Extraction of Deoiled Walnut Dietary Fibers and Effects of Particle Sizes on the Physiochemical Properties

Abstract

In this study, chemical method and enzymatic method were used to extract dietary fibers (DF) from deoiled walnut residues, and the resulting fibers were named as CDF and EDF respectively. The CDF and EDF were characterized by FTIR, XRAD and SEM. Moreover, the effects of particle sizes on physiochemical properties of CDF and EDF were explored. The results indicated that enzymatic method inhibited the structural damage of walnut DF, thus increasing the external surface area of EDF. Besides, particle sizes affected the physiochemical properties (water retention capacity (WRC), oil retention capacity (ORC), water swelling capacity (WSC) and apparent viscosity) of walnut DF. Particle sizes ranged from 180 to 450 µm, CDF and EDF had higher WRC. In the particle sizes range of 125–180 µm, CDF and EDF had higher ORC and apparent viscosity. These results might provide a reference for the utilization of walnut DF in food industry.

Introduction

Dietary fiber (DF) is an ingredient widely existing in human diets and has been early seen as the seventh nutrient, in addition to carbon hydrates, proteins, lipids, vitamins, minerals and water. Although DF do not contribute any energy to human body, it has a variety of beneficial effects on human health. For example, previous investigations showed that the adequate consumption of DF was closely related to the improvement of body immunity, metabolism and the decreased risks of some diseases (e.g., colon cancer, obesity, hypertension and diabetes) (Adorian et al., 2016; Benítez-Páez et al., 2016; Deehan and Walter, 2016; Feng et al., 2017; Ge et al., 2016). Moreover, a recently published report also showed that the uptake of DF could enhance the tolerance of mice against food allergies (Tan et al., 2016). Conversely, the inadequate uptake of DF may cause serious health problems. (Desai et al., 2016) suggested that the deprivation of DF could promote the lethal colitis of mice, because the practice of DF deprivation increased the accessibility of mucus-eroding microbiota to the epithelial tissues in mice intestine tract (Desai et al., 2016).

Compared to the excessive consumption of fat and lipid, DF consumption is under relatively low levels. Due to unsavory taste, DF is not consumed solely. Rather, DF is usually used as a potential additive to improve the sensory qualities and texture of some food products such as cake and pasta (Ben Jeddou et al., 2017; Cappa and Alamprese, 2017; Eshak, 2016). Generally, DF can be extracted from plenty of plant resources, including carrots (Chau et al., 2004), orange peels (Garau et al., 2007), lemons (Karaman et al., 2017), soybeans (Cheng et al., 2012), black bean coats (Feng et al., 2017), the cores of maize straw (Lv et al., 2017), etc. In addition, some novel plant resources are emerging, such as sugarcane (Zhuang et al., 2016), bamboo shoots (Zheng et al., 2017), papaya peel (Zhang et al., 2017), rice bran (Wang et al., 2016), waterleaf (Andarwulan et al., 2015), Moringa seeds (Anudeep et al., 2016), coffee silverskin (Behrouzian et al., 2016), cumin (Ma M. M. and Mu T. H., 2016 b). Different parent plant resources and extraction procedures may lead to the significant differences in structure of the resulting DFs, which can further affect its physiochemical properties (Macagnan et al., 2016). At present, DF is classified into two types in terms of its solubility, namely soluble dietary fiber (SDF) and insoluble dietary fiber (IDF), respectively (Dai and Chau, 2017). The former mainly include pectin and gum, and the later were composed of cellulose, hemicellulose and lignin.

As a highly-valued edible oil, walnut oil has been proven to possess significant health effects, due to its high unsaturated fatty acid contents (approximately 92%) (Chung et al., 2016). Moreover, attractive ardor and taste of walnut oil also attract more and more interests all around the world, leading to increased walnut cultivation. For instance, China, one of the largest walnut-cultivating countries, had a walnut planting area of approximately 2.5 million hm² and a yield of over 2.4 million tons in 2013, according to a report. With the rapid development of walnut processing industry, walnut producers prefer to process walnut into edible oils instead of selling them as raw nuts, so that they can obtain higher economic returns. That leads to the production of thousands of tons of walnut residues, which are often lowly utilized. According to our previous analysis, deoiled walnut residues included around 40% of protein and 20% of dietary fiber, making walnut residues an excellent DF and protein resources. Consequently, the objective of present work was to study the effects of different extraction methods including chemical method and enzymatic method on the physiochemical properties of walnut dietary fiber. Besides, the effects of different particle sizes on the physiochemical properties of the resulting fibers were also explored. These findings may provide some information to help obtain walnut dietary fiber with desired properties.

