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. 2019 Nov 18;57(4):1421–1429. doi: 10.1007/s13197-019-04177-8

Physicochemical, functional, and microstructural properties of modified insoluble dietary fiber extracted from rose pomace

Yuanyuan He 1,2, Wen Li 1, Xiaoyu Zhang 1, Taotao Li 1, Difeng Ren 1,✉, Jun Lu 2,✉
PMCID: PMC7054496  PMID: 32180638

Abstract

Rose pomace, a by-product of the essential oil extraction process, is rich in dietary fiber. Insoluble dietary fiber (IDF) extracted from rose pomace was modified by enzymatic hydrolysis (EH) and ultrasound-assisted enzymatic hydrolysis (UEH) methods, and their physicochemical, functional, and microstructural properties were studied. The results showed that EH treatment performed better in the yield of soluble dietary fiber and the glucose adsorption capacity than UEH which contributed to better oil-holding, swelling, cation-exchange, and cholesterol adsorption capacities. Moreover, cellulose, hemicellulose, and lignin were detected based on Fourier transform infrared spectra and X-ray diffraction patterns. Scanning electron microscopy revealed that IDF had a shaly surface with a loose block structure after modification. In conclusion, different modification degrees have respective advantages, and modified IDF from rose pomace could be utilized in the food industry as a new source of functional ingredients, as well as to increase the economic value of rose products.

Electronic supplementary material

The online version of this article (10.1007/s13197-019-04177-8) contains supplementary material, which is available to authorized users.

Keywords: Modification, Dietary fiber, Rose pomace, Enzymatic hydrolysis, Physicochemical properties

Introduction

Rose (Rosa rugosa Thunb.), a member of the Rosaceae family, is widely distributed in many countries in Europe, Asia, and Africa. Rose is rich in bioactive substances, such as pigments, polyphenols, flavonoids, and fatty oils (Xie and Zhang 2012), and has positive effects on the regulation of liver function and blood circulation, as well as in the treatment of diabetes, gastrointestinal tract disorders, and chronic inflammatory diseases (Lee et al. 2008; Czyzowska et al. 2015). Therefore, rose is not only a medicinal material but also a considerable raw material in the perfume and food industries due to its essential oil and aroma (Zhao et al. 2016; Xiao et al. 2019). However, the yield of essential oil is meager (0.03–0.04%) (Manouchehri et al. 2018), resulting in large amounts of rose by-products. As the food industry is forced to emphasize recovery, recycling, and upgrading of waste, the processing of rose pomace, which consists mainly of dietary fiber, is still limited to merely drying and cutting to make fuel or feed.

Dietary fiber is generally regarded as a group of carbohydrate polymers that are non-hydrolysable and energy-free in the human small intestine and play an irreplaceable role in human health (Zhu et al. 2018). It consists of soluble dietary fiber (SDF) and IDF, both of which increase the volume and weight of food without significantly affecting the calorie content and thus increase satiety and reduce appetite, while also balancing the pH in the small intestine, which helps to inhibit harmful bacteria and protect the digestive system (Spiller et al. 2001). IDF has a better anti-obesity effect than SDF and can promote lipid adsorption and discharge (Isken et al. 2010), while also increasing insulin sensitivity to reduce the risk of type 2 diabetes (Robertson et al. 2012). These functions base on swelling and adsorption capabilities determined by the microstructure of dietary fiber. Another study suggested the importance of the SDF: IDF ratio for health, and concluded that SDF as 30–50% of total dietary fiber for daily intake presented optimal proportions to obtain the physiological effects associated with both fractions (Grigelmo-Miguel and Martin-Belloso 1999). However, the SDF content in many natural materials is too low to fulfill this requirement. Modification treatments have been shown to not only alter the SDF: IDF ratio, but also improve the physicochemical and functional properties of dietary fiber. These treatments break down glycosidic bonds in the high-molecular-weight components of the fiber, thus changing the network structure, exposing more groups, and significantly converting IDF into SDF. Therefore, modified dietary fiber has improved physical and functional properties and increased yield of SDF. Many studies have compared the effects of different modification methods on the properties of dietary fiber (Xie et al. 2017; Niu et al. 2018; Yu et al. 2018). However, the effects of combined methods have not been thoroughly examined concerning the varying degrees of modification.

