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The dependency of rheological properties of Plantago lanceolata seed mucilage as a novel source of hydrocolloid on mono- and di-valent salts
Journal Pre-proofs The dependency of rheological properties of Plantago lanceolata seed mucilage as a novel source of hydrocolloid on mono- and di-valent salts Mohammad Ali Hesarinejad, Elhamalsadat Shekarforoush, Farnaz Rezayian Attar, Sajad Ghaderi PII: DOI: Reference:
S0141-8130(19)34552-0 https://doi.org/10.1016/j.ijbiomac.2019.10.093 BIOMAC 13592
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International Journal of Biological Macromolecules
Received Date: Revised Date: Accepted Date:
18 June 2019 6 October 2019 9 October 2019
Please cite this article as: M. Ali Hesarinejad, E. Shekarforoush, F. Rezayian Attar, S. Ghaderi, The dependency of rheological properties of Plantago lanceolata seed mucilage as a novel source of hydrocolloid on mono- and di-valent salts, International Journal of Biological Macromolecules (2019), doi: https://doi.org/10.1016/ j.ijbiomac.2019.10.093
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The dependency of rheological properties of Plantago
1
lanceolata seed mucilage as a novel source of hydrocolloid
2
on mono- and di-valent salts
3
Mohammad Ali Hesarinejad1*, Elhamalsadat Shekarforoush2, Farnaz Rezayian Attar3, Sajad
4
Ghaderi4
5 6
Department of Food Processing, Research Institute of Food Science and Technology, Mashhad, Iran
7
University of Copenhagen, Department of Food Science, Rolighedsvej 30, DK-1958, Copenhagen, Denmark
8
1 2
3
Department of Food Science and Technology, Ferdowsi University of Mashhad, Mashhad, Iran 4
Faculty of Health and Nutrition, Yasuj University of Medical Science (YUMS), Yasuj, Iran
9 10 11
Abstract
12
The effects of NaCl and CaCl2 (0-200 mM) on the rheological properties of Plantago
13
lanceolata seed mucilage (PLSM) as a novel potential source of polysaccharide gum were
14
investigated in this study. Furthermore, FTIR analysis was measured to supply more
15
structural information. The FTIR spectra revealed that PLSM with the presence of glycoside
16
bonds and carboxyl and hydroxyl groups behave like a typical polyelectrolyte. It was
17
observed that the gum solutions exhibited viscoelastic properties under the given conditions
18
and the addition of salts had significant affection on the rheological parameters of gum
19
solutions. The weak gel-like behavior (0.1
20
types and ionic strengths. The limiting values of strain mostly increased with enhance cation
21
concentration due to the intermolecular interaction and therefore increase the stiffness of gum
22
solutions in the concentrated domain. The frequency sweep results showed that developing
23
ion concentration had a positive effect on the viscoelasticity of gum solutions which Ca2+ was
24
more effective than Na+. Tanglertpaibul&Rao model showed the highest efficiency to
25
evaluate the intrinsic viscosity of PLSM for all co-solutes. The results of this study could be
26
useful when considering the effects of salts on food systems.
27 28
Keywords: Plantago lanceolata seed mucilage; Salt; Rheology.
29 30
*
corresponding author: [email protected]; [email protected]
1. Introduction
31
Polysaccharide gums are mainly long chain complex polymers with hydrophilic nature to
32
categorize based on their sources to improve specific functional properties of food, cosmetic
33
and pharmaceutical products by rheology, solubility, water and oil binding capacity, gelling
34
and emulsifying activity [1]. Plant-based gums are mainly complex polysaccharides from
35
different parts of plants [2]. In comparison to other gums from animal and microbial sources,
36
they are more demanded due to their safe green nature and proper customer feedback [3].
37
Gum refers to any various viscous or sticky materials that are exuded by certain plants
38
while mucilage is a thick gluey substance produced by some plants and microorganisms.
39
Mucilage is a physiological product and is often found in various parts of plants. Mucilage is
40
an edible material and is also applied in medicine. Plantago lanceolata seed mucilage
41
(PLSM) is seed polysaccharide mucilage is extracted from the Plantago lanceolata. It is a
42
species of flowering plant in the plantain family Plantaginaceae. P. lanceolata is native to
43
Eurasia however; it is widely distributed all over the world [4,5]. PLSM mentioned as novel
44
polysaccharide mucilage by Hesarinejad et al.[6], which is included arabinoxylomannan-type
45
polysaccharide with 69.42% Mannose, 11.98% Arabinose, 7.42% Xylose, and 11.18% other
46
monosaccharides contained Rhamnose, Galactose, Glucose, and Uronic acids. They also
47
stated that PLSM behaved like a typical polyelectrolyte mucilage due to the presence of
48
glycoside bonds and hydroxyl, carboxyl and the relatively moderate amount of acidic
49
polysaccharides [6]. Therefore, the addition of cations should be a notable effect on the
50
dynamic rheological properties of ionic nature of PLSM. The cations affect the gel strength
51
or viscosity with an impression on the balance of attractive and repulsive forces between the
52
molecules. This effect, which is explored by the rheological properties, has been widely
53
applied for the characterization of mucilage [2,7–10].
