- Email: [email protected]
Comparative study between cold and hot water extracted polysaccharides from Plantago ovata seed husk by using rheological methods
Food Hydrocolloids 101 (2020) 105465
Contents lists available at ScienceDirect
Food Hydrocolloids journal homepage: http://www.elsevier.com/locate/foodhyd
Comparative study between cold and hot water extracted polysaccharides from Plantago ovata seed husk by using rheological methods Peiyuan Zhou a, b, Mohamed Eid a, b, Wenfei Xiong a, b, Cong Ren a, b, Tingyang Ai a, b, Ziyu Deng a, b, Jing Li a, b, Bin Li a, b, * a b
College of Food Science and Technology, Huazhong Agricultural University, Wuhan, 430070, Hubei, China Key Laboratory of Environment Correlative Dietology (Huazhong Agricultural University), Ministry of Education, Wuhan, 430070, Hubei, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Plantago ovata Polysaccharide Rheological properties Viscoelastic Sol-gel transition Critical concentration
Cold and hot water extracted polysaccharides (CP and HP) were isolated from Plantago ovata seed husk at 25 and 85 � C, respectively. Their rheological properties were systematically investigated and compared. Different con centrations and temperatures were studied to evaluate the viscoelastic properties of the two fractions. It was found that CP solutions behaved as viscous fluids, while HP solutions preferred to exhibit as gel-like structure. The sol-gel transition process of HP was evaluated and the gel point was determined to be 4.8 mg/mL by WinterChambon method in water at 25 � C. The steady flow sweep investigated the viscosity properties of CP solutions and two critical concentrations were found, which locating the transition from the dilute to semi-dilute region, and from the semi-dilute to concentrated region. In addition, the two fractions could form thermo-reversible weak gels, and the gelation temperatures of them increased with the increase of concentration. It is evident that both polysaccharides have significant potential for application in food industry.
1. Introduction Plantago ovata (Psyllium) seed husk has a long history of utilization as medicine and pharmacological supplements due to its rich content of bioactive polysaccharides (Hussain, Muhammad, Jantan, & Bukhari, 2016; Madgulkar, Rao, & Warrier, 2015). For the past few years, psyl lium has attracted an increasing amount of interest caused of its swelling and gelling properties (Hussain et al., 2016). Natural and modified psyllium polysaccharides were investigated as pharmaceutical excipi ents in controlled drug delivery systems (Bhatia & Ahuja, 2015; Iqbal, Akbar, Hussain, Saghir, & Sher, 2011; Pawar & Varkhade, 2014). In addition, psyllium polysaccharides were widely used as stabilizers and thickeners in food industry, such as ice creams (Dhar, Kaul, Sareen, & Koul, 2005), yogurt (Ladjevardi, Gharibzahedi, & Mousavi, 2015), and biodegradable edible films (Ahmadi, Kalbasi-Ashtari, Oromiehie, Yar mand, & Jahandideh, 2012). These increasing applications above are highly dependent on rheological properties of psyllium. Thus, expand ing the database on rheological properties of psyllium polysaccharides is critical to broaden their application range. According to previous studies, the separated polysaccharides from psyllium displayed markedly different rheological properties (Guo, Cui,
Wang, & Young, 2008; Yu et al., 2017). Three main polysaccharide fractions were extracted from psyllium: cold water fraction (CWF), hot water fraction (HWF) and alkaline fraction (AF) (Fischer et al., 2004; Guo et al., 2008; Laidlaw & Percival, 1949, 1950; Marlett & Fischer, 2003; Yu et al., 2017). Their structures are identified and compared, and many literatures illustrated the variation in rheological properties due to the structure variance. (Guo et al., 2008; Marlett & Fischer, 2003; Yu et al., 2017). It has been found that these fractions are all highly branched arabinoxylans, which composed of a xylan backbone with a variety of side chains attached at O-3 and/or O-2 positions (Yu et al., 2017). The side chains include arabinose, xylose, and oligosaccharides consisting of arabinose and/or xylose as well as rhamnose with terminal galacturonic acid (Edwards, Chaplin, Blackwood, & Dettmar, 2003; Kennedy, Sandhu, & Southgate, 1979; Sandhu, Hudson, & Kennedy, 1981; Yu et al., 2017). Although the three fractions have a similar main structure, remarkably CWF has more complex linkages and large amounts of rhamnose and uronic acids (Guo et al., 2008; Yu et al., 2017). Furthermore, its side chains are long and irregular, while the HWF and AF have short and regular side chains (Guo et al., 2008). These different structural features could be the reason for the higher solubility of CWF and better gelling properties of HWF and AF (Guo et al., 2008; Yu et al.,
* Corresponding author. College of Food Science and Technology, Huazhong Agricultural University, Wuhan, 430070, Hubei, China. E-mail address: [email protected] (B. Li). https://doi.org/10.1016/j.foodhyd.2019.105465 Received 1 August 2019; Received in revised form 18 October 2019; Accepted 24 October 2019 Available online 31 October 2019 0268-005X/© 2019 Elsevier Ltd. All rights reserved.
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
exhibited gel-like behaviors. But the study of rheological properties was insufficient to give exhaustive information on each fraction. The rheological properties of psyllium polysaccharides depend on many factors such as concentration, temperature, ionic strength, pH etc. (Farahnaky et al., 2010; Guo et al., 2009; Haque et al., 1993). Among these factors, concentration and temperature are the primary ones. It has been found that the gel-forming fractions can form a weak gel at room temperature, and the strength of gel increased with the increase of concentration (Farahnaky et al., 2010; Haque et al., 1993). However, the sol-gel transition process and the critical gelation concentration were hardly studied. Furthermore, the gel-forming fractions were determined to form physical gels which exhibited no thermal hysteresis during heating and cooling procedure (Farahnaky et al., 2010; Guo et al., 2009; Haque et al., 1993; Yu et al., 2017). Moreover, the different gel-forming fractions have different sensitivities to temperature, the melting tem perature point (Tm) of 1% HW fraction was around 41 � C, whereas the Tm of 1% alkaline fraction was more than 85 � C (Yu et al., 2017). The gelation and melting process of different fractions under different con centrations need to be studied systematically. On the other hand, the study on rheological properties of cold water fraction is limited. But it is an indispensable part of a comprehensive understanding of psyllium polysaccharides. In the present work, we focused on the cold (25 � C) and hot (85 � C) water extracted polysaccharides (CP and HP) from psyllium due to their high content and convenient extraction method. The objective of this work is to investigate the rheological properties of CP and HP and their dependency to concentration and temperature. Furthermore, the solu tion behavior of CP and the sol-gel transition process of HP in water were evaluated for the first time. We also compared the structures, molecular weights and uronic acid content of the two fractions, to get a further explanation for the difference in their rheological properties. These works are useful for the design of products based on psyllium poly saccharides, since insight in how the different concentrations and tem peratures affected the rheological properties, which can help to predict the stability and the final textural of formulated food.
Table 1 Parameters of molecular weight and uronic acid content of CP and HP. Mwa (g/moL) CP HP a b c
5
6.3 � 10 2.1 � 106
Mnb (g/moL) 5
2.4 � 10 5.9 � 105
Rg(z)c (nm)
Uronic acid (% w/w)
20 33
18.50% 3.50%
Mw: Molecular weight was measured by SEC-MALLS. Mn: Number average molecular weight was measured by SEC-MALLS. Rg(z): z-average radius of gyration was measured by SEC-MALLS.
Fig. 1. FT-IR spectra of CP and HP.
2017). However, most of the rheological properties analysis in literatures were based on the gel-forming properties of crude water extract or alkaline extract fractions (Farahnaky, Askari, Majzoobi, & Mesbahi, 2010; Guo, Cui, Wang, Goff, & Smith, 2009; Haque, Richardson, Morris, & Dea, 1993). But these studies could not provide accurate information on the rheological properties of different psyllium polysaccharides based on changed extraction method. Therefore, a systematical rheo logical study for each polysaccharide is required. Recently, Yu et al. (2017) found that cold water (25 � C) fraction displayed a viscoelastic fluid response, while hot water (65 � C) fraction and alkaline fraction
2. Materials and methods 2.1. Materials Psyllium (Plantago ovata) husk powder (98%) was obtained from Abhyuday Industries (India). Dextran standard (40 kDa) was from Aladdin. All other chemicals and reagents were from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and of analytical grade. All aqueous solutions were prepared with reverse osmosis treated deionized
Fig. 2. Strain sweep of CP (A) and HP (B) at different concentrations (25 � C). 2
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
Fig. 3. Dynamic rheological behaviors of the two fractions versus frequency at different concentrations (25 � C): viscoelastic moduli (G0 & G00 ) of CP (A) and HP (B), loss factor (tanδ) of CP (C) and HP (D), and complex viscosity (η*) of CP (E) and HP (F).
water (DI water).
anhydrous ethanol and dried in a vacuum oven to obtain crude cold water extracted polysaccharide. After that the crude polysaccharide was redissolved in DI water and deproteinized according to Sevage method (Sevage, 1934). The resulting solution was subsequently dialysed at room temperature for 72 h against DI water and freeze dried to obtain purified cold water extracted polysaccharide (CP). The remaining solid phase was washed twice with DI water at 25 � C, then dispersed in 4000 mL of DI water at 85 � C for 2 h under constant stirring. The sus pension was filtered and centrifuged at 10,000 g for 30 min. The resulting solution was concentrated, precipitated by ethanol, deprotei nized, dialysed and freeze dried in a similar manner as CP to obtain hot
2.2. Extraction of polysaccharides The polysaccharides were extracted by sequential procedure ac cording to the work (Yu et al., 2017) with several modifications. Briefly, psyllium powder (10 g) were dispersed in DI water (2000 mL) at 25 � C for 4 h under constant stirring, then centrifuged at 10,000 g for 30 min. The supernatant was concentrated in a rotary evaporator under reduced pressure at 55 � C and then precipitated by ethanol at a final concen tration of 80% for 12 h. The precipitate was washed twice with 3
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
Scientific™ OMNIC™ (OMNIC™ Series Software, Madison, WI, USA) program.