Biological and physical treatments are used to modify dietary fiber from different food sources (Yu et al. 2018). For instance, Huang et al. (2017) modified IDF from garlic straw with ultrasonic processing and reported significant improvements in functional and physicochemical properties. Yu et al. (2018) compared the properties of IDF extracted from carrot pomace with complex enzymes, ultrafine comminution, and high hydrostatic pressure treatments, and showed that the complex enzyme method contributed most to the yield of SDF and the cholesterol adsorption capacity of IDF. Wen et al. (2017) studied the effects of enzyme and enzyme-micronization treatments on the structural and functional properties of rice bran dietary fiber (RBDF), and concluded that a combination of cellulase and xylanase with enzyme-micronization treatments not only increased the SDF content significantly but also improved the cholesterol and sodium taurocholate absorption capacity and the swelling capacity of RBDF. Cheng et al. (2017) also reported that the optimized enzyme mixture hydrolyzed cellulose and hemicellulose components effectively to obtain a higher proportion of SDF from vegetable and fruit dregs and improved the properties. However, modification of dietary fiber from rose pomace has not been widely studied.

In this study, IDF, as the main component of dietary fiber from rose pomace, was modified using optimized enzymatic hydrolysis (EH) and ultrasound-assisted enzymatic hydrolysis (UEH) methods. The water-holding, oil-holding, swelling, cation-exchange, glucose adsorption, and cholesterol adsorption capacities were measured to evaluate the physicochemical and functional properties. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were performed to examine the effects of modification treatments on structural characteristics. This study provides experimental support for the reuse of rose by-products as a new source of dietary fiber in functional foods and other sections of the food industry.

Materials and methods

Materials

Rose pomace was supplied by Weichang Yixiang Rose Planting Specialized Cooperative (Chengde, Hebei, China) after the hot water extraction of essential oil. Heat-resistant α-amylase (40,000 U/g), amyloglucosidase (100,000 U/mL), and xylanase (100,000 U/g) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Alcalase 2.4 L (2.4 AU/g) was supplied by Novozymes (Tianjin, China). Cellulase (100,000 U/g) was purchased from Beijing Baiaolaibo Technology Co., Ltd. (Beijing, China). Lecithin was purchased from Beijing Aoboxing Bio-Tech Co., Ltd. (Beijing, China). Lard and peanut oil were purchased from a local market in Beijing. o-Phthalaldehyde was purchased from Sinopharm Chemical Reagent (Shanghai, China). 3,5-Dinitrosalicylic acid (DNS) was supplied by Tianjin Jinke Fine Chemicals Research Institute (Tianjin, China). Sodium hydroxide, sodium chloride, glucose, ethanol, acetone, sulfuric acid, glacial acetic acid, hydrochloric acid, and other reagents were of analytical grade and were purchased from Beijing Chemical Works (Beijing, China).

Preparation and modification of rose pomace

Rose pomace was dried in an air oven (DHG-903385-III; Shanghai Xinmiao Medical Devices Manufacturing Co., Ltd., Shanghai, China) at 65 °C for 3 h, ground into a powder, and then sieved through a 60-mesh screen. Dietary fiber was extracted with the enzymatic–gravimetric procedure according to the AOAC 991.43 method (1995) and the method of Xie et al. (2017) with some modifications. Briefly, 1 g of pomace powder was dispersed in 40 mL of MES-Tris buffer (0.05 M, pH 8.2) and stirred for 0.5 h. Then, 200 U of the heat-resistant α-amylase solution was added to the suspension, followed by incubation at 90 °C for 30 min with constant stirring. The temperature was then decreased to 60 °C, and 0.48 AU of Alcalase 2.4 L was added to the mixture, followed by incubation with stirring for 30 min. Next, hydrolyzation was performed by adding 500 U of amyloglucosidase at 60 °C for 30 min with stirring (pH 4.5, adjusted with 0.5 M HCl). The suspension was filtered and precipitated with 95% (v/v) preheated ethanol, 78% (v/v) preheated ethanol, acetone, and distilled water twice, followed by vacuum filtration. Four volumes of 95% ethanol were added to the supernatant, left to stand at room temperature for 12 h, and then centrifuged for 15 min at 5000 rpm to obtain SDF. The sediment was dried in an air oven at 65 °C for 3 h to obtain IDF.