54
The influence of cations on rheological behavior is important not only to identify
55
whether the polysaccharide mucilage behaves as polyelectrolyte but also to determine
56
functional rheological attributes. Despite the great potential of the effect of cations on
57
functionalities of this rarely investigated mucilage, no study has been introduced on its
58
rheological behavior when commonly used salts such as NaCl and CaCl2 are present. The
59
literature review indicates there is no information on the action of cations on the rheological
60
behavior of this novel mucilage. Therefore, in the present study, we report for the first time
61
the influence of different concentrations of some common salts in foods (i.e. NaCl and
62
CaCl2) on intrinsic viscosity and dynamic rheological properties of PLSM to use in the
63
development of foods.
64
65
2. Materials and methods
66
2.1.
67
Materials
P. lanceolata seeds were purchased by the medical plant market in Mashhad, Iran. The
68
cleaned seeds were packed in plastic shopping bags and kept in a dry and cool place. NaCl
69
and CaCl2 were obtained from Merck (Darmstadt, Germany) and AppliChem (Darmstadt,
70
Germany), respectively.
71 72
2.2.
Extraction of Plantago lanceolata seed mucilage
73
All over the extraction time (1 h), the seed–water (1:20, seed: water ratio) slurry was
74
stirred continuously with a mechanical mixing paddle. Husks of the seeds were separated
75
using a centrifuge (27-cm basket and 1-mm mesh). After the seeds were discarded, the
76
dispersion was freeze-dried, milled and sieved using a mesh 18 sifter. The powder was kept
77
in an air-tight test tube at room temperature for the experiments. Chemical analysis showed
78
that the PLSM contained 4.01% protein, 3.17% moisture, 5.47% ash and no fat content. The
79
total carbohydrate content was 87.35% [6]. All chemicals were in analytical grade.
80 81
2.3.
Fourier transform infrared spectroscopy (FTIR)
FTIR spectra of PLSM were recorded on a Bruker IFS66 spectrometer in the spectral range 4000–550 cm−1.
82 83 84 85
2.4.
Small amplitude oscillatory shear measurements
86
PLSM was dissolved in deionized water at a concentration of 1% w/v. Then the various
87
concentrations of NaCl and CaCl2 (0, 15, 50, 100 and 200 mM) were added with stirring for
88
30 min. Finally, the solutions left at 4°C overnight (complete hydration), after stirred by
89
roller shaker for 24 h at room temperature. The Rheological properties of PLSM solutions in
90
water were examined by low amplitude oscillatory shear measurements using HAAKE
91
MARS III rheometer (Thermo Scientific, Karlsruhe, Germany) equipped with a Peltier plate
92
for temperature control. A parallel plate sensor (35 mm diameter) was used. The RheoWin
93
software 3.61 (Thermo Fisher Scientific) was evaluated the data. The experiments were
94
performed at least in triplicate.
95
First, the LVE region must be determined by performing an amplitude sweep measurements (0.01–100%) at a constant frequency (1 Hz) and temperature (25 °C).
96 97
Frequency sweep tests were carried to specify the viscoelastic properties of PLSM in
98
presence of salts at a constant shear stress of 1 Pa (within the LVE region) and frequency
99
sweeps (0.1 to 100 Hz). In this test, a strain of 0.2% was applied to make the minimum
100
disturb of the network of the PLSM solution.
101 102
2.5.
Dilute solution properties
103
2.5.1.
Intrinsic viscosity determination
104
The viscosity of PLSM solutions was determined using a size 75 Cannon-Ubbelohde
105
capillary viscometer (Cannon Instruments Co., Germany; viscometer constant, k=0.01875
106
mm2s-2) immersed in a thermostatic water bath under accurate temperature control (±0.1°C).
107
All the measurements were reported as average values± standard deviation. The relative
108
viscosity (ηrel) and specific viscosity (ηsp) were calculated as follows
109
rel
sp
0
0 rel 1 0
(1)
110
(2)
111
where η is dynamic viscosity (i.e. corrected for density) of PLSM solution and η 0 is the dynamic viscosity of solvent (de-ionized water).
