction in the  1D, the yield of SDF significantly (p < 0.05) increased with a solid-liquid ratio from 1:5 to 1:15 (w/v), which might be due to the increase of the molecule movement rate.The SDF yield decreased when the solid-liquid ratio attained 1:20 (w/v), as more protein and starch components would be dissolved at a high solid-liquid ratio.The purity of SDF increased from 1:5 to 1:10 (w/v) and decreased with further changes in the ratio.It was reported that the solid-liquid ratio had a similar effect on the extraction of SDF from Rosa Roxburghii Tratt [30].Although the maximum yield of 31.83 ± 0.06% was obtained at the solid-liquid ratio of 1:15 (w/v), the maximum purity of 94.40 ± 0.85% was observed at 1:10 (w/v).Considering the economic cost, the solid-liquid ratio of 1:15 (w/v) was selected for hot water extraction, and the yield of SDF was 31.83 ± 0.06%, while the purity was 93.17 ± 0.32%.
In conclusion, the optimum conditions of hot water extraction were extraction solution of pH 8.0, extraction temperature of 80 • C, time of 120 min, and a solid-liquid ratio of 1:15 (w/v), which allowed 31.84 ± 0.31% of SDF yield and 93.28 ± 0.27% of SDF purity in the confirmation experiments.Compared with the yield of 12.01 ± 0.51% and purity of 73-78% of rice bran SDF extraction using the traditional hot water method [14], the SDF extraction efficiency by optimized hot water extraction has been improved.

Ultrasound-Assisted Enzymatic Extraction
According to preliminary experiments, the ultrasound-assisted enzymatic treatment has no significant effect on the purity of SDF from the glutinous rice bran compared with the hot water extraction because of the efficiency of alcohol precipitation and deproteinization.Therefore, the yield of SDF was seriously considered in the ultrasound-assisted enzymatic method based on the optimization of cellulase dosage and ultrasonic time using the solidliquid ratio of 1:15 (w/v).

Effect of Cellulase Dosage on SDF Yield
In the experiment, SDF was extracted under the most suitable cellulase pH of 4.8, an ultrasonic temperature of 50 • C, an ultrasonic power of 50 W, and ultrasonic time of 25 min.The effect of cellulase dosage of 3 × 10 3 , 5 × 10 3 , 7 × 10 3 , 9 × 10 3 , and 11 × 10 3 U/g on the yield of SDF was investigated, respectively.As seen in Figure 2A, the yield of SDF increased significantly (p < 0.05) with the addition of cellulase from 3 to 9 × 10 3 U/g, and the SDF yield decreased after the maximum value of 32.87 ± 0.35% with further increase of the cellulase dosage.It was reported that the substrate could not be fully hydrolyzed at the lower addition of cellulase; furthermore, the slow dissolution rate of cellulose and substances also resulted in the lower yield, while the higher addition of enzyme would affect the polymerization degree of SDF [31], which would also reduce the yield by increasing extraction difficulty.According to the study of Chen et al. [32], the addition of cellulase had a similar effect on the extraction of corn silk polysaccharides.Hence, the optimum cellulase dosage was 9 × 10 3 U/g, and the yield of SDF reached 32.87 ± 0.35%.

Effect of Ultrasonic Time on SDF Yield
Under the conditions of pH 4.8, ultrasonic temperature of 50 °C, ultrasonic power of 50 W, and cellulase dosage of 9 × 10 3 U/g, the effect of ultrasonic time of 5, 15, 25, 35, and 45 min on the yield of SDF was investigated, respectively.With the extension of ultrasonic time, the yield of SDF gradually increased in the ultrasonic time range from 5 to 35 min (Figure 2B), where a yield of 34.89 ± 0.06% was reached at 35 min; then the yield of SDF decreased with the increase of time.Since the tightly bonded structure inside the plant cells was not completely loose and the enzymatic hydrolysis could not be accelerated [33], the low yield was obtained during the shorter ultrasonic treatment.However, with an extension of ultrasonic time, the structure of SDF would be destroyed by the strong vibration generated by the ultrasonic equipment [34], resulting in a decline in the yield.In conclusion, the optimum ultrasonic time was 35 min, and the yield of SDF was 34.87 ± 0.55%.
In summary, the highest SDF yield of 34.87 ± 0.55% could be reached under the optimum conditions of ultrasound-assisted enzymatic extraction with a cellulase dosage of 9 × 10 3 U/g, an ultrasonic temperature of 50 °C, ultrasonic power of 50 W, and ultrasonic time of 35 min.