Based on the optimization of the modification conditions by response surface methodology (Supplementary Material), 230 U of cellulase and 900 U of xylanase per gram of rose pomace were added at the end of the EH process with constant stirring for 2 h. All other treatments were as above to obtain enzymatic hydrolysis-dietary fiber (EH-DF). In order to obtain ultrasound-assisted enzymatic hydrolysis-dietary fiber (UEH-DF), ultrasound pretreatment (KQ3200DE; Kunshan Ultrasound Instrument Co., Ltd., Jiangsu, China) at 150 W for 30 min was applied when the powder was dispersed in MES-Tris buffer, and all following treatments were the same as those described for EH-DF.

Physicochemical properties

Water holding capacity

The water-holding capacity (WHC) of IDF was determined according to the method described by Wang et al. (2013) with some modifications. Briefly, 0.50 g (m0) of dried IDF sample was weighed accurately and mixed with distilled water at a ratio of 1:100 (w/w), then added to a 50 mL centrifuge tube which was carefully weighed (m1). After equilibration at room temperature for 1 h, the sample was centrifuged at 5000 rpm for 15 min. The total weight of the sediment and the tube was recorded (m2) after decantation to remove the supernatant. WHC was calculated as the amount of water held by the IDF sample (g/g dry weight) as follows:

WHCg/g=m2-m1m0 1

Oil holding capacity

The oil-holding capacity (OHC) of IDF was measured according to the method of Sangnark and Noomhom (2003) with modifications. Briefly, 0.50 g of the IDF sample was weighed (m1) and mixed with 10 g of lard or peanut oil, and then incubated in a centrifuge tube at 37 °C for 1 h. After centrifugation at 5000 rpm for 20 min, the free oil was removed carefully, and the residue was weighed accurately (m2). OHC was determined as the weight of oil retained by each IDF sample as follows:

OHCg/g=m2-m1m1 2

Swelling capacity

Swelling capacity (SC) of IDF samples was examined according to the method described by Mokni Ghribi et al. (2015) with slight modifications. Briefly, 0.50 g of IDF sample (m0) was weighed accurately, added to a 25 mL measuring cylinder, mixed with 20 mL of distilled water, and left to stand for 24 h at room temperature. The volumes before (V1) and after (V2) standing were recorded, and SC of IDF was expressed as volumetric change of IDF dry sample after water swelling as follows:

SCmL/g=V2-V1m0 3

Functional properties

Cation exchange capacity

Cation-exchange capacity (CEC) was measured by mixing 0.50 g of the IDF sample with 50 mL of 0.1 M HCl, stirred thoroughly and stand for 24 h at room temperature, then filtered with filter paper using distilled water to wash down the acid. Then, 0.20 g of the residue was dried and added to a conical flask with 80 mL of 15% (w/v) NaCl with constant stirring, compared with a blank containing distilled water. Several drops of phenolphthalein were added, and the solution was titrated with 0.1 M NaOH. The volume of NaOH required to change the pH of the suspension was recorded (Benítez et al. 2011). CEC was expressed as follows:

CECmM/g=0.1×V1-V0md 4

where V1 is the titrated volume of NaOH of the sample (mL), V0 is the titrated volume of NaOH of the blank (mL), 0.1 is the concentration of NaOH (M), and md is the dry weight of the sample (g).