Table 2 Power law parameters of storage (G0 ) and loss (G00 ) moduli for CP and HP. G0 ¼ k1ωn1
Concentration (w/v) CP 0.1–1 rad s 1 1.5% 2% 3% 4% 1–10 rad s 1 1.5% 2% 3% 4% 10–100 rad s 1 1.5% 2% 3% 4% HP 0.1–10 rad s 1 0.3% 0.4% 0.5% 0.6% 0.8% 1% 1.5%
G00 ¼ k2ωn2 k2
n2
R2
0.9988 0.9999 0.9995 0.9990
0.54 1.07 5.15 14.96
0.94 0.88 0.75 0.65
0.9992 0.9993 0.9987 0.9980
1.05 0.96 0.76 0.64
0.9992 0.9988 0.9984 0.9983
0.58 1.15 5.47 15.66
0.77 0.69 0.54 0.46
0.9993 0.9987 0.9981 0.9979
0.47 0.66 4.86 16.69
0.59 0.71 0.54 0.46
0.9689 0.9990 0.9980 0.9981
0.89 1.77 7.83 21.02
0.61 0.52 0.40 0.34
0.9993 0.9990 0.9989 0.9990
0.07 0.45 1.23 2.76 9.63 20.16 36.06
0.38 0.22 0.19 0.18 0.17 0.16 0.18
0.9980 0.9978 0.9971 0.9923 0.9862 0.9770 0.9904
0.04 0.16 0.35 0.74 2.19 3.87 8.46
0.43 0.23 0.14 0.12 0.10 0.09 0.13
0.9962 0.9847 0.9839 0.9944 0.9994 0.9985 0.9994
k1
n1
R
0.11 0.30 2.49 9.87
1.31 1.21 0.99 0.85
0.12 0.34 2.77 10.64
2
2.5. Molecular weight measurement The molecular weights of the two fractions were determined by a size exclusion chromatography system, which equipped with a multi-angle laser light scattering (wavelength of 663.6 nm, eighteen angles) (Dawn Heleos II, Wyatt, USA), a refractive index detector (OPTILAB TrEX, Wyatt, USA), a OHpak SB-G guard column (6 mm � 50 mm) and a OHpak SB-805 HQ column (8 mm � 300 mm) (Shodex, Tokyo, Japan). The two fractions were dissolved (0.2 mg/mL) in 0.1 M NaNO3 with 0.02% (w/w) NaN3 under overnight stirring at room temperature and then filtered through 0.45 μm membrane before injection. The system was eluted with 0.1 M NaNO3 solution containing 0.02% (w/w) NaN3 and a flow rate of 0.4 mL/min. The column temperature was 50 � C. The sample solution was injected through a 1000 μL full loop. Data was collected and analyzed using the ASTRA (Version 6.1.1.84) software, normalized with dextran narrow standard (Mw ¼ 40,000). A refractive index increment (dn/dc) of 0.146 was used in the calculations. 2.6. Rheological measurements The rheological tests of CP and HP were performed on a DHR-2 rheometer (TA Instruments, USA) at 25 � C with a Peltier temperature control unit on the bottom plate. After loading the sample, the edge of the geometry was carefully trimmed and filled with low viscosity sili cone oil to prevent water evaporation. All samples were equilibrated for 5 min before testing. 2.6.1. Small amplitude oscillatory shear (SAOS) measurements 2.6.1.1. Sample preparation. CP was dissolved in DI water with different concentrations (1.5, 2, 3, 4% w/v) at 25 � C, while HP was dissolved in DI water with a various concentration (0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.5% w/ v) at 85 � C for 2 h. All samples were placed at room temperature over night to complete the hydration. 2.6.1.2. Strain and frequency sweep. A parallel plate geometry (60 mm diameter, 0.5 mm gap) was used for CP and 0.3% (w/v) HP, while HP (0.4–1.5% w/v) were tested by a 40 mm parallel plate (0.5 mm gap). Dynamic strain sweep measurements were carried out at 1 Hz to determine the linear viscoelastic regime (LVR) with a strain range from 0.1 to 1000% at 25 � C. Afterwards, dynamic frequency sweep ranging from 0.1 to 100 rad s 1 were measured at 25 � C in a constant strain of 10%.
Fig. 4. Loss tangent tanδ as a function of polysaccharide concentration c for HP in water at 25 � C and various angular frequencies (1–10 rad s 1). Cgel is the gel point.
2.6.1.3. Temperature sweep. The temperature sweep measurements were performed at the constant strain of 10%, which was well within the linear viscoelastic region, while the frequency was fixed at 1 Hz. The sample was measured by linear increase in temperature from 5 to 50 � C for CP and 5–80 � C for HP, then linearly cooling down to 5 � C. The heating and cooling steps were performed at rate of 1.5 and 2 � C/min for CP and HP, respectively. A parallel plate geometry (40 mm diameter, 0.5 mm gap) was used for the measurement.
water extracted polysaccharide (HP). 2.3. Uronic acids analysis Total uronic acid content of the two fractions was estimated by and sulfuric acid-carbazole spectrophotometric method using galacturonic acid as standard (Taylor, 1993).
2.6.2. Steady shear flow In the steady shear test, a series of CP solutions (0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.45, 0.5, 0.6, 0.75, 1, 1.25, 1.5, 2, 2.5% w/v) in DI water were prepared. A cone and plate geometry (1-degree angle) with 60 mm diameter and 0.5 mm gap was used. The measurements were performed at shear rates from 0.01 to 1000 s 1 at 25 � C. The viscosity data were fitted with the Cross model (Eq. (1)). For pseudoplastic fluid, the infinite viscosity tends to zero when the shear rate is infinite.
2.4. Fourier transform infrared (FT-IR) spectroscopy FT-IR spectra were acquired on a Nicolet iS50 FT-IR spectrometer (Thermo Scientific, USA) with a GoldenGate diamond attenuated total reflectance (ATR) accessory at room temperature (referenced against air). A region from 400 to 4000 cm 1 was used for scanning at 4 cm 1 resolution using 64 scans. Spectra were analyzed by the Thermo 4
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
Fig. 5. Temperature sweep results and loss tangent tanδ of CP: the gel melting process (A and C) and gelation process (B and D).
η ¼ η∞ þ
η0
η∞
1 þ ðτγ_ Þm
characterized by SEC-MALLS analysis. As shown in Table 1, the massaverage molecular mass (Mw) and number-average molecular mass (Mn) of CP were 6.3 � 105 g/mol and 2.4 � 105 g/mol respectively, while 2.1 � 106 g/mol and 5.9 � 105 g/mol for HP. The z-average radius of gyration, Rg(z), was 20 nm for CP and 33 nm for HP. However, our results were different to those of the similar polysaccharides which dissolved in 0.04 M KOH at 50 � C (Yu et al., 2017). They found that the molecular weight of the cold water (25 � C) fraction was higher than those of the hot water (60 � C) fraction and 0.2 M KOH fraction, which is contrary to our results. Therefore, these discrepancies can not only be explained easily by different sources or methods, but also the different treatment in extracting and purifying process. Nevertheless, the Mw of HP was similar to other water-soluble polysaccharides extracted from Plantaginaceae such as Ispaghula husk (2.2 � 106 g/mol) (Edwards et al., 2003), Plantago major seeds (1.2 � 106 g/mol) (Behbahani et al., 2017), Plantago asiatica seeds (1.8 � 106 g/mol) (J. Yin et al., 2012) and Plantago notata Lagasca seeds (2.3 � 106 g/mol) (Benaoun et al., 2017).
(1)
η is the apparent viscosity, η∞ is the infinite-rate viscosity, η0 is the zerorate viscosity, τ is the time constant and m is the dimensionless exponent that captures the strength of shear thinning effect (Yu, Yakubov, Mar tínez-Sanz, Gilbert, & Stokes, 2018). 3. Results and discussion 3.1. Determination of uronic acid contents In early research, the uronic acid content (18%) of cold water (25 � C) extracted polysaccharide was much higher than the content (3%) of hot water (90–95 � C) extracted polysaccharide from the residue (Laidlaw & Percival, 1949, 1950). Similar results were found in our work. CP had a higher uronic acid content (18.5%), which was about five times that of HP (3.5%) (Table 1). This indicated that CP was more like an anionic polysaccharide, while HP was a neutral polysaccharide. This result explained that water-extracted psyllium polysaccharide was a mixture of a polyuronide and a neutral arabinoxylan rather than a single species of polysaccharide (Fischer et al., 2004; Kennedy et al., 1979). Further more, the abundance (18%) of negatively charged GalAp residues in CP is a plausible explanation for its higher solubility compared to HP in water at 25 � C (Yu et al., 2017).
3.3. FT-IR spectroscopy The FT-IR spectra of the two fractions were displayed in Fig. 1. It was observed that the two fractions had similar typical bands and peaks of polysaccharides. Both of them displayed a strong and broad absorption peak at around 3360 cm 1 corresponding to the hydroxyl (-OH) stretching vibration as well as water adsorption (Adel, El-Wahab, Ibra him, & Al-Shemy, 2010). The weak signal obtained at 2921 cm 1 was attributed to the asymmetric vibration of C–H group (Benaoun et al., 2017). According to reports, absorption bands around 1600 cm 1 and
3.2. Molecular weight The molecular weights of both fractions in DI water were 5
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
Fig. 6. Temperature sweep results and loss tangent tanδ of HP: the gel melting process (A and C) and gelation process (B and D).
Fig. 7. Steady flow behaviors at different concentrations (A) and log-log plot of the zero-shear specific viscosity (ηsp,0) versus the space occupancy c [η] (B) for CP in water at 25 � C.
1400 cm 1 (1607 and 1412 cm 1 for CP, 1632 and 1420 cm 1 for HP) were typical of the asymmetrical and symmetrical stretching vibration of carboxylate groups (–COO–) from acid residues (Behbahani et al., 2017; Petera et al., 2015), while the absorption band at 1730 cm 1 indicated the presence of uronic acid. Furthermore, CP had a stronger absorption at 1730 cm 1 than HP due to its higher uronic acid content, corresponding to the previous results of uronic acid content analysis.
Moreover, the peak at 1370 cm 1 could correspond to ester carbonyl – O) of the carboxylic function of d-GalA (Petera et al., 2015). groups (C– The signal peak observed at 1036 cm 1 resulted from the stretching vibration of the pyranose ring (J. Y. Yin, Nie, Zhou, Wan, & Xie, 2010). Finally, the characteristic absorption at 896 cm 1 suggested the β configuration of the main sugar units of both CP and HP (Benaoun et al., 2017). 6
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
referred to as the frequency exponents and can be used to describe the viscoelastic characteristics of polysaccharide solutions (Razmkhah, Razavi, & Mohammadifar, 2017). The values of k1, k2, n1, n2 and R2 were summarized in Table 2. For CP solutions, the values of both n1 and n2 decreased with the increment of concentration during different frequency ranges (Table 2). At low frequencies (0.1–1 rad s 1) with lower concentrations (1.5–2% w/v), the dependence of viscoelastic moduli for CP was close to that of dilute solution (n1 ¼ 2, n2 ¼ 1) (Wang & Cui, 2005), and smaller values of n1 and n2 were obtained at higher frequencies and concentrations, indicating the formation of viscous gels for CP (Wu et al., 2018). It was reported that the strength and nature of entangled weak gels could be estimated by n1, of which low values (near zero) are assigned to elastic gels while viscous gels close to 1 (Wu et al., 2018). According to low values of n1 and n2 for HP solutions, they exhibited slight frequency dependency, suggesting that HP solutions tend to form elastic gels. The gel strength increased with the increase of concentration, as observed from the decrease in the “n1” value, which was in consistency with the result from Fig. 3B. The values of k1 were much higher than those of k2, which also confirming the weak gel-like structure of HP (Razavi, Cui, & Ding, 2016; Razmkhah et al., 2017; Shi, Wang, Li, & Adhikari, 2013). In general, tanδ <1 suggests predominantly elastic behavior while tanδ >1 indicates primarily viscous behavior (Xu, Zhang, Liu, Sun, & Wang, 2016). It was also reported that polymer systems can be classified into three by tanδ values: high values for dilute solutions, 0.2–0.3 for amorphous polymers and low values (close to 0.01) for gels or glassy crystalline polymers (Wu et al., 2018). Fig. 3C showed the change of tanδ versus frequency under different concentrations of CP. As can be observed, tanδ decreased with the increment of concentration and fre quency. At low frequency range (0.1–10 rad s 1), viscous behaviors were obtained with the tanδ value was higher than 1. As frequency increased, elastic behaviors were obtained for CP at higher concentra tions (3–4% w/v) with tanδ value was between 0.6 and 1, which inferred the intermediate behavior of CP between a dilute solution and an elastic gel, thus giving rise to weak formation of gel. In contrary, as shown in Fig. 3D, the tanδ values for HP with various concentrations were smaller than 1 and had a weak dependence on frequency, suggesting that HP formed weak gels where the elasticity dominates these systems (Zhang et al., 2007). The tanδ values at low concentrations (0.3–0.4% w/v) were much higher than those at higher concentrations (0.5–1.5% w/v) which were between 0.1 and 0.5, indicating that the high concentration samples may display an intermediate behavior between a weak and an elastic gel. Fig. 3E and F showed that the complex dynamic viscosity (η*) of the two fractions increased as the concentration increasing, and both frac tions displayed a shear dependent flow behavior as η* decreased with increase in frequency. That is, both fractions had non-Newtonian shearthinning behavior, which is common in other polysaccharides (Hesar inejad, Koocheki, & Razavi, 2014; Wu et al., 2018). In addition, compared to the marked linear decrease of η* for HP, the η* of CP initially showed a relatively small reduction and then reduced rapidly at higher frequencies.
