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Microwave assisted extraction with three modifications on structural and functional properties of soluble dietary fibers from grapefruit peel
Journal Pre-proof Microwave assisted extraction with three modifications on structural and functional properties of soluble dietary fibers from grapefruit peel
Jiapan Gan, Ziyan Huang, Qiang Yu, Guanyi Peng, Yi Chen, Jianhua Xie, Shaoping Nie, Mingyong Xie PII:
S0268-005X(19)31647-9
DOI:
https://doi.org/10.1016/j.foodhyd.2019.105549
Reference:
FOOHYD 105549
To appear in:
Food Hydrocolloids
Received Date:
23 July 2019
Accepted Date:
27 November 2019
Please cite this article as: Jiapan Gan, Ziyan Huang, Qiang Yu, Guanyi Peng, Yi Chen, Jianhua Xie, Shaoping Nie, Mingyong Xie, Microwave assisted extraction with three modifications on structural and functional properties of soluble dietary fibers from grapefruit peel, Food Hydrocolloids (2019), https://doi.org/10.1016/j.foodhyd.2019.105549
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof
Microwave assisted extraction with three modifications on structural and functional properties of soluble dietary fibers from grapefruit peel
Jiapan Gana, Ziyan Huangb, Qiang Yua,*, Guanyi Penga, Yi Chena, Jianhua Xiea, Shaoping Niea, Mingyong Xiea
a
State Key Laboratory of Food Science and Technology, China-Canada Joint Lab of
Food Science and Technology (Nanchang), Nanchang University, 235 Nanjing East Road, Nanchang 330047, China b
School of Food Science and Technology, Nanchang University, Nanchang 330031,
China
*Corresponding to: Associate Professor Qiang Yu, PhD Tel.&Fax: +86 791-88304447-8330 Email: [email protected]
Journal Pre-proof Abstract Microwave-sodium hydroxide treatment (MST), microwave-enzymatic treatment (MET) and microwave-ultrasonic treatment (MUT) were used to treat grapefruit peel, then the structural and functional properties of extracted soluble dietary fibers (SDF), named as MST-SDF, MET-SDF and MUT-SDF, were investigated. The scanning electron microscopy, molecular weight, X-ray diffraction, FT-IR, thermal properties and monosaccharide composition were used to determine the structural properties. Compared to the grapefruit peel SDF without treatment (WT-SDF) and pure microwave treatment (PMT-SDF), MST-SDF, MET-SDF and MUT-SDF showed a more complex and loose structure. Moreover, they possessed higher molecular weight, crystallinity and thermal stability, and more diverse monosaccharide composition. In addition, functional properties, including water holding capacity (WHC), oil holding capacity (OHC), cholesterol adsorption capacity (CAC), glucose adsorption capacity (GAC) and nitrite ion adsorption capacity (NIAC), were enhanced by the three different modification methods. Especially, MUT-SDF showed the highest WHC, OHC, CAC, GAC and NIAC as compared with the other samples. In summary, the present study suggested that the MUT could be used as the ideal modification method for grapefruit peel SDF, and MUT-SDF has great potential for the application in functional food industry. Keywords: Grapefruit peel; Soluble dietary fiber; Microwave; Structural properties; Functional properties
Journal Pre-proof 1. Introduction Grapefruit, one of the favorite Chinese foods that is highly appreciated by consumers, contains many phytochemicals and nutrients that contribute to a healthy diet (El Kantar, et al., 2019). Grapefruit is consumed in both the fruit market and for processing (Zheng, Zhang, Quan, Zheng, & Xi, 2016). During the processing of grapefruit, peels are the primary byproduct. The peels are dumped into landfills as industrial waste or used as fertilizer (Lei, et al., 2015), which result in great waste of resources. However, grapefruit peels can be used as a potential and high-quality source of natural ingredients, such as dietary fiber. Dietary fiber (DF), classified as a wide range of indigestible nutrients derived from plant cell walls, is known as the seventh important nutrient for organisms (Fan, et al., 2017; Liu, et al., 2019). DF has numerous benefits for our health, such as reducing risk of cancer, diabetes, and asthma (Brownlee, 2011; J. Chen, et al., 2015). According to the solution capacity in water, DF can be divided into two types: soluble dietary fiber (SDF) and insoluble dietary fiber (IDF) (Jia, et al., 2019). Compared with the IDF, SDF has superior beneficial properties for human health (Huang, He, Zou, & Zhuang, 2016), such as better solubility, water/oil holding capacity, surface or interfacial properties, binding capacity for the destructive molecules (Andrzej, Sabina, Jaroslaw, & Yuge, 2018; Moczkowska, Karp, Niu, & Kurek, 2019). However, in the natural plant cell walls, insoluble dietary fiber accounts for a large proportion, while the proportion of soluble dietary fiber is very low (H. Chen, et al., 2018). Thus, finding the appropriate method to convert more IDFs into SDFs is extremely
Journal Pre-proof important. Microwave treatment is gradually receiving the attention of researchers as a modification method. It could form dipole rotation of the solvent and raise the solvent temperature quickly, leading to the increase of compound solubility (Ahmad & Langrish, 2012). In addition, microwave energy can directly penetrate the wall material to reach the center of the sample, making the sample heated more evenly and quickly, thus save more time and energy (Jamaluddin, et al., 2013; Marra, Bonis, & Ruocco, 2010). More importantly, microwave could increase intracellular pressure in fresh tissue rupturing the cell wall and release active ingredients (Wei, et al., 2019). Sodium hydroxide treatment could cause swelling of plant cell wall, β-elimination of major components and change of micro-morphology of cellulose. Ultrasonic treatment could result in plant cell wall cavitation, which will affect the structure and morphology of carbohydrate polymer (Bagherian, Ashtiani, Fouladitajar, & Mohtashamy, 2011), leading to better dissolution of its components. Enzymatic hydrolysis can destroy the chemical chains of lignin, cellulose and hemicellulose in the cell wall (Santala, Kiran, Sozer, Poutanen, & Nordlund, 2014). All these methods can change the microstructure and composition of DFs, resulting in both desirable and undesirable effects on structural and functional properties (Peerajit, Chiewchan, & Devahastin, 2012). However, to our knowledge, there is no information about the comparison regarding the effect of microwave treatment combined with sodium hydroxide, enzymatic, ultrasonic methods on the properties of SDF from grapefruit peel.