Effect of Ultrasonic Time on SDF Yield
Under the conditions of pH 4.8, ultrasonic temperature of 50 • C, ultrasonic power of 50 W, and cellulase dosage of 9 × 10 3 U/g, the effect of ultrasonic time of 5, 15, 25, 35, and 45 min on the yield of SDF was investigated, respectively.With the extension of ultrasonic time, the yield of SDF gradually increased in the ultrasonic time range from 5 to 35 min (Figure 2B), where a yield of 34.89 ± 0.06% was reached at 35 min; then the yield of SDF decreased with the increase of time.Since the tightly bonded structure inside the plant cells was not completely loose and the enzymatic hydrolysis could not be accelerated [33], the low yield was obtained during the shorter ultrasonic treatment.However, with an extension of ultrasonic time, the structure of SDF would be destroyed by the strong vibration generated by the ultrasonic equipment [34], resulting in a decline in the yield.In conclusion, the optimum ultrasonic time was 35 min, and the yield of SDF was 34.87 ± 0.55%.
In summary, the highest SDF yield of 34.87 ± 0.55% could be reached under the optimum conditions of ultrasound-assisted enzymatic extraction with a cellulase dosage of 9 × 10 3 U/g, an ultrasonic temperature of 50 • C, ultrasonic power of 50 W, and ultrasonic time of 35 min.

FTIR
The information relating to chemical bonds and groups in the structure of the organic molecule could be detected by using Fourier transform infrared spectra.Since the typical SDF spectra extracted by hot water have been reported [35], only the SDF extracted by ultrasound-assisted enzymatic was determined by infrared spectroscopy in this study, and the similar spectrum obtained indicated the consistency of composition.As shown in Figure 3, the typical transitions of SDF were observed at 3500-3000, 1700-1500, and 1150-700 cm −1 regions from the SDF sample extracted by the ultrasound-assisted enzymatic method.A characteristic broad and intense peak was observed at 3299 cm −1 , which was related to the stretching of the hydrogen bonds between H and O atoms in the hydroxyl groups, and was mainly associated with cellulose and hemicelluloses [36].The weak peak around 2930 cm −1 was due to the C-H stretching and bending vibrations, which were assigned as the stretching of C-H groups, and it was the typical peak for polysaccharide-based polymers.Compared with dietary fibers from cactus rackets by hot water extraction [37], the hydrogen bond intensity formed between and within molecules of SDF extracted by the ultrasound-assisted enzymatic method was not higher.The stretching vibration peak of glucuronic C=O appeared at 1651.91 cm −1 , indicating the existence of glucuronic acid.The absorption peaks at 1018.58 cm −1 and 1077.96cm −1 were induced by two C-O stretching vibrations of the pyranose ring, one of which was C-O-H, and the other might be hemicellulose and cellulose C-O-C [38], which was usually reported as arabinose and xylan.The wavelength range of 950-1200 cm −1 was considered to be the "fingerprint" area of carbohydrates because it could identify the main chemical groups, such as C-O-C, β-glycosidic linkages, etc.The weak peaks at 898 cm −1 were observed, which was the bending vibration characteristic peak of the β-Pyran ring C-H, and it could be inferred that the SDF contains β-Type glycosidic bonds [39].
Separations 2023, 10, x FOR PEER REVIEW 9 xylan.The wavelength range of 950-1200 cm −1 was considered to be the "fingerprint" of carbohydrates because it could identify the main chemical groups, such as C-O-C glycosidic linkages, etc.The weak peaks at 898 cm −1 were observed, which was the be ing vibration characteristic peak of the β-Pyran ring C-H, and it could be inferred tha SDF contains β-Type glycosidic bonds [39].The color difference instrument could quantify the color into a value, and the ∆E could accurately reflect the difference in the displayed color between two samples.The ∆E = 2.91 (Table 1) indicated that the colors of SDF samples obtained from both two extractions were similar.The SDF obtained by hot water extraction process was recorded as B-SDF, and the SDF extracted by the ultrasoundassisted enzymatic was recorded as A-SDF.
Based on the a and b values, the SDF extracted by hot water was brownish gray, while the SDF extracted by the ultrasound-assisted enzymatic method was beige, which might be attributed to the difference in extraction pH [40].In addition, it could be seen from Table 1 that the L value of the SDF obtained by the ultrasound-assisted enzymatic method was higher, suggesting an enhancement of brightness.In a short, the SDF extracted by ultrasound-assisted enzymatic method would be more conducive to the development and production of new beverage products with good appearance.