Table 3 The fitting parameters of reduced Cross model for CP. Concentration (w/v)
η0 (Pa s)
0.1% 0.15% 0.2% 0.25% 0.3% 0.4% 0.45% 0.5% 0.6% 0.75% 1% 1.25% 1.5% 2% 2.5%
0.0016 0.0022 0.0029 0.0034 0.0041 0.0059 0.0071 0.0092 0.0139 0.0220 0.0530 0.1486 0.3907 1.0600 5.1417
τ (ms) 0.0268 0.0148 0.0161 0.0512 0.0735 0.0816 0.1282 0.1750 0.3413 0.6130 1.3426 3.3691 13.0920 16.3895 171.014
R2
m 0.6731 0.5130 0.4594 0.6210 0.6540 0.5565 0.5779 0.5018 0.4538 0.6467 0.7254 0.7002 0.5509 0.5924 0.4984
0.9809 0.9944 0.9939 0.9978 0.9993 0.9998 0.9996 0.9981 0.9991 0.9954 0.9961 0.9963 0.9977 0.9986 0.9985
3.4. Dynamic oscillatory rheology 3.4.1. Strain sweep measurements Fig. 2 showed that both G0 and G00 of the two fractions initially remained constant during the linear viscoelastic region (LVR), and diminished when critical strain was exceeded. The critical strain reflects the deformability of the mucilage. Obviously, the LVR of HP was larger than that of CP, which inferred HP had a higher resistance to deforma tion. Furthermore, as expected an increase in both G0 and G00 was observed with polysaccharide concentration increasing. As shown in Fig. 2A, CP displayed a viscous response at lower concentrations (1.5–3% w/v) since G00 was higher than G0 . When the concentration reached 4% (w/v), G00 was almost equal to G0 indicating that a weak gel was formed. By contrast, HP displayed an elastic behavior within the tested concentration range (0.3–1.5% w/v) since G0 is always higher than G00 (Fig. 2B). It was very interesting that HP fraction was easily gelled at low concentration (lower than 0.5% w/v, 25 � C), whereas few general polymers showed similar phenomenon. 3.4.2. Frequency sweep measurements The dependences of G0 , G00 , tanδ and η* on frequency under different concentrations at 25 � C were employed to characterize viscoelastic be haviors of both fractions (Fig. 3). For CP solutions (Fig. 3A), viscous behaviors (G0 < G00 ) were obtained at lower concentrations (1.5–2% w/ v) within the test frequency range; while at higher concentrations (3–4% w/v), crossover points of G0 and G00 occurred, appearing a viscous response (G0 < G00 ) below the point and an elastic response (G00 < G0 ) above. Moreover, the crossover point moved to lower frequency with concentration increasing, indicating that molecules were more likely to interact with each other and generate a temporary entangled network at �n, Mun ~ oz, higher concentrations (Nwokocha & Williams, 2016; Rinco �n, & Alfaro, 2014; Zhang, Xu, Xu, & Zhang, 2007). On the Ramírez, Gala other hand, HP displayed a gel-like behavior. Both G0 and G00 showed slightly frequency dependency with G0 exceeding G00 under different concentrations (0.3–1.5% w/v) throughout the accessible range of fre quency (Fig. 3B). This behavior is typical for ‘weak gels’ such as xanthan gum (Edwin R Morris, 1990). Although the dependences of the two fractions on frequency decreased with concentration increasing, HP displayed a less depen dence on frequency than CP, which may be due to the stronger resistance to deformation of its weak gel network. The dependences of G0 and G00 for the two fractions on frequency were demonstrated by the analysis of power law equation ((2) and (3)): G ¼ k1 ωn1
(2)
G00 ¼ k1 ωn1
(3)
0
3.5. Determination of the critical gelation concentration for HP Traditionally, the crossover of G0 and G00 was used as an indicator of the gel point. But it is not valid in our work. It was found that HP so lution exhibited a visible liquid behavior at the low concentration (0.3% w/v) while the G0 was slightly higher than G00 and no crossover occurred. Therefore, a reliable and valid method was required to determine the get point. It was reported that a scaling law of G0 (ω) ~ G00 (ω) ~ ωn (0 < n < 1) or G0 (ω)/G00 (ω) ¼ tanδ ¼ tan (nπ/2) at the gel point was found for all gelling systems (Li & Aoki, 1997). The frequency-independence of loss tangent in the vicinity of the gel point has been widely used to determine the gel point for chemical and physical gels (Zhang et al., 2007). This method was used to determine
Where k1 and k2 are constants, ω is the frequency, n1 and n2 may be 7
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
the sol-gel transition concentration from a frequency plot (1–10 rad s 1) of tanδ vs concentration of HP in water at 25 � C (Fig. 4). It was observed that all curves intersected at a common point corresponding to 4.8 mg/mL, which was defined as the critical gelation concentration Cgel. When the concentration exceeded Cgel, tanδ values were indepen dent of the frequency, suggesting that gel formation occurred.
shear rate became increasingly pronounced. The typical curves of CP solutions at higher concentrations presented shear-thinning behavior since the apparent viscosity dropped with increasing shear rate. How ever, at low shear rate, little or no decrease was observed. It may be explained that there was a dynamic equilibrium between disentangle ments and entanglements at low shear rate, then the formation of new entanglements was impeded by greater shear forces disrupting network at high shear rate (Wu et al., 2018). The flow characteristics of CP were presented in Table 3. The η0 and τ increased with concentration, which indicated that concentration increment resulted in higher consistency and pseudoplasticity for CP in solution. It can be explained that the free movement of individual molecular chains were restricted by increasing chain entanglements at higher concentrations, therefore, the in teractions between molecules are intensified, which enhanced the con sistency and pseudoplastic behaviors of CP (Wu et al., 2018). The double logarithmic plot of the zero-shear specific viscosity (ηsp,0) versus the space occupancy of CP in water at 25 � C was shown in Fig. 7B. Most of the past studies reported that only two regions (the dilute and concentrated solution) were observed for most disordered poly saccharide solutions in this type of plot (E Rw Morris, Cutler, Ross-Murphy, Rees, & Price, 1981) And the fitting curves of the two regions intercepted at a point namely the critical overlap concentration, C*, with the slopes of 1.4 and 3.3 respectively (E Rw Morris et al., 1981). However, in some other research, three domains were reported, corre sponding to the dilute, semi-dilute and concentrated regions (Tao & Feng, 2012), and the three slopes values were within 0.91–1.3, 1.16–2.1, and 2.24–5 respectively (Arvidson, Rinehart, & Gadala-Maria, 2006; Lazaridou, Biliaderis, & Izydorczyk, 2003; Tao et al., 2012; Yu et al., 2018). Similarly, in our results (Fig. 7B), the data were fitted to three linear lines with the slopes of 0.9, 2.1 and 4.8 respectively, and the intrinsic viscosity [η] of CP in water at 25 � C was determined to be 10.2 dL/g (Supplementary Fig. S1). The C* and C** of CP were calcu lated at 3.7 and 9.2 mg/mL, while the dimensionless coil overlap pa rameters, C*[η] and C**[η], were 3.8 and 9.4 respectively. The results were quite consistent with other random coil polysaccharides mentioned before.
3.6. Temperature sweep measurements In addition to concentration, temperature was also proved to have influence on the gelation and gel melting process of polysaccharides from psyllium (Farahnaky et al., 2010; Haque et al., 1993; Yu et al., 2017). Temperature sweep results of the two fractions with different concentrations were shown in Figs. 5 and 6. It was observed that both G0 and G00 of the two fractions decreased with increasing temperature in the melting process, while increased with decreasing temperature in the gelation process. But the change rate of G0 was higher than G00 resulting in an intersection, which was defined as the melting temperature (Tm) or the gelation temperature (Tg) (Edwin R Morris, Nishinari, & Rinaudo, 2012). Additionally, it is possible to note that both fractions had thermo-reversible properties in the investigated temperature range, as the heating curve was almost coincided with the cooling curve. This phenomenon was similar to high acyl gellan (Yang et al., 2019). Furthermore, the Tm and Tg exhibited a dependence on concentra tion. As shown in Fig. 5, when CP concentration was 4% (w/v), Tg was determined to be 25 � C, then decreased to 10 � C as concentration decreased to 1.5% (w/v). A similar dependence on concentration was found in the results of HP (Fig. 6), the Tg and Tm increased from 52 to 76.5 � C with the concentration increasing from 0.5 to 1.2% (w/v). Since earlier work established that “long-live” hydrogen bonds and dynamic polymer entanglements both contributed to the formation of weak gel (Yu et al., 2018). It was plausible that polysaccharide solutions per formed a gel-like behavior below Tg was attributed to more hydrogen bonding and less movement of polymers, which may facilitate the intermolecular associations. The tanδ from temperature sweep was also used to illustrate the variation of viscoelasticity caused by temperature change. As shown in Fig. 5C and D, the tanδ values of CP increased with temperature. When the temperature exceeded 25 � C, all CP solutions exhibited a viscous behavior as the tanδ was higher than 1. Furthermore, the slope of the tanδ curve decreased with concentration, indicating the solution exhibited a more elastic behavior. In contrast, at low concentrations (0.5–0.6% w/v), the tanδ values increased linearly with temperature and rose abruptly when reaching Tg (Fig. 6C and D). Furthermore, at higher concentrations, with the temperature increasing, a platform was observed at the range of 30–55 � C, which indicating that the network was in a dynamic equilibrium between disentanglements and entan glements. As the temperature further heightened, the tanδ values increased sharply, suggesting that the intermolecular associations were further destroyed by the acceleration movement of the polymers.
4. Conclusions In this study, we extracted two different polysaccharides from psyl lium husk with cold (25 � C) and hot water (85 � C). The uronic acid contents of CP was about 5 times that of HP and the molecular weight of CP is lower than HP. Therefore, these two fractions displayed completely different rheological properties. It was found that CP behaved more like a liquid, while HP tended to be an elastic solid. Furthermore, the viscoelastic behaviors of both fractions had a strong dependence on concentration and temperature. The gel strength of HP and the apparent viscosity of CP were increased with concentration increasing. In the solgel transition process of HP, the critical gelation concentration was found to be 4.8 mg/mL. Meanwhile, three regimes were found in CP solutions, corresponding to the dilute, semi-dilute and concentrated regions, with two critical concentrations, C* (3.7 mg/mL) and C** (9.2 mg/mL). In addition, both fractions had thermo-reversible properties within the testing temperature range and the Tm and Tg increased with increasing polysaccharide concentration. This paper explored the different rheological properties of the two polysaccharides, which will lay the practical foundation for their future application in food products.