Journal Pre-proof In
this
study,
microwave-sodium
hydroxide
treatment
(MST),
microwave-enzymatic treatment (MET) and microwave-ultrasonic treatment (MUT) were used to treat grapefruit peel, then the structural and functional properties of extracted soluble dietary fibers (SDF), named as MST-SDF, MET-SDF and MUT-SDF, were investigated, aiming to find the ideal modification method for SDF extracted from grapefruit peel. 2. Materials and methods 2.1 Materials The fresh grapefruit (cv. guanximiyou) were purchased from Jiangxi, China. The peels were collected and cut into pieces, then dried in a constant temperature blast drying oven at 60℃ for 36 h, followed by crushing and screening through the 100 mesh sifter to obtain grapefruit peel powder. The cellulase (Item number: C8270; 3000 U/g) was purchased from Solarbio (Beijing, China). Heat-stable α-amylase (2.0 × 105 U/g) was obtained from Aladdin Biotechnology Co., Ltd (Shanghai, China). Papain (5.0 × 104 U/g) was purchased from Pangbo Bioengineering Co., Ltd respectively (Guizhou, China). Other chemicals used in the experiments were analytical grade. 2.2 Modification of grapefruit peel fiber 2.2.1 Preparation of WT-SDF 10 g of grapefruit peel powder was extracted by distilled water (1:10, w/v) in the water bath with continuous stirring at room temperature for 2 h. After centrifugation at 4800 rmp for 15 min, the supernatant was collected and mixed with four times
Journal Pre-proof volumes of 95% ethyl alcohol for 12 h. Then the solution was centrifuged at 4800 rmp for 10 min to collect the precipitate, which was dissolved in distilled water to removal of ethanol by rotary evaporation. Finally the mixture was freeze-dried to obtain the SDF. 2.2.2 Preparation of PMT-SDF 3 g of grapefruit peel powder was mixed with 30 mL distilled water (1:10, w/v) in a polyfluoroalkoxy (PFA) tube. Each time eight tubes were treated at 500W and 80°C for 40 min using a microwave cracker (CEM Mars-6, USA). After the same treatment described in 2.2.1, PMT-SDF was obtained. 2.2.3 Preparation of MST, MET and MUT SDFs 5 g microwave-treated sample was collected to mix with sodium hydroxide (1%, w/w), stirring (200 rmp) using a magnetic stirrer in a water bath (50°C) environment for 30 min. Then the mixture went through the same procedure described in 2.2.1, the MST-SDF was obtained. To get the MET-SDF, 240 mg cellulase (3000 U/g) was blended to 5 g microwave-treated sample (cellulase/substrate ratio of 30 U/g) stirring for 240 min at pH 4.5, 50°C. Then heat-stable α-amylase (1% g/g) was added to the mixture at pH 5.5 and incubated in a water bath at 95°C for 30 min. After the temperature of the solution dropped to 60°C, papain (0.05% g/g) was added to the mixture at pH 6.0 and incubated in a water bath at 60°C for 30 min. The rest of the steps are the same as described in 2.2.1. To prepare MUT-SDF, 5 g microwave-treated sample were placed in glass flask and subjected to ultrasonic treatment. Sonicator JY92-Ⅱ(Ningbo Scientz. Biotechnology Co. LTD) was used at
Journal Pre-proof power 200 W for 10 min at room temperature (25°C). Then the SDF extracted was named as MUT-SDF. 2.2.4 Chemical analysis of different treated samples Soluble, insoluble and total dietary fiber (TDF) contents were measured by the AOAC method 991.43 (AOAC, 1996). The content of moisture, ash, protein and fat in the SDF samples were measured by the Chinese national standard method (GB5009.3-2016, GB5009.4-2016, GB5009.5-2016 and GB5009.6-2016). 2.3 Scanning electron microscopy (SEM) The morphology and microstructure investigation of the SDF samples were carried out using scanning electron microscopy (JSM 6701F, JEOL Ltd, Japan). The Freeze-dried SDF samples were placed on double-sided tape and coated with a thin gold layer. The images were collected at an accelerating voltage of 10.0 kV. Micrographs were recorded at 100× and 1000× magnifications. 2.4 Molecular weight (Mw) The molecular weight of SDF samples were analyzed by using a high-performance gel permeation chromatography (HPGPC) method described by the previous report (Xie, et al., 2010). Ultra-pure water containing 0.02 (w/w, %) NaN3 was used as the mobile phase, Dextran T standard (Mw: 10 kDa, 40 kDa, 70 kDa, 500 kDa, 2,000 kDa) and glucose were used to produce the standard curve. The freeze-dried samples and dextran standards were dissolved in the mobile phase (1 mg/mL). Before injecting into the chromatographic system, the solution was filtered through a 0.22 μm membrane filter. Molecular weight of SDF samples were estimated
Journal Pre-proof according to the standard curve obtained from T-series Dextran standard of known molecular weight. 2.5 X-ray diffraction (XRD) The XRD patterns of the SDF samples were obtained by an X-ray diffraction (D8 Advance, Bruker, Germany) instrument at the operating voltage and current of 30 kV and 20 mA, respectively. The diffraction angle (2θ) was scanned from 5° to 50°. The crystallinity index (Ic) of SDF samples was calculated from the diffraction intensity data according to Segal's method ( Segal, C., Martin, Jr., & Conrad, 1959). 2.6 Fourier transfer-infrared spectrometry (FT-IR) Differences in the molecular structure of the SDF samples were carried out by a Nicolet 5700 Fourier-transform infrared spectrophotometer (FTIR, Nicolet, USA) in the range from 400 to 4000 cm-1 with a resolution of 4 cm-1 and 32 scans. Each SDF sample (5 mg) was mixed with 100 mg KBr, then the mixture was ground and tableted. Scans were compared with a blank KBr background. 2.7 Thermal properties The
thermal
properties
of
the
SDF
samples
were
determined
with
thermogravimetric analysis (TGA) according to the method (Khatkar, Barak, & Mudgil, 2013) with some modification. The TGA of the SDF samples (6 mg) were performed in a nitrogen atmosphere using a thermos gravimetric analyzer (TGA 4000, PerkinElmer, Waltham, MA, USA) at a heating rate of 20°C/min over a temperature range of 50-600°C. The high-purity nitrogen gas flow of 20 mL/min was used as the purge gas prior to the experiment.
Journal Pre-proof 2.8 Monosaccharide composition The monosaccharide compositions of the SDF samples were analyzed by high-performance anion exchange chromatography (HPAEC, Dionex ICS-5000, Thermo Fisher Scientific Massachusetts, USA) equipped with pulse amperometric detection (PAD). A CarboPacTM PA20 guard column (4 mm x 50 mm) and a CarboPacTM PA20 analytical column (4 mm x 250 mm) were connected in series for the analysis. A Chromeleon 6.8 (Thermo Fisher Scientific, Massachusetts, USA) software was used for data collection and processing. The freeze-dried samples (5 mg) were hydrolyzed by 0.5 mL of 12 M H2SO4 in a sealed glass tube, stirred using a magnetic stirrer in ice bath for 30 min, then the H2SO4 was diluted to 2 M for another 2 h of hydrolysis in a silicone oil bath. Finally 2.5 mL ultra-pure water was added to the tube, keep stirring in oil bath at 105°C for 4 h. After the solution cooled to room temperature, the concentration of the mixture was adjusted to 100 μg/mL. The solution was filtered through a 0.22 μm syringe filter before injection into the system. 2.9 Functional properties 2.9.1 Water holding capacity (WHC) The freeze-dried samples (W1, 1 g) were hydrated with 25 mL ultra-pure water in a centrifuge tube at room temperature for 2 h. The residue was immediately collected and weighted (W2), after centrifugation at 4800 rmp for 10 min. The WHC was calculated using the following Eq. (1): WHC (g/g) = (W2-W1)/W1 2.9.2 Oil holding capacity (OHC)
(1)
Journal Pre-proof The freeze-dried samples (M1, 1 g) were mixed with 25 mL soybean oil in a centrifuge tube at the room temperature for 2 h. After centrifugation at 4800 rmp for 10 min, the supernatant (free oil) was removed and the residue was immediately weighted (M2). The OHC was calculated using the following Eq. (2): OHC (g/g) = (M2-M1)/M1
(2)
2.9.3 Cholesterol adsorption capacity (CAC) CAC was determined by the method (Jia, et al., 2019) with some modifications. The egg yolk (10 mL) was mixed with distilled water (90 mL) and then whipped it into an emulsion. The SDF sample (0.1 g) was mixed with 5 mL emulsion and shaken at the room temperature for 2 h. After centrifugation at 4800 rmp for 10 min, 1 mL of the supernatant was collected and diluted 10 times with glacial acetic acid. Then 0.4 mL solution was taken to determine the cholesterol content. 5mL egg yolk emulsion without the SDF sample was used as the blank. The CAC was calculated using the following Eq. (3): CAC (mg/g) = (m1−m2)/w1
(3)
Where m1 and m2 were the weight of cholesterol in the solution before and after adsorption, w1 was the weight of SDF sample. 2.9.4 Glucose adsorption capacity (GAC) The GAC was evaluated based on a previously described method with slightly modified (Niu, Li, Xia, Hou, & Xu, 2018). A freeze-dried SDF sample (0.1 g) was mixed with 5 mL glucose solution (0.5 mg/mL). The mixture was placed at room temperature for 2 h. After centrifugation at 4800 rmp for 10 min, 0.5 mL of the