112 113
The intrinsic viscosity [] was experimentally calculated by measurement of viscosity at
114
very low concentrations. Measurements were made at different concentrations and
115
extrapolated to infinite dilution using both Huggins (Eqn. 3) and Kraemer (Eqn. 4) models
116
[11,12]:
117
sp
k H C
(3)
118
ln rel 2 k K C C
(4)
119
C
Here, kH, kK and C are the Huggins constant, the Kraemer constant and concentration,
120 121
respectively. The intrinsic viscosity can also be calculated by measuring the slope of relative viscosity
122
or specific viscosity vs. concentration. In fact, there are some developed equations (Eqns. 5-
123
8) to determine the intrinsic viscosity as follows:
124 125
Tanglertpaibul and Rao [13]:
rel 1 C Higiro, Herald, and Alavi [14]:
(5)
126 127
rel e C
rel
1
1 C
(6)
128
(7)
129 130
Fedors [15]: 1 1 2 rel 2 1
1 1 C C Max
(8)
Where, C is the polymer concentration (g/dL), Cmax is a factor indicating Fedors
131
132 133
concentration limit.
134
2.5.2.
Estimation of the molecular conformation
135
The molecular conformation of a polysaccharides can be estimated from the following
136
equation (Eqn. 9). 'b' component is the slope of a double logarithmic plot of ηsp versus
137
concentration [16].
138
sp aC b
(9)
139 140
2.5.3.
Determination of chain stiffness parameter
141
The salt tolerance (S) was calculated from the following equation (Eqn. 10) which is the
142
slope of intrinsic viscosity at different ionic strengths versus the inverse square root of ionic
143
strength (I-0.5) [8,17]:
144
SI 0.5 (10) where, is the intrinsic viscosity at infinite ionic strength. S parameter can be used as a measure of chain stiffness.
145 146 147 148
2.6.
Statistical analysis
149
All determinations were performed in triplicate and results were reported as the mean
150
values ± of standard deviations. Rheological properties of polysaccharide mucilage solutions
151
were defined by applying linear regression method based on minimizing sum of squares in
152
Excel Microsoft Office software (v.2013). Significant differences (p < 0.05) among the
153
rheological parameter of polysaccharide mucilage solutions were evaluated with analysis of
154
variance (one-way ANOVA), using the SPSS (version 16.0) program.
155 156
3. Results and discussion
157
3.1.
158
FTIR
Figure 1 shows the FTIR spectrum of PLSM in the region of 4000–550 cm–1 at room
159
temperature (25°C). The peak in the region of 3000-3600 cm−1 was ascribed to O-H
160
stretching, and the band at 1656 cm−1 related to the COO− asymmetrical stretching of the
161
hydrogen-bonded carboxylic groups. In Figure 1, the region between 1150 and 950 cm−1 was
162
attributed to the vibrations of C-O, C-O-C glycosidic and C-O-H bonds. The two peaks 881
163
and 776 cm–1 may also be assigned as =CH out of plane. Similar spectra were obtained for
164
other mucilages such as basil, tamarind, and Plantago major seed mucilage [18–21].
165 166
167 Figure 1. FTIR spectra and the main absorption peaks and their tentative assignments of
PLSM.
168 169 170
3.2. Dynamic rheological measurements
171
3.2.1. Strain sweep measurements
172
The effect of ionic strength on the elastic and loss modulus, strain (γL), and tan δLVE of
173
PLSM solutions are summarized in Table 1. The strain at which storage modulus decreased
174
sharply is defined as the critical strain. The critical strain is reflected in the maximum
175
deformability which the hydrocolloid could retain without structural collapse [22]. In
176
addition, most solid foods have LVE region in the range of 0.1-2% [23].
177
The G′ of PLSM solution remaining constant at strain about 1.3% at low concentrations
178
of CaCl2. With increasing divalent cation concentration, the γL increased to more than 2%
179
(Table 1). This shows that increasing CaCl2 concentration increased the resistance to elastic
180
deformation (yield strain) of PLSM solution due to the interchains interaction to increase in
181
the stiffness of mucilage solutions in the concentrated domain at the presence of divalent
182
[24].
183
The γL of mucilage solution was at the highest level in the presence of 100 mM NaCl,
184
and then the γL was decreased at 200 mM of mono-valent salt. This decrease might be
185
attributed to the progressive suppression of intermolecular charge–charge repulsion and
186
consequent contraction of the polysaccharide molecules [25]. Similar reduction was reported
187
by Sherahi et al. [26] for Descurainia sophia seed gum. It has been reported that the affinity
188
of PLSM for cations is proportional to the charge/ion radius ratios and the small ions with
189
high charge have a stronger affinity for chain binding sites [27–29].