Glucose adsorption capacity

Glucose adsorption capacity (GAC) was determined by adding 0.20 g of IDF sample to 10 mL of glucose solution with concentrations of 10 mM, 50 mM, 100 mM, and 200 mM, respectively, stirred, and incubated in a water bath at 37 °C for 6 h. The mixture was centrifuged at 4000 rpm for 20 min. The DNS colorimetric method was used to determine the level of glucose in the supernatant. GAC was calculated as the variance of glucose concentration with the addition of IDF to the solution as follows (Chau et al. 2003):

GACg/g=m1-m2m0 5

where m1 = the content (g) of glucose in the solution before adsorption, m2 = the content (g) of glucose in the solution after adsorption, and m0 = the weight of IDF sample (g).

Cholesterol adsorption capacity

Cholesterol adsorption capacity (CAC) was determined according to the procedure reported by Zhang et al. (2011) with some modifications. Briefly, the IDF sample (m0) was added to a yolk solution (10 mM, w/v) at a ratio of 1:25. The pH was adjusted to 2.0 or 7.0, and the mixtures were then stirred and incubated in a water bath at 37 °C for 2 h, followed by centrifugation at 4000 rpm for 15 min. The colorimetric method was used, and the optical density of the supernatant at 550 nm was measured to determine the cholesterol content in the yolk solution (m1). A yolk solution without IDF was prepared in the same manner as a control, and the cholesterol content was also measured (m2). CAC was determined as the variance of cholesterol content absorbed by IDF as follows:

CACmg/g=m2-m1m0 6

Microstructure analysis

Fourier transform infrared spectroscopy

FTIR spectra of IDF samples were obtained at room temperature using a Nicolet 6700 FTIR Spectrometer (Thermo Fisher Scientific, Madison, WI, USA). The sample was ground and passed through a 60 mesh screen, mixed with KBr powder (1:50, w/w) under infrared irradiation, and pressed into a tablet. Spectra were collected in the range of 4000–400 cm−1 with a resolution of 4 cm−1.

X-ray diffraction

XRD patterns were determined using a Bruker D8 Advance X-ray diffractometer (Bruker, Beijing, China). Briefly, the IDF sample was ground and passed through a 60 mesh screen, compacted, and scanned at a rate of 4°/min with a step width of 0.02° within the range of 5°–70°.

Scanning electron microscopy

IDF samples were fully dried, and a small amount was adhered to double-sided conductive tape on the sample stage that had been cleaned with ethanol. Samples were observed using a Hitachi S-3400N II SEM (Hitachi Ltd., Tokyo, Japan) at 5 kV after coating with Au–Pd using an ion sputtering plating apparatus. SEM images were collected at 1200 × magnification.

Statistical analysis

Measurements were conducted in triplicate. The data were examined by analysis of variance using SPSS 22.0 (IBM Corp., Armonk, NY, USA) and presented as the mean ± standard deviation. Differences among means were considered significant at the 95% level (P < 0.05).

Results and discussion

Chemical compositions of dietary fiber samples

Based on response surface methodology, the optimized EH modification conditions were 230 U/g of cellulase, 940 U/g of xylanase, and hydrolysis time of 2.03 h (Supplementary Material). The total, insoluble, and soluble dietary fiber contents (% dry weight) of the dietary fiber samples extracted from rose pomace were corrected for moisture, remaining protein, and ash contents (Table 1). The SDF contents of the modified dietary fiber (EH-DF, 15.41%; UEH-DF, 13.59%) were significantly increased compared with unmodified dietary fiber (4.45%). Moreover, the increase in SDF content (10.96%) was greater with EH than UEH treatment. IDF contents were also increased after modification treatments (EH-DF, 61.17%; UEH-DF, 64.54%) compared with unmodified dietary fiber (41.12%). These results indicated the same trend in total dietary fiber. There were no significant differences in moisture or ash contents of dietary fiber samples associated with modifications (P > 0.05). These results indicated that EH treatment efficiently promoted the transformation from insoluble to soluble fiber, while the combined UEH method may lead to overtreatment and result in the loss of SDF.