Journal Pre-proof supernatant was transferred into a glass tube. Ultra-pure water was added to the tube until the volume reached to 3 mL and then mixed with 2 mL dinitro-salicylate (DNS) color development reagent. The mixture was incubated in a water bath at 100°C for 6 min with continuous shaking. After the solution dropped to room temperature, the residual concentration of glucose was measured at 520 nm using a TU-1810DAPC spectrophotometer (Persee, Beijing, China), and quantified based on the standard curve. The GAC was calculated using the following Eq. (4): GAC (mg/g) = (m1−m2)/w1
(4)
Where m1 and m2 were the weight of glucose in the solution before and after adsorption, w1 was the weight of the SDF sample. 2.9.5 Nitrite ion adsorption capacity (NIAC) The measurement of NIAC was determined by the method (Luo, et al. 2018) with some modification. The freeze-dried sample (0.1 g) was mixed with 5 mL NaNO2 solution (20 μg/mL) with the pH was adjusted to 7.0 and 2.0, to simulate the environment in the small intestine and stomach. The mixture was placed at room temperature for 2 h. After centrifugation at 4800 rmp for 10 min, 0.4 mL of the supernatant was transferred into a glass tube. Ultra-pure water was added to the tube until the volume reached to 2 mL and then 2 mL p-aminobenzene sulfonic acid (4 μg/mL) and 1 mL hydrochloride naphthodiamide (2 μg/mL) were added to the mixture. After the solution reacted in the dark for 30 min, the concentration of NaNO2 was measured at 538 nm by a TU-1810DAPC spectrophotometer (Persee, Beijing, China), and quantified based on the standard curve. The NIAC was calculated using
Journal Pre-proof the following Eq. (5): NIAC (μg/g) = (m1−m2)/w1
(5)
Where m1 and m2 were the weight of NaNO2 in the solution before and after adsorption, w1 was the weight of the SDF sample. 2.10 Statistical analyses All experiments were performed in triplicate and the results were expressed as mean ± standard deviation (S.D.). Statistical analysis was carried out by IBM SPSS statistical software (version 21.0, SPSS Inc., Chicago, IL, USA). P value < 0.05 was considered to be statistically significant. 3. Results and discussion 3.1 Extraction yield and composition of SDF The extraction yield and composition of SDF samples were listed in the Table 1. After treatment, the yield of PMT-SDF (7.94±0.20 %), MST-SDF (17.19±0.35 %), MET-SDF (9.13± 0.17 %) and MUT-SDF (8.35± 0.08 %) were increased, which might be due to the microwave energy could increase intracellular pressure, rupture the cell wall and release more active ingredients. In addition, the result showed that the IDF have turned to SDF after treatment. Moreover, the results showed that all samples had a high SDF purity. 3.2 SEM observation The SEM images of SDF samples were illustrated in Fig. 1. The WT-SDF sample structure was tight and disordered with a lot of irregular filaments. After microwave treatment, the filament became a sheet-like structure with a large number of holes on
Journal Pre-proof its surface. Compared with microwave treatment, the structure of MST-SDF was shrinking and became more disordered, which might be due to the strong corrosiveness of sodium hydroxide. The structure of MET-SDF turned to be multi-layer filaments and the pores were basically disappeared compared to PMT-SDF, which might be because the sheet structure was hydrolyzed by the enzyme treatment. From the micrographs of the MUT-SDF, the structure was tighter and the shape of the pore became messy, but the number of the pore increased dramatically and appeared more dense and multi-layered. This special structure could increase the relative surface area, and lead to the increase of WHC, OHC, CAC, GAC, NIAC. 3.3 Mw measurement HPGPC was applied to investigate the difference between SDF samples in molecular weight, the results were showed in the Fig. 2. From the results, the average Mw of all SDF samples were ranged from 0.16 to 620 kDa, and the highest proportion were around 620 kDa. As shown in the Fig. 2 (A), six peaks were observed in the WT-SDF, while only five peaks were found in the PMT-SDF, MST-SDF, MET-SDF and MUT-SDF samples. At the same time, the proportion of high molecular weight of PMT-SDF, MST-SDF, MET-SDF and MUT-SDF samples were increased compared to that of WT-SDF, which might be due to the microwave treatment combined two component peaks thus increase molecular weight. From the figures, the elution times of all SDF samples peaks were similar, which were around 12.4, 14.1, 14.9, 16.5 and 18.3 min. Due to different modification methods, the five SDF samples had slightly different values but similar molecular weight distributions. The highest molecular
Journal Pre-proof weight was observed in MUT-SDF (614.1 kDa), which might be because ultrasonic treatment made the cell wall broke more thoroughly, and the soluble dietary fiber dissolved more completely (Khodaei & Karboune, 2013). 3.4 XRD analysis The XRD results of the SDFs were listed in the Fig. 3. All the samples had the characteristic crystalline peaks at 21.18° 2θ, except that the MST-SDF sample had new characteristic crystalline peak at 12.61° 2θ, indicating that the crystalline structure of MST-SDF sample was significantly changed after treatment. The crystallinity index of the WT-SDF was 19.18%, and significantly decreased to 13.75% in PMT-SDF by microwave treatment, suggested that the crystal structure of SDF was damaged seriously by the microwave. However, the crystallinity index of the MST-SDF (27.89%), MET-SDF (20.16%), MUT-SDF (21.67%) were increased compared with that of PMT-SDF. The reason for the increasing of the crystallinity might be because the combined treatment could make the molecular rearrangement more regular. 3.5 FT-IR analysis The FT-IR spectra of SDF samples from different modification methods were recorded from 400 cm-1 to 4000 cm-1. From the results shown in the Fig. 4, there were several similar characteristic peaks in all SDF samples spectra. The strong and wide peak in the range of 3000-3650 cm-1 was assigned to the -OH stretching. The peaks at 1618.7 cm-1 and 1415.7 cm-1 might be the characteristic absorption of C=O bond stretch in asymmetric and symmetric, respectively. Particularly, the peaks at
Journal Pre-proof 1000-1300 cm-1 were distributed to the C-O stretching vibration, which might be the blending vibration of primary alcohols or the C-O-C and C-O-H of sugar ring (H. Chen, et al., 2018). Moreover, the peak at 917.8 cm-1 in all samples was indicative of stretching vibration of β-glycosidic linkages in polysaccharides. Except the similar characteristic peaks described above, there were some different peaks observed in the MST-SDF. Compared with other samples, the MST-SDF sample had a peak intensity reduction at the 2926.7 cm-1 that was C-H stretching vibrations from the methylene group of polysaccharides (Mengmei & Taihua, 2016), which might be due to the breakage of the bonding in the molecule by sodium hydroxide treatment. The characteristic peaks at 1740-1760 cm-1 (C-O stretching of -COOH) were observed in all samples except the MST-SDF sample. This phenomenon might be due to the hydroxyl group on the carboxylic acid was replaced by a sodium atom. In addition, the spectra of the MST-SDF sample had a red shift phenomenon in the position of some peaks, which indicated the microwave-sodium hydroxide treatment destroyed the structure of organic molecules severely. 3.6 Thermal properties The TGA results of the SDF samples were showed in the Fig. 5. WT-SDF, PMT-SDF, MET-SDF and MUT-SDF had the similar curve patterns, which can be divided into three stages. For the first stage 50-200°C, the weight of the samples were slightly decreased. As observed from the second stage between 200-400°C, a rapid weight loss could be detected, which might be due to the polysaccharide pyrolytic decomposition. At the final stage 400-600°C, the weight loss of the samples became
Journal Pre-proof slowly owing to the thermal decomposition of char. The residual mass for the WT-SDF, PMT-SDF, MET-SDF and MUT-SDF were 29.19%, 26.10%, 28.06% and 28.65%, respectively. However, the weight loss of the MST-SDF sample could be divided into four stages. The first stage at 50-240°C, the mass of MST-SDF sample dropped slowly. The rate of weight loss increased dramatically when it went to the second stage 240-340°C. At the third stage 340-500°C, the rate of weight loss decreased. The mass of the MST-SDF remained stable at the last stage 500-600°C. The residual mass for MST-SDF was significantly increased to 42.03 % compared with that of WT-SDF, PMT-SDF, MET-SDF and MUT-SDF, which might be related with its high crystallinity index. The result was consistent with the previous study (Luo, et al., 2018) that the high crystallinity sample has high thermal stability. 3.7 Monosaccharide composition analysis The monosaccharide composition of SDF samples were shown in the Table 2. The amount of Rhamnose in the WT-SDF sample was set as 1, to which all other monosaccharides were compared. As shown in the Table 2, eight kinds of monosaccharides, including Rha, Ara, Gal, Glu, Xyl, Man, Fru, Gala, were found in the PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. However, mannose was not detected in the WT-SDF. This might be because the microwave energy broke the glycosidic bond and formed new monosaccharide in the SDF samples. After microwave treatment, the content of Gal, Glu and Fru were decreased. The PMT-SDF had the highest Xyl content, while the MST-SDF had the highest Ara content. Meanwhile, the lowest Rha content was observed in the MET-SDF, but the