Materials and Methods

Materials    The deoiled walnut residues were kindly provided by a local company (Xi'an, China), and the variety name is ‘Xiangling’, one of the widest cultivated walnut varieties in China. After the residual oil was removed using petroleum ether as the extractant, the residues were stored at 4°C for further analysis.

All chemical reagents for analysis were of analytical grade, and were purchased from Jingbo biological technology Co., Ltd. (Xi'an, China). For FTIR analysis, KBr was of spectroscopical grade and purchased from Sigma Aldrich (St. Louis, MO, USA). Besides, alkali protease, papain, α-amylase and glucoamylase, whose origin creature were Bacillus licheniformis, Carica papaya L., Bacillus licheniformis and Aspergillus niger, respectively, were purchased from Sigma Aldrich and used according to the specifications. The rapeseed oil was purchased from local market.

Preparation of walnut dietary fiber    The extraction of walnut dietary fiber was carried out using chemical and enzymatic methods, and the resulting DFs were named as CDF and EDF, respectively. The chemical extraction process was performed by the method of Ma and Mu (2016 b) with some modifications. The deoiled walnut residues were dissolved into 1 M NaOH solution with a solid-liquid ratio of 1: 10 (g/mL). Then, the mixture was incubated at 55°C for 4 h under moderate stirring (79-2 Bidirectional magnetic heating stirrer, Jintan Fuhua Instrumrnt Co., Ltd., China). After filtrating the mixture with 0.45 µm filter membrane, the residues were collected and washed thoroughly with deionized water until neutral, immediately. Then 1 M HCl solution with a solid-liquid ratio of 1: 10 was added into the residues, after which the mixture was moderately oscillated at room temperature (25°C) for 30 min. Subsequently, the mixture was filtrated. And the residues were collected and washed using deionized water to neutral. Finally, the residues were dried in an air blowing thermostatic oven (GZX-9146 MBE, Shanghai Bo Xun Industrial Co., Ltd. medical equipment factory, China) at 45°C until the moisture content was lower than 10%.

The enzymatic extraction process was carried out by the method of Ma and Mu (2016 b) with some modification. Briefly, after the deoiled walnut residues were mixed with 10-fold volume (g/mL) deionized water, 1 M NaOH was added to adjust the pH of the solution to approximately 6.5–7.0. Subsequently, α-amylase (4000 µ/g) was added to give an enzyme concentration of 3%. After fully mixing, the solution was incubated under a hot water bath (60°C) for 2 h, after which, 3% of the papain (7000 µ/g) was added. After the solution was continually incubated for 2 h, 1 M NaOH solution was used to adjust the pH of the solution to approximately 8.0. Then, 3% of alkali protease (4000 µ/g) was added, and then the solution was incubated at 55°C under moderate oscillation for 2 h. Subsequently, 1 M HCl solution was used to acidify the solution to pH 4.0–4.2, after which 3% of glucoamylase (4000 µ/g) was added and the matrix was slightly oscillated at 60°C water-bath for 30 min. Finally, the solution was heated up to 100°C and kept for 3 min to inactivate the enzymes. Then the solution was filtrated when it cooled down to room temperature. The residues were collected and washed thoroughly with deionized water until neutral, then dried in an air blowing thermostatic oven at 45°C until the moisture content was lower than 10%.

Basic component determination    After the preparation of CDF and EDF, the basic components, including protein content, DF content, water content and ash content were determined. The protein content was determined by using an Automatic kieldahl instrument (Kjeltec 8400 Automatic kieldahl apparatus, Volkswagen Corporation, Swedish). The DF content was determined by the method of Arrigoni E et al. (1986) with some modifications. The moisture content was determined with a drying oven (GZX-9146 MBE, Shanghai Bo Xun Industrial Co., Ltd. medical equipment factory, China). The ash content was determined using a Muffle furnace (DC-B, Beijing Kewei Yongxing Instrument Company, China). Meanwhile, the deoiled walnut residues served as the reference.