3.7. Steady flow sweep of CP In previous studies, the focus was on dynamic viscoelastic properties of gel-forming fractions from psyllium (Farahnaky et al., 2010; Guo et al., 2009; Haque et al., 1993). There was almost no study on steady flow behavior, this may closely relate to their brittle gel structures, which were not suitable for steady flow sweep. Recently, the rheological behavior of two gel-forming fractions were investigated using steady shear flow under non-gelled conditions (Yu et al., 2018). Although CP displayed a typical viscoelastic fluid response, research about the steady shear flow testing of CP were scarce. Therefore, we discussed the vis cosity properties of CP using steady shear flow in this section. The viscosity shear rate profiles of aqueous CP solutions under different concentrations (0.1–2.0% w/v) at 25 � C were shown in Fig. 7A. As concentration increased, the dependence of the apparent viscosity on
Declaration of competing interest There is no conflict of interests among authors. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 31772015), 8
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
Technical Innovation Program of Hubei Province (Program No. 2017ABA150) and Hubei Provincial Natural Science Foundation for Innovative Group (2019CFA011).
Lazaridou, A., Biliaderis, C., & Izydorczyk, M. (2003). Molecular size effects on rheological properties of oat β-glucans in solution and gels. Food Hydrocolloids, 17 (5), 693–712. Li, L., & Aoki, Y. (1997). Rheological images of poly (vinyl chloride) gels. 1. The dependence of sol gel transition on concentation. Macromolecules, 30(25), 7835–7841. Madgulkar, A. R., Rao, M. R., & Warrier, D. (2015). Characterization of psyllium (Plantago ovata) polysaccharide and its uses. Polysaccharides: Bioactivity and biotechnology, 871–890. Marlett, J. A., & Fischer, M. H. (2003). The active fraction of psyllium seed husk. Proceedings of the Nutrition Society, 62(1), 207–209. Morris, E. R. (1990). Shear-thinning of ‘random coil’polysaccharides: Characterisation by two parameters from a simple linear plot. Carbohydrate Polymers, 13(1), 85–96. Morris, E. R., Cutler, A., Ross-Murphy, S., Rees, D., & Price, J. (1981). Concentration and shear rate dependence of viscosity in random coil polysaccharide solutions. Carbohydrate Polymers, 1(1), 5–21. Morris, E. R., Nishinari, K., & Rinaudo, M. (2012). Gelation of gellan–a review. Food Hydrocolloids, 28(2), 373–411. Nwokocha, L. M., & Williams, P. A. (2016). Solution characteristics and thermorheology of Prosopis africana seed polysaccharide. Food Hydrocolloids, 56, 201–206. Pawar, H., & Varkhade, C. (2014). Isolation, characterization and investigation of Plantago ovata husk polysaccharide as superdisintegrant. International Journal of Biological Macromolecules, 69, 52–58. Petera, B., Delattre, C., Pierre, G., Wadouachi, A., Elboutachfaiti, R., Engel, E., et al. (2015). Characterization of arabinogalactan-rich mucilage from Cereus triangularis cladodes. Carbohydrate Polymers, 127, 372–380. Razavi, S. M. A., Cui, S. W., & Ding, H. (2016). Structural and physicochemical characteristics of a novel water-soluble gum from Lallemantia royleana seed. International Journal of Biological Macromolecules, 83, 142–151. Razmkhah, S., Razavi, S. M. A., & Mohammadifar, M. A. (2017). Dilute solution, flow behavior, thixotropy and viscoelastic characterization of cress seed (Lepidium sativum) gum fractions. Food Hydrocolloids, 63, 404–413. Rinc� on, F., Mu~ noz, J., Ramírez, P., Gal� an, H., & Alfaro, M. C. (2014). Physicochemical and rheological characterization of Prosopis juliflora seed gum aqueous dispersions. Food Hydrocolloids, 35, 348–357. Sandhu, J. S., Hudson, G. J., & Kennedy, J. F. (1981). The gel nature and structure of the carbohydrate of ispaghula husk ex Plantago ovata Forsk. Carbohydrate Research, 93 (2), 247–259. Sevage, M. (1934). Mechanism of the respiration of pneumococci II. Biochemistry Journal, 273, 419–429. Shi, A.-M., Wang, L.-J., Li, D., & Adhikari, B. (2013). Characterization of starch films containing starch nanoparticles. Part 2: Viscoelasticity and creep properties. Carbohydrate Polymers, 96(2), 602–610. Tao, Y., & Feng, D. (2012). Dilute solution and rheological properties of hyperbranched polysaccharide from Pleurotus tuber-regium sclerotia. Food Hydrocolloids, 28(1), 151–158. Taylor, K. A. (1993). A colorimetric method for the quantitation of galacturonic acid. Applied Biochemistry and Biotechnology, 43(1), 51–54. Wang, Q., & Cui, S. W. (2005). Understanding the physical properties of food polysaccharides. Food Carbohydrates: Chemistry, physical properties, and applications, 162–214. Wu, Y., Guo, R., Cao, N., Sun, X., Sui, Z., & Guo, Q. (2018). A systematical rheological study of polysaccharide from Sophora alopecuroides L. seeds. Carbohydrate Polymers, 180, 63–71. Xu, J.-L., Zhang, J.-C., Liu, Y., Sun, H.-J., & Wang, J.-H. (2016). Rheological properties of a polysaccharide from floral mushrooms cultivated in Huangshan Mountain. Carbohydrate Polymers, 139, 43–49. Yang, X., Hou, Y., Gong, T., Sun, L., Xue, J., & Guo, Y. (2019). Concentration-dependent rheological behavior and gelation mechanism of high acyl gellan aqueous solutions. International Journal of Biological Macromolecules, 131, 959–970. Yin, J., Lin, H., Li, J., Wang, Y., Cui, S. W., Nie, S., et al. (2012). Structural characterization of a highly branched polysaccharide from the seeds of Plantago asiatica L. Carbohydrate Polymers, 87(4), 2416–2424. Yin, J. Y., Nie, S. P., Zhou, C., Wan, Y., & Xie, M. Y. (2010). Chemical characteristics and antioxidant activities of polysaccharide purified from the seeds of Plantago asiatica L. Journal of the Science of Food and Agriculture, 90(2), 210–217. Yu, L., Yakubov, G. E., Martínez-Sanz, M., Gilbert, E. P., & Stokes, J. R. (2018). Rheological and structural properties of complex arabinoxylans from Plantago ovata seed mucilage under non-gelled conditions. Carbohydrate Polymers, 193, 179–188. Yu, L., Yakubov, G. E., Zeng, W., Xing, X., Stenson, J., Bulone, V., et al. (2017). Multilayer mucilage of Plantago ovata seeds: Rheological differences arise from variations in arabinoxylan side chains. Carbohydrate Polymers, 165, 132–141. Zhang, Y., Xu, X., Xu, J., & Zhang, L. (2007). Dynamic viscoelastic behavior of triple helical Lentinan in water: Effects of concentration and molecular weight. Polymer, 48 (22), 6681–6690.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.foodhyd.2019.105465. References Adel, A. M., El-Wahab, Z. H. A., Ibrahim, A. A., & Al-Shemy, M. T. (2010). Characterization of microcrystalline cellulose prepared from lignocellulosic materials. Part I. Acid catalyzed hydrolysis. Bioresource Technology, 101(12), 4446–4455. Ahmadi, R., Kalbasi-Ashtari, A., Oromiehie, A., Yarmand, M.-S., & Jahandideh, F. (2012). Development and characterization of a novel biodegradable edible film obtained from psyllium seed (Plantago ovata Forsk). Journal of Food Engineering, 109(4), 745–751. Arvidson, S. A., Rinehart, B. T., & Gadala-Maria, F. (2006). Concentration regimes of solutions of levan polysaccharide from Bacillus sp. Carbohydrate Polymers, 65(2), 144–149. Behbahani, B. A., Yazdi, F. T., Shahidi, F., Hesarinejad, M. A., Mortazavi, S. A., & Mohebbi, M. (2017). Plantago major seed mucilage: Optimization of extraction and some physicochemical and rheological aspects. Carbohydrate Polymers, 155, 68–77. Benaoun, F., Delattre, C., Boual, Z., Ursu, A. V., Vial, C., Gardarin, C., et al. (2017). Structural characterization and rheological behavior of a heteroxylan extracted from Plantago notata Lagasca (Plantaginaceae) seeds. Carbohydrate Polymers, 175, 96–104. Bhatia, M., & Ahuja, M. (2015). Psyllium arabinoxylan: Carboxymethylation, characterization and evaluation for nanoparticulate drug delivery. International Journal of Biological Macromolecules, 72, 495–501. Dhar, M., Kaul, S., Sareen, S., & Koul, A. (2005). Plantago ovata: Genetic diversity, cultivation, utilization and chemistry. Plant Genetic Resources, 3(2), 252–263. Edwards, S., Chaplin, M. F., Blackwood, A. D., & Dettmar, P. W. (2003). Primary structure of arabinoxylans of ispaghula husk and wheat bran. Proceedings of the Nutrition Society, 62(1), 217–222. Farahnaky, A., Askari, H., Majzoobi, M., & Mesbahi, G. (2010). The impact of concentration, temperature and pH on dynamic rheology of psyllium gels. Journal of Food Engineering, 100(2), 294–301. Fischer, M. H., Yu, N., Gray, G. R., Ralph, J., Anderson, L., & Marlett, J. A. (2004). The gel-forming polysaccharide of psyllium husk (Plantago ovata Forsk). Carbohydrate Research, 339(11), 2009–2017. Guo, Q., Cui, S. W., Wang, Q., Goff, H. D., & Smith, A. (2009). Microstructure and rheological properties of psyllium polysaccharide gel. Food Hydrocolloids, 23(6), 1542–1547. Guo, Q., Cui, S. W., Wang, Q., & Young, J. C. (2008). Fractionation and physicochemical characterization of psyllium gum. Carbohydrate Polymers, 73(1), 35–43. Haque, A., Richardson, R. K., Morris, E. R., & Dea, I. C. (1993). Xanthan-like ‘weak gel’ rheology from dispersions of ispaghula seed husk. Carbohydrate Polymers, 22(4), 223–232. Hesarinejad, M. A., Koocheki, A., & Razavi, S. M. A. (2014). Dynamic rheological properties of lepidium perfoliatum seed gum: Effect of concentration, temperature and heating/cooling rate. Food Hydrocolloids, 35, 583–589. Hussain, M. A., Muhammad, G., Jantan, I., & Bukhari, S. N. A. (2016). Psyllium arabinoxylan: A versatile biomaterial for potential medicinal and pharmaceutical applications. Polymer Reviews, 56(1), 1–30. Iqbal, M. S., Akbar, J., Hussain, M. A., Saghir, S., & Sher, M. (2011). Evaluation of hotwater extracted arabinoxylans from ispaghula seeds as drug carriers. Carbohydrate Polymers, 83(3), 1218–1225. Kennedy, J. F., Sandhu, J. S., & Southgate, D. A. (1979). Structural data for the carbohydrate of ispaghula husk ex Plantago ovata Forsk. Carbohydrate Research, 75, 265–274. Ladjevardi, Z. S., Gharibzahedi, S. M. T., & Mousavi, M. (2015). Development of a stable low-fat yogurt gel using functionality of psyllium (Plantago ovata Forsk) husk gum. Carbohydrate Polymers, 125, 272–280. Laidlaw, R., & Percival, E. (1949). 345. Studies on seed mucilages. Part III. Examination of a polysaccharide extracted from the seeds of Plantago ovata forsk. Journal of the Chemical Society, 1600–1607. Laidlaw, R., & Percival, E. (1950). 101. Studies of seed mucilages. Part V. Examination of a polysaccharide extracted from the seeds of Plantago ovata forsk by hot water. Journal of the Chemical Society, 528–534.