Journal Pre-proof MUT-SDF had the lowest Man content in the treated samples. This phenomenon might be because the different treatment methods had different effects on macromolecules in the SDF samples. 3.8 Functional properties 3.8.1 WHC and OHC The WHC and OHC of grapefruit peel SDFs subjected to different treatments were showed in the Fig. 6. High WHC SDF could change the viscosity of certain food and prevent food from shrinking (Elleuch, et al., 2011). From the Fig. 6 (A), the WHC decreased after samples were microwave treated. However, compared with other treated samples, the MST-SDF and MUT-SDF showed a rebound of WHC. Especially the MUT-SDF displayed the highest WHC (11.39±0.09 g/g), maybe because the special functional properties of ultrasound could result in a loose porous structure of SDF. The OHC of SDF is important for food applications, such as preventing fat loss during cooking (Schneeman, 1999) and helping people to remove excess fat. As shown in the Fig. 6 (B), the OHC of WT-SDF was (18.23±0.13 g/g), while PMT-SDF (13.21±0.12 g/g) and MST-SDF (11.12±0.08 g/g) significantly decreased owing to the destruction of the pore structure. Nevertheless, MUT-SDF exerted the highest OHC (25.01±0.04 g/g) since its complex and numerous pores like structure. These results suggested that the pore-like structure could significantly affect the OHC of SDFs. 3.8.2 CAC CAC is an important functional property of SDF, which has been proven to be
Journal Pre-proof able to decrease the cardiovascular disease risk and serum cholesterol levels in human body (Nsor-Atindana, Zhong, & Mothibe, 2012). The CAC of the samples were showed in Fig. 6 (C). From the results, the WT-SDF showed the lowest CAC (2.69±0.12 mg/g). After microwave treatment, the CAC of PMT-SDF significantly increased (24.93±0.02 mg/g). Compared with the PMT-SDF, the CAC of MST-SDF and MET-SDF decreased. In the treated samples, the lowest CAC was observed in the MST-SDF (12.86±0.09 mg/g) while the highest CAC was showed by the MUT-SDF (26.48±0.08 mg/g), which might be due to the MUT-SDF had larger specific surface area and more complex structure to bind more cholesterol. 3.8.3 GAC GAC is another important functional property of SDF, which could combine with glucose in intestinal juice thus led to the decrease of postprandial blood glucose levels. The GAC of different SDF samples were showed in the Fig. 6 (D). Although all the SDF samples were found to be effective in adsorbing glucose, the MUT-SDF still showed the strongest adsorption capacity (24.42±0.06 mg/g). The results suggested that the MUT-SDF had a superior hypoglycemic effect in vitro. 3.8.4 NIAC In the environment of gastric acid, nitrite reacts with secondary amines, tertiary amines and amides in food to form strong carcinogen N-nitrosamine, which can also enter the fetus through the placenta and have teratogenic effects on the fetus (Bruning-Fann, C. S., Kaneene, & J. B., 1993). Therefore, the NIAC of SDF is one kind of meaningful functional property. As shown in Fig. 7, we investigated the
Journal Pre-proof adsorption capacity of nitrite ions for modified grapefruit peel SDFs in simulated gastric (PH=2) and small intestine (PH=7) environments. The results showed that the NIAC of SDF samples in the gastric environment were significantly higher than that of the small intestine environment. In addition, the MUT-SDF showed the highest NIAC in both PH=2 (219.43±0.33 μg/g) and PH=7 (9.41±0.14 μg/g), which might be due to the MUT-SDF sample had better structure and massive explosion of functional groups to interaction with more nitrite ion. 4. Conclusions In this study, we used three modification methods to modify grapefruit peel and investigated the structural and functional properties of SDF extracted from modified grapefruit peel. MST-SDF, MET-SDF and MUT-SDF exhibited more complex and loose structure higher molecular weight, crystallinity and thermal stability, and more diverse monosaccharide composition than that of WT-SDF and PMT-SDF. The modification methods not only improved the structural characteristics of the SDF, but also improved functional properties, especially the MUT-SDF showed the highest binding capacity for the water, oil, cholesterol, glucose and nitrite ion. Therefore, MUT-SDF had a great potential for the application in functional food industry, and MUT could improve the economic benefit and utilization rate of the grapefruit peel. Acknowledgements This work was supported by the Key Research and Development Program of Jiangxi Province of China (20171BBF60041), National Natural Science Foundation of China (31701603), and Research Project of State Key Laboratory of Food Science and
Journal Pre-proof Technology (SKLF-ZZA-201611).
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Microwave-assisted
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releases
the
antioxidant
polysaccharides from seabuckthorn (Hippophae rhamnoides L.) berries. International Journal of Biological Macromolecules, 123, 280-290. Xie, J. H., Xie, M.-Y., Nie, S.-P., Shen, M.-Y., Wang, Y.-X., & Li, C. (2010) Isolation, chemical composition and antioxidant activities of a water-soluble polysaccharide from Cyclocarya paliurus (Batal.) Iljinskaja. Food Chemistry, 119(4), 1626-1632. Zheng, H., Zhang, Q., Quan, J., Zheng, Q., & Xi, W. (2016). Determination of sugars, organic acids, aroma components, and carotenoids in grapefruit pulps. Food Chemistry, 205, 112-121.
Journal Pre-proof Figure Captions Fig. 1. SEM micrographs of SDFs: WT-SDF (A, a), PMT-SDF (B, b), MST-SDF (C, c), MET-SDF (D, d) and MUT-SDF (E, e). Magnification = 100× (A-E) and 1000× (a-e). Fig. 2. Gel permeation chromatogram profiles of WT-SDF (A), PMT-SDF (B), MST-SDF (C), MET-SDF (D) and MUT-SDF (E). Fig. 3. The X-ray diffraction patterns of WT-SDF, PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. Fig. 4. FT-IR spectras of WT-SDF, PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. Fig. 5. Thermal properties of WT-SDF, PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. Fig. 6. The WHC (A), OHC (B), CAC (C), GAC (D) of WT-SDF, PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. Results were expressed as means ± standard deviation (n = 3). Different letters denote significantly different in the group, (P < 0.05). Fig. 7. The NIAC of WT-SDF, PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. Results were expressed as means ± standard deviation (n = 3). Different letters denote significantly different in the group, (P < 0.05).
1
Table 1 Extraction yield and composition of SDF samples (g/100 g) SDF yield
2
IDF yield
TDF
SDF
composition
Moisture
Ash
Protein
Fat
Uronic acid
Purity
WT
3.62±0.13e
59.77±0.38a
62.39±0.41a
4.56±0.12b
1.78±0.02c
2.08±0.12c
1.12±0.04b 28.67±0.19e
90.46±0.56d
PMT
7.94±0.20d
55.47±0.49b
63.41±0.36a
3.41±0.04d
1.57±0.05d
1.81±0.09d
1.04±0.07d 38.39±0.25b
92.17±0.62b
MST
17.19±0.35a 46.19±0.39e
63.38±0.32a
5.36±0.07a
2.36±0.09a
2.56±0.08b
1.27±0.06a 39.53±0.23a
88.45±0.47e
MET
9.13±0.17b
53.23±0.32d
62.36±0.47a
3.38±0.05e
1.87±0.05b
3.08±0.06a
1.08±0.05c 36.94±0.28c
90.59±0.51c
MUT
8.35±0.08c
55.02±0.27c
63.37±0.36a
3.68±0.09c
1.35±0.01e
1.27±0.03e
0.98±0.06e 36.84±0.27d
92.72±0.53a
Results were expressed as means ± standard deviation (n = 3). Different letters denote significantly different in the column, (P < 0.05).
4
Table 2 Monosaccharides composition ratio and relative mole % of SDF samples Monosaccharide
WT-SDF
PMT-SDF
MST-SDF
MET-SDF
MUT-SDF
Rhamnose
ratio 1.00
mole % 2.73
ratio 1.03
mole % 2.72
ratio 1.56
mole % 4.35
ratio 0.80
mole % 2.51
ratio 1.10
mole % 3.44
Arabinose
7.58
20.68
9.02
23.82
13.76
38.40
6.57
20.64
8.75
27.38
Galactose
4.02
10.97
2.77
7.31
2.39
6.67
3.64
11.44
2.83
8.85
Glucose
12.73
34.73
10.83
28.60
7.14
19.93
10.54
33.11
9.11
28.50
Xylose
2.72
7.42
3.44
9.08
1.97
5.50
1.18
3.71
2.80
8.76
Mannose
ND
0
2.16
5.70
1.65
4.61
1.37
4.30
1.34
4.19
Fructose
3.28
8.95
0.83
2.19
3.20
8.93
1.87
5.87
1.34
4.19
Galacturonic acid
5.32
14.52
7.79
20.57
4.16
11.61
5.86
18.41
4.69
14.67
5
ND: not detected. Ratio means the amount of different monosaccharides in different samples comparted to the amount of rhamnose in the
6
WT-SDF sample. Mole % means the relative mole % of different monosaccharides in the same sample.
Journal Pre-proof Declaration of interests The authors declare no conflict of interest in this work.
Journal Pre-proof These authors contributed equally to this work.
Journal Pre-proof Figure 1
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Journal Pre-proof Figure 7
Journal Pre-proof Highlights
MST-SDF, MET-SDF or MUT-SDF were prepared by microwave assisted with sodium hydroxide, enzymatic or ultrasonic treatment.
