Physical properties of mucilage polysaccharides from Dioscorea opposita Thunb

Food Chemistry 311 (2020) 126039

Contents lists available at ScienceDirect

Food Chemistry journal homepage: www.elsevier.com/locate/foodchem

Physical properties of mucilage polysaccharides from Dioscorea opposita Thunb

T

Fanyi Maa, Ruijiao Wanga, Xiaojing Lia, Wenyi Kanga, Alan E. Bellb, Dongbao Zhaoa, ⁎ ⁎ Xiuhua Liua, , Weizhe Chenc, a

Key Laboratory of Natural Medicine and Immune-Engineering of Henan Province, College of Chemistry and Chemical Engineering, National R&D Center for Edible Fungus Processing Technology, School of Life Sciences, Henan University, Kaifeng 475004, China b Department of Food and Nutritional Science, University of Reading, Whiteknights, Reading RG6 6AP, UK c Xuchang Quality and Technical Supervision and Test Center, Xuchang 461000, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Chinese yam Mucilage Polysaccharides Rheology Emulsification

The physical properties of the mucilage polysaccharides of Dioscorea opposita (DOMP) were investigated in this study. The monosaccharide and amino acid contents, and molecular weight were determined, and morphology was observed. The rheological and emulsifying properties of different concentrations of DOMP were determined at acidic and basic pH (pH 5.0 and 9.0). The glucose and protein contents were 11.05% and 13.39%, respectively, and the average molecular weight was 9062 Da. The DOMP particles were spheres of 0.18 μm diameter, which aggregated in solution. The viscosity of DOMP decreased gradually with increase in shear rate, which was indicative of pseudoplastic characteristics. DOMP showed relatively better emulsification properties than Konjac glucomannan (KGM). The particle size of DOMP decreased and its emulsifying properties improved under both acidic and basic conditions. These results suggested that DOMP can be used as a natural processing agent for improving the mouth-feel of food.

1. Introduction Dioscorea opposita Thunb., an edible Chinese yam (CY), is an important tropical tuber crop that acts as the staple food and natural medicine in many parts of the world, and is especially used in traditional Chinese medicine (Hariprakash & Nambiasn, 1996; Yang, Lu, & Hwang, 2003). D. opposita is used for treating haemorrhoids, poor appetite, chronic diarrhoea, asthma, dry coughs, frequent or uncontrollable urination, emotional instability, and diabetes (Chan & Ng, 2013; Choi, Koo, & Hwang, 2004). The viscous mucilage of D. opposita (DOM) is composed mainly of mannan-protein macromolecules (Kiho, Hara, & Ukai, 1985; Misaki, Ito, & Harada, 1972; Ohtani & Murakami, 1991). According to our previous study, DOM is a highly branched neutral polysaccharide-protein complex of 143 kDa, which mainly consists of glucose, mannose, fructose, xylose, and proteins (Ma, 2017b). The mucilaginous material can be used in the food industry as thickeners, stabilisers, gelling agents, syneresis controllers, emulsifiers, suspension stabilisers, and prebiotic (Lucey, 2002; Nikoofar, Hojjatoleslami, & Shariaty, 2013).

Currently, the demand for new and natural additives is increasing, as they can be used as natural emulsifiers or thickeners for human consumption instead of synthetic additives that pose risk to human health (Avila-de la Rosa, Alvarez-Ramirez, Vernon-Carter, CarrilloNavas, & Pérez-Alonso, 2015). Previously, we have investigated the chemical composition and characteristics of DOM; however, the rheological and emulsifying properties of DOM polysaccharides (DOMP) are not well-documented. The aim of this study was to evaluate the viscoelastic behaviour and emulsification properties at the oil–water interface of DOMP. We believe that this may provide information regarding the structures formation by DOMP molecules at the interface. Subsequently, this can promote the formulation of stable and functional emulsions suitable for human consumption.

Abbreviations: CY, Chinese yam; DOM, Dioscorea opposita mucilage; DOMP, Dioscorea opposita mucilage polysaccharides; MW, molecular weight; KGM, konjac glucomannan; O/W, oil/water; PDI, polydispersity index ⁎ Corresponding authors. E-mail addresses: [email protected] (X. Liu), [email protected] (W. Chen). https://doi.org/10.1016/j.foodchem.2019.126039 Received 6 August 2019; Received in revised form 21 November 2019; Accepted 6 December 2019 Available online 17 December 2019 0308-8146/ © 2019 Elsevier Ltd. All rights reserved.

Food Chemistry 311 (2020) 126039

F. Ma, et al.

2. Materials and methods

acids in DOMP.

2.1. Materials

2.3.6. Determination of molecular weight (MW) As previously described by Ma (2018), the weight-average MW (Mw) and MW polydispersity (Mw/Mn) of DOMP samples were measured using a high-performance size-exclusion chromatography column (OHpak SB-802.5 HQ column (8.0 mm × 300 mm, Shodex Co., Japan)) attached to multiangle laser light scattering and refractive index detector (HPSEC-MALLS-RID, Wyatt Technology Co., USA).

Fresh D. opposita Thunb. was purchased from Bao He Tang (Jiaozuo) Pharmaceutical Co. Ltd., Jiaozuo City, Henan province, China. All reagents and standard samples were purchased from Sigma-Aldrich Co. Ltd., and Tianjin Kemiou Chemical Reagent Co. Ltd., China. Konjac glucomannan, (KGM; LOT: Z01J8X39049) was purchased from Yuanye Co. Ltd., Shanghai, China. All chemicals used were of analytical grade.