Structural analysis (FTIR, X-ray diffraction and SEM)    Approximately 5 mg of the dry walnut dietary fiber were mixed with KBr (1: 100) and pressed into a semitransparent pellet for FTIR analysis. The scanning spectrum was performed within middle infrared spectrum (4000-400 cm⁻¹), with 256 scans and 4 cm⁻¹ resolution (Khor et al., 2017). Besides, all obtained spectra were smoothed to eliminate the noise interference by using OMNIC software.

To investigate the crystalline structure of the extracted dietary fibers, X-ray diffraction measurements were carried out by using a Powder X-ray Diffractometer (D8 Advance, Germany). Before X-ray measurement, the fibers were ground to powder form, and then the powder was scanned from 10° to 60° at a step size of 0.02° and a time interval of 5 s per step.

The microstructure of the CDF and EDF was examined by using a scanning electron microscope (TM3030, Hitachi Co. Ltd., Kyoto, Japan) (Karaman et al., 2017). The scanning process was performed in high vacuum mode, with 10 KV accelerating voltage.

Physiochemical and functional properties of CDF and EDF with different particle sizes    The CDF and EDF were smashed using a pulverizer (FW-200D, Tianjin Xin Bo De Instrument Co., Ltd., China). After that, the smashed fibers were sieved by using different mesh-sized sieves (98, 98–110, 110–125, 125–154, 154–180, 180–280, 280–450 µm). Consequently, the fibers with different particles were obtained, and their weights were respectively recorded. Besides, the weight percentages of the sieved fibers over the total weight were also evaluated accordingly.

Water retention capacity (WRC)    The WRC of the CDF and EDF was determined as the method described by Behrouzian et al. (2016) and Eshak (2016) with slight modifications. Briefly, 0.5 g dietary fiber was added into a centrifuge tube, after which 8 mL of deionized water was added and the tube was vigorously oscillated so as to completely mix the matrix. After placing the tube at room temperature for 18 h, the tube was centrifuged at 4000×g for 20 min. The supernatant was removed and then the weight of the precipitate was recorded as m₁. Subsequently, the precipitate was dried at 105°C until constant weight, and the weight was recorded as m². The WRC was calculated according the following equation:   

Water swelling capacity (WSC)    The WSC value was determined as the method described by Lv et al. (2017) and Zhou et al. (2011) with some modifications. A 0.5 g of dietary fiber was added in a test tube, and then the volume of the fiber was recorded as V₁. Subsequently, 10 mL of deionized water was added into the tube, after which the tube was balanced for 18 h, and then the volume of the matrix was recorded as V₂. The WSC value was calculated by the following equation:   

Oil retention capacity (ORC)    The ORC was determined as the method described by Ben et al.. (2017) and Fu et al. (2010) with slight modifications. A 0.2 g walnut dietary fiber was added into a centrifuge tube, and the total weight of the tube was recorded as m₁. After adding 5 mL rapeseed oil into the tube, the tube was placed at room temperature for 18 h so that the fiber can fully absorb the oil. Then, the mixture was centrifuged at 4000×g for 10 min. After removing the supernatant oil, the weight of the residues was recorded as m₂. The ORC was calculated according to the following equation:   

Cation absorption capacity    The cation absorption capacity of CDF and EDF was determined according to the following method: 1.0 g dietary fiber was added into a test tube, after which 5 mL 0.1 M HCl solution was added and the solution was fully mixed and balanced at room temperature for 24 h. After the solution was filtrated using a vacuum pump (0.1 MPa), the residues were collected and washed with deionized water until Cl⁻ was completely removed. Subsequently, the residues were dried using a air-drying oven until constant weight (50°C, approximately 6 h). Then, 0.25 g of the dried samples was added into 100 mL NaCl solution to give a concentration of approximately 15% (w/v), and the mixture was moderately stirred for 5 min. After adding 0.5 mL of 0.1 M NaOH solution into the mixture, the mixture was immediately stirred and the pH of the mixture solution was detected using a pH meter (PHS-25, Shanghai instrument and electrical scientific instrument Limited by Share Ltd, China). The pH was recorded after each addition of NaOH solution and until the pH was nearly constant.

Glucose inhibitory capacity    The glucose inhibitory capacity was determined using Chau's method (Chau et al., 2004). Briefly, after washing the dietary fiber with absolute ethanol, the fiber was dried at 45°C for 2 h so as to remove the residual ethanol. A 0.2 g of walnut dietary fiber was precisely weighed and added into a tube, in which 10 mL of 150 mM glucose solution had been added previously. After the mixture was fully mixed, the mixture was dialyzed using a dislysis bag (12000 MWCO) for 60 min. In the dialysis process, the glucose concentration in dialyzate was monitored every 10 min. Besides, the 10 mL of 150 mM glucose solution without dietary fiber added served as the control.