190
An increase in the divalent salt concentration resulted in viscoelastic modulus reduction,
191
as expected for such polyelectrolytes. According to Medina-Torres, Brito-De La Fuente,
192
Torrestiana-Sanchez, & Katthain [30], the addition of positive ions reduces repulsion forces
193
with the expansion of molecule in a negatively charged polyelectrolyte molucles leads to a
194
viscoelastic modulus decline. The G′ and G″ reductions were more dependent on the Ca2+ ion
195
rather than Na+ concentration. These results suggest that PLSM is a negatively charged
196
polyelectrolyte molecule. With the addition of divalent salt, the values of elastic and loss
197
modulus decreased which indicates that structural strength (G′LVE) of the system was
198
diminished. A similar trend was reported for quince seed mucilage and xanthan gum by
199
Turkoz et al.[31] and Rezagholi et al. [32], respectively.
200
A tan δ less than 1 indicates predominantly elastic behavior. As shown in Table 1, the tan
201
δ values for PLSM in the presence of mono and divalent salts were smaller than unity (0.17 –
202
0.23), indicating that the solutions were more elastic than viscous. Tan δ in the numerical
203
range of 0.2–0.3 is reported for amorphous polymers [33]. As indicated by tan δ values, the
204
elasticity of the PLSM slightly increased with increasing concentrations of NaCl and CaCl 2
205
from 15 to 200 mM.
206 207
Table 1. Viscoelastic parameters for PLSM solutions in the presence of cations at different concentrations, as quantified by strain sweep experiments at frequency of 1 Hz. Cosolute Water NaCl
Salt Conc. (mM) 15 50
G'LVE (Pa) 157.4±8.5b 117.8±9.0a 487.9±6.2d
G''LVE (Pa) 32.3±3.3ab 28.1±4.6a 109.8±8.0d
γL (%) 1.04 1.17 1.21
Tan δLVE 0.20 0.23 0.22
208 209
CaCl2
100 200 15 50 100 200
1517.0±11.5e 1833.2±9.4g 231.2±8.7c 498.1±12.0d 1634.3±9.6f 2009.6±13.4h
323.5±11.0f 419.8±4.4g 46.2±3.4b 94.6±6.3c 277.8±5.7e 361.7±13.1fg
1.28 1.23 1.29 1.59 1.89 2.21
0.21 0.22 0.20 0.19 0.17 0.18
The given values represent the average of three independent measurements. Different letters (a-g) in the same column indicate significant differences at 5%.
210 211 212
3.2.2. Frequency sweep measurements
213
The rheological properties are affected by the addition of salts in PLSM solutions as
214
polyelectrolyte solution. The value of elastic modulus was always higher than loss modulus
215
and no crossover point occurred, representing typical weak gel-like behavior (Fig. 2).
216
Therefore, PLSM solutions behave more elastic than viscous; it means that the deformation
217
will mostly be elastic and recoverable.
218
As shown in Figure 2, the rheological characteristics of PLSM were influenced by the
219
addition of mono- and divalent salts due to the ionic nature of the biopolymer [28]. These
220
rheological parameters changed more harshly with the adding of calcium ions rather than
221
sodium ions. The less frequency dependency and higher increase in the elastic modulus when
222
the CaCl2 is added, demonstrate the change in the network structure of PLSM from
223
concentrated solutions to elastic gels [22,34].
224 225
226
227 Figure 2. Dynamic mechanical spectra (closed symbols, storage modulus, G′; open symbols, loss modulus, G′′) measured at 0.2% strain, in the presence of different NaCl and CaCl 2 concentrations. All measurements were made at 25 °C.
228 229 230 231
The G′ and G″ of PLSM in the presence of CaCl2 slightly increased with the addition of
232
salts to 200 mM. The presence of ions may promote interactions between chains and thus an
233
increase in the viscosity [29]. It seems that Ca2+ could modify the network structure of PLSM
234
through cross-linking with carboxyl groups. Similar observations have been reported by Lin
235
et al. [2] for mulberry leaf hydrocolloids. It was also reported by Goh et al. (2006) that the
236
polysaccharide is related to the double-helices which form a three-dimensional gel network
237
[35]. Rodrıguez-Hernandez, Durand, Garnier, Tecante, & Doublier stated that the stronger
238
carboxylate–cation2+–carboxylate interactions evolve a higher capacity of adjacent helices
239
cross-linking when divalent ions are involved [36]. Similar to these promoted inter-chain
240
interactions and increased junction zones were reported for alginate, pectin and gellan gum
241
[29,37–39].