Solubility (S)
As shown in Figure 4, the solubility of SDF was affected by the dissolution temperature significantly.The solubility of SDF extracted by the two methods showed an upward trend with the increase in dissolution temperature, which was probably due to the degradation of cellulose and the exposure of the molecular groups [41].It seemed that the solubility of SDF obtained by the ultrasonic coenzyme treatment was improved because of the function of cellulase, which was more conducive to the combination with water molecules.
Separations 2023, 10, x FOR PEER REVIEW 10 of 14 dation of cellulose and the exposure of the molecular groups [41].It seemed that the solubility of SDF obtained by the ultrasonic coenzyme treatment was improved because of the function of cellulase, which was more conducive to the combination with water molecules.

WHC and SC
The WHC and SC of glutinous rice bran SDFs after different treatments are shown in Figure 5.It was reported that the WHC of SDF was positively correlated with satiety in

WHC and SC
The WHC and SC of glutinous rice bran SDFs after different treatments are shown in Figure 5.It was reported that the WHC of SDF was positively correlated with satiety in the stomach, suggesting that high WHC would be beneficial to the reduction of food intake [42].From Figure 5, the WHC of B-SDF and A-SDF reached 1.81 ± 0.08 and 2.12 ± 0.10 (g/g), respectively.The WHC of A-SDF has been well improved by an increase of 17.1%.During the ultrasonic treatment, the particle size of fibers would be reduced, and the surface area would be decreased, while the particles were thinner, leading to an increase in WHC because of the good ability to hold water [43].The SC of SDF would enhance the sense of satiety after eating [44] and improve th texture and viscosity in food applications.As shown in Figure 5, the SC of B-SDF and A SDF were 0.16 ± 0.12 and 0.23 ± 0.10 (mL/g), respectively.The SC of A-SDF has been we improved by an increase of 43.8%, suggesting that the pore-like structure induced by th ultrasonic treatment could significantly affect the SC of SDF [45].

Conclusions
To extend the soluble dietary fiber source, the hot water extraction and ultrasound assisted enzymatic method have been applied in the glutinous rice bran extraction.Th optimum hot water extraction method was established with an SDF yield of 31.83 ± 0.06% and purity of 93.28 ± 0.27% under the conditions of extraction solution of pH 8.0, extrac tion temperature of 80 °C, extraction time of 120 min, and a solid-liquid ratio of 1:15 (w/v The SDF yield could improve to 34.87 ± 0.55% by using the ultrasound-assisted enzymat method under the optimum cellulase treatment.Although similar yields and compos tions of SDF were obtained by the two methods, the ultrasound-assisted cellulase trea ment might affect the structure of SDF, which improved the whiteness, solubility, wate holding, and swelling capacities, suggesting that the SDF would have a potential in th food development as a useful ingredient.More details about the structural modificatio of SDF should be further explored, and our current results introducing the hot water an ultrasound-assisted enzymatic methods can be applied in the SDF extraction from glut nous rice bran. Author Contributions: H.C.: Investigation, Methodology, Visualization, Writing original draf The SC of SDF would enhance the sense of satiety after eating [44] and improve the texture and viscosity in food applications.As shown in Figure 5, the SC of B-SDF and A-SDF were 0.16 ± 0.12 and 0.23 ± 0.10 (mL/g), respectively.The SC of A-SDF has been well improved by an increase of 43.8%, suggesting that the pore-like structure induced by the ultrasonic treatment could significantly affect the SC of SDF [45].

Conclusions
To extend the soluble dietary fiber source, the hot water extraction and ultrasoundassisted enzymatic method have been applied in the glutinous rice bran extraction.The optimum hot water extraction method was established with an SDF yield of 31.83 ± 0.06% and purity of 93.28 ± 0.27% under the conditions of extraction solution of pH 8.0, extraction temperature of 80 • C, extraction time of 120 min, and a solid-liquid ratio of 1:15 (w/v).The SDF yield could improve to 34.87 ± 0.55% by using the ultrasound-assisted enzymatic method under the optimum cellulase treatment.Although similar yields and compositions of SDF were obtained by the two methods, the ultrasound-assisted cellulase treatment might affect the structure of SDF, which improved the whiteness, solubility, water holding, and swelling capacities, suggesting that the SDF would have a potential in the food development as a useful ingredient.More details about the structural modification of SDF should be further explored, and our current results introducing the hot water and ultrasound-assisted enzymatic methods can be applied in the SDF extraction from glutinous rice bran.