Table 1.

Components, water holding capacity (WHC), swelling capacity (SC), oil-holding capacity (OHC) and cation-exchange capacity (CEC) of insoluble dietary fiber (IDF) from rose pomace (% dry weight)

Properties Unmodified IDF EH-IDF UEH-IDF
Moisture (%) 9.40 ± 0.51a 8.42 ± 0.62a 8.20 ± 0.60a
Ash (%) 4.09 ± 0.37a 3.72 ± 0.29a 3.54 ± 0.48a
TDF (%) 46.30 ± 0.40b 77.30 ± 1.28a 78.90 ± 1.40a
IDF (%) 41.12 ± 1.15b 61.17 ± 2.35a 64.54 ± 1.42a
SDF (%) 4.45 ± 0.81b 15.41 ± 1.05a 13.59 ± 1.19a
WHC (g/g) 8.32 ± 0.35b 10.45 ± 0.58a 11.09 ± 0.60a
SC (mL/g) 6.67 ± 0.06c 8.33 ± 0.45b 9.76 ± 0.49a
OHC (g peanut oil/g) 3.13 ± 0.18c 4.47 ± 0.11b 4.87 ± 0.23a
OHC (g lard/g) 3.34 ± 0.20c 4.88 ± 0.22b 5.55 ± 0.20a
CEC (mM/g) 0.62 ± 0.01c 0.75 ± 0.01b 0.86 ± 0.02a
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EH enzymatic hydrolysis, UEH ultrasound-assisted enzymatic hydrolysis

Data were expressed by mean ± standard deviation (n = 3). Values with different letters are significantly different (P < 0.05)

Physicochemical properties of IDF samples

The WHC, OHC, and SC of IDF from rose pomace subjected to different modification treatments are listed in Table 1. The capacities of modified IDF were enhanced compared with the unmodified sample. However, ultrasonic treatment did not lead to a significant difference in WHC (P > 0.05), though a higher value was contributed. It is generally considered that modification treatment enlarges the spaces between molecules, loosens the network structure, and exposes more polar groups and binding sites, thereby contributing to better capacities for combining water and oil. However, side effects occur if the particle size is too small, as the spatial structure can become damaged, abilities of interception and combination were descended as the specific surface area increases, leading to the decreases in WHC, SC, and OHC (Xie et al. 2017).

WHC is defined as the sum of free water and bound water in dietary fiber by physical means, and is influenced by environmental conditions and physicochemical properties of the fiber, such as ionic strength and structural morphology. High WHC improves food quality by reducing syneresis, which is often required in functional foods. In contrast to WHC, SC measures the variation in the total volume of dietary fiber and water and is closely related to the temperature, particle charge, source, and components of fiber (Karaman et al. 2017). Changes in the surface properties of IDF lead to exposure of more hydrophilic groups and interactions between crystalline and amorphous regions in the molecules (Elleuch et al. 2011). The SC of UEH-IDF (9.76 ± 0.49 mL/g) was 46.32% greater than that of unmodified IDF (6.67 ± 0.06 mL/g), and combined treatment showed a better result than single treatment.

Meanwhile, the animal-OHC was higher than vegetable-OHC and increased with modification. The animal-OHC of unmodified IDF was 6.7% higher than the vegetable-OHC, while it increased to 14.0% for UEH-IDF. These observations suggested better adsorption of saturated fatty acids by IDF than unsaturated fatty acids. Ultrasonic treatment had a positive influence on OHC, but its effect was not as significant as compound-enzymatic hydrolysis. OHC was probably influenced by the chemical structure, surface properties, electric charge density, and particle hydrophobicity of dietary fiber modified in different ways (Fernánde-López et al. 2009).

In summary, the properties outlined above indicated the suitability of dietary fiber for use in the food industry for modifying viscosity and texture, adsorbing fat from emulsified products, avoiding dehydration in food processing and preservation, and other health-related functions (Karaman et al. 2017).