242
G′, G″ and Tanδ at the frequency of 1 Hz for samples containing mono and divalent salts
243
are arranged in Table 2. The G′ and G″ have steadily increased with the addition of mono and
244
divalent salts. It is believed that Na+ ion may create indirect cross-linking with the assistance
245
of water [2]. By shielding the electrostatic repulsion of the carboxylate groups, polyanion–
246
cation–water–cation–polyanion linkages between the adjacent chains of other linkages could
247
be evolved [29,40]. As mentioned before, the G′ of PLSM increased with the addition of 15 -
248
200 mM CaCl2. In justifying this phenomenon, the researchers have stated that the stronger
249
carboxylate–cation–carboxylate interactions, rendering a higher capacity of adjacent helices
250
cross-linking when divalent cations are involved [36]. As a result, an increase in the elastic
251
modulus may be observed in the presence of salts. Comparing the effect of NaCl and CaCl2,
252
on G′ of PLSM solutions showed that calcium ions had greater effects than sodium ions.
253
The tan δ is a beneficial parameter to evaluate the viscoelasticity of a sample at a given
254
frequency. The tan δ values lower than one means that the sample is predominantly elastic.
255
As shown in Table 2, the tan δ values of the samples in the presence of different
256
concentrations of cations were lower than 1, indicating that the solutions are more elastic than
257
viscous.
258 259
Table 2. The viscoelastic parameters of PLSM solutions in the presence of different concentrations of cations as quantified by frequency sweep experiments (frequency: 1 Hz). Cosolute
NaCl
CaCl2
Salt Conc. (mM) 15 50 100 200 15 50 100 200
G' (Pa) 159.6±1.0a 227.6±0.9a 100.8±0.6b 337.8±2.2e 241.3±1.0a 350.1±0.9a 408.5±0.6b 542.0±1.4c
G'' (Pa) 32.9±0.8a 48.9±0.9ab 19.1±0.7b 67.2±0.9d 52.0±0.8a 85.0±0.9ab 96.9±0.7b 117.5±0.1c
260 261
Tan δ 0.20 0.21 0.19 0.20 0.21 0.24 0.23 0.21
262
3.3. Dilute solution properties
263
In dilute solutions, the intrinsic viscosity is related to the dimension and conformation of
264
the macromolecular chains in a particular solvent because the biopolymer chains are
265
separated [41]. Therefore, intrinsic viscosity determination provides deep insights into the
266
molecular characteristics of a biopolymer [42]. The intrinsic viscosity of PLSM solutions at
267
different NaCl and CaCl2 concentrations were calculated using six models (Table 3). The
268
Tanglertpaibul & Rao model had a higher coefficient of determination than other models. A
269
similar result was observed by Sherahi et al. [26] for Descurainia sophia seed gum in the
270
presence of NaCl and CaCl2.
271
The intrinsic viscosity of PLSM was observed to be 16.58 dl/g in deionized water at 25
272
°C. This value was lower than those reported for Alyssum homolocarpum seed gum (18.33
273
dl.g-1)[43] , Balangu seed gum (72.3 dl.g-1) [10], Sage seed gum (24.32 dl.g-1) [44] and
274
Tragacanthin (19.60 dl.g-1) [45]. However, the intrinsic viscosity of PLSM was higher than
275
the values of Basil seed gum (8.38 dl.g-1) [46], Plantago major seed mucilage (14.24 dl.g-1)
276
[21], and Mucuna flagellipes seed gum (7.90 dl.g-1) [47]. Considering that the intrinsic
277
viscosity is related to molecular properties including molecular weight, molecular shape,
278
voluminosity, and chain conformation of polysaccharide molecules [48]; the difference in
279
hydrocolloid structure and the rheological behavior is entirely different from one
280
hydrocolloid solution to another [49].
281
The intrinsic viscosities appeared to have a decreasing trend as the ionic strength
282
increased (Table 3). In the case of Na+ ion, an increase in the ionic strength from 0 to 50 mM
283
caused an abrupt drop in PLSM intrinsic viscosity from 16.85±0.14 to 14.78±0.17 dl/g.
284
Increasing the ionic strength of PLSM solutions containing Ca2+ to 50 mM, decreased the
285
intrinsic viscosity of PLSM to 14.40±0.09 dl/g (Table 3). These findings show that divalent
286
salts had more ability to reduce the intrinsic viscosity of PLSM solution than mono-valent
287
salts [8,28]. However, increasing the ionic strength to 100 mM for Ca2+ and Na+ caused
288
PLSM solution to precipitate completely. This decline may be due to the shielding effect of
289
charges on the macromolecular chains [50]. Probably, this effect will become predominant by
290
increasing the ionic strength of the solution, therefore the aggregation between molecular
291
species and a diminution in intrinsic viscosity will be observed. Similar results were reported
292
for sage seed gum [51], Balangu seed gum [10], Descurainia sophia seed gum [26], Prunus
293
armeniaca gum [7], hydroxypropyl methyl cellulose [52], guar [53], κ-carrageenan [54],
294
locust bean and xanthan gum [41,53] when the salt concentration (in both mono and divalent
295
cations) increased. The impact of calcium ion on intrinsic viscosity was more considerable
296
than that induced by sodium ion, indicating monovalent salt was less effective in decreasing
297
molecular dimensions than divalent one. This is presumably because of the molecular cross-
298
linking between PLSM and Ca2+ and the occurrence of some aggregation, which resulted in a
299
greater amount of molecular contraction [55].