2.3.7. Fourier transform infrared spectroscopy (FT-IR) DOMP was analysed using FT-IR (Vertex 70, Bruker, Germany) in the spectral range of 400–4000 cm−1.

2.2. Preparation of DOMP DOMP was extracted as previously described by Zhang, Wang, Liu, and Li (2016) with minor modifications. Briefly, fresh D. opposita was peeled, weighed, sliced, and homogenised in an industrial blender for 5 min. After centrifugation at 4,000 r/min for 5 min, DOM (the supernatant) was collected and poured into ethanol such that the concentration of ethanol was 75%. The precipitate was obtained and lyophilised for 3 days to a constant weight to determine DOMP yield. DOMP was stored in vacuum desiccators over phosphorus pentoxide until further use.

2.3.8. Scanning electron microscopy (SEM) A thermal field emission scanning electron microscope (JSM-7001F, JEOL Ltd., Japan) was used to inspect the morphology of DOMP. 2.3.9. X-ray diffraction (XRD) An X-ray powder diffractometer (Bruker D8 Advance, Germany) was used to determine the degree of starch crystallinity with Ni- filtered Cu Kα radiation (λ = 1.78901 Å). The DOMP samples were exposed to the X-ray beam from the X-ray generator running at 40 kV and 40 mA and readings were recorded at the scanning regions of the diffraction angle 4-90° (2θ) at room temperature.

2.3. Analytical methods 2.3.1. Yield of extracted DOMP D. opposita were peeled, weighed, and grounded. DOMP was lyophilised and weighed to determine the DOMP yield (%) using the following formula:

DOM yield (%) =

2.4. Rheological and emulsification properties of DOMP 2.4.1. Sample preparation Six samples were prepared: DOMP-native, DOMP-5, DOMP-9, DOMP-N-7, DOMP-5–7, and DOMP-9–7. As previously described by Ma (2017b), DOMP was dispersed (10% w/v) by adding the required amount of sample to deionised water (pH 7.0, resistivity: 18 Ω·m) with gentle stirring at room temperature (20 °C). The solutions were further degassed under vacuum to remove any entrapped air bubbles. DOMP samples were prepared by either dialysing overnight at 4 °C (native) or dialysing against phosphate-buffered solutions of different pH (0.3 M, pH 5.0 and pH 9.0) overnight at 4 °C to equilibrate to the required pH. A part of the samples (DOMP-native, DOMP-5 and DOMP-9) was lyophilised and stored in vacuum desiccators over phosphorus pentoxide for further study. The remaining samples were then dialysed against several changes of deionised water for 36 h at 4 °C. No change in sample volume was observed. DOMP-N-7, DOMP-5–7, and DOMP-9–7 samples were freeze-dried and stored in vacuum desiccators over phosphorus pentoxide for further study. a) Rheological sample preparation: Each of the six samples was separately dissolved in deionised water (pH 7.0, resistivity: 18 Ω·m) at different concentrations (2–10% w/v) with gentle stirring at room temperature (20 °C) until dispersion. b) Sample preparation for emulsification: Each sample of DOMP-native, DOMP-5, DOMP-9, DOMP-N-7, DOMP-5–7, and DOMP-9–7 was separately dissolved in deionised water (pH 7.0, resistivity: 18 Ω·m) at different concentrations (0.2%, 0.4%, 0.6%, 0.8%, and 1.0% w/v) with gentle stirring at room temperature (20 °C) until dispersion. The medium-chain triglyceride (MCT) was used as the oil sample, and 1:1 ratio of DOMP: MCT was used according to Ma (2017b).

Weight of dried DOMP (g) × 100 Weight of fresh yam without peels (g)

2.3.2. pH determination DOMP (1% w/v) was prepared and the pH meter (ZD-2A, Dapu Instrument, Shanghai, China) was calibrated using standard solutions of known pH (4.00, 6.86, and 9.18). The pH of the sample solutions was read directly from the instrument and the mean value of three consecutive measurements was recorded. 2.3.3. Determination of glucose and protein content Glucose and protein contents were determined using the phenolsulphuric acid method and Coomassie Brilliant Blue method, respectively (Bradford, 1976; Dubois, Gilles, Hamilton, Reders, & Smith, 1956). 2.3.4. Determination of monosaccharides As previously described by Wang, Zhao, Pu, and Luan (2016), 1phenyl-3-methyl-5-pyrazolone (PMP) derivatisation and high-performance liquid chromatography (HPLC, Agilent, 1260, USA) was used for determination of monosaccharide content using an Agilent Eclipse Plus C18 column (4.6 × 250 mm, 5 μm). Nine standards (Ludger Co. Ltd.), including arabinose, rhamnose, galactose, glucose, mannose, xylose, ribose, galacturonic acid, and glucuronic acid were used to determine the monosaccharide content in DOMP samples. Chromatographic separation was performed using 85:15 (v/v) ratio of 0.1 mol·L-1 phosphate buffer (pH 6.7) and acetonitrile as the mobile phase at a flow rate of 1.0 mL·min−1. The temperature of the column was maintained at 25 °C and absorbance was measured using a variable-wavelength UV–visible detector (VWD) at 245 nm.