9
Contents lists available at ScienceDirect
Food Hydrocolloids journal homepage: http://www.elsevier.com/locate/foodhyd
Comparative study between cold and hot water extracted polysaccharides from Plantago ovata seed husk by using rheological methods Peiyuan Zhou a, b, Mohamed Eid a, b, Wenfei Xiong a, b, Cong Ren a, b, Tingyang Ai a, b, Ziyu Deng a, b, Jing Li a, b, Bin Li a, b, * a b
College of Food Science and Technology, Huazhong Agricultural University, Wuhan, 430070, Hubei, China Key Laboratory of Environment Correlative Dietology (Huazhong Agricultural University), Ministry of Education, Wuhan, 430070, Hubei, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Plantago ovata Polysaccharide Rheological properties Viscoelastic Sol-gel transition Critical concentration
Cold and hot water extracted polysaccharides (CP and HP) were isolated from Plantago ovata seed husk at 25 and 85 � C, respectively. Their rheological properties were systematically investigated and compared. Different con centrations and temperatures were studied to evaluate the viscoelastic properties of the two fractions. It was found that CP solutions behaved as viscous fluids, while HP solutions preferred to exhibit as gel-like structure. The sol-gel transition process of HP was evaluated and the gel point was determined to be 4.8 mg/mL by WinterChambon method in water at 25 � C. The steady flow sweep investigated the viscosity properties of CP solutions and two critical concentrations were found, which locating the transition from the dilute to semi-dilute region, and from the semi-dilute to concentrated region. In addition, the two fractions could form thermo-reversible weak gels, and the gelation temperatures of them increased with the increase of concentration. It is evident that both polysaccharides have significant potential for application in food industry.
1. Introduction Plantago ovata (Psyllium) seed husk has a long history of utilization as medicine and pharmacological supplements due to its rich content of bioactive polysaccharides (Hussain, Muhammad, Jantan, & Bukhari, 2016; Madgulkar, Rao, & Warrier, 2015). For the past few years, psyl lium has attracted an increasing amount of interest caused of its swelling and gelling properties (Hussain et al., 2016). Natural and modified psyllium polysaccharides were investigated as pharmaceutical excipi ents in controlled drug delivery systems (Bhatia & Ahuja, 2015; Iqbal, Akbar, Hussain, Saghir, & Sher, 2011; Pawar & Varkhade, 2014). In addition, psyllium polysaccharides were widely used as stabilizers and thickeners in food industry, such as ice creams (Dhar, Kaul, Sareen, & Koul, 2005), yogurt (Ladjevardi, Gharibzahedi, & Mousavi, 2015), and biodegradable edible films (Ahmadi, Kalbasi-Ashtari, Oromiehie, Yar mand, & Jahandideh, 2012). These increasing applications above are highly dependent on rheological properties of psyllium. Thus, expand ing the database on rheological properties of psyllium polysaccharides is critical to broaden their application range. According to previous studies, the separated polysaccharides from psyllium displayed markedly different rheological properties (Guo, Cui,
Wang, & Young, 2008; Yu et al., 2017). Three main polysaccharide fractions were extracted from psyllium: cold water fraction (CWF), hot water fraction (HWF) and alkaline fraction (AF) (Fischer et al., 2004; Guo et al., 2008; Laidlaw & Percival, 1949, 1950; Marlett & Fischer, 2003; Yu et al., 2017). Their structures are identified and compared, and many literatures illustrated the variation in rheological properties due to the structure variance. (Guo et al., 2008; Marlett & Fischer, 2003; Yu et al., 2017). It has been found that these fractions are all highly branched arabinoxylans, which composed of a xylan backbone with a variety of side chains attached at O-3 and/or O-2 positions (Yu et al., 2017). The side chains include arabinose, xylose, and oligosaccharides consisting of arabinose and/or xylose as well as rhamnose with terminal galacturonic acid (Edwards, Chaplin, Blackwood, & Dettmar, 2003; Kennedy, Sandhu, & Southgate, 1979; Sandhu, Hudson, & Kennedy, 1981; Yu et al., 2017). Although the three fractions have a similar main structure, remarkably CWF has more complex linkages and large amounts of rhamnose and uronic acids (Guo et al., 2008; Yu et al., 2017). Furthermore, its side chains are long and irregular, while the HWF and AF have short and regular side chains (Guo et al., 2008). These different structural features could be the reason for the higher solubility of CWF and better gelling properties of HWF and AF (Guo et al., 2008; Yu et al.,
* Corresponding author. College of Food Science and Technology, Huazhong Agricultural University, Wuhan, 430070, Hubei, China. E-mail address: [email protected] (B. Li). https://doi.org/10.1016/j.foodhyd.2019.105465 Received 1 August 2019; Received in revised form 18 October 2019; Accepted 24 October 2019 Available online 31 October 2019 0268-005X/© 2019 Elsevier Ltd. All rights reserved.
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
exhibited gel-like behaviors. But the study of rheological properties was insufficient to give exhaustive information on each fraction. The rheological properties of psyllium polysaccharides depend on many factors such as concentration, temperature, ionic strength, pH etc. (Farahnaky et al., 2010; Guo et al., 2009; Haque et al., 1993). Among these factors, concentration and temperature are the primary ones. It has been found that the gel-forming fractions can form a weak gel at room temperature, and the strength of gel increased with the increase of concentration (Farahnaky et al., 2010; Haque et al., 1993). However, the sol-gel transition process and the critical gelation concentration were hardly studied. Furthermore, the gel-forming fractions were determined to form physical gels which exhibited no thermal hysteresis during heating and cooling procedure (Farahnaky et al., 2010; Guo et al., 2009; Haque et al., 1993; Yu et al., 2017). Moreover, the different gel-forming fractions have different sensitivities to temperature, the melting tem perature point (Tm) of 1% HW fraction was around 41 � C, whereas the Tm of 1% alkaline fraction was more than 85 � C (Yu et al., 2017). The gelation and melting process of different fractions under different con centrations need to be studied systematically. On the other hand, the study on rheological properties of cold water fraction is limited. But it is an indispensable part of a comprehensive understanding of psyllium polysaccharides. In the present work, we focused on the cold (25 � C) and hot (85 � C) water extracted polysaccharides (CP and HP) from psyllium due to their high content and convenient extraction method. The objective of this work is to investigate the rheological properties of CP and HP and their dependency to concentration and temperature. Furthermore, the solu tion behavior of CP and the sol-gel transition process of HP in water were evaluated for the first time. We also compared the structures, molecular weights and uronic acid content of the two fractions, to get a further explanation for the difference in their rheological properties. These works are useful for the design of products based on psyllium poly saccharides, since insight in how the different concentrations and tem peratures affected the rheological properties, which can help to predict the stability and the final textural of formulated food.
Table 1 Parameters of molecular weight and uronic acid content of CP and HP. Mwa (g/moL) CP HP a b c
5
6.3 � 10 2.1 � 106
Mnb (g/moL) 5
2.4 � 10 5.9 � 105
Rg(z)c (nm)
Uronic acid (% w/w)
20 33
18.50% 3.50%
Mw: Molecular weight was measured by SEC-MALLS. Mn: Number average molecular weight was measured by SEC-MALLS. Rg(z): z-average radius of gyration was measured by SEC-MALLS.
Fig. 1. FT-IR spectra of CP and HP.
2017). However, most of the rheological properties analysis in literatures were based on the gel-forming properties of crude water extract or alkaline extract fractions (Farahnaky, Askari, Majzoobi, & Mesbahi, 2010; Guo, Cui, Wang, Goff, & Smith, 2009; Haque, Richardson, Morris, & Dea, 1993). But these studies could not provide accurate information on the rheological properties of different psyllium polysaccharides based on changed extraction method. Therefore, a systematical rheo logical study for each polysaccharide is required. Recently, Yu et al. (2017) found that cold water (25 � C) fraction displayed a viscoelastic fluid response, while hot water (65 � C) fraction and alkaline fraction
2. Materials and methods 2.1. Materials Psyllium (Plantago ovata) husk powder (98%) was obtained from Abhyuday Industries (India). Dextran standard (40 kDa) was from Aladdin. All other chemicals and reagents were from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and of analytical grade. All aqueous solutions were prepared with reverse osmosis treated deionized
Fig. 2. Strain sweep of CP (A) and HP (B) at different concentrations (25 � C). 2
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
Fig. 3. Dynamic rheological behaviors of the two fractions versus frequency at different concentrations (25 � C): viscoelastic moduli (G0 & G00 ) of CP (A) and HP (B), loss factor (tanδ) of CP (C) and HP (D), and complex viscosity (η*) of CP (E) and HP (F).
water (DI water).
anhydrous ethanol and dried in a vacuum oven to obtain crude cold water extracted polysaccharide. After that the crude polysaccharide was redissolved in DI water and deproteinized according to Sevage method (Sevage, 1934). The resulting solution was subsequently dialysed at room temperature for 72 h against DI water and freeze dried to obtain purified cold water extracted polysaccharide (CP). The remaining solid phase was washed twice with DI water at 25 � C, then dispersed in 4000 mL of DI water at 85 � C for 2 h under constant stirring. The sus pension was filtered and centrifuged at 10,000 g for 30 min. The resulting solution was concentrated, precipitated by ethanol, deprotei nized, dialysed and freeze dried in a similar manner as CP to obtain hot
2.2. Extraction of polysaccharides The polysaccharides were extracted by sequential procedure ac cording to the work (Yu et al., 2017) with several modifications. Briefly, psyllium powder (10 g) were dispersed in DI water (2000 mL) at 25 � C for 4 h under constant stirring, then centrifuged at 10,000 g for 30 min. The supernatant was concentrated in a rotary evaporator under reduced pressure at 55 � C and then precipitated by ethanol at a final concen tration of 80% for 12 h. The precipitate was washed twice with 3
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
Scientific™ OMNIC™ (OMNIC™ Series Software, Madison, WI, USA) program.