300 301
3.3.1. Molecular conformation
302
The most predominant polysaccharide conformation in dilute solution is the random coil
303
structure in which the molecules oscillate sequentially via Brownian motion [56]. The single
304
polysaccharide coils move freely in a dilute solution because they are far enough apart and
305
have little effect on each other [57]. The double logarithm plot of specific viscosity against
306
concentration is used to determine the coil overlap parameter and the dilute Newtonian
307
domain [58].
308
The slopes of the master curves in the dilute domain and concentrated regime are usually
309
about 1.4 and 3.3, respectively [58]. The power equation slope (b), berry number (C[η]) and
310
slope of master curves of PLSM solutions at various ion types and concentrations were
311
shown (Table 4). The slopes of master curves were in the range of 1.17 to 1.28 and 1.20 to
312
1.30 when different concentrations of NaCl and CaCl2 were added to PLSM solutions,
313
respectively. Hence, it can be observed that all PLSM solutions were in the dilute region
314
without molecular entanglements and coil overlapping at all co-solutes concentrations.
315
Furthermore, the Berry number laid within the range of 0.42–1.62 at all tested
316
conditions, displaying once again that no molecular entanglements and coil overlaps occurred
317
(Table 4). When the Berry number exceeds unity, the molecular entanglement and coil-
318
overlapping start to occur in the concentrated domain [59].
319
Some researchers also stated that in the dilute domain, the slope of the power-law model
320
(b value) higher than unity is concerned with entanglement [58] or random coil conformation
321
[56], while the lower ones pertain to the rod-like conformation [34]. The b values of PLSM
322
solutions changed from 1.22 to 1.31 and from 1.25 to 1.36 in the presence of NaCl and CaCl2,
323
respectively (Table 4). It represented that the molecular conformation of PLSM is probably
324
random coil in the presence of these cations [60–62]. This may be due to the shielding effect
325
of charges on polyelectrolyte chains [26]. Moreover, the b value increased when ions
326
concentration increased, which expressed that they were able to promote the random coil
327
conformation of PLSM. Hence, the molecular conformation of PLSM was still random coil in
328
the presence of these cation salts. Similar results were obtained for cress seed gum [61],
329
Balangu seed gum [10] and Descurainia sophia seed gum [26]. In contrast to these
330
observations, Lai & Chiang reported that b value for hsian-tsao leaf gum ranged from 0.78 to
331
0.8 in the dilute regime and concluded that the molecular conformation was more rod-like
332
rather than the random coil [34].
333
334
Table 3. Intrinsic viscosity values determined by five models for PLSM solutions at different concentrations of NaCl and CaCl2 Cosolute
Salt Conc. (mM)
Water
NaCl
CaCl2
Huggins (Eqn. 3)
Kraemer (Eqn. 4)
Tang. & Rao (Eqn. 5)
Higiro 1(Eqn. 6)
Higiro 2 (Eqn. 7)
Fedor (Eqn. 8)
[η]
R2
[η]
R2
[η]
R2
[η]
R2
[η]
R2
[η]
R2
0
12.36±0.22
0.93
12.08±0.10
0.94
16.85±0.14
0.98
10.00±0.07
0.97
9.41±0.09
0.93
9.47±0.19
0.97
10
12.19±0.17
0.90
11.81±0.11
0.91
15.64±0.18
0.98
9.64±0.13
0.97
9.15±0.13
0.94
8.97±0.14
0.94
15
11.88±0.10
0.90
11.44±0.10
0.84
15.04±0.15
0.98
9.20±0.10
0.98
8.83±0.11
0.90
8.81±0.09
0.92
50
11.73±0.11
0.87
10.69±0.10
0.79
14.78±0.17
0.98
8.85±0.09
0.98
8.61±0.10
0.88
8.60±0.15
0.92
100
-
-
-
-
-
-
-
-
-
-
-
-
200
-
-
-
-
-
-
-
-
-
-
-
-
10
11.89±0.11
0.87
11.41±0.17
0.89
15.39±0.10
0.98
9.81±0.08
0.98
9.24±0.11
0.94
9.12±0.17
0.91
15
11.71±0.12
0.81
10.57±0.11
0.83
14.94±0.11
0.98
9.49±0.10
0.98
8.86±0.14
0.94
8.93±0.12
0.94
50
11.34±0.15
0.77
10.33±0.10
0.80
14.40±0.09
0.98
8.99±0.09
0.97
8.69±0.10
0.91
8.74±0.14
0.91
100
-
-
-
-
-
-
-
-
-
-
-
-
200
-
-
-
-
-
-
-
-
-
-
-
-
335 336 337