2.4.2. Rheological measurements The rheological properties of the samples were investigated using a rotatory rheometer (TA-DHR2, TA Instruments, USA) with a 60 mm cone plate (2°). Oscillation frequency measurements were performed with a strain of 5% and angular frequency ranging from 100 to 0.1 rad/ s. Dynamic strain sweep measurements were performed to determine the linear viscoelastic regimen with strain range from 0.01 to 100%. The testing conditions were selected to avoid exceeding the linear viscoelastic limits of any of the samples studied (no evidence of any

2.3.5. Determination of amino acids As previously described by Waqas et al. (2015), an amino acid analyser (L-8900 amino acid analyser, Japan) and Shim-pack amino-Na column (4.5 × 60 mm, Shimadzu) were used to identify the amino 2

Food Chemistry 311 (2020) 126039

F. Ma, et al.

comparable with those of our previous study on D. opposita mucilage (DOM) (Ma, 2017b). The yield of DOMP was approximately 5.71%, which was less than the expected yield of DOM (8.18%). The glucose and protein contents were 11.05% and 13.39%, respectively. Compared to the corresponding contents of DOM (approximately 16% glucose and 2.78% protein), the glucose content of DOMP was slightly reduced, whereas the protein content increased significantly. These results might be caused by the insolubility of DOMP which presents large molecular weight and complexity of the structure. Some glucose and monosaccharide could be surrounded by protein fractions, which led to the difficulty of hydrolysis. Meanwhile, during ethanol precipitation, large amounts of protein were denatured, and the smaller molecular weight proteins were precipitated along with the polysaccharides, making DOMP a polysaccharide-protein complex. Mannose and glucose accounted for > 85% of the polysaccharides, and the thickness and texture of DOMP was similar to that of KGM. KGM, a competitive control sample in this study, is a natural polysaccharide, which links β-D-mannose and β-D-glucose via β-1,3- or β1,4-glycosidic linkages (Harding, Smith, Lawson, Gahler, & Wood, 2011; Lu, 2015). Arginine was the most abundant amino acid in DOMP, accounting for approximately 66.91% of the total amino acid content, which was consistent with the results of a previous study on amino acid content in polysaccharide and mucilage from D. opposita (Ma, 2017a& b). Recently, Claybaugh (2014) showed that L-arginine supplementation improved insulin sensitivity with preserved renal function of patients with type II diabetes by altering the nitric oxide pathway. The molecular weight of DOMP was approximately 9 kDa, distributed mainly from 4 kDa to 20 kDa, which was considerably smaller than that of DOM. A previous study has shown the molecular weight of DOM to be 143 kDa (mainly distributed from 10 kDa to 100 kDa), and approximately 14% of its components were larger than 200 kDa (Ma, 2017b). Zhang et al. (2016) observed that the large molecular weight of mucilage (> 105) may affect its ingestion and clinical application. Therefore, DOMP with smaller molecular weight might be beneficial for gastrointestinal ingestion and for applications in food or pharmaceutical industries.

structural breakdown from the preliminary stress amplitude scans, i.e. linear viscoelastic behaviour). Dynamic flow sweep measurements of the storage modulus (G’) and loss modulus (G”) were performed with shear rate from 0.01 to 100 1/s. Samples were loaded onto the rheometer and allowed to equilibrate to the measuring temperature (25 ± 1 °C, ≈0.5 min). For each test, approximately 2 mL sample was transferred onto the plate and the measurements were made over the frequency range of 0.1–10 Hz (strain 0.01%). 2.4.3. Droplet distribution measurements The droplet diameters (z-average) and distribution (polydispersity index, PDI) and zeta-potential of the emulsions were measured using Malvern zeta-potential (Malvern-NanoZS90, Malvern Ltd., UK). To obtain comparable and representative data, the results were recorded as the averages of six replicates ± standard deviation (SD). Statistical analysis was performed using a paired t-test, and letters a to z indicated statistical significance (P < 0.05). 3. Results and discussion 3.1. Compositions of DOMP Table 1 shows the characterisation of DOMP in terms of yield, pH, glucose, protein, monosaccharide, and amino acid contents, and molecular weight distribution. The results obtained in this study were Table 1 Monosaccharides and amino acid content, and molecular weight of DOMP. DOMP Yield (%) Glucose content (%) Protein content (%) pH

5.71 ± 0.59 11.05 ± 0.87 13.39 ± 0.49 6.58 ± 0.07

Monosaccharides (%) Arabinose Galactose Glucose Mannose Rhamnose Ribose Xylose Galacturonic acid Glucuronic acid

3.33 0.35 23.45 62.52 0.42 0.07 0.42 0.01 0.02

Amino acids (%) Alanine (ALA) Arginine (ARG) Aspartic acid (ASP) Cysteine (CYS) Glutamic acid (GLU) Glycine (GLY) Histidine (HIS) Isoleucine (ILE) Leucine (LEU) Lysine (LYS) Methionine (MET) Phenylalanine (PHE) Proline (PRO) Serine (SER) Threonine (THR) Tyrosine (TYR) Valine (VAL)

2.70 66.91 2.24 0.05 0.53 1.27 0.69 0.11 0.18 3.39 0.05 0.16 0.13 15.50 5.82 0.07 0.22

Molecular distributions Molecular weight (Da) PDI Distributions (kDa) <4 4–10 10–20 20–100