Table 2 Power law parameters of storage (G0 ) and loss (G00 ) moduli for CP and HP. G0 ¼ k1ωn1
Concentration (w/v) CP 0.1–1 rad s 1 1.5% 2% 3% 4% 1–10 rad s 1 1.5% 2% 3% 4% 10–100 rad s 1 1.5% 2% 3% 4% HP 0.1–10 rad s 1 0.3% 0.4% 0.5% 0.6% 0.8% 1% 1.5%
G00 ¼ k2ωn2 k2
n2
R2
0.9988 0.9999 0.9995 0.9990
0.54 1.07 5.15 14.96
0.94 0.88 0.75 0.65
0.9992 0.9993 0.9987 0.9980
1.05 0.96 0.76 0.64
0.9992 0.9988 0.9984 0.9983
0.58 1.15 5.47 15.66
0.77 0.69 0.54 0.46
0.9993 0.9987 0.9981 0.9979
0.47 0.66 4.86 16.69
0.59 0.71 0.54 0.46
0.9689 0.9990 0.9980 0.9981
0.89 1.77 7.83 21.02
0.61 0.52 0.40 0.34
0.9993 0.9990 0.9989 0.9990
0.07 0.45 1.23 2.76 9.63 20.16 36.06
0.38 0.22 0.19 0.18 0.17 0.16 0.18
0.9980 0.9978 0.9971 0.9923 0.9862 0.9770 0.9904
0.04 0.16 0.35 0.74 2.19 3.87 8.46
0.43 0.23 0.14 0.12 0.10 0.09 0.13
0.9962 0.9847 0.9839 0.9944 0.9994 0.9985 0.9994
k1
n1
R
0.11 0.30 2.49 9.87
1.31 1.21 0.99 0.85
0.12 0.34 2.77 10.64
2
2.5. Molecular weight measurement The molecular weights of the two fractions were determined by a size exclusion chromatography system, which equipped with a multi-angle laser light scattering (wavelength of 663.6 nm, eighteen angles) (Dawn Heleos II, Wyatt, USA), a refractive index detector (OPTILAB TrEX, Wyatt, USA), a OHpak SB-G guard column (6 mm � 50 mm) and a OHpak SB-805 HQ column (8 mm � 300 mm) (Shodex, Tokyo, Japan). The two fractions were dissolved (0.2 mg/mL) in 0.1 M NaNO3 with 0.02% (w/w) NaN3 under overnight stirring at room temperature and then filtered through 0.45 μm membrane before injection. The system was eluted with 0.1 M NaNO3 solution containing 0.02% (w/w) NaN3 and a flow rate of 0.4 mL/min. The column temperature was 50 � C. The sample solution was injected through a 1000 μL full loop. Data was collected and analyzed using the ASTRA (Version 6.1.1.84) software, normalized with dextran narrow standard (Mw ¼ 40,000). A refractive index increment (dn/dc) of 0.146 was used in the calculations. 2.6. Rheological measurements The rheological tests of CP and HP were performed on a DHR-2 rheometer (TA Instruments, USA) at 25 � C with a Peltier temperature control unit on the bottom plate. After loading the sample, the edge of the geometry was carefully trimmed and filled with low viscosity sili cone oil to prevent water evaporation. All samples were equilibrated for 5 min before testing. 2.6.1. Small amplitude oscillatory shear (SAOS) measurements 2.6.1.1. Sample preparation. CP was dissolved in DI water with different concentrations (1.5, 2, 3, 4% w/v) at 25 � C, while HP was dissolved in DI water with a various concentration (0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.5% w/ v) at 85 � C for 2 h. All samples were placed at room temperature over night to complete the hydration. 2.6.1.2. Strain and frequency sweep. A parallel plate geometry (60 mm diameter, 0.5 mm gap) was used for CP and 0.3% (w/v) HP, while HP (0.4–1.5% w/v) were tested by a 40 mm parallel plate (0.5 mm gap). Dynamic strain sweep measurements were carried out at 1 Hz to determine the linear viscoelastic regime (LVR) with a strain range from 0.1 to 1000% at 25 � C. Afterwards, dynamic frequency sweep ranging from 0.1 to 100 rad s 1 were measured at 25 � C in a constant strain of 10%.
Fig. 4. Loss tangent tanδ as a function of polysaccharide concentration c for HP in water at 25 � C and various angular frequencies (1–10 rad s 1). Cgel is the gel point.
2.6.1.3. Temperature sweep. The temperature sweep measurements were performed at the constant strain of 10%, which was well within the linear viscoelastic region, while the frequency was fixed at 1 Hz. The sample was measured by linear increase in temperature from 5 to 50 � C for CP and 5–80 � C for HP, then linearly cooling down to 5 � C. The heating and cooling steps were performed at rate of 1.5 and 2 � C/min for CP and HP, respectively. A parallel plate geometry (40 mm diameter, 0.5 mm gap) was used for the measurement.
water extracted polysaccharide (HP). 2.3. Uronic acids analysis Total uronic acid content of the two fractions was estimated by and sulfuric acid-carbazole spectrophotometric method using galacturonic acid as standard (Taylor, 1993).
2.6.2. Steady shear flow In the steady shear test, a series of CP solutions (0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.45, 0.5, 0.6, 0.75, 1, 1.25, 1.5, 2, 2.5% w/v) in DI water were prepared. A cone and plate geometry (1-degree angle) with 60 mm diameter and 0.5 mm gap was used. The measurements were performed at shear rates from 0.01 to 1000 s 1 at 25 � C. The viscosity data were fitted with the Cross model (Eq. (1)). For pseudoplastic fluid, the infinite viscosity tends to zero when the shear rate is infinite.
2.4. Fourier transform infrared (FT-IR) spectroscopy FT-IR spectra were acquired on a Nicolet iS50 FT-IR spectrometer (Thermo Scientific, USA) with a GoldenGate diamond attenuated total reflectance (ATR) accessory at room temperature (referenced against air). A region from 400 to 4000 cm 1 was used for scanning at 4 cm 1 resolution using 64 scans. Spectra were analyzed by the Thermo 4
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
Fig. 5. Temperature sweep results and loss tangent tanδ of CP: the gel melting process (A and C) and gelation process (B and D).
η ¼ η∞ þ
η0
η∞
1 þ ðτγ_ Þm
characterized by SEC-MALLS analysis. As shown in Table 1, the massaverage molecular mass (Mw) and number-average molecular mass (Mn) of CP were 6.3 � 105 g/mol and 2.4 � 105 g/mol respectively, while 2.1 � 106 g/mol and 5.9 � 105 g/mol for HP. The z-average radius of gyration, Rg(z), was 20 nm for CP and 33 nm for HP. However, our results were different to those of the similar polysaccharides which dissolved in 0.04 M KOH at 50 � C (Yu et al., 2017). They found that the molecular weight of the cold water (25 � C) fraction was higher than those of the hot water (60 � C) fraction and 0.2 M KOH fraction, which is contrary to our results. Therefore, these discrepancies can not only be explained easily by different sources or methods, but also the different treatment in extracting and purifying process. Nevertheless, the Mw of HP was similar to other water-soluble polysaccharides extracted from Plantaginaceae such as Ispaghula husk (2.2 � 106 g/mol) (Edwards et al., 2003), Plantago major seeds (1.2 � 106 g/mol) (Behbahani et al., 2017), Plantago asiatica seeds (1.8 � 106 g/mol) (J. Yin et al., 2012) and Plantago notata Lagasca seeds (2.3 � 106 g/mol) (Benaoun et al., 2017).
(1)
η is the apparent viscosity, η∞ is the infinite-rate viscosity, η0 is the zerorate viscosity, τ is the time constant and m is the dimensionless exponent that captures the strength of shear thinning effect (Yu, Yakubov, Mar tínez-Sanz, Gilbert, & Stokes, 2018). 3. Results and discussion 3.1. Determination of uronic acid contents In early research, the uronic acid content (18%) of cold water (25 � C) extracted polysaccharide was much higher than the content (3%) of hot water (90–95 � C) extracted polysaccharide from the residue (Laidlaw & Percival, 1949, 1950). Similar results were found in our work. CP had a higher uronic acid content (18.5%), which was about five times that of HP (3.5%) (Table 1). This indicated that CP was more like an anionic polysaccharide, while HP was a neutral polysaccharide. This result explained that water-extracted psyllium polysaccharide was a mixture of a polyuronide and a neutral arabinoxylan rather than a single species of polysaccharide (Fischer et al., 2004; Kennedy et al., 1979). Further more, the abundance (18%) of negatively charged GalAp residues in CP is a plausible explanation for its higher solubility compared to HP in water at 25 � C (Yu et al., 2017).
3.3. FT-IR spectroscopy The FT-IR spectra of the two fractions were displayed in Fig. 1. It was observed that the two fractions had similar typical bands and peaks of polysaccharides. Both of them displayed a strong and broad absorption peak at around 3360 cm 1 corresponding to the hydroxyl (-OH) stretching vibration as well as water adsorption (Adel, El-Wahab, Ibra him, & Al-Shemy, 2010). The weak signal obtained at 2921 cm 1 was attributed to the asymmetric vibration of C–H group (Benaoun et al., 2017). According to reports, absorption bands around 1600 cm 1 and
3.2. Molecular weight The molecular weights of both fractions in DI water were 5
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
Fig. 6. Temperature sweep results and loss tangent tanδ of HP: the gel melting process (A and C) and gelation process (B and D).
Fig. 7. Steady flow behaviors at different concentrations (A) and log-log plot of the zero-shear specific viscosity (ηsp,0) versus the space occupancy c [η] (B) for CP in water at 25 � C.
1400 cm 1 (1607 and 1412 cm 1 for CP, 1632 and 1420 cm 1 for HP) were typical of the asymmetrical and symmetrical stretching vibration of carboxylate groups (–COO–) from acid residues (Behbahani et al., 2017; Petera et al., 2015), while the absorption band at 1730 cm 1 indicated the presence of uronic acid. Furthermore, CP had a stronger absorption at 1730 cm 1 than HP due to its higher uronic acid content, corresponding to the previous results of uronic acid content analysis.
Moreover, the peak at 1370 cm 1 could correspond to ester carbonyl – O) of the carboxylic function of d-GalA (Petera et al., 2015). groups (C– The signal peak observed at 1036 cm 1 resulted from the stretching vibration of the pyranose ring (J. Y. Yin, Nie, Zhou, Wan, & Xie, 2010). Finally, the characteristic absorption at 896 cm 1 suggested the β configuration of the main sugar units of both CP and HP (Benaoun et al., 2017). 6
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
referred to as the frequency exponents and can be used to describe the viscoelastic characteristics of polysaccharide solutions (Razmkhah, Razavi, & Mohammadifar, 2017). The values of k1, k2, n1, n2 and R2 were summarized in Table 2. For CP solutions, the values of both n1 and n2 decreased with the increment of concentration during different frequency ranges (Table 2). At low frequencies (0.1–1 rad s 1) with lower concentrations (1.5–2% w/v), the dependence of viscoelastic moduli for CP was close to that of dilute solution (n1 ¼ 2, n2 ¼ 1) (Wang & Cui, 2005), and smaller values of n1 and n2 were obtained at higher frequencies and concentrations, indicating the formation of viscous gels for CP (Wu et al., 2018). It was reported that the strength and nature of entangled weak gels could be estimated by n1, of which low values (near zero) are assigned to elastic gels while viscous gels close to 1 (Wu et al., 2018). According to low values of n1 and n2 for HP solutions, they exhibited slight frequency dependency, suggesting that HP solutions tend to form elastic gels. The gel strength increased with the increase of concentration, as observed from the decrease in the “n1” value, which was in consistency with the result from Fig. 3B. The values of k1 were much higher than those of k2, which also confirming the weak gel-like structure of HP (Razavi, Cui, & Ding, 2016; Razmkhah et al., 2017; Shi, Wang, Li, & Adhikari, 2013). In general, tanδ <1 suggests predominantly elastic behavior while tanδ >1 indicates primarily viscous behavior (Xu, Zhang, Liu, Sun, & Wang, 2016). It was also reported that polymer systems can be classified into three by tanδ values: high values for dilute solutions, 0.2–0.3 for amorphous polymers and low values (close to 0.01) for gels or glassy crystalline polymers (Wu et al., 2018). Fig. 3C showed the change of tanδ versus frequency under different concentrations of CP. As can be observed, tanδ decreased with the increment of concentration and fre quency. At low frequency range (0.1–10 rad s 1), viscous behaviors were obtained with the tanδ value was higher than 1. As frequency increased, elastic behaviors were obtained for CP at higher concentra tions (3–4% w/v) with tanδ value was between 0.6 and 1, which inferred the intermediate behavior of CP between a dilute solution and an elastic gel, thus giving rise to weak formation of gel. In contrary, as shown in Fig. 3D, the tanδ values for HP with various concentrations were smaller than 1 and had a weak dependence on frequency, suggesting that HP formed weak gels where the elasticity dominates these systems (Zhang et al., 2007). The tanδ values at low concentrations (0.3–0.4% w/v) were much higher than those at higher concentrations (0.5–1.5% w/v) which were between 0.1 and 0.5, indicating that the high concentration samples may display an intermediate behavior between a weak and an elastic gel. Fig. 3E and F showed that the complex dynamic viscosity (η*) of the two fractions increased as the concentration increasing, and both frac tions displayed a shear dependent flow behavior as η* decreased with increase in frequency. That is, both fractions had non-Newtonian shearthinning behavior, which is common in other polysaccharides (Hesar inejad, Koocheki, & Razavi, 2014; Wu et al., 2018). In addition, compared to the marked linear decrease of η* for HP, the η* of CP initially showed a relatively small reduction and then reduced rapidly at higher frequencies.