Table 4. Some molecular parameters of PLSM solutions in the presence of NaCl and CaCl2 Solvent
water
b (Power equation slope) Berry number (C[η]) Master curve slope
NaCl
CaCl2
10
15
50
100
200
10
15
50
100
200
1.18
1.22
1.27
1.31
-
-
1.25
1.29
1.36
-
-
0.48-1.53
0.46-1.44
0.44-1.39
0.43-1.36
-
-
0.45-1.42
0.43-1.62
0.42-1.33
-
-
1.15
1.17
1.21
1.28
-
-
1.20
1.24
1.30
-
-
338 339
3.3.2. Estimation of the chain stiffness parameters
340
Based on Eq. 10, a plot of [η] vs. I-0.5 was outlined to determine the stiffness parameter
341
(S) and intrinsic viscosity for PLSM solutions at infinite ionic strength ([η]∞) of Na+ and Ca2+
342
ions (Figure 3). It can be seen that there was a linear trend for both ions studied (R2 > 0.93),
343
following the relationship explained by Smidsrød & Haug [17]. In this regard, [η]∞ was found
344
to be 14.03 dl/g and 14.28 dl/g for PLSM solutions in the presence of Na+ and Ca2+ ions,
345
respectively.
346
The values of chain stiffness for PLSM in NaCl and CaCl2 solutions were 0.16 and 0.11,
347
respectively. The value of chain stiffness parameter for PLSM in NaCl solution was lower
348
than tragacantin (0.6), Balangu seed gum (0.346) and Sage seed gum (0.381) that reported by
349
Mohammadifar et al., Amini & Razavi, and Yousefi et al., respectively [10,45,51]. In
350
addition, in CaCl2 solution, this parameter was lower than that for Balangu seed gum (0.507)
351
[10] and Sage seed gum (0.821) [51] which shows that PLSM had a rather flexible
352
conformation. The higher values of stiffness parameter for divalent ion indicate that it made
353
more interactions in PLSM chain than monovalent one.
354 355
356 Figure 3. Dependence of intrinsic viscosity ([η]) on inverse square root of ionic strength (I−0.5) for PLSM solutions.
357 358 359
4. Conclusion
360
In this study, the effects of NaCl and CaCl2 salts on the dilute solution and dynamic
361
rheological properties of PLSM solutions were investigated in order to shed light on their
362
behaviors in real systems. The small amplitude oscillatory shear measurements of PLSM
363
solutions at all ion types and concentrations represented weak gel-like behavior. The tan δ
364
values showed that PLSM can form weak elastic gels throughout the specified frequency
365
range at the presence of both ions studied. The G′ and G″ of PLSM solutions in the presence
366
of CaCl2 and NaCl steadily increased with the addition of salts. It also showed that calcium
367
ions had greater effects than sodium ions. Rheological measurements of PLSM in the dilute
368
region at the presence of CaCl2 and NaCl revealed that increasing the ion strength led to a
369
diminution in the intrinsic viscosity effectively. In other words, the intrinsic viscosities
370
appear to have a decreasing trend as ionic strength raised. Tanglertpaibul & Rao equation was
371
the best model for specifying the intrinsic viscosity of PLSM solutions at different ion types
372
and ionic strengths. The obtained b values for PLSM solutions at the evaluated conditions
373
were within the range of 1.18 –1.36, demonstrating that the molecular conformation of PLSM
374
is probably random coil. Overall, it can be concluded that the solvent quality diminished
375
significantly by adding salt regardless of the salt type. These results could be useful for
376
applying this novel hydrocolloid as a replacer for other commercial plant hydrocolloids in
377
food products.
378 379
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Highlights
548 549
The intrinsic viscosity of PLSM was decreased by an increase in salt (NaCl and CaCl2) concentration. Calcium ions had a more pronounced effect on PLSM molecular parameters in comparison with sodium ions. PLSM solutions represented weak gel-like behavior at all tested ion types and concentrations. The viscoelastic moduli of PLSM steadily increased with the addition of salts.