3.2. Characteristics of DOMP Fig. 1(a) shows the FTIR of DOMP. According to Andrade, Nunes, and Pereira (2015), the wide band at 3417 cm−1 is indicative of hydroxyl groups (–OH) with stretching vibration. The peaks at 2925 cm−1 and 2862 cm−1 are indicative of CH bond with stretching vibration. The wavenumber between 1700 and 1600 cm−1 corresponded to the carbonyl (C]O) group with stretching vibration in carboxylic acids, aldehydes, and ketones. The wavenumber at 1447 cm−1 indicated the presence of the CeOeH group of carboxylic acids with bending vibration (Kong et al., 2015). The peak at 1247 cm−1 indicated the presence of the methyl group (CH3) with symmetrical bending vibration. The peak at 1069 cm−1 was possibly contributed by CeOeC pyranose stretching vibration. Compared to the FTIR of DOM, the FITR of DOMP showed an additional peak at 923 cm−1, which was indicative of β-D-glucose pyranose bending vibration (Ma, 2017b). Therefore, according to results, the proposed structure of mucilage polysaccharide could consist of 1-3-ɑ-D-glucose, 1-4-β-D-glucose and 1-2-ɑ-mannose, and the polysaccharide chain may emerge as a spiral shape, which is required further investigations. The surface morphology of DOMP is shown in Fig. 1(b). Previous studies have suggested that the surface topography, structure, and properties of polysaccharides can be influenced by the conditions of extraction, purification, and preparation (Nep & Conway, 2010). The DOMP particles were spherical with average diameter of 0.18 μm and formed aggregates. Unlike DOM, DOMP showed squamous structure and resembled a cracked film (Ma, 2017b). Fig. 1 (c) shows the XRD angular diffraction intensity plots of DOMP from 4° to 90°. The graph after 40° has not been shown due to absence of peaks till 90°. A broad

9062 1.208 6.00% 62.25% 28.50% 3.25%

3

Food Chemistry 311 (2020) 126039

F. Ma, et al.

Fig. 1. (a) Fourier transform infrared spectra of DOMP; (b) scanning electron microscopic image of DOMP; (c) X-ray diffraction of DOMP from 4 to 40° (The diffraction from 40° to 90° is not shown due to absence of peaks).

of the protein-polysaccharide complex and changed its rheological behaviour. The elastic and viscous properties of the materials were measured and used to determine their rheological behaviour (McClements, 2005). Fig. 2(b) and (c) show the storage modulus (elastic modulus, G′) and loss modulus (viscous modulus, G′′), respectively, as a function of frequency for the DOMP solutions (10% w/v) at 25 °C. Generally, the materials showed elastic behaviour (gel-like) when G′ > G′′, whereas they were viscous (liquid-like) when G′ < G′′ (Karimi & Moharmmadifor, 2014). Strong gel properties were observed if G′ dominated over G′′, whereas weak gel properties were observed if G′ was close to G′′ (Williams & Phillips, 2009). Therefore, Fig. 2(b) and (c) indicated that all the DOMP samples showed weak gel and more liquidlike properties.

peak spanning 2θ values from 15° to 40° indicated that DOMP forms an amorphous body (Dimopoulou, Ritzoulis, & Panayiiotou, 2015).

3.3. Rheological behaviour of DOMP 3.3.1. Effect of concentration on DOMP The dependence of shear viscosity (η) of the DOMP solution (10% w/v) on shear rate was analysed at 25 °C over the range of 1 to 100 s−1 (Fig. 2(a)) and for different concentrations ranging from 2% to 10% (w/ v). Fig. 2 shows the results obtained with 10% (w/v) DOMP solution, and the other concentrations of DOMP showed the same trend (data not shown). The viscosity of DOMP decreased gradually with increase in shear rate. The DOMP-N and DOMP-N-7 solution flowed indicative of its pseudoplastic characteristics (shear-thinning), however, the rheological behaviour of DOMP-N-7 was tended to Newtonian flow. All other samples (DOMP-5, DOMP-9, DOMP-5-7 and DOMP-9-7) after the acidic and alkaline treatments presents Newtonian behaviour. The transition from non-Newtonian flow to Newtonian flow behaviour can reveal that the structural change. The degree of space occupancy of polymers can be normalized by multiplying concentration and intrinsic viscosity (Williams & Phillips, 2009); that is to say, with the decrease of viscosity in this study, the molecular weight and branches were reduced. The DOMP-N-7 was dialysed over 36 h with several changes of deionised water, and therefore, it contained smaller molecules after dialysis, resulting in more liquid behaviour. The viscosities of DOMP samples after pH treatments (DOMP-5, DOMP-9, DOMP-5-7 and DOMP-9-7) were similar to that of DOMP-N-7, which shows the Newtonian flow behaviour. These results also indicated that pH treatment can denature proteins and change the structure of DOMP, which altered the structure

3.4. Emulsification properties of DOMP 3.4.1. Particle diameters and stability of DOMP solutions The detailed quantitative information on the droplet size distribution was helpful to learn more about the properties of emulsions (Horne, 1995; McClements, 2005). Table 2 shows the droplet sizes (a) and zeta-potential (b) of KGM and DOMP solutions at different concentrations (0.2%, 0.4%, 0.6%, 0.8%, and 1.0% w/v). The samples included DOMP-N, pH-treated DOMP (DOMP-5, and DOMP-9), and DOMP neutralised after pH treatment (DOMP-N-7, DOMP-5-7, and DOMP-9-7). The droplet sizes of KGM and DOMP-native generally increased with concentration, and there was no significant difference between the solutions at 0.2% w/v. However, the droplet size of DOMPnative increased dramatically from 1.86 μm to 12.54 μm, which