Table 3 The fitting parameters of reduced Cross model for CP. Concentration (w/v)
η0 (Pa s)
0.1% 0.15% 0.2% 0.25% 0.3% 0.4% 0.45% 0.5% 0.6% 0.75% 1% 1.25% 1.5% 2% 2.5%
0.0016 0.0022 0.0029 0.0034 0.0041 0.0059 0.0071 0.0092 0.0139 0.0220 0.0530 0.1486 0.3907 1.0600 5.1417
τ (ms) 0.0268 0.0148 0.0161 0.0512 0.0735 0.0816 0.1282 0.1750 0.3413 0.6130 1.3426 3.3691 13.0920 16.3895 171.014
R2
m 0.6731 0.5130 0.4594 0.6210 0.6540 0.5565 0.5779 0.5018 0.4538 0.6467 0.7254 0.7002 0.5509 0.5924 0.4984
0.9809 0.9944 0.9939 0.9978 0.9993 0.9998 0.9996 0.9981 0.9991 0.9954 0.9961 0.9963 0.9977 0.9986 0.9985
3.4. Dynamic oscillatory rheology 3.4.1. Strain sweep measurements Fig. 2 showed that both G0 and G00 of the two fractions initially remained constant during the linear viscoelastic region (LVR), and diminished when critical strain was exceeded. The critical strain reflects the deformability of the mucilage. Obviously, the LVR of HP was larger than that of CP, which inferred HP had a higher resistance to deforma tion. Furthermore, as expected an increase in both G0 and G00 was observed with polysaccharide concentration increasing. As shown in Fig. 2A, CP displayed a viscous response at lower concentrations (1.5–3% w/v) since G00 was higher than G0 . When the concentration reached 4% (w/v), G00 was almost equal to G0 indicating that a weak gel was formed. By contrast, HP displayed an elastic behavior within the tested concentration range (0.3–1.5% w/v) since G0 is always higher than G00 (Fig. 2B). It was very interesting that HP fraction was easily gelled at low concentration (lower than 0.5% w/v, 25 � C), whereas few general polymers showed similar phenomenon. 3.4.2. Frequency sweep measurements The dependences of G0 , G00 , tanδ and η* on frequency under different concentrations at 25 � C were employed to characterize viscoelastic be haviors of both fractions (Fig. 3). For CP solutions (Fig. 3A), viscous behaviors (G0 < G00 ) were obtained at lower concentrations (1.5–2% w/ v) within the test frequency range; while at higher concentrations (3–4% w/v), crossover points of G0 and G00 occurred, appearing a viscous response (G0 < G00 ) below the point and an elastic response (G00 < G0 ) above. Moreover, the crossover point moved to lower frequency with concentration increasing, indicating that molecules were more likely to interact with each other and generate a temporary entangled network at �n, Mun ~ oz, higher concentrations (Nwokocha & Williams, 2016; Rinco �n, & Alfaro, 2014; Zhang, Xu, Xu, & Zhang, 2007). On the Ramírez, Gala other hand, HP displayed a gel-like behavior. Both G0 and G00 showed slightly frequency dependency with G0 exceeding G00 under different concentrations (0.3–1.5% w/v) throughout the accessible range of fre quency (Fig. 3B). This behavior is typical for ‘weak gels’ such as xanthan gum (Edwin R Morris, 1990). Although the dependences of the two fractions on frequency decreased with concentration increasing, HP displayed a less depen dence on frequency than CP, which may be due to the stronger resistance to deformation of its weak gel network. The dependences of G0 and G00 for the two fractions on frequency were demonstrated by the analysis of power law equation ((2) and (3)): G ¼ k1 ωn1
(2)
G00 ¼ k1 ωn1
(3)
0
3.5. Determination of the critical gelation concentration for HP Traditionally, the crossover of G0 and G00 was used as an indicator of the gel point. But it is not valid in our work. It was found that HP so lution exhibited a visible liquid behavior at the low concentration (0.3% w/v) while the G0 was slightly higher than G00 and no crossover occurred. Therefore, a reliable and valid method was required to determine the get point. It was reported that a scaling law of G0 (ω) ~ G00 (ω) ~ ωn (0 < n < 1) or G0 (ω)/G00 (ω) ¼ tanδ ¼ tan (nπ/2) at the gel point was found for all gelling systems (Li & Aoki, 1997). The frequency-independence of loss tangent in the vicinity of the gel point has been widely used to determine the gel point for chemical and physical gels (Zhang et al., 2007). This method was used to determine
Where k1 and k2 are constants, ω is the frequency, n1 and n2 may be 7
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
the sol-gel transition concentration from a frequency plot (1–10 rad s 1) of tanδ vs concentration of HP in water at 25 � C (Fig. 4). It was observed that all curves intersected at a common point corresponding to 4.8 mg/mL, which was defined as the critical gelation concentration Cgel. When the concentration exceeded Cgel, tanδ values were indepen dent of the frequency, suggesting that gel formation occurred.
shear rate became increasingly pronounced. The typical curves of CP solutions at higher concentrations presented shear-thinning behavior since the apparent viscosity dropped with increasing shear rate. How ever, at low shear rate, little or no decrease was observed. It may be explained that there was a dynamic equilibrium between disentangle ments and entanglements at low shear rate, then the formation of new entanglements was impeded by greater shear forces disrupting network at high shear rate (Wu et al., 2018). The flow characteristics of CP were presented in Table 3. The η0 and τ increased with concentration, which indicated that concentration increment resulted in higher consistency and pseudoplasticity for CP in solution. It can be explained that the free movement of individual molecular chains were restricted by increasing chain entanglements at higher concentrations, therefore, the in teractions between molecules are intensified, which enhanced the con sistency and pseudoplastic behaviors of CP (Wu et al., 2018). The double logarithmic plot of the zero-shear specific viscosity (ηsp,0) versus the space occupancy of CP in water at 25 � C was shown in Fig. 7B. Most of the past studies reported that only two regions (the dilute and concentrated solution) were observed for most disordered poly saccharide solutions in this type of plot (E Rw Morris, Cutler, Ross-Murphy, Rees, & Price, 1981) And the fitting curves of the two regions intercepted at a point namely the critical overlap concentration, C*, with the slopes of 1.4 and 3.3 respectively (E Rw Morris et al., 1981). However, in some other research, three domains were reported, corre sponding to the dilute, semi-dilute and concentrated regions (Tao & Feng, 2012), and the three slopes values were within 0.91–1.3, 1.16–2.1, and 2.24–5 respectively (Arvidson, Rinehart, & Gadala-Maria, 2006; Lazaridou, Biliaderis, & Izydorczyk, 2003; Tao et al., 2012; Yu et al., 2018). Similarly, in our results (Fig. 7B), the data were fitted to three linear lines with the slopes of 0.9, 2.1 and 4.8 respectively, and the intrinsic viscosity [η] of CP in water at 25 � C was determined to be 10.2 dL/g (Supplementary Fig. S1). The C* and C** of CP were calcu lated at 3.7 and 9.2 mg/mL, while the dimensionless coil overlap pa rameters, C*[η] and C**[η], were 3.8 and 9.4 respectively. The results were quite consistent with other random coil polysaccharides mentioned before.
3.6. Temperature sweep measurements In addition to concentration, temperature was also proved to have influence on the gelation and gel melting process of polysaccharides from psyllium (Farahnaky et al., 2010; Haque et al., 1993; Yu et al., 2017). Temperature sweep results of the two fractions with different concentrations were shown in Figs. 5 and 6. It was observed that both G0 and G00 of the two fractions decreased with increasing temperature in the melting process, while increased with decreasing temperature in the gelation process. But the change rate of G0 was higher than G00 resulting in an intersection, which was defined as the melting temperature (Tm) or the gelation temperature (Tg) (Edwin R Morris, Nishinari, & Rinaudo, 2012). Additionally, it is possible to note that both fractions had thermo-reversible properties in the investigated temperature range, as the heating curve was almost coincided with the cooling curve. This phenomenon was similar to high acyl gellan (Yang et al., 2019). Furthermore, the Tm and Tg exhibited a dependence on concentra tion. As shown in Fig. 5, when CP concentration was 4% (w/v), Tg was determined to be 25 � C, then decreased to 10 � C as concentration decreased to 1.5% (w/v). A similar dependence on concentration was found in the results of HP (Fig. 6), the Tg and Tm increased from 52 to 76.5 � C with the concentration increasing from 0.5 to 1.2% (w/v). Since earlier work established that “long-live” hydrogen bonds and dynamic polymer entanglements both contributed to the formation of weak gel (Yu et al., 2018). It was plausible that polysaccharide solutions per formed a gel-like behavior below Tg was attributed to more hydrogen bonding and less movement of polymers, which may facilitate the intermolecular associations. The tanδ from temperature sweep was also used to illustrate the variation of viscoelasticity caused by temperature change. As shown in Fig. 5C and D, the tanδ values of CP increased with temperature. When the temperature exceeded 25 � C, all CP solutions exhibited a viscous behavior as the tanδ was higher than 1. Furthermore, the slope of the tanδ curve decreased with concentration, indicating the solution exhibited a more elastic behavior. In contrast, at low concentrations (0.5–0.6% w/v), the tanδ values increased linearly with temperature and rose abruptly when reaching Tg (Fig. 6C and D). Furthermore, at higher concentrations, with the temperature increasing, a platform was observed at the range of 30–55 � C, which indicating that the network was in a dynamic equilibrium between disentanglements and entan glements. As the temperature further heightened, the tanδ values increased sharply, suggesting that the intermolecular associations were further destroyed by the acceleration movement of the polymers.
4. Conclusions In this study, we extracted two different polysaccharides from psyl lium husk with cold (25 � C) and hot water (85 � C). The uronic acid contents of CP was about 5 times that of HP and the molecular weight of CP is lower than HP. Therefore, these two fractions displayed completely different rheological properties. It was found that CP behaved more like a liquid, while HP tended to be an elastic solid. Furthermore, the viscoelastic behaviors of both fractions had a strong dependence on concentration and temperature. The gel strength of HP and the apparent viscosity of CP were increased with concentration increasing. In the solgel transition process of HP, the critical gelation concentration was found to be 4.8 mg/mL. Meanwhile, three regimes were found in CP solutions, corresponding to the dilute, semi-dilute and concentrated regions, with two critical concentrations, C* (3.7 mg/mL) and C** (9.2 mg/mL). In addition, both fractions had thermo-reversible properties within the testing temperature range and the Tm and Tg increased with increasing polysaccharide concentration. This paper explored the different rheological properties of the two polysaccharides, which will lay the practical foundation for their future application in food products.