The surface and internal structure of MST-SDF, MET-SDF, MUT-SDF samples became more complex.
The crystallinity and thermal stability of MST-SDF, MET-SDF, MUT-SDF samples were improved.
The MUT-SDF exhibited significantly superiority on the binding capacity for water, oil, cholesterol, glucose and nitrite ion.
The MUT-SDF has great potential for the application in functional food industry.
Jiapan Gan, Ziyan Huang, Qiang Yu, Guanyi Peng, Yi Chen, Jianhua Xie, Shaoping Nie, Mingyong Xie PII:
S0268-005X(19)31647-9
DOI:
https://doi.org/10.1016/j.foodhyd.2019.105549
Reference:
FOOHYD 105549
To appear in:
Food Hydrocolloids
Received Date:
23 July 2019
Accepted Date:
27 November 2019
Please cite this article as: Jiapan Gan, Ziyan Huang, Qiang Yu, Guanyi Peng, Yi Chen, Jianhua Xie, Shaoping Nie, Mingyong Xie, Microwave assisted extraction with three modifications on structural and functional properties of soluble dietary fibers from grapefruit peel, Food Hydrocolloids (2019), https://doi.org/10.1016/j.foodhyd.2019.105549
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Journal Pre-proof
Microwave assisted extraction with three modifications on structural and functional properties of soluble dietary fibers from grapefruit peel
Jiapan Gana, Ziyan Huangb, Qiang Yua,*, Guanyi Penga, Yi Chena, Jianhua Xiea, Shaoping Niea, Mingyong Xiea
a
State Key Laboratory of Food Science and Technology, China-Canada Joint Lab of
Food Science and Technology (Nanchang), Nanchang University, 235 Nanjing East Road, Nanchang 330047, China b
School of Food Science and Technology, Nanchang University, Nanchang 330031,
China
*Corresponding to: Associate Professor Qiang Yu, PhD Tel.&Fax: +86 791-88304447-8330 Email: [email protected]
Journal Pre-proof Abstract Microwave-sodium hydroxide treatment (MST), microwave-enzymatic treatment (MET) and microwave-ultrasonic treatment (MUT) were used to treat grapefruit peel, then the structural and functional properties of extracted soluble dietary fibers (SDF), named as MST-SDF, MET-SDF and MUT-SDF, were investigated. The scanning electron microscopy, molecular weight, X-ray diffraction, FT-IR, thermal properties and monosaccharide composition were used to determine the structural properties. Compared to the grapefruit peel SDF without treatment (WT-SDF) and pure microwave treatment (PMT-SDF), MST-SDF, MET-SDF and MUT-SDF showed a more complex and loose structure. Moreover, they possessed higher molecular weight, crystallinity and thermal stability, and more diverse monosaccharide composition. In addition, functional properties, including water holding capacity (WHC), oil holding capacity (OHC), cholesterol adsorption capacity (CAC), glucose adsorption capacity (GAC) and nitrite ion adsorption capacity (NIAC), were enhanced by the three different modification methods. Especially, MUT-SDF showed the highest WHC, OHC, CAC, GAC and NIAC as compared with the other samples. In summary, the present study suggested that the MUT could be used as the ideal modification method for grapefruit peel SDF, and MUT-SDF has great potential for the application in functional food industry. Keywords: Grapefruit peel; Soluble dietary fiber; Microwave; Structural properties; Functional properties
Journal Pre-proof 1. Introduction Grapefruit, one of the favorite Chinese foods that is highly appreciated by consumers, contains many phytochemicals and nutrients that contribute to a healthy diet (El Kantar, et al., 2019). Grapefruit is consumed in both the fruit market and for processing (Zheng, Zhang, Quan, Zheng, & Xi, 2016). During the processing of grapefruit, peels are the primary byproduct. The peels are dumped into landfills as industrial waste or used as fertilizer (Lei, et al., 2015), which result in great waste of resources. However, grapefruit peels can be used as a potential and high-quality source of natural ingredients, such as dietary fiber. Dietary fiber (DF), classified as a wide range of indigestible nutrients derived from plant cell walls, is known as the seventh important nutrient for organisms (Fan, et al., 2017; Liu, et al., 2019). DF has numerous benefits for our health, such as reducing risk of cancer, diabetes, and asthma (Brownlee, 2011; J. Chen, et al., 2015). According to the solution capacity in water, DF can be divided into two types: soluble dietary fiber (SDF) and insoluble dietary fiber (IDF) (Jia, et al., 2019). Compared with the IDF, SDF has superior beneficial properties for human health (Huang, He, Zou, & Zhuang, 2016), such as better solubility, water/oil holding capacity, surface or interfacial properties, binding capacity for the destructive molecules (Andrzej, Sabina, Jaroslaw, & Yuge, 2018; Moczkowska, Karp, Niu, & Kurek, 2019). However, in the natural plant cell walls, insoluble dietary fiber accounts for a large proportion, while the proportion of soluble dietary fiber is very low (H. Chen, et al., 2018). Thus, finding the appropriate method to convert more IDFs into SDFs is extremely
Journal Pre-proof important. Microwave treatment is gradually receiving the attention of researchers as a modification method. It could form dipole rotation of the solvent and raise the solvent temperature quickly, leading to the increase of compound solubility (Ahmad & Langrish, 2012). In addition, microwave energy can directly penetrate the wall material to reach the center of the sample, making the sample heated more evenly and quickly, thus save more time and energy (Jamaluddin, et al., 2013; Marra, Bonis, & Ruocco, 2010). More importantly, microwave could increase intracellular pressure in fresh tissue rupturing the cell wall and release active ingredients (Wei, et al., 2019). Sodium hydroxide treatment could cause swelling of plant cell wall, β-elimination of major components and change of micro-morphology of cellulose. Ultrasonic treatment could result in plant cell wall cavitation, which will affect the structure and morphology of carbohydrate polymer (Bagherian, Ashtiani, Fouladitajar, & Mohtashamy, 2011), leading to better dissolution of its components. Enzymatic hydrolysis can destroy the chemical chains of lignin, cellulose and hemicellulose in the cell wall (Santala, Kiran, Sozer, Poutanen, & Nordlund, 2014). All these methods can change the microstructure and composition of DFs, resulting in both desirable and undesirable effects on structural and functional properties (Peerajit, Chiewchan, & Devahastin, 2012). However, to our knowledge, there is no information about the comparison regarding the effect of microwave treatment combined with sodium hydroxide, enzymatic, ultrasonic methods on the properties of SDF from grapefruit peel.