Figure 1 .
Figure 1.Effects of pH (A), water stripping temperature (B), extraction time (C), and solid-liquid ratio (D) on yield and purity of SDF.Different letters on top of the error bar indicate significant (p < 0.05) differences among samples treated under different conditions.

Figure 1 .
Figure 1.Effects of pH (A), water stripping temperature (B), extraction time (C), and solid-liquid ratio (D) on yield and purity of SDF.Different letters on top of the error bar indicate significant (p < 0.05) differences among samples treated under different conditions.3.1.4.Effect of Solid-Liquid Ratio on SDF Yield and Purity The effect of solid-liquid ratios (1:5, 1:8, 1:10, 1:15, and 1:20 (w/v)) on the extraction rate of SDF was studied under the conditions of pH 8.0, an extraction temperature of 80 • C, and extraction time of 120 min.Both the molecule movement rate and solvent density could be affected by solid-liquid ratios [29].As shown in Figure1D, the yield of SDF significantly (p < 0.05) increased with a solid-liquid ratio from 1:5 to 1:15 (w/v), which might be due to the increase of the molecule movement rate.The SDF yield decreased when the solid-liquid ratio attained 1:20 (w/v), as more protein and starch components would be dissolved at a high solid-liquid ratio.The purity of SDF increased from 1:5 to 1:10 (w/v) and decreased with further changes in the ratio.It was reported that the solid-liquid ratio had a similar effect on the extraction of SDF from Rosa Roxburghii Tratt[30].Although the maximum yield of 31.83 ± 0.06% was obtained at the solid-liquid ratio of 1:15 (w/v), the maximum purity of 94.40 ± 0.85% was observed at 1:10 (w/v).Considering the economic cost, the solid-liquid ratio of 1:15 (w/v) was selected for hot water extraction, and the yield of SDF was 31.83 ± 0.06%, while the purity was 93.17 ± 0.32%.In conclusion, the optimum conditions of hot water extraction were extraction solution of pH 8.0, extraction temperature of 80 • C, time of 120 min, and a solid-liquid ratio of 1:15 (w/v), which allowed 31.84 ± 0.31% of SDF yield and 93.28 ± 0.27% of SDF purity in the confirmation experiments.Compared with the yield of 12.01 ± 0.51% and purity of 73-78% of rice bran SDF extraction using the traditional hot water method[14], the SDF extraction efficiency by optimized hot water extraction has been improved.

Figure 2 .
Figure 2. The influence of cellulase dosage (A) and ultrasonic time (B) on the extraction yield of ultrasound-assisted enzymatic extraction on SDF.Different letters on top of the error bar indicate significant (p < 0.05) differences among samples treated under different conditions.

Figure 2 .
Figure 2. The influence of cellulase dosage (A) and ultrasonic time (B) on the extraction yield of ultrasound-assisted enzymatic extraction on SDF.Different letters on top of the error bar indicate significant (p < 0.05) differences among samples treated under different conditions.

Figure 3 .
Figure 3. FT-IR spectra of SDF extracted from glutinous rice bran by the ultrasound-assisted e matic method.

Figure 3 .
Figure 3. FT-IR spectra of SDF extracted from glutinous rice bran by the ultrasound-assisted enzymatic method.

Figure 4 .
Figure 4. Effect of temperature on solubility of SDF extracted from glutinous rice bran by different methods.B-SDF extracted by the hot water extraction process, and A-SDF extracted by the ultrasound-assisted enzymatic method.Different letters on the top of the column indicate significant differences (p < 0.05).

Figure 4 .
Figure 4. Effect of temperature on solubility of SDF extracted from glutinous rice bran by different methods.B-SDF extracted by the hot water extraction process, and A-SDF extracted by the ultrasoundassisted enzymatic method.Different letters on the top of the column indicate significant differences (p < 0.05).

Separations 2023 , 1 Figure 5 .
Figure 5.Comparison of swelling capacity and water holding capacity extracted from glutinous ric bran by different methods.B-SDF extracted by the hot water extraction process, and A-SDF ex tracted by the ultrasound-assisted enzymatic method.Different letters on the top of the colum indicate significant differences (p < 0.05).

Figure 5 .
Figure 5.Comparison of swelling capacity and water holding capacity extracted from glutinous rice bran by different methods.B-SDF extracted by the hot water extraction process, and A-SDF extracted by the ultrasound-assisted enzymatic method.Different letters on the top of the column indicate significant differences (p < 0.05).

Table 1 .
Color difference analysis of SDF extracted from glutinous rice bran by different methods.