DPPH scavenging capacity    The DPPH scavenging capacity of walnut dietary fiber was evaluated by the method of Nisar et al. (2017) with slight modifications. In brief, the different amounts of walnut dietary fiber were dissolved into deionized water, allowing the concentrations of the walnut dietary fibers to be 1.0 mg/mL, 1.5 mg/mL, 2.0 mg/mL, 2.5 mg/mL, 3.0 mg/mL, respectively. Then, 0.2 mM DPPH solution was prepared, and 2 mL DPPH solution was added into 2 mL walnut fiber solution. After moderate stirring, the absorbance of the mixture solution was recorded at 517 nm. Meanwhile, the deionized water served as the blank group, and the ascorbic acid of analytic grade served as the control group.

Statistical analysis    All experimental data were analyzed by SPSS 13.0 software (SPSS Inc., USA) and the result was expressed as means ± standard deviation. Moreover, the significant differences between mean values were calculated by Duncan's multiple test at a significance level of p < 0.05.

Results and Discussion

The component analysis of the walnut dietary fiber    Unlike other dietary fiber resources, deoiled walnut residues contain a large amount of protein, thus the removal of protein is very crucial to the extraction of walnut dietary fiber. Generally, many methods can be used to separate protein, including isoelectric precipitation, metal ionic precipitation and gel permeation chromatography. Among these methods, isoelectric precipitation had an immense potential to be used in food industry, because the method is easy to manipulate and is inexpensive. Enzymatic extraction, however, has some advantages over chemical extraction. For example, enzymatic methods can hinder the structural damage of dietary fiber, due to mild reaction conditions. Therefore, we adopted chemical and enzymatic methods to extract and separate dietary fiber from deoiled walnut residues.

To evaluate the extraction efficiency of chemical and enzymatic methods, we firstly compared the basic components of the resulting dietary fibers, and the results were exhibited in Table 1. It can be observed that compared with the deoiled walnut residues which consisted mainly of protein (59.57%) and dietary fiber (18.62%), CDF and EDF had significantly lower (p < 0.05) protein contents, with a value of 3.41% and 5.25%, respectively. This indicated that the chemical and enzymatic methods can effectively remove the protein from deoiled walnut residues. Besides, the DF content of EDF was higher than that of CDF, and there were significant difference between them (p < 0.05). Meanwhile, the ash content of CDF was higher than that of EDF, and there were significant difference between them (p < 0.05). Moreover, the yield of the walnut dietary fiber prepared by the two methods had no significant difference (p < 0.05), which all indicated that the extraction efficiency of chemical and enzymatic methods was equal. Furthermore, as showed in Table 1, the difference of the moisture contents was insignificant (p < 0.05).

Table 1. Basic compositions of walnut dietary fibers and deoiled walnut residues

	yield (%)	Dietary fiber content (%)	Protein content (%)	Ash content (%)	Moisture content (%)
Deoiled walnut residues		18.62±1.65c	59.57±0.78a	3.86±0.54a	6.54±0.53b
CDF	10.47±1.87a	75.34±3.16b	3.41±0.04c	3.34±0.62a	7.63±0.65ab
EDF	11.85±1.46a	81.15±2.87a	5.25±0.12b	1.76±0.24b	7.98±0.49a

The different superscript letters represent significant difference (p < 0.05).

Structural Characteristics on FTIR characteristcs    To investigate the structural differences of the resulting fibers, we performed the FTIR analysis and the results were presented in Fig. 1. It can be observed that the spectral profiles of CDF and EDF were roughly similar except a few characteristic absorption bands. Clearly, CDF and EDF had three mutual absorption bands, located around 2925 cm⁻¹, 1620 cm⁻¹ and 1050 cm⁻¹. Besides, two marked absorption peaks located approximately 3305 cm⁻¹ and 1537 cm⁻¹, respectively, were observed in the spectrum curve of EDF, indicating a significant structural difference between CDF and EDF.

Fig. 1.

Spectroscopy results of CDF and EDF.