Fig. 2. Rheological behaviour of DOMP solution (10% w/v) at 25 °C. (a) Shear-dependent viscosity (η) of DOMP; (b) shear storage modulus (G’) as a function of angular frequency (ω) of DOMP; (c) shear loss modulus (G”) as a function of angular frequency (ω) of DOMP. 4

Food Chemistry 311 (2020) 126039

F. Ma, et al.

Table 2 Droplet diameter (μm) and zeta-potential (mV) of different concentrations of konjac glucomannan (KGM) and Dioscorea opposita mucilage polysaccharides (DOMP) samples at different concentrations. (a) Droplet diameter (μm) and polydispersity index (PDI) of KGM and DOMP solutions at different concentrations Droplet diameters (z-average in μm ± standard deviation with mean PDI in parentheses) Concentrations (% w/v) 0.2% KGM DOMP-native DOMP-5 DOMP-9 DOMP-N-7 DOMP-5–7 DOMP-9–7

1.58 1.86 2.18 0.78 4.74 4.36 3.81

0.4% ± ± ± ± ± ± ±

a

0.21 (0.44) 0.06a (0.28) 0.11 (0.24) 0.04 (0.02) 0.26 (0.14) 0.19i (0.03) 0.42 (0.08)

1.77 2.90 1.59 2.37 6.57 5.01 4.53

0.6% ± ± ± ± ± ± ±

ab

0.11 (0.68) 0.12 (0.51) 0.06be (0.02) 0.14 (0.05) 0.04 h (0.47) 0.75i (0.26) 0.14i (0.40)

(b) Zeta-potential (mV) of KGM and DOMP solutions at different concentrations KGM −8.27 ± 2.4 −9.74 ± 1.3 DOMP-native −53.1 ± 1.1 −69.6 ± 0.2 DOMP-5 −21.2 ± 4.0 −19.7 ± 0.6 DOMP-9 −53.8 ± 6.2 −50.2 ± 4.4 DOMP-N-7 −35.7 ± 2.3 −52.8 ± 0.9 DOMP-5–7 −52.4 ± 5.7 −53.9 ± 4.8 DOMP-9–7 −30.4 ± 4.5 −35.8 ± 4.8

0.8% c

1.0%

3.00 ± 0.44 (0.36) 4.59 ± 0.18 (0.59) 3.25 ± 0.01c (0.45) 1.55 ± 0.08 g (0.29) 6.30 ± 0.36 h (0.08) 15.76 ± 2.98 (0.20) 6.83 ± 0.16 (0.34)

2.57 ± 0.68 (0.49) 11.59 ± 0.54 (0.33) df 1.83 ± 0.13 (0.06) 1.43 ± 0.06 g (0.47) 10.78 ± 0.31d (0.043) 42.20 ± 1.30 (0.42) 8.68 ± 0.77 (0.13)

2.91 ± 0.10 cd (0.51) 12.54 ± 0.23 (0.46) 1.49 ± 0.07ef (0.45) 1.58 ± 0.43 (0.32) 10.93 ± 0.08 (0.02) 54.79 ± 1.83 (0.32) 20.99 ± 1.50 (0.04)

−3.71 −64.8 −18.2 −42.9 −35.3 −57.1 −60.4

0.3 ± −63.4 −12.8 −38.9 −35.1 −64.8 −59.1

2.5 ± −73.1 −17.0 −37.9 −55.9 −71.6 −67.0

± ± ± ± ± ± ±

0.8 1.1 0.8 8.2 2.4 3.0 7.4

abcd

0.8 ± 0.9 ± 0.6 ± 5.1 ± 3.1 ± 4.4 ± 3.2

0.5 ± 1.0 ± 1.5 ± 5.8 ± 7.5 ± 1.5 ± 2.4

Note: Data are reported as mean of six replicates; results are presented as mean ± standard deviation; paired values with superscript letters e through i indicate NOT significant difference (P > 0.05).

indicated that DOMP-native flocculated more easily than KGM. The droplet diameters decreased after acidic and alkaline treatment, suggesting that extreme pH may denature the proteins and alter the functional groups, thereby changing the structure of DOMP that had been dispersed and hydrolysed to manosaccharide, amino acids or short chain of polysaccharides with amino acids. According to Wu, Eskin, Cui, and Pokharel (2015), most polysaccharides are slightly acidic in water, which supports our observation that the pH of DOMP was 6.58 (Table 1). Introduction of H+ into the solution may cause repulsion among polysaccharide chains, and more OH− groups may become available to combine with the dissociated H+ in solution, which may render flocculation of DOMP-5 and DOMP-9 solutions difficult, thereby promoting the formation of smaller droplets. DOMP-native, DOMP-5, and DOMP-9 were dialysed against several changes of deionised water for 36 h at 4 °C, and lyophilised to obtain DOMP-N-7, DOMP-5-7, and DOMP-9-7 samples, respectively. The droplet diameters of DOMP-N-7 were larger than those of DOMP-native, suggesting that larger molecules might have been removed during dialysis. The droplet sizes of DOMP-5-7 and DOMP-9-7 were significantly larger than their corresponding samples and considerably larger than those of DOMP-native and DOMP-N-7. The pH values of DOMP-5-7 and DOMP-9-7 changed back to 7.0 after dialysis; however, the structures of DOMP did not change back to the original form. The stereochemical reactions of the polysaccharide chain can occur unhindered in the absence of extra H+ and OH−. Hence, DOMP-5-7 and DOMP-9-7 flocculated easily and coalesced together, resulting in larger molecules. Table 2(b) shows the zeta-potential of different concentrations of KGM and DOMP solutions. Zeta-potential is generally used as an indicator of the stability of emulsions. An emulsion is considered stable if the absolute value of zeta-potential is > 30; in contrast, the hydrocolloid shows unstable behaviour if the absolute value is < 30 (Williams & Phillips, 2009). The zeta-potential of KGM solutions ranged from −8.27 mV to 2.5 mV, whereas those of the DOMP solutions ranged from −53.1 mV to −73.1 mV. Therefore, the KGM solutions were considered unstable, whereas, the DOMP-native solutions were stable. After pH treatment, the zeta-potential of DOMP-5 decreased significantly, whereas that of DOMP-9 reduced slightly. Nakauma (2008) studied the stability of hydrocolloids after pH treatment, and observed that pH change may alter zeta-potential without affecting stability.