550 551 552 553 554 555 556 557
S0141-8130(19)34552-0 https://doi.org/10.1016/j.ijbiomac.2019.10.093 BIOMAC 13592
To appear in:
International Journal of Biological Macromolecules
Received Date: Revised Date: Accepted Date:
18 June 2019 6 October 2019 9 October 2019
Please cite this article as: M. Ali Hesarinejad, E. Shekarforoush, F. Rezayian Attar, S. Ghaderi, The dependency of rheological properties of Plantago lanceolata seed mucilage as a novel source of hydrocolloid on mono- and di-valent salts, International Journal of Biological Macromolecules (2019), doi: https://doi.org/10.1016/ j.ijbiomac.2019.10.093
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© 2019 Published by Elsevier B.V.
The dependency of rheological properties of Plantago
1
lanceolata seed mucilage as a novel source of hydrocolloid
2
on mono- and di-valent salts
3
Mohammad Ali Hesarinejad1*, Elhamalsadat Shekarforoush2, Farnaz Rezayian Attar3, Sajad
4
Ghaderi4
5 6
Department of Food Processing, Research Institute of Food Science and Technology, Mashhad, Iran
7
University of Copenhagen, Department of Food Science, Rolighedsvej 30, DK-1958, Copenhagen, Denmark
8
1 2
3
Department of Food Science and Technology, Ferdowsi University of Mashhad, Mashhad, Iran 4
Faculty of Health and Nutrition, Yasuj University of Medical Science (YUMS), Yasuj, Iran
9 10 11
Abstract
12
The effects of NaCl and CaCl2 (0-200 mM) on the rheological properties of Plantago
13
lanceolata seed mucilage (PLSM) as a novel potential source of polysaccharide gum were
14
investigated in this study. Furthermore, FTIR analysis was measured to supply more
15
structural information. The FTIR spectra revealed that PLSM with the presence of glycoside
16
bonds and carboxyl and hydroxyl groups behave like a typical polyelectrolyte. It was
17
observed that the gum solutions exhibited viscoelastic properties under the given conditions
18
and the addition of salts had significant affection on the rheological parameters of gum
19
solutions. The weak gel-like behavior (0.1
20
types and ionic strengths. The limiting values of strain mostly increased with enhance cation
21
concentration due to the intermolecular interaction and therefore increase the stiffness of gum
22
solutions in the concentrated domain. The frequency sweep results showed that developing
23
ion concentration had a positive effect on the viscoelasticity of gum solutions which Ca2+ was
24
more effective than Na+. Tanglertpaibul&Rao model showed the highest efficiency to
25
evaluate the intrinsic viscosity of PLSM for all co-solutes. The results of this study could be
26
useful when considering the effects of salts on food systems.
27 28
Keywords: Plantago lanceolata seed mucilage; Salt; Rheology.
29 30
*
corresponding author: [email protected]; [email protected]
1. Introduction
31
Polysaccharide gums are mainly long chain complex polymers with hydrophilic nature to
32
categorize based on their sources to improve specific functional properties of food, cosmetic
33
and pharmaceutical products by rheology, solubility, water and oil binding capacity, gelling
34
and emulsifying activity [1]. Plant-based gums are mainly complex polysaccharides from
35
different parts of plants [2]. In comparison to other gums from animal and microbial sources,
36
they are more demanded due to their safe green nature and proper customer feedback [3].
37
Gum refers to any various viscous or sticky materials that are exuded by certain plants
38
while mucilage is a thick gluey substance produced by some plants and microorganisms.
39
Mucilage is a physiological product and is often found in various parts of plants. Mucilage is
40
an edible material and is also applied in medicine. Plantago lanceolata seed mucilage
41
(PLSM) is seed polysaccharide mucilage is extracted from the Plantago lanceolata. It is a
42
species of flowering plant in the plantain family Plantaginaceae. P. lanceolata is native to
43
Eurasia however; it is widely distributed all over the world [4,5]. PLSM mentioned as novel
44
polysaccharide mucilage by Hesarinejad et al.[6], which is included arabinoxylomannan-type
45
polysaccharide with 69.42% Mannose, 11.98% Arabinose, 7.42% Xylose, and 11.18% other
46
monosaccharides contained Rhamnose, Galactose, Glucose, and Uronic acids. They also
47
stated that PLSM behaved like a typical polyelectrolyte mucilage due to the presence of
48
glycoside bonds and hydroxyl, carboxyl and the relatively moderate amount of acidic
49
polysaccharides [6]. Therefore, the addition of cations should be a notable effect on the
50
dynamic rheological properties of ionic nature of PLSM. The cations affect the gel strength
51
or viscosity with an impression on the balance of attractive and repulsive forces between the
52
molecules. This effect, which is explored by the rheological properties, has been widely
53
applied for the characterization of mucilage [2,7–10].
54
The influence of cations on rheological behavior is important not only to identify
55
whether the polysaccharide mucilage behaves as polyelectrolyte but also to determine
56
functional rheological attributes. Despite the great potential