Functional properties of IDF samples

Table 1 shows the CEC of dietary fiber from rose pomace with different degrees of modification via NaOH titration.d The curves of different samples were similar, but the volumes of NaOH required to reach the same pH differed. The NaOH volume required to change the suspension from acidic to alkaline increased significantly with the degree of modification (unmodified IDF, 0.62 ± 0.01 mM/g; EH-IDF, 0.75 ± 0.01 mM/g; UEH-IDF, 0.86 ± 0.02 mM/g), indicating that modification resulted in more cations and improved CEC.

Carboxyl, hydroxy, amidogen, and other groups in macromolecules confer the properties of a weak acidic cation exchange resin to dietary fiber, resulting in a transient decrease of ion concentration in the body after consumption, leading to changes in reduction/oxidation potential and intracellular osmotic pressure, thereby contributing to the regulation of blood pressure. Higher CEC has also been suggested to result from a higher content of uric acid in vivo (Dronnet et al. 1998), as supported by the significant relationship between CEC and uric acid (r = 0.99) (Huang and Ma 2016). Besides, CEC contributes to the destabilization and disintegration of lipid emulsions and reduces the diffusion and absorption of lipids and cholesterol in a synergistic effect with CAC in decreasing the utilization of cholesterol in vivo (Furda 1990).

As shown in Fig. 1, the GAC of IDF samples increased significantly with increasing glucose concentration. EH modification showed a positive influence on GAC, while ultrasonic treatment led to a lower adsorption volume than the control, suggesting that structural damage of dietary fiber due to overtreatment could reduce the glucose binding ability. The GAC of unmodified IDF increased tenfold with a fourfold increase in glucose concentration. The ratio of adsorption volume to glucose concentration within the same sample remained stable at high concentrations, which was consistent with a previous study showing that dietary fiber could effectively capture glucose and delay its release in high-concentration environments, indicating physiological functions of reducing instantaneous blood glucose concentration and maintaining postprandial blood glucose in vivo (Ahmed et al. 2011). However, UEH-IDF exhibited the lowest GAC, indicating an adverse effect of overtreatment to the dietary fiber.

Fig. 1.

Fig. 1

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Glucose adsorption capacity (GAC) of rose insoluble dietary fiber (IDF) samples with different modification degrees. EH enzymatic hydrolysis, UEH ultrasound-assisted enzymatic hydrolysis. Data were expressed by mean ± standard deviation (n = 3). Values with different letters are significantly different (P < 0.05)

Previous studies have shown that the GAC of dietary fiber varies with species and modification methods. The GAC of carrot pomace dietary fiber modified by ultrafine comminution showed better performance than that subjected to enzymatic modification and reached 2.634 mM/g. The exposure of polar and nonpolar groups enhances the interaction between dietary fiber and glucose molecules, as affected by the type and number of side-chain groups exposed under different pressures. Moreover, the network of dietary fiber is considered to form a strong physical barrier to entrap glucose molecules (Saikia and Mahanta 2016), delaying diffusion and postponing absorption in the gastrointestinal tract.

The CAC in vitro test evaluated the adsorption capacity of dietary fiber in environments mimicking the stomach (pH 2) and intestine (pH 7). As shown in Fig. 2, UEH modification significantly increased the CAC of IDF, and the CAC of all samples at pH 7 (unmodified IDF, 13.24 ± 0.24 mg/g; EH-IDF, 13.42 ± 0.10 mg/g; UEH-IDF, 14.50 ± 0.14 mg/g) were higher than those at pH 2 (unmodified IDF, 12.01 ± 0.27 mg/g; EH-IDF, 12.79 ± 0.29 mg/g; UEH-IDF, 13.98 ± 0.05 mg/g), indicating that the mechanism of binding was not only physical, but also chemical. The CAC was influenced by the source of materials and treatment methods used. The enzymatic method showed better performance concerning the CAC of IDF from carrot pomace than physical treatments, reaching a level of around 37 mg/g (Yu et al. 2018). This result was consistent with the XRD results (see “XRD patterns” section), as combined treatment of UEH would cause a more considerable disruption of the structure of dietary fiber than single treatment. CAC represents the effect of dietary fiber on lowering blood pressure and blood lipid levels, supporting the development of functional foods with effects related to cardiovascular health.