Several factors may affect the zeta-potential under different pH conditions, such as amino acid composition and their isoelectric points, polysaccharide solubility and conformation, and molecular degradation under extreme conditions (Wu et al., 2015). Although the zeta-potential value after acidic and alkaline treatments was not be able to indicate the stability of emulsions, the results could show the ionic changes which may imply the structures change of DOMP. Extreme conditions may result in conformational changes or depolymerisation of the polysaccharide chains. The zeta-potential values of DOMP-N-7, DOMP5-7, and DOMP-9-7 were all over | ± 30|, which indicated that the properties of DOMP-5 changed back to the original values after dialysis. 3.4.2. Emulsification properties of DOMP with MCT Table 3(a) shows the droplet sizes (z-average, μm) and PDI of emulsions stabilised by KGM, DOMP-native, pH-treated DOMP samples (DOMP-5 and DOMP-9), and DOMP neutralised after pH treatment (DOMP-N-7, DOMP-5–7, and DOMP-9–7) with MCT. Compared to the emulsion of KGM and MCT, the emulsion made using DOMP-native and MCT showed smaller droplet diameters, indicating that DOMP had better emulsification properties at lower concentrations. After pH treatment, the droplet sizes of both emulsions made using DOMP-5 and DOMP-9 with MCT were reduced significantly. Owing to changes in the polysaccharide chains of DOMP samples, such as the disengagement of spiral structure, the emulsifying properties of DOMP-5 and DOMP-9 were altered, because of which they were difficult to coagulate. After dialysing for 36 h, the droplet size of the emulsion made using DOMP-N-7 and MCT was similar to that observed at low concentration, but was significantly smaller than that of DOMP-native with MCT at higher concentration. The results suggested that 36 h dialysis dispersed the molecules, and some molecules were dialysed out through the membrane. The stability of the emulsions was affected by the concentration and presence of ionic surfactants (Fun, Simon, & Sjöblom, 2010). The emulsions made using DOMP-5-7/DOMP-9-7 with MCT contained relatively larger droplet sizes, which indicated that dialysis recovered the emulsification properties of DOMP-N-5/DOMP-9-7 and MCT. Changes in ion (H+ and OH−) concentration may also be responsible for this phenomenon, which rendered aggregation of large droplets difficult. Knowledge regarding the stability of dispersed systems or emulsions is a prerequisite for understanding the mechanisms functional at the interface (Lagevin & Monroy, 2010). Therefore, the zeta-potential, an 5

Food Chemistry 311 (2020) 126039

F. Ma, et al.

Table 3 Droplet diameter (μm) and zeta-potential (mV) of emulsions made from different concentrations of Dioscorea opposita mucilage polysaccharides (DOMP) and medium-chain triglyceride (MCT). (a) Droplet diameter (μm) and polydispersity index (PDI) of emulsions made from different concentrations of DOMP and MCT Droplet diameters (z-average in μm ± standard deviation with mean PDI in parentheses) Concentrations (% w/v) 0.2% MCT KGM + MCT DOMP-native + MCT DOMP-5 + MCT DOMP-9 + MCT DOMP-N-7 + MCT DOMP-5-7 + MCT DOMP-9-7 + MCT

6.01 2.50 2.12 1.61 2.21 1.44 2.55 1.77

0.4% ± ± ± ± ± ± ± ±

0.06 (0.52) 0.04a (0.26) 0.10d (0.20) 0.06 (0.02) 0.09d (0.51) 0.23 (0.41) 0.41adk (0.32) 0.15 (0.36)

6.31 6.86 2.67 1.22 1.60 2.16 1.62 3.56

0.6% ± ± ± ± ± ± ± ±

0.10 (0.21) 0.49 (0.32) 0.14 (0.37) 0.01f (0.46) 0.13 h (0.04) 0.34j (0.21) 0.56fk (0.37) 0.12 (0.47)

6.16 7.38 4.87 1.28 1.54 2.12 2.71 7.05

0.8% ± ± ± ± ± ± ± ±

0.99 (0.47) 1.38b (0.43) 0.07 (0.10) 0.04f (0.24) 0.13hi (0.04) 0.15j (0.28) 0.14 (0.40) 0.09b (0.20)