In general, the absorption band located at around 3300 cm⁻¹ is attributed to the stretching vibrations of O-H bonds (Feng et al., 2017). The absorption peaks appeared at 2925 cm⁻¹ is attributed the C-H vibrations of methylene groups of the fiber (Ma M. M. and Mu T. H., 2016 a). From Fig. 1, it can be observed that the intensity and width of the absorption bands at around 3300 cm⁻¹ was significantly lower as compared with EDF, indicating that the walnut fiber may be partly degraded during alkaline treatments. The relatively weak band occurred at 2925 cm⁻¹, which caused by C-H stretching vibrations, may indicate the presence of the polysaccharide compounds (Feng et al., 2017). Furthermore, the absorption peak of EDF at around 1660 cm⁻¹, together with the peak occurred at 1537 cm⁻¹ and 1056 cm⁻¹, may indicated the presence of benzene ring structure (Ma M. M. and Mu T. H., 2016 a). These findings implied that EDF contain a large amount of lignin compared with CDF. It also indicated that chemical extraction may lead to the degradation of lignin.

XRAD measurement    The XRAD patterns of walnut dietary fibers were shown in Fig. 2. It was cleared that both CDF and EDF had a significant bread-shape diffraction peak at around 2θ=20°, indicated that the fibers was amorphous or semicrystalline structure (Ma M. M. and Mu T. H., 2016 b). Besides, the results indicated that EDF had a lower 2θ value (−19.26°) than CDF (−21.52°), suggested that enzymatic extraction allowed a more compact crystal structure of the extracted walnut fiber.

Fig. 2.

XRAD results of CDF and EDF.

As discussed above, EDF can hinder the degradation of lignin. Thus, EDF had a higher lignin content, which can enhance the binding capacity between cellulose molecules, because lignin contains more negative charge groups leading to hydrogen bonding. Consequently, EDF had a more compact crystalline structure.

SEM observation    The microstructure of dietary fiber, mainly surface morphology and porosity, had a significant effect on its physicochemical and functional properties (Feng et al., 2017). Thus, the SEM scanning and the results are displayed in Fig. 3. It can be observed that the deoiled walnut residues exhibited many small global particles and continuous sheet-like structure (Fig. 3A). These global particles may be protein aggregates. During the chemical and enzymatic extraction processes, these protein aggregates were greatly removed, as shown in Table 1, and therefore the global particles were not observed in CDF and EDF (Fig. 3B & C). Moreover, compared with CDF, EDF has obvious helix structure, indicated that enzymatic extraction decreased the structural damage of walnut dietary fiber, significantly, which was in line with the XRAD measurement result. Furthermore, the orderly helix structure facilitates the increase of the external surface area of EDF, which may have some effects on the physiochemical properties of the fiber.

Fig. 3.

SEM scanning results of walnut dietary fibers (A), CDF (B), EDF (C).

Effect of particle sizes on the physiochemical and functional properties of CDF and EDF    The physiochemical and functional properties of dietary fiber are closely related to its particle sizes. Thus, CDF and EDF was grinded by a pulverizer, and then the effects of different particle sizes on the physicochemical properties were explored. The particle size distribution is displayed in Fig. 4. It can be observed that both CDF and EDF had a main particle sizes within the range of 154–180 µm. An increase and/or decrease of particle size led to decreased distribution percentages of CDF and EDF.

Fig. 4.

Particle size distributions and their weight percentages of CDF and EDF.

The physiochemical properties of dietary fiber mainly include water retention capacity (WRC), oil retention capacity (ORC), water swelling capacity (WSC) and apparent viscosity. Understanding this information is the first step to utilize dietary fiber. For example, the dietary fiber with a high ORC value can be used as an additive in food products containing high oil levels. Highly viscous dietary fibers has been proven to be a good additive to modify the viscosity of some formulated foods (Cappa and Alamprese, 2017). As shown in Fig. 5, it can be observed that CDF and EDF had higher WRC, ORC, WSC and apparent viscosity than the deoiled walnut residues. Besides, CDF and EDF had higher WRC, ORC, WSC and apparent viscosity within the particle sizes range of 125–180 µm. An increase and/or decrease of particle sizes could result in the significant decrease of these physiochemical properties. Roughly, CDF and EDF had higher WRC values in the particle sizes range of 180–450 µm, higher ORC and apparent viscosity values in the particle sizes range of 125–180 µm, higher WSC value in the particle sizes range of 154–280 µm.