Journal Pre-proof In
this
study,
microwave-sodium
hydroxide
treatment
(MST),
microwave-enzymatic treatment (MET) and microwave-ultrasonic treatment (MUT) were used to treat grapefruit peel, then the structural and functional properties of extracted soluble dietary fibers (SDF), named as MST-SDF, MET-SDF and MUT-SDF, were investigated, aiming to find the ideal modification method for SDF extracted from grapefruit peel. 2. Materials and methods 2.1 Materials The fresh grapefruit (cv. guanximiyou) were purchased from Jiangxi, China. The peels were collected and cut into pieces, then dried in a constant temperature blast drying oven at 60℃ for 36 h, followed by crushing and screening through the 100 mesh sifter to obtain grapefruit peel powder. The cellulase (Item number: C8270; 3000 U/g) was purchased from Solarbio (Beijing, China). Heat-stable α-amylase (2.0 × 105 U/g) was obtained from Aladdin Biotechnology Co., Ltd (Shanghai, China). Papain (5.0 × 104 U/g) was purchased from Pangbo Bioengineering Co., Ltd respectively (Guizhou, China). Other chemicals used in the experiments were analytical grade. 2.2 Modification of grapefruit peel fiber 2.2.1 Preparation of WT-SDF 10 g of grapefruit peel powder was extracted by distilled water (1:10, w/v) in the water bath with continuous stirring at room temperature for 2 h. After centrifugation at 4800 rmp for 15 min, the supernatant was collected and mixed with four times
Journal Pre-proof volumes of 95% ethyl alcohol for 12 h. Then the solution was centrifuged at 4800 rmp for 10 min to collect the precipitate, which was dissolved in distilled water to removal of ethanol by rotary evaporation. Finally the mixture was freeze-dried to obtain the SDF. 2.2.2 Preparation of PMT-SDF 3 g of grapefruit peel powder was mixed with 30 mL distilled water (1:10, w/v) in a polyfluoroalkoxy (PFA) tube. Each time eight tubes were treated at 500W and 80°C for 40 min using a microwave cracker (CEM Mars-6, USA). After the same treatment described in 2.2.1, PMT-SDF was obtained. 2.2.3 Preparation of MST, MET and MUT SDFs 5 g microwave-treated sample was collected to mix with sodium hydroxide (1%, w/w), stirring (200 rmp) using a magnetic stirrer in a water bath (50°C) environment for 30 min. Then the mixture went through the same procedure described in 2.2.1, the MST-SDF was obtained. To get the MET-SDF, 240 mg cellulase (3000 U/g) was blended to 5 g microwave-treated sample (cellulase/substrate ratio of 30 U/g) stirring for 240 min at pH 4.5, 50°C. Then heat-stable α-amylase (1% g/g) was added to the mixture at pH 5.5 and incubated in a water bath at 95°C for 30 min. After the temperature of the solution dropped to 60°C, papain (0.05% g/g) was added to the mixture at pH 6.0 and incubated in a water bath at 60°C for 30 min. The rest of the steps are the same as described in 2.2.1. To prepare MUT-SDF, 5 g microwave-treated sample were placed in glass flask and subjected to ultrasonic treatment. Sonicator JY92-Ⅱ(Ningbo Scientz. Biotechnology Co. LTD) was used at
Journal Pre-proof power 200 W for 10 min at room temperature (25°C). Then the SDF extracted was named as MUT-SDF. 2.2.4 Chemical analysis of different treated samples Soluble, insoluble and total dietary fiber (TDF) contents were measured by the AOAC method 991.43 (AOAC, 1996). The content of moisture, ash, protein and fat in the SDF samples were measured by the Chinese national standard method (GB5009.3-2016, GB5009.4-2016, GB5009.5-2016 and GB5009.6-2016). 2.3 Scanning electron microscopy (SEM) The morphology and microstructure investigation of the SDF samples were carried out using scanning electron microscopy (JSM 6701F, JEOL Ltd, Japan). The Freeze-dried SDF samples were placed on double-sided tape and coated with a thin gold layer. The images were collected at an accelerating voltage of 10.0 kV. Micrographs were recorded at 100× and 1000× magnifications. 2.4 Molecular weight (Mw) The molecular weight of SDF samples were analyzed by using a high-performance gel permeation chromatography (HPGPC) method described by the previous report (Xie, et al., 2010). Ultra-pure water containing 0.02 (w/w, %) NaN3 was used as the mobile phase, Dextran T standard (Mw: 10 kDa, 40 kDa, 70 kDa, 500 kDa, 2,000 kDa) and glucose were used to produce the standard curve. The freeze-dried samples and dextran standards were dissolved in the mobile phase (1 mg/mL). Before injecting into the chromatographic system, the solution was filtered through a 0.22 μm membrane filter. Molecular weight of SDF samples were estimated
Journal Pre-proof according to the standard curve obtained from T-series Dextran standard of known molecular weight. 2.5 X-ray diffraction (XRD) The XRD patterns of the SDF samples were obtained by an X-ray diffraction (D8 Advance, Bruker, Germany) instrument at the operating voltage and current of 30 kV and 20 mA, respectively. The diffraction angle (2θ) was scanned from 5° to 50°. The crystallinity index (Ic) of SDF samples was calculated from the diffraction intensity data according to Segal's method ( Segal, C., Martin, Jr., & Conrad, 1959). 2.6 Fourier transfer-infrared spectrometry (FT-IR) Differences in the molecular structure of the SDF samples were carried out by a Nicolet 5700 Fourier-transform infrared spectrophotometer (FTIR, Nicolet, USA) in the range from 400 to 4000 cm-1 with a resolution of 4 cm-1 and 32 scans. Each SDF sample (5 mg) was mixed with 100 mg KBr, then the mixture was ground and tableted. Scans were compared with a blank KBr background. 2.7 Thermal properties The
thermal
properties
of
the
SDF
samples
were
determined
with
thermogravimetric analysis (TGA) according to the method (Khatkar, Barak, & Mudgil, 2013) with some modification. The TGA of the SDF samples (6 mg) were performed in a nitrogen atmosphere using a thermos gravimetric analyzer (TGA 4000, PerkinElmer, Waltham, MA, USA) at a heating rate of 20°C/min over a temperature range of 50-600°C. The high-purity nitrogen gas flow of 20 mL/min was used as the purge gas prior to the experiment.
Journal Pre-proof 2.8 Monosaccharide composition The monosaccharide compositions of the SDF samples were analyzed by high-performance anion exchange chromatography (HPAEC, Dionex ICS-5000, Thermo Fisher Scientific Massachusetts, USA) equipped with pulse amperometric detection (PAD). A CarboPacTM PA20 guard column (4 mm x 50 mm) and a CarboPacTM PA20 analytical column (4 mm x 250 mm) were connected in series for the analysis. A Chromeleon 6.8 (Thermo Fisher Scientific, Massachusetts, USA) software was used for data collection and processing. The freeze-dried samples (5 mg) were hydrolyzed by 0.5 mL of 12 M H2SO4 in a sealed glass tube, stirred using a magnetic stirrer in ice bath for 30 min, then the H2SO4 was diluted to 2 M for another 2 h of hydrolysis in a silicone oil bath. Finally 2.5 mL ultra-pure water was added to the tube, keep stirring in oil bath at 105°C for 4 h. After the solution cooled to room temperature, the concentration of the mixture was adjusted to 100 μg/mL. The solution was filtered through a 0.22 μm syringe filter before injection into the system. 2.9 Functional properties 2.9.1 Water holding capacity (WHC) The freeze-dried samples (W1, 1 g) were hydrated with 25 mL ultra-pure water in a centrifuge tube at room temperature for 2 h. The residue was immediately collected and weighted (W2), after centrifugation at 4800 rmp for 10 min. The WHC was calculated using the following Eq. (1): WHC (g/g) = (W2-W1)/W1 2.9.2 Oil holding capacity (OHC)
(1)
Journal Pre-proof The freeze-dried samples (M1, 1 g) were mixed with 25 mL soybean oil in a centrifuge tube at the room temperature for 2 h. After centrifugation at 4800 rmp for 10 min, the supernatant (free oil) was removed and the residue was immediately weighted (M2). The OHC was calculated using the following Eq. (2): OHC (g/g) = (M2-M1)/M1
(2)
2.9.3 Cholesterol adsorption capacity (CAC) CAC was determined by the method (Jia, et al., 2019) with some modifications. The egg yolk (10 mL) was mixed with distilled water (90 mL) and then whipped it into an emulsion. The SDF sample (0.1 g) was mixed with 5 mL emulsion and shaken at the room temperature for 2 h. After centrifugation at 4800 rmp for 10 min, 1 mL of the supernatant was collected and diluted 10 times with glacial acetic acid. Then 0.4 mL solution was taken to determine the cholesterol content. 5mL egg yolk emulsion without the SDF sample was used as the blank. The CAC was calculated using the following Eq. (3): CAC (mg/g) = (m1−m2)/w1
(3)
Where m1 and m2 were the weight of cholesterol in the solution before and after adsorption, w1 was the weight of SDF sample. 2.9.4 Glucose adsorption capacity (GAC) The GAC was evaluated based on a previously described method with slightly modified (Niu, Li, Xia, Hou, & Xu, 2018). A freeze-dried SDF sample (0.1 g) was mixed with 5 mL glucose solution (0.5 mg/mL). The mixture was placed at room temperature for 2 h. After centrifugation at 4800 rmp for 10 min, 0.5 mL of the