(b) Zeta-potential (mV) of emulsions made from different concentrations of DOMP and MCT MCT −18.6 ± 2.14 −39.4 ± 1.9 −41.8 ± 2.2 KGM + MCT −24.0 ± 2.8 −20.8 ± 4.0 −17.8 ± 3.8 DOMP-native + MCT −44.6 ± 1.8 −46.9 ± 2.1 −55.4 ± 0.5 DOMP-5 + MCT −27.5 ± 0.3 −29.7 ± 1.6 −27.3 ± 3.0 DOMP-9 + MCT −59.1 ± 7.5 −51.7 ± 5.4 −56.2 ± 1.9 DOMP-N-7 + MCT −41.0 ± 3.9 −38.1 ± 4.8 −35.3 ± 3.3 DOMP-5-7 + MCT −64.4 ± 7.1 −54.3 ± 6.1 −49.3 ± 7.7 DOMP-9-7 + MCT −48.9 ± 5.9 −53.5 ± 2.5 −59.8 ± 3.5

6.30 7.93 7.15 0.95 0.94 5.14 7.43 5.84

1.0% ± ± ± ± ± ± ± ±

0.05 (0.22) 3.45 (0.33) 0.13 (0.30) 0.01 g (0.32) 0.06 g (0.02) 0.43 (0.26) 018 (0.11) 0.23 (0.27)

−36.6 ± 1.9 −9.2 ± 1.8 −58.4 ± 0.4 −14.8 ± 1.9 −49.4 ± 5.4 −41.2 ± 7.6 −72.0 ± 1.8 −46.9 ± 1.9

5.65 ± 0.02 (0.52) 10.19 ± 0.73c (0.51) 9.03 ± 0.10ce (0.36) 0.94 ± 0.06 g (0.35) 1.44 ± 0.03i (0.24) 8.92 ± 0.04e (0.25) 16.48 ± 3.36 l (0.11) 16.79 ± 0.37 l (0.21) −47.4 ± 1.2 −3.2 ± 1.5 −59.0 ± 1.4 −20.0 ± 1.6 −48.4 ± 0.5 −47.1 ± 3.8 −52.7 ± 2.7 −55.5 ± 1.7

Note: Data are reported as mean of six replicates; results are presented as mean ± standard deviation; paired values with superscript letters a through l indicate NOT significant difference (P > 0.05).

References

indicator of the emulsion stability, was investigated and the results are shown in Table 3(b). The zeta-potential of 0.2% w/v MCT alone was approximately −18.6 mV, which indicated instability. However, in the concentration range of 0.4–1.0%, MCT alone showed higher zeta-potential, which did not indicate the stability of the solutions, but reflected the ion charges of MCT. The zeta-potential values of the emulsions made using different concentrations of KGM and MCT were in the range of −24.0 mV to −3.2 mV, which was less than | ± 30|, indicating instability. The zeta-potential values of the DOMP-5 emulsions were in the range of −29.7 mV to −14.8 mV, suggesting that the emulsions were less stable. The rest of the emulsions made using DOMP-native, DOMP-9, DOMP-N-7, DOMP-5-7, and DOMP-9-7 with MCT showed relatively higher absolute values. However, according to Nakauma (2008), pH may alter zeta-potential without affecting stability. Thus, zeta-potential is supplementary for determining the stability of the emulsion.

Andrade, L. A., Nunes, C. A., & Pereira, J. (2015). Relationship between the chemical components of taro rhizome mucilage and its emulsifying property. Food Chemistry, 178, 331–338. Avila-de la Rosa, G., Alvarez-Ramirez, J., Vernon-Carter, E. J., Carrillo-Navas, H., & Pérez-Alonso, C. (2015). Viscoelasticity of chia (Salvia hispanica L.) seed mucilage dispersion in the vicinity of an oil-water interface. Food Hydrocolloids, 49, 200–207. Bradford, M. M. (1976). A refined and sensitive method for the quantification of microgram quantities of protein utilizing the principle of protein-dye binding. Analytical Biochemistry, 72, 248–254. Chan, Y. S., & Ng, T. B. (2013). A lectin with highly potent inhibitory activity toward breast cancer cells from edible tubers of Dioscorea opposita cv. Nagaimo. PloS One, 8, e54212. Choi, E. M., Koo, S. J., & Hwang, J. K. (2004). Immune cell stimulating activity of mucopolysaccharide isolated from yam (Dioscorea batatas). Journal of Ethnopharmacology, 91, 1–6. Claybaugh, T., et al. (2014). L-arginine supplementation in type II diabetic rats preserves renal function and improves insulin sensitivity by altering the nitric oxide pathway. International Journal of Endocrinology, 2014, 171546. Dimopoulou, M., Ritzoulis, C., & Panayiiotou, C. (2015). Surface characterization of okra hydrocolloid extract by inverse gas chromatography (IGC). Colloids and Surfaces A: Physicochemical and Engineering Aspects, 475, 37–43. Dubois, M., Gilles, K. A., Hamilton, J. K., Reders, P. A., & Smith, F. (1956). Colorimetric method for determination of sugars and related substances. Analytical Chemistry, 28, 350–356. Fun, Y., Simon, S., & Sjöblom, J. (2010). Interfacial shear rheology of asphaltenes at oilwater interface and its relation to emulsion stability: Influence of concentration, solvent aromaticity and nonionic surfactant. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 366, 120–128. Harding, S. E., Smith, I. H., Lawson, C. J., Gahler, R. J., & Wood, S. (2011). Studies on macromolecular interactions in ternary mixtures of konjac glucomannan, xanthan gum and sodium alginate. Carbohydrate Polymers, 83, 329–338. Hariprakash, S. C., & Nambiasn, B. (1996). Carbohydrate metabolism during dormancy and sprouting in yam (Dioscorea) tubers: Changes in carbohydrate constituents in yam (Dioscorea) tubers during dormancy and sprouting. Journal of Agricultural and Food Chemistry, 44, 3066–3069. Horne, D.S. (1995). Light scattering studies of colloid stability and gelation. In E., Dickinson (Ed). New physico-chemical techniques for the characterization of complex food system (pp. 240–267). Cambridge: Blackie Academic & Professional. Karimi, N., & Moharmmadifor, M. A. (2014). Role of water soluble and water swellable fractions of gum tragacanth on stability and characteristic of model oil in water emulsion. Food Hydrocolloids, 24, 42–49. Kiho, T., Hara, C., & Ukai, S. (1985). A glucomannan from the tubers of Dioscorea japonica Thunb. Chemical & Pharmaceutical Bulletin, 33(1), 270–275. Kong, L., Yu, L., Feng, T., Yin, X., Liu, T., & Dong, L. (2015). Physicochemical characterization of the polysaccharide from Bletilla striata: Effect of drying method. Carbohydrate Polymers, 125, 1–8. Lagevin, D., & Monroy, F. (2010). Interfacial rheology of polyeletrolytes and polymer monolayers at the air-water interface. Current Opinion on Colloid Interface Science, 15, 283–293.