Fig. 5.

Effects of particle sizes on WRC (A), ORC (B), apparent viscosity (C) and WSC (D) of CDF and EDF.

These findings can be explained by the external surface areas of CDF and EDF and their molecular structures. Generally, the larger the particle sizes, the smaller the external surface area. Hence, as the particle sizes decreased, the ORC, WSC and apparent viscosity values were increased, due to the increased eternal surface areas. However, when particle sizes were beyond 125 µm, the ORC, WSC and apparent viscosity values were significantly decreased, probably because the pulverizing process led to the structural damage of the dietary fiber.

Effect of particle sizes on the cation absorption capacity of CDF and EDF    As shown in Fig. 6, CDF exhibited higher caption absorption capacity compared with EDF. However, there was no significant difference of caption absorption capacity between EDF and the deoiled walnut residues (control). Besides, as the particle sizes increased, the cation absorption capacity of CDF was observed to be slightly decreased; however, the particle sizes showed no significant effects on the cation absorption capacity of EDF.

Fig. 6.

Effects of particle sizes on cation exchange capacity of CDF (A) and EDF (B).

Effect of particle sizes on the glucose inhibition capacity of CDF and EDF    As presented in Fig. 7, Moreover, it can be observed that CDF and EDF had higher glucose inhibition capacity than the deoiled walnut residues. For CDF, the best glucose inhibition capacity was obtained within the particle sizes range of 98–125 µm. However, the best glucose inhibition capacity of EDF can be observed in the particle sizes below 450 µm.

Fig. 7.

Effects of particle sizes on glucose inhibitory capacity of CDF (A) and EDF (B).

DPPH scavenging capacity    DPPH scavenging experiment was carried out to explore the in vitro anti-oxidation activity of CDF and EDF. As presented in Fig. 8., the DPPH scavenging capacity of both CDF and EDF showed significant concentration-dependent behavior. At the same concentration, the DPPH scavenging capacity of EDF was always higher than that of CDF, indicated that EDF had higher antioxidant activities.

Fig. 8.

Effects of DPPH scavenging capacity of CDF (A) and EDF (B).

Conclusion

CDF and EDF were obtained by treating deoiled walnut residues with chemical method (alkaline treatment) and enzymatic method, respectively. In addition to investigating the structural characteristics of CDF and EDF by means of FTIR, XRAD, SEM, the effects of particle sizes on the physiochemical and function properties of CDF and EDF were explored. Compared with chemical extraction method, enzymatic method can hinder the structural damage of walnut dietary fiber, significantly, allowing EDF to have a larger external area. Besides, the results indicated that particle sizes had a significant effect on the physiochemical properties of walnut dietary fiber. Within the particle sizes range of 180–450 µm, CDF and EDF had higher WRC values; in the particle sizes range of 125–180 µm, higher ORC and apparent viscosity values can be obtained. An increase and/or decrease of particle sizes led to the decreased physiochemical properties. Furthermore, particle sizes has significant effects on the cation absorption capacity of CDF but no significant effects on EDF. These findings may provide some information to help obtain walnut dietary fiber with desired properties.

Acknowledgements    This work was financially supported by the Shaanxi Characteristic Industry Innovation Chain (2018TSCXL-NY-06-02) and Shaanxi Characteristic Fruit Storage and Preservation Key Technology Integration and Demonstration (2018)