Fig. 2.

Fig. 2

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Cholesterol adsorption capacity (CAC) of rose insoluble dietary fiber (IDF) samples with different modification degrees. EH enzymatic hydrolysis; UEH ultrasound-assisted enzymatic hydrolysis. Data were expressed by mean ± standard deviation (n = 3). Values with different letters are significantly different (P < 0.05)

FTIR spectra

The FTIR spectra of IDF samples are shown in Fig. 3. The broad absorption peak at 3500–3200 cm−1 was formed by tensile vibration of O–H in alcohol, phenol, and other hydrogen bonds, indicating the combination of hydrogen and the hydroxyl groups in cellulose and hemicellulose. The peak observed in the region of 2920–2930 cm−1 proved the C–H stretching in the methylene groups of polysaccharides (Wang et al. 2016). These two peaks were observed in the spectra of all samples, indicating the typical structures of cellulose and hemicellulose (Ma and Mu 2016).

Fig. 3.

Fig. 3

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Fourier transform infrared spectroscopy (FTIR) spectra of rose insoluble dietary fiber (IDF) samples with different modification degrees. A: unmodified IDF; B: EH, enzymatic hydrolysis; C: UEH, ultrasound-assisted enzymatic hydrolysis

The characteristic absorption peak near 1657 cm−1 proved the aromatic benzene in lignin (Xie et al. 2017), while another study regarded the increase in peak intensity due to the absorption of moisture by fiber (Wen et al. 2017). The bands at 1736 and 1514 cm−1 were assigned to characteristic bending or stretching vibrations of the different groups from cellulose and characteristic bending or stretching of different groups of lignin (Zhao et al. 2013). The peak at 1618–1657 cm−1 was stronger for UEH-IDF than other samples, with two weaker shoulder peaks at 1736 cm−1 and 1514 cm−1, suggesting the transfer of C–O from carbonyl and amide groups to the benzene ring in lignin under ultrasonic treatment.

Each sample had a small peak at 1371–1379 cm−1 (1371, 1371, and 1379 cm−1) representing C–H bending (Ullah et al. 2017), a peak at 1236–1248 cm−1 (1236, 1244, and 1248 cm−1) indicating C–O stretching of carboxylic acid, and a sharp peak at 1014–1034 cm−1 (1014, 1030, and 1034 cm−1) corresponding to the O–H group in lignin or C–OH bending in hemicellulose (Ramadoss and Muthukumar 2016).

The peak intensities of UEH-IDF were generally weaker than the other samples, indicating that smaller phenolic compounds were converted from lignin and intermolecular hydrogen bonds in cellulose were ruptured after ultrasonic processing (Xie et al. 2017). Moreover, the spectra of the three samples were redshifted along with the degree of modification, indicating that the structures of molecules were disrupted after modification, which was consistent with the previous report (Chen et al. 2015).

XRD patterns

The XRD patterns of IDF are shown in Fig. 4. Generally, all of the samples had similar patterns, with main peaks around 14–15 Å, 21–22 Å, 23.5–24.5 Å, and 30–31 Å in all samples. Karaman et al. (2017) suggested that peaks at around 14–15 Å and 21–22 Å may be derived from cellulose crystals. Cellulose has a crystalline structure due to hydrogen bonding interactions and Van der Waals forces between adjacent molecules. In contrast, amorphous regions consist of noncrystalline cellulose, hemicellulose, and lignin. The peak position and peak width of all samples were not significantly different, suggesting that the ordered structure of the crystalline region of the cellulose in IDF was not disrupted (Huang et al. 2017).

Fig. 4.