4. Conclusion In this study, we investigated the physical properties of DOMP, including its composition, morphology, and rheological and emulsification properties. The results suggested that DOMP possesses pseudoplastic characteristics and showed relatively better emulsification properties than KGM. DOMP can be considered a natural food processing agent for improving the sensory attributes and taste of food. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 31701553 and 31800288), China Postdoctoral Science Foundation (No. 2018T110731 and 2018M632787), Foundation of Educational Department of Henan Province (No. 19B350001). 6

Food Chemistry 311 (2020) 126039

F. Ma, et al.

set yoghurt. International Journal of Farming and Allied Sciences, 2, 864–865. Ohtani, K., & Murakami, K. (1991). Structure of mannan fractionated from water-soluble mucilage of nagaimo (Dioscorea batatas dence). Agricultural and Biological Chemistry, 55, 2413–2414. Wang, Q., Zhao, X., Pu, J., & Luan, X. (2016). Influences of acidic reaction and hydrolytic conditions on monosaccharide composition analysis of acidic, neutral and basic polysaccharides. Carbohydrate Polymers, 143, 296–300. Waqas, M., Khan, A. L., Hamayun, M., Shahzad, R., Kim, Y. H., & Choi, I. J. (2015). Endophytic infection alleviates biotic stress in sunflower through regulation of defence hormones, antioxidants and functional amino acids. European Journal of Plant Pathology, 141, 803–824. Williams, P. A., & Phillips, G. O. (2009). Handbook of Hydrocolloids (2nd ed.). Cambridge: Woodhead Publishing Limited. Wu, Y., Eskin, N. A. M., Cui, W., & Pokharel, B. (2015). Emulsifying properties of water soluble yellow mustard mucilage: A comparative study with gum arabic and citrus pectin. Food Hydrocolloids, 47, 191–196. Yang, D., Lu, T., & Hwang, L. S. (2003). Isolation and identification of steroidal saponins in Taiwanese yam cultivar (Dioscorea pseudojaponica Yamamoto). Journal of Agricultural and Food Chemistry, 51, 6438–6444. Zhang, Z., Wang, X., Liu, C., & Li, J. (2016). The degradation, antioxidant and antimutagenic activity of the mucilage polysaccharide from Dioscorea opposita. Carbohydrate Polymers, 150, 227–231.

Lu, M., et al. (2015). Controlled release of anthocyanins from oxidized konjac glucomannan microspheres stabilized by chitosan oligosaccharides. Food Hydrocolloids, 51, 476–485. Lucey, J. A. (2002). Formation and physical properties of milk protein gels. Journal of Dairy Science, 85, 281–294. Ma, F., et al. (2017b). Chemical components and emulsification properties of mucilage from Dioscorea opposita Thunb. Food Chemistry, 228, 315–322. Ma, F., et al. (2017a). Rheological properties of polysaccharides from Dioscorea opposita Thunb. Food Chemistry, 227, 64–72. Ma, F., et al. (2018). Characterisation of the mucilage polysaccharides from Dioscorea opposita Thunb. with enzymatic hydrolysis. Food Chemistry, 245, 13–21. McClements, D. J. (2005). Food emulsions: Principles, practices, and techniques (2nd ed.). USA: CRC Press. Misaki, A., Ito, T., & Harada, T. (1972). Constitutional studies on the mucilage of “yamanoimo”, Dioscorea batatas Decne, forma Tsukune: Isolation and structure of a mannan. Agricultural Biology and Chemistry, 36, 761–771. Nakauma, M., et al. (2008). Comparison of sugar beet pectin, soybean soluble polysaccharide, and gum arabic as food emulsifier. 1. Effect of concentration, pH, and salts on the emulsifying properties. Food Hydrocolloids, 22, 1254–1267. Nep, E. I., & Conway, B. R. (2010). Characterization of grewia gum, a potential pharmaceutical excipient. Journal of Excipients and Food Chemicals, 1, 30–40. Nikoofar, E., Hojjatoleslami, M., & Shariaty, M. A. (2013). Surveying the effect of quince seed mucilage as a fat replacer on texture and physicochemical properties of semi fat

7