References

•  Adorian,  T. J.,  Goulart,  F. R.,  Mombach,  P. I.,  de Menezes Lovatto,  N.,  Dalcin,  M.,  Molinari,  M.,  Lazzari,  R., and  da Silva,  L. P. (2016). Effect of different dietary fiber concentrates on the metabolism and indirect immune response in silver catfish. Anim. Feed Sci. Technol., 215, 124-132.
•  Andarwulan,  N.,  Faridah,  D. N.,  Prabekti,  Y. S.,  Fadhilatunnur,  H.,  Mualim,  L.,  Aziz,  S. A., and  Cisneros-Zevallos,  L. (2015). Dietary fiber content of waterleaf (Talinum triangulare (Jacq.) Willd) cultivated with organic and conventional fertilization in different seasons. Am. J. Plant Sci., 6, 334-343.
•  Anudeep,  S.,  Prasanna,  V. K.,  Adya,  S. M., and  Radha,  C. (2016). Characterization of soluble dietary fiber from Moringa oleifera seeds and its immunomodulatory effects. Int. J. Biol. Macromol., 91, 656-662.
•  Arrigoni,  E.,  Caprez,  A.,  Amadò,  R., and  Neukom,  H. (1986). Chemical composition and physical properties of modified dietary fibre sources. Food Hydrocoll., 1, 57-64.
•  Behrouzian,  F.,  Amini,  A. M.,  Alghooneh,  A., and  Razavi,  S.  Mohammad  A. (2016). Characterization of dietary fiber from coffee silverskin: An optimization study using response surface methodology. Bioactive Carbohydrates and Dietary Fibre, 8, 58-64.
•  Ben Jeddou,  K.,  Bouaziz,  F.,  Zouari-Ellouzi,  S.,  Chaari,  F.,  Ellouz-Chaabouni,  S.,  Ellouz-Ghorbel,  R., and  Nouri-Ellouz,  O. (2017). Improvement of texture and sensory properties of cakes by addition of potato peel powder with high level of dietary fiber and protein. Food Chem., 217, 668-677.
•  Benítez-Páez,  A.,  Gómez Del Pulgar,  E. M.,  Kjølbæk,  L.,  Brahe,  L. K.,  Astrup,  A.,  Larsen,  L., and  Sanz,  Y. (2016). Impact of dietary fiber and fat on gut microbiota re-modeling and metabolic health. Trends Food Sci. Technol., 57, 201-212.
•  Cappa,  C. and  Alamprese,  C. (2017). Brewer's spent grain valorization in fiber-enriched fresh egg pasta production: Modelling and optimization study. LWT-Food Sci. Technol., 82, 464-470.
•  Chau,  C.-F.,  Chen,  C.-H., and  Lee,  M.-H. (2004). Comparison of the characteristics, functional properties, and in vitro hypoglycemic effects of various carrot insoluble fiber-rich fractions. LWT, Food Sci. Technol. 37, 155-160.
•  Cheng,  Z.,  Yang,  R., and  Zhao,  W. (2012). Gel properties of ribbonfish (Trichiurus haumela) surimi gels with soybean dietary fiber Induced by high pressure and heating. Int. J. Food Eng., 7, 639-646.
•  Chung,  J.,  Kim,  Y.-S.,  Lee,  J.,  Lee,  J. H.,  Choi,  S.-W., and  Kim,  Y. (2016). Compositional analysis of walnut lipid extracts and properties as an anti-cancer stem cell regulator via suppression of the self-renewal capacity. Food Sci. Biotechnol., 25, 623-629.
•  Dai,  F. J. and  Chau,  C. F. (2017). Classification and regulatory perspectives of dietary fiber. J. Food Drug Anal., 25, 37-42.
•  Deehan,  E. C. and  Walter,  J. (2016). The fiber gap and the disappearing gut microbiome: Implications for human nutrition. Trends Endocrinol. Metab., 27, 239-242.
•  Desai,  M. S.,  Seekatz,  A. M.,  Koropatkin,  N. M.,  Kamada,  N.,  Hickey,  C. A.,  Wolter,  M.,  Pudlo,  N. A.,  Kitamoto,  S.,  Terrapon,  N.,  Muller,  A.,  Young,  V. B.,  Henrissat,  B.,  Wilmes,  P.,  Stappenbeck,  T. S.,  Nunez,  G., and  Martens,  E. C. (2016). A dietary fiber-deprived gut microbiota degrades the colonic mucus barrier and enhances pathogen susceptibility. Cell, 167, 1339-1353. e21.
•  Eshak,  N. S. (2016). Sensory evaluation and nutritional value of balady flat bread supplemented with banana peels as a natural source of dietary fiber. Ann. Agric. Sci., 61, 229-235.
•  Feng,  Z.,  Dou,  W.,  Alaxi,  S.,  Niu,  Y., and  Yu,  L. (2017). Modified soluble dietary fiber from black bean coats with its rheological and bile acid binding properties. Food Hydrocolloids, 62, 94-101.