3.7. Steady flow sweep of CP In previous studies, the focus was on dynamic viscoelastic properties of gel-forming fractions from psyllium (Farahnaky et al., 2010; Guo et al., 2009; Haque et al., 1993). There was almost no study on steady flow behavior, this may closely relate to their brittle gel structures, which were not suitable for steady flow sweep. Recently, the rheological behavior of two gel-forming fractions were investigated using steady shear flow under non-gelled conditions (Yu et al., 2018). Although CP displayed a typical viscoelastic fluid response, research about the steady shear flow testing of CP were scarce. Therefore, we discussed the vis cosity properties of CP using steady shear flow in this section. The viscosity shear rate profiles of aqueous CP solutions under different concentrations (0.1–2.0% w/v) at 25 � C were shown in Fig. 7A. As concentration increased, the dependence of the apparent viscosity on
Declaration of competing interest There is no conflict of interests among authors. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 31772015), 8
P. Zhou et al.
Food Hydrocolloids 101 (2020) 105465
Technical Innovation Program of Hubei Province (Program No. 2017ABA150) and Hubei Provincial Natural Science Foundation for Innovative Group (2019CFA011).
Lazaridou, A., Biliaderis, C., & Izydorczyk, M. (2003). Molecular size effects on rheological properties of oat β-glucans in solution and gels. Food Hydrocolloids, 17 (5), 693–712. Li, L., & Aoki, Y. (1997). Rheological images of poly (vinyl chloride) gels. 1. The dependence of sol gel transition on concentation. Macromolecules, 30(25), 7835–7841. Madgulkar, A. R., Rao, M. R., & Warrier, D. (2015). Characterization of psyllium (Plantago ovata) polysaccharide and its uses. Polysaccharides: Bioactivity and biotechnology, 871–890. Marlett, J. A., & Fischer, M. H. (2003). The active fraction of psyllium seed husk. Proceedings of the Nutrition Society, 62(1), 207–209. Morris, E. R. (1990). Shear-thinning of ‘random coil’polysaccharides: Characterisation by two parameters from a simple linear plot. Carbohydrate Polymers, 13(1), 85–96. Morris, E. R., Cutler, A., Ross-Murphy, S., Rees, D., & Price, J. (1981). Concentration and shear rate dependence of viscosity in random coil polysaccharide solutions. Carbohydrate Polymers, 1(1), 5–21. Morris, E. R., Nishinari, K., & Rinaudo, M. (2012). Gelation of gellan–a review. Food Hydrocolloids, 28(2), 373–411. Nwokocha, L. M., & Williams, P. A. (2016). Solution characteristics and thermorheology of Prosopis africana seed polysaccharide. Food Hydrocolloids, 56, 201–206. Pawar, H., & Varkhade, C. (2014). Isolation, characterization and investigation of Plantago ovata husk polysaccharide as superdisintegrant. International Journal of Biological Macromolecules, 69, 52–58. Petera, B., Delattre, C., Pierre, G., Wadouachi, A., Elboutachfaiti, R., Engel, E., et al. (2015). Characterization of arabinogalactan-rich mucilage from Cereus triangularis cladodes. Carbohydrate Polymers, 127, 372–380. Razavi, S. M. A., Cui, S. W., & Ding, H. (2016). Structural and physicochemical characteristics of a novel water-soluble gum from Lallemantia royleana seed. International Journal of Biological Macromolecules, 83, 142–151. Razmkhah, S., Razavi, S. M. A., & Mohammadifar, M. A. (2017). Dilute solution, flow behavior, thixotropy and viscoelastic characterization of cress seed (Lepidium sativum) gum fractions. Food Hydrocolloids, 63, 404–413. Rinc� on, F., Mu~ noz, J., Ramírez, P., Gal� an, H., & Alfaro, M. C. (2014). Physicochemical and rheological characterization of Prosopis juliflora seed gum aqueous dispersions. Food Hydrocolloids, 35, 348–357. Sandhu, J. S., Hudson, G. J., & Kennedy, J. F. (1981). The gel nature and structure of the carbohydrate of ispaghula husk ex Plantago ovata Forsk. Carbohydrate Research, 93 (2), 247–259. Sevage, M. (1934). Mechanism of the respiration of pneumococci II. Biochemistry Journal, 273, 419–429. Shi, A.-M., Wang, L.-J., Li, D., & Adhikari, B. (2013). Characterization of starch films containing starch nanoparticles. Part 2: Viscoelasticity and creep properties. Carbohydrate Polymers, 96(2), 602–610. Tao, Y., & Feng, D. (2012). Dilute solution and rheological properties of hyperbranched polysaccharide from Pleurotus tuber-regium sclerotia. Food Hydrocolloids, 28(1), 151–158. Taylor, K. A. (1993). A colorimetric method for the quantitation of galacturonic acid. Applied Biochemistry and Biotechnology, 43(1), 51–54. Wang, Q., & Cui, S. W. (2005). Understanding the physical properties of food polysaccharides. Food Carbohydrates: Chemistry, physical properties, and applications, 162–214. Wu, Y., Guo, R., Cao, N., Sun, X., Sui, Z., & Guo, Q. (2018). A systematical rheological study of polysaccharide from Sophora alopecuroides L. seeds. Carbohydrate Polymers, 180, 63–71. Xu, J.-L., Zhang, J.-C., Liu, Y., Sun, H.-J., & Wang, J.-H. (2016). Rheological properties of a polysaccharide from floral mushrooms cultivated in Huangshan Mountain. Carbohydrate Polymers, 139, 43–49. Yang, X., Hou, Y., Gong, T., Sun, L., Xue, J., & Guo, Y. (2019). Concentration-dependent rheological behavior and gelation mechanism of high acyl gellan aqueous solutions. International Journal of Biological Macromolecules, 131, 959–970. Yin, J., Lin, H., Li, J., Wang, Y., Cui, S. W., Nie, S., et al. (2012). Structural characterization of a highly branched polysaccharide from the seeds of Plantago asiatica L. Carbohydrate Polymers, 87(4), 2416–2424. Yin, J. Y., Nie, S. P., Zhou, C., Wan, Y., & Xie, M. Y. (2010). Chemical characteristics and antioxidant activities of polysaccharide purified from the seeds of Plantago asiatica L. Journal of the Science of Food and Agriculture, 90(2), 210–217. Yu, L., Yakubov, G. E., Martínez-Sanz, M., Gilbert, E. P., & Stokes, J. R. (2018). Rheological and structural properties of complex arabinoxylans from Plantago ovata seed mucilage under non-gelled conditions. Carbohydrate Polymers, 193, 179–188. Yu, L., Yakubov, G. E., Zeng, W., Xing, X., Stenson, J., Bulone, V., et al. (2017). Multilayer mucilage of Plantago ovata seeds: Rheological differences arise from variations in arabinoxylan side chains. Carbohydrate Polymers, 165, 132–141. Zhang, Y., Xu, X., Xu, J., & Zhang, L. (2007). Dynamic viscoelastic behavior of triple helical Lentinan in water: Effects of concentration and molecular weight. Polymer, 48 (22), 6681–6690.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.foodhyd.2019.105465. References Adel, A. M., El-Wahab, Z. H. A., Ibrahim, A. A., & Al-Shemy, M. T. (2010). Characterization of microcrystalline cellulose prepared from lignocellulosic materials. Part I. Acid catalyzed hydrolysis. Bioresource Technology, 101(12), 4446–4455. Ahmadi, R., Kalbasi-Ashtari, A., Oromiehie, A., Yarmand, M.-S., & Jahandideh, F. (2012). Development and characterization of a novel biodegradable edible film obtained from psyllium seed (Plantago ovata Forsk). Journal of Food Engineering, 109(4), 745–751. Arvidson, S. A., Rinehart, B. T., & Gadala-Maria, F. (2006). Concentration regimes of solutions of levan polysaccharide from Bacillus sp. Carbohydrate Polymers, 65(2), 144–149. Behbahani, B. A., Yazdi, F. T., Shahidi, F., Hesarinejad, M. A., Mortazavi, S. A., & Mohebbi, M. (2017). Plantago major seed mucilage: Optimization of extraction and some physicochemical and rheological aspects. Carbohydrate Polymers, 155, 68–77. Benaoun, F., Delattre, C., Boual, Z., Ursu, A. V., Vial, C., Gardarin, C., et al. (2017). Structural characterization and rheological behavior of a heteroxylan extracted from Plantago notata Lagasca (Plantaginaceae) seeds. Carbohydrate Polymers, 175, 96–104. Bhatia, M., & Ahuja, M. (2015). Psyllium arabinoxylan: Carboxymethylation, characterization and evaluation for nanoparticulate drug delivery. International Journal of Biological Macromolecules, 72, 495–501. Dhar, M., Kaul, S., Sareen, S., & Koul, A. (2005). Plantago ovata: Genetic diversity, cultivation, utilization and chemistry. Plant Genetic Resources, 3(2), 252–263. Edwards, S., Chaplin, M. F., Blackwood, A. D., & Dettmar, P. W. (2003). Primary structure of arabinoxylans of ispaghula husk and wheat bran. Proceedings of the Nutrition Society, 62(1), 217–222. Farahnaky, A., Askari, H., Majzoobi, M., & Mesbahi, G. (2010). The impact of concentration, temperature and pH on dynamic rheology of psyllium gels. Journal of Food Engineering, 100(2), 294–301. Fischer, M. H., Yu, N., Gray, G. R., Ralph, J., Anderson, L., & Marlett, J. A. (2004). The gel-forming polysaccharide of psyllium husk (Plantago ovata Forsk). Carbohydrate Research, 339(11), 2009–2017. Guo, Q., Cui, S. W., Wang, Q., Goff, H. D., & Smith, A. (2009). Microstructure and rheological properties of psyllium polysaccharide gel. Food Hydrocolloids, 23(6), 1542–1547. Guo, Q., Cui, S. W., Wang, Q., & Young, J. C. (2008). Fractionation and physicochemical characterization of psyllium gum. Carbohydrate Polymers, 73(1), 35–43. Haque, A., Richardson, R. K., Morris, E. R., & Dea, I. C. (1993). Xanthan-like ‘weak gel’ rheology from dispersions of ispaghula seed husk. Carbohydrate Polymers, 22(4), 223–232. Hesarinejad, M. A., Koocheki, A., & Razavi, S. M. A. (2014). Dynamic rheological properties of lepidium perfoliatum seed gum: Effect of concentration, temperature and heating/cooling rate. Food Hydrocolloids, 35, 583–589. Hussain, M. A., Muhammad, G., Jantan, I., & Bukhari, S. N. A. (2016). Psyllium arabinoxylan: A versatile biomaterial for potential medicinal and pharmaceutical applications. Polymer Reviews, 56(1), 1–30. Iqbal, M. S., Akbar, J., Hussain, M. A., Saghir, S., & Sher, M. (2011). Evaluation of hotwater extracted arabinoxylans from ispaghula seeds as drug carriers. Carbohydrate Polymers, 83(3), 1218–1225. Kennedy, J. F., Sandhu, J. S., & Southgate, D. A. (1979). Structural data for the carbohydrate of ispaghula husk ex Plantago ovata Forsk. Carbohydrate Research, 75, 265–274. Ladjevardi, Z. S., Gharibzahedi, S. M. T., & Mousavi, M. (2015). Development of a stable low-fat yogurt gel using functionality of psyllium (Plantago ovata Forsk) husk gum. Carbohydrate Polymers, 125, 272–280. Laidlaw, R., & Percival, E. (1949). 345. Studies on seed mucilages. Part III. Examination of a polysaccharide extracted from the seeds of Plantago ovata forsk. Journal of the Chemical Society, 1600–1607. Laidlaw, R., & Percival, E. (1950). 101. Studies of seed mucilages. Part V. Examination of a polysaccharide extracted from the seeds of Plantago ovata forsk by hot water. Journal of the Chemical Society, 528–534.
9