Journal Pre-proof supernatant was transferred into a glass tube. Ultra-pure water was added to the tube until the volume reached to 3 mL and then mixed with 2 mL dinitro-salicylate (DNS) color development reagent. The mixture was incubated in a water bath at 100°C for 6 min with continuous shaking. After the solution dropped to room temperature, the residual concentration of glucose was measured at 520 nm using a TU-1810DAPC spectrophotometer (Persee, Beijing, China), and quantified based on the standard curve. The GAC was calculated using the following Eq. (4): GAC (mg/g) = (m1−m2)/w1
(4)
Where m1 and m2 were the weight of glucose in the solution before and after adsorption, w1 was the weight of the SDF sample. 2.9.5 Nitrite ion adsorption capacity (NIAC) The measurement of NIAC was determined by the method (Luo, et al. 2018) with some modification. The freeze-dried sample (0.1 g) was mixed with 5 mL NaNO2 solution (20 μg/mL) with the pH was adjusted to 7.0 and 2.0, to simulate the environment in the small intestine and stomach. The mixture was placed at room temperature for 2 h. After centrifugation at 4800 rmp for 10 min, 0.4 mL of the supernatant was transferred into a glass tube. Ultra-pure water was added to the tube until the volume reached to 2 mL and then 2 mL p-aminobenzene sulfonic acid (4 μg/mL) and 1 mL hydrochloride naphthodiamide (2 μg/mL) were added to the mixture. After the solution reacted in the dark for 30 min, the concentration of NaNO2 was measured at 538 nm by a TU-1810DAPC spectrophotometer (Persee, Beijing, China), and quantified based on the standard curve. The NIAC was calculated using
Journal Pre-proof the following Eq. (5): NIAC (μg/g) = (m1−m2)/w1
(5)
Where m1 and m2 were the weight of NaNO2 in the solution before and after adsorption, w1 was the weight of the SDF sample. 2.10 Statistical analyses All experiments were performed in triplicate and the results were expressed as mean ± standard deviation (S.D.). Statistical analysis was carried out by IBM SPSS statistical software (version 21.0, SPSS Inc., Chicago, IL, USA). P value < 0.05 was considered to be statistically significant. 3. Results and discussion 3.1 Extraction yield and composition of SDF The extraction yield and composition of SDF samples were listed in the Table 1. After treatment, the yield of PMT-SDF (7.94±0.20 %), MST-SDF (17.19±0.35 %), MET-SDF (9.13± 0.17 %) and MUT-SDF (8.35± 0.08 %) were increased, which might be due to the microwave energy could increase intracellular pressure, rupture the cell wall and release more active ingredients. In addition, the result showed that the IDF have turned to SDF after treatment. Moreover, the results showed that all samples had a high SDF purity. 3.2 SEM observation The SEM images of SDF samples were illustrated in Fig. 1. The WT-SDF sample structure was tight and disordered with a lot of irregular filaments. After microwave treatment, the filament became a sheet-like structure with a large number of holes on
Journal Pre-proof its surface. Compared with microwave treatment, the structure of MST-SDF was shrinking and became more disordered, which might be due to the strong corrosiveness of sodium hydroxide. The structure of MET-SDF turned to be multi-layer filaments and the pores were basically disappeared compared to PMT-SDF, which might be because the sheet structure was hydrolyzed by the enzyme treatment. From the micrographs of the MUT-SDF, the structure was tighter and the shape of the pore became messy, but the number of the pore increased dramatically and appeared more dense and multi-layered. This special structure could increase the relative surface area, and lead to the increase of WHC, OHC, CAC, GAC, NIAC. 3.3 Mw measurement HPGPC was applied to investigate the difference between SDF samples in molecular weight, the results were showed in the Fig. 2. From the results, the average Mw of all SDF samples were ranged from 0.16 to 620 kDa, and the highest proportion were around 620 kDa. As shown in the Fig. 2 (A), six peaks were observed in the WT-SDF, while only five peaks were found in the PMT-SDF, MST-SDF, MET-SDF and MUT-SDF samples. At the same time, the proportion of high molecular weight of PMT-SDF, MST-SDF, MET-SDF and MUT-SDF samples were increased compared to that of WT-SDF, which might be due to the microwave treatment combined two component peaks thus increase molecular weight. From the figures, the elution times of all SDF samples peaks were similar, which were around 12.4, 14.1, 14.9, 16.5 and 18.3 min. Due to different modification methods, the five SDF samples had slightly different values but similar molecular weight distributions. The highest molecular
Journal Pre-proof weight was observed in MUT-SDF (614.1 kDa), which might be because ultrasonic treatment made the cell wall broke more thoroughly, and the soluble dietary fiber dissolved more completely (Khodaei & Karboune, 2013). 3.4 XRD analysis The XRD results of the SDFs were listed in the Fig. 3. All the samples had the characteristic crystalline peaks at 21.18° 2θ, except that the MST-SDF sample had new characteristic crystalline peak at 12.61° 2θ, indicating that the crystalline structure of MST-SDF sample was significantly changed after treatment. The crystallinity index of the WT-SDF was 19.18%, and significantly decreased to 13.75% in PMT-SDF by microwave treatment, suggested that the crystal structure of SDF was damaged seriously by the microwave. However, the crystallinity index of the MST-SDF (27.89%), MET-SDF (20.16%), MUT-SDF (21.67%) were increased compared with that of PMT-SDF. The reason for the increasing of the crystallinity might be because the combined treatment could make the molecular rearrangement more regular. 3.5 FT-IR analysis The FT-IR spectra of SDF samples from different modification methods were recorded from 400 cm-1 to 4000 cm-1. From the results shown in the Fig. 4, there were several similar characteristic peaks in all SDF samples spectra. The strong and wide peak in the range of 3000-3650 cm-1 was assigned to the -OH stretching. The peaks at 1618.7 cm-1 and 1415.7 cm-1 might be the characteristic absorption of C=O bond stretch in asymmetric and symmetric, respectively. Particularly, the peaks at
Journal Pre-proof 1000-1300 cm-1 were distributed to the C-O stretching vibration, which might be the blending vibration of primary alcohols or the C-O-C and C-O-H of sugar ring (H. Chen, et al., 2018). Moreover, the peak at 917.8 cm-1 in all samples was indicative of stretching vibration of β-glycosidic linkages in polysaccharides. Except the similar characteristic peaks described above, there were some different peaks observed in the MST-SDF. Compared with other samples, the MST-SDF sample had a peak intensity reduction at the 2926.7 cm-1 that was C-H stretching vibrations from the methylene group of polysaccharides (Mengmei & Taihua, 2016), which might be due to the breakage of the bonding in the molecule by sodium hydroxide treatment. The characteristic peaks at 1740-1760 cm-1 (C-O stretching of -COOH) were observed in all samples except the MST-SDF sample. This phenomenon might be due to the hydroxyl group on the carboxylic acid was replaced by a sodium atom. In addition, the spectra of the MST-SDF sample had a red shift phenomenon in the position of some peaks, which indicated the microwave-sodium hydroxide treatment destroyed the structure of organic molecules severely. 3.6 Thermal properties The TGA results of the SDF samples were showed in the Fig. 5. WT-SDF, PMT-SDF, MET-SDF and MUT-SDF had the similar curve patterns, which can be divided into three stages. For the first stage 50-200°C, the weight of the samples were slightly decreased. As observed from the second stage between 200-400°C, a rapid weight loss could be detected, which might be due to the polysaccharide pyrolytic decomposition. At the final stage 400-600°C, the weight loss of the samples became
Journal Pre-proof slowly owing to the thermal decomposition of char. The residual mass for the WT-SDF, PMT-SDF, MET-SDF and MUT-SDF were 29.19%, 26.10%, 28.06% and 28.65%, respectively. However, the weight loss of the MST-SDF sample could be divided into four stages. The first stage at 50-240°C, the mass of MST-SDF sample dropped slowly. The rate of weight loss increased dramatically when it went to the second stage 240-340°C. At the third stage 340-500°C, the rate of weight loss decreased. The mass of the MST-SDF remained stable at the last stage 500-600°C. The residual mass for MST-SDF was significantly increased to 42.03 % compared with that of WT-SDF, PMT-SDF, MET-SDF and MUT-SDF, which might be related with its high crystallinity index. The result was consistent with the previous study (Luo, et al., 2018) that the high crystallinity sample has high thermal stability. 3.7 Monosaccharide composition analysis The monosaccharide composition of SDF samples were shown in the Table 2. The amount of Rhamnose in the WT-SDF sample was set as 1, to which all other monosaccharides were compared. As shown in the Table 2, eight kinds of monosaccharides, including Rha, Ara, Gal, Glu, Xyl, Man, Fru, Gala, were found in the PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. However, mannose was not detected in the WT-SDF. This might be because the microwave energy broke the glycosidic bond and formed new monosaccharide in the SDF samples. After microwave treatment, the content of Gal, Glu and Fru were decreased. The PMT-SDF had the highest Xyl content, while the MST-SDF had the highest Ara content. Meanwhile, the lowest Rha content was observed in the MET-SDF, but the