•  Fu,  Q.,  Li,  L.,  Yu,  X., and  Li,  B. (2010). In vitro binding of bile salts by different insoluble dietary fiber extracted from brewers' spent grain. Int. J. Food Eng., 6.
•  Garau,  M. C.,  Simal,  S.,  Rosselló,  C., and  Femenia,  A. (2007). Effect of air-drying temperature on physico-chemical properties of dietary fibre and antioxidant capacity of orange (Citrus aurantium v. Canoneta) by-products. Food Chem., 104, 1014-1024.
•  Ge,  X.,  Tian,  H.,  Ding,  C.,  Gu,  L.,  Wei,  Y.,  Gong,  J.,  Zhu,  W.,  Li,  N., and  Li,  J. (2016). Fecal microbiota transplantation in combination with soluble dietary fiber for treatment of slow transit constipation: A pilot study. Arch. Med. Res., 47, 236-242.
•  Karaman,  E.,  Yılmaz,  E., and  Tuncel,  N. B. (2017). Physicochemical, microstructural and functional characterization of dietary fibers extracted from lemon, orange and grapefruit seeds press meals. Bioactive Carbohydrates and Dietary Fibre, 11, 9-17.
•  Khor,  C. M.,  Ng,  W. K.,  Chan,  K. P., and  Dong,  Y. (2017). Preparation and characterization of quercetin/dietary fiber nanoformulations. Carbohydr. Polym., 161, 109-117.
•  Lv,  J. S.,  Liu,  X. Y.,  Zhang,  X. P., and  Wang,  L. S. (2017). Chemical composition and functional characteristics of dietary fiber-rich powder obtained from core of maize straw. Food Chem., 227, 383-389.
•  Ma,  M. M. and  Mu,  T. H. (2016a). Modification of deoiled cumin dietary fiber with laccase and cellulase under high hydrostatic pressure. Carbohydr. Polym., 136, 87-94.
•  Ma,  M. M. and  Mu,  T. H. (2016b). Effects of extraction methods and particle size distribution on the structural, physicochemical, and functional properties of dietary fiber from deoiled cumin. Food Chem., 194, 237-246.
•  Macagnan,  F. T.,  da Silva,  L. P., and  Hecktheuer,  L. H. (2016). Dietary fibre: The scientific search for an ideal definition and methodology of analysis, and its physiological importance as a carrier of biORCtive compounds. Food Res. Int., 85, 144-154.
•  Nisar,  T.,  Wang,  Z. C.,  Yang,  X.,  Tian,  Y.,  Iqbal,  M., and  Guo,  Y. (2017). Characterization of citrus pectin films integrated with clove bud essential oil: Physical, thermal, barrier, antioxidant and antibacterial properties. Int. J. Biol. Macromol., 106, 670-680.
•  Tan,  J.,  McKenzie,  C.,  Vuillermin,  P. J.,  Goverse,  G.,  Vinuesa,  C. G.,  Mebius,  R. E.,  Macia,  L., and  Mackay,  C. R. (2016). Dietary fiber and bacterial SCFA enhance oral tolerance and protect against food allergy through diverse cellular pathways. Cell Rep., 15, 2809-2824.
•  Wang,  J.,  Suo,  G.,  de Wit,  M.,  Boom,  R. M., and  Schutyser,  M. A. I. (2016). Dietary fibre enrichment from deoiledrice bran by dry fractionation. J. Food Eng., 186, 50-57.
•  Zhang,  W.,  Zeng,  G.,  Pan,  Y.,  Chen,  W.,  Huang,  W.,  Chen,  H., and  Li,  Y. (2017). Properties of soluble dietary fiber-polysaccharide from papaya peel obtained through alkaline or ultrasound-assisted alkaline extraction. Carbohydr. Polym., 172, 102-112.
•  Zheng,  J.,  Wu,  J.,  Dai,  Y.,  Kan,  J., and  Zhang,  F. (2017). Influence of bamboo shoot dietary fiber on the rheological and textural properties of milk pudding. LWT, Food Sci. Technol., 84, 364-369.
•  Zhou,  X. L.,  Qian,  Y. F.,  Zhou,  Y. M., and  Zhang,  R. (2011). Effect of enzymatic extraction treatment on physicochemical properties, microstructure and nutrient composition of tartary buckwheat bran: A new source of antioxidant dietary fiber. Adv Mat Res., 396–398, 2052-2059.
•  Zhuang,  X.,  Jiang,  X.,  Han,  M.,  Kang,  Z.-l.,  Zhao,  L.,  Xu,  X.-l., and  Zhou,  G.-h. (2016). Influence of sugarcane dietary fiber on water states and microstructure of myofibrillar protein gels. Food Hydrocoll., 57, 253-261.