Fig. 4

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X-ray diffraction (XRD) patterns of rose insoluble dietary fiber (IDF) samples with different modification degrees. A: unmodified IDF; B: EH, enzymatic hydrolysis; C: UEH, ultrasound-assisted enzymatic hydrolysis. 2θ represents diffraction angle

The lower peak intensities in the two modified samples were probably due to EH treatment. However, some new high-intensity peaks appeared in UEH-IDF, indicating the marked influence of physical modification treatments on the formation of new crystallization. Further studies are required to determine the precise mechanism underlying this effect. Amorphous compounds, such as lignin and hemicellulose, could be removed by effective treatments, and in turn, increase the crystallinity index. A similar study showed that microwave treatments enhanced the crystallinity of orange seeds (Karaman et al. 2017).

SEM analysis

SEM images of IDF are shown in Fig. 5. Unmodified IDF had a tube-like structure, which transformed into blocks with folds like shales after EH modification. Structural homogenization was further enhanced after ultrasonic treatment, and the structure became even looser. The above results indicated that cellulase and xylanase had significant effects on the structure of IDF, degrading the compact structure of large molecules, and promoting the transformation of IDF into SDF. The structure was further expanded with increased interspaces after ultrasound-assisted treatment. Another study also indicated that although ultrasound did not hydrolyze dietary fiber into soluble sugars, it caused the dietary fiber to be more readily hydrolyzed by enhancing the accessible surface area and destroying the cellulose–hemicellulose–lignin matrix (Ramadoss and Muthukumar 2016). The above characteristics were consistent with the results of the adsorption capacities of IDF and the results of a previous study by Yu et al. (2018). Combined modification methods, such as biophysical treatment, could break the fiber matrix, markedly alter the particle size and surface area, and thus lead to a discontinuous and loosened structure (Wen et al. 2017).

Fig. 5.

Fig. 5

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Scanning electron microscopy (SEM) images (1200 ×) of unmodified rose insoluble dietary fiber (IDF) (a), enzymatic hydrolysis (EH) modified rose IDF (b) and ultrasound-assisted enzymatic hydrolysis (UEH) modified rose IDF (c)

Conclusion

This study set out to assess the physicochemical, functional, and microstructural properties of dietary fiber modified by EH and UEH. The optimized combination of cellulase and xylanase conducted to an effective yield of SDF from rose pomace. Compared with EH, which significantly enhanced the physicochemical and functional properties, ultrasonic pretreatment contributed to better WHC, OHC, SC, CEC, and CAC but resulted in lowering GAC. Enzymatic modification could effectively hydrolyze cellulose and hemicellulose components, loosen the microstructure and enhance the surface area; while physical method like ultrasound could exert a sharper influence on the microstructure of IDF. Overall, EH could significantly increase the yield of SDF and improved the properties of dietary fiber from rose pomace as a modification method more economical than UEH. This study provides experiment assistance to the utilization of rose by-products as a new source of dietary fiber.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Supplementary material 1 (DOCX 799 kb) (799.8KB, docx)

Acknowledgements

This work was supported by Grants from the National Key R&D Program of China (No. 2018YFD0400900); the Fundamental Research Funds for the Central Universities (No. 2018BLRD08); and the National Undergraduate Innovation and Entrepreneurship Training Program (No. 201710022024).

Abbreviations

DF

Dietary fiber

IDF

Insoluble dietary fiber

SDF

Soluble dietary fiber

EH

Enzymatic hydrolysis

UEH

Ultrasound-assisted enzymatic hydrolysis

WHC

Water-holding capacity

OHC

Oil-holding capacity

SC

Swelling capacity

CEC

Cation-exchange capacity

GAC

Glucose adsorption capacity

CAC

Cholesterol adsorption capacity

FTIR

Fourier transform infrared

XRD

X-ray diffraction

SEM

Scanning electron microscopy

RBDF

Rice bran dietary fiber

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Footnotes

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Contributor Information

Difeng Ren, Email: rendifeng@bjfu.edu.cn.

Jun Lu, Email: lujun@cnif.cn.

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