Journal Pre-proof MUT-SDF had the lowest Man content in the treated samples. This phenomenon might be because the different treatment methods had different effects on macromolecules in the SDF samples. 3.8 Functional properties 3.8.1 WHC and OHC The WHC and OHC of grapefruit peel SDFs subjected to different treatments were showed in the Fig. 6. High WHC SDF could change the viscosity of certain food and prevent food from shrinking (Elleuch, et al., 2011). From the Fig. 6 (A), the WHC decreased after samples were microwave treated. However, compared with other treated samples, the MST-SDF and MUT-SDF showed a rebound of WHC. Especially the MUT-SDF displayed the highest WHC (11.39±0.09 g/g), maybe because the special functional properties of ultrasound could result in a loose porous structure of SDF. The OHC of SDF is important for food applications, such as preventing fat loss during cooking (Schneeman, 1999) and helping people to remove excess fat. As shown in the Fig. 6 (B), the OHC of WT-SDF was (18.23±0.13 g/g), while PMT-SDF (13.21±0.12 g/g) and MST-SDF (11.12±0.08 g/g) significantly decreased owing to the destruction of the pore structure. Nevertheless, MUT-SDF exerted the highest OHC (25.01±0.04 g/g) since its complex and numerous pores like structure. These results suggested that the pore-like structure could significantly affect the OHC of SDFs. 3.8.2 CAC CAC is an important functional property of SDF, which has been proven to be
Journal Pre-proof able to decrease the cardiovascular disease risk and serum cholesterol levels in human body (Nsor-Atindana, Zhong, & Mothibe, 2012). The CAC of the samples were showed in Fig. 6 (C). From the results, the WT-SDF showed the lowest CAC (2.69±0.12 mg/g). After microwave treatment, the CAC of PMT-SDF significantly increased (24.93±0.02 mg/g). Compared with the PMT-SDF, the CAC of MST-SDF and MET-SDF decreased. In the treated samples, the lowest CAC was observed in the MST-SDF (12.86±0.09 mg/g) while the highest CAC was showed by the MUT-SDF (26.48±0.08 mg/g), which might be due to the MUT-SDF had larger specific surface area and more complex structure to bind more cholesterol. 3.8.3 GAC GAC is another important functional property of SDF, which could combine with glucose in intestinal juice thus led to the decrease of postprandial blood glucose levels. The GAC of different SDF samples were showed in the Fig. 6 (D). Although all the SDF samples were found to be effective in adsorbing glucose, the MUT-SDF still showed the strongest adsorption capacity (24.42±0.06 mg/g). The results suggested that the MUT-SDF had a superior hypoglycemic effect in vitro. 3.8.4 NIAC In the environment of gastric acid, nitrite reacts with secondary amines, tertiary amines and amides in food to form strong carcinogen N-nitrosamine, which can also enter the fetus through the placenta and have teratogenic effects on the fetus (Bruning-Fann, C. S., Kaneene, & J. B., 1993). Therefore, the NIAC of SDF is one kind of meaningful functional property. As shown in Fig. 7, we investigated the
Journal Pre-proof adsorption capacity of nitrite ions for modified grapefruit peel SDFs in simulated gastric (PH=2) and small intestine (PH=7) environments. The results showed that the NIAC of SDF samples in the gastric environment were significantly higher than that of the small intestine environment. In addition, the MUT-SDF showed the highest NIAC in both PH=2 (219.43±0.33 μg/g) and PH=7 (9.41±0.14 μg/g), which might be due to the MUT-SDF sample had better structure and massive explosion of functional groups to interaction with more nitrite ion. 4. Conclusions In this study, we used three modification methods to modify grapefruit peel and investigated the structural and functional properties of SDF extracted from modified grapefruit peel. MST-SDF, MET-SDF and MUT-SDF exhibited more complex and loose structure higher molecular weight, crystallinity and thermal stability, and more diverse monosaccharide composition than that of WT-SDF and PMT-SDF. The modification methods not only improved the structural characteristics of the SDF, but also improved functional properties, especially the MUT-SDF showed the highest binding capacity for the water, oil, cholesterol, glucose and nitrite ion. Therefore, MUT-SDF had a great potential for the application in functional food industry, and MUT could improve the economic benefit and utilization rate of the grapefruit peel. Acknowledgements This work was supported by the Key Research and Development Program of Jiangxi Province of China (20171BBF60041), National Natural Science Foundation of China (31701603), and Research Project of State Key Laboratory of Food Science and
Journal Pre-proof Technology (SKLF-ZZA-201611).
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Microwave-assisted
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Journal Pre-proof Figure Captions Fig. 1. SEM micrographs of SDFs: WT-SDF (A, a), PMT-SDF (B, b), MST-SDF (C, c), MET-SDF (D, d) and MUT-SDF (E, e). Magnification = 100× (A-E) and 1000× (a-e). Fig. 2. Gel permeation chromatogram profiles of WT-SDF (A), PMT-SDF (B), MST-SDF (C), MET-SDF (D) and MUT-SDF (E). Fig. 3. The X-ray diffraction patterns of WT-SDF, PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. Fig. 4. FT-IR spectras of WT-SDF, PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. Fig. 5. Thermal properties of WT-SDF, PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. Fig. 6. The WHC (A), OHC (B), CAC (C), GAC (D) of WT-SDF, PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. Results were expressed as means ± standard deviation (n = 3). Different letters denote significantly different in the group, (P < 0.05). Fig. 7. The NIAC of WT-SDF, PMT-SDF, MST-SDF, MET-SDF and MUT-SDF. Results were expressed as means ± standard deviation (n = 3). Different letters denote significantly different in the group, (P < 0.05).
1
Table 1 Extraction yield and composition of SDF samples (g/100 g) SDF yield
2
IDF yield
TDF
SDF
composition
Moisture
Ash
Protein
Fat
Uronic acid
Purity
WT
3.62±0.13e
59.77±0.38a
62.39±0.41a
4.56±0.12b
1.78±0.02c
2.08±0.12c
1.12±0.04b 28.67±0.19e
90.46±0.56d
PMT
7.94±0.20d
55.47±0.49b
63.41±0.36a
3.41±0.04d
1.57±0.05d
1.81±0.09d
1.04±0.07d 38.39±0.25b
92.17±0.62b
MST
17.19±0.35a 46.19±0.39e
63.38±0.32a
5.36±0.07a
2.36±0.09a
2.56±0.08b
1.27±0.06a 39.53±0.23a
88.45±0.47e
MET
9.13±0.17b
53.23±0.32d
62.36±0.47a
3.38±0.05e
1.87±0.05b
3.08±0.06a
1.08±0.05c 36.94±0.28c
90.59±0.51c
MUT
8.35±0.08c
55.02±0.27c
63.37±0.36a
3.68±0.09c
1.35±0.01e
1.27±0.03e
0.98±0.06e 36.84±0.27d
92.72±0.53a
Results were expressed as means ± standard deviation (n = 3). Different letters denote significantly different in the column, (P < 0.05).
4
Table 2 Monosaccharides composition ratio and relative mole % of SDF samples Monosaccharide
WT-SDF
PMT-SDF
MST-SDF
MET-SDF
MUT-SDF
Rhamnose
ratio 1.00
mole % 2.73
ratio 1.03
mole % 2.72
ratio 1.56
mole % 4.35
ratio 0.80
mole % 2.51
ratio 1.10
mole % 3.44
Arabinose
7.58
20.68
9.02
23.82
13.76
38.40
6.57
20.64
8.75
27.38
Galactose
4.02
10.97
2.77
7.31
2.39
6.67
3.64
11.44
2.83
8.85
Glucose
12.73
34.73
10.83
28.60
7.14
19.93
10.54
33.11
9.11
28.50
Xylose
2.72
7.42
3.44
9.08
1.97
5.50
1.18
3.71
2.80
8.76
Mannose
ND
0
2.16
5.70
1.65
4.61
1.37
4.30
1.34
4.19
Fructose
3.28
8.95
0.83
2.19
3.20
8.93
1.87
5.87
1.34
4.19
Galacturonic acid
5.32
14.52
7.79
20.57
4.16
11.61
5.86
18.41
4.69
14.67
5
ND: not detected. Ratio means the amount of different monosaccharides in different samples comparted to the amount of rhamnose in the
6
WT-SDF sample. Mole % means the relative mole % of different monosaccharides in the same sample.
Journal Pre-proof Declaration of interests The authors declare no conflict of interest in this work.
Journal Pre-proof These authors contributed equally to this work.
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Journal Pre-proof Highlights
MST-SDF, MET-SDF or MUT-SDF were prepared by microwave assisted with sodium hydroxide, enzymatic or ultrasonic treatment.
The surface and internal structure of MST-SDF, MET-SDF, MUT-SDF samples became more complex.
The crystallinity and thermal stability of MST-SDF, MET-SDF, MUT-SDF samples were improved.
The MUT-SDF exhibited significantly superiority on the binding capacity for water, oil, cholesterol, glucose and nitrite ion.
The MUT-SDF has great potential for the application in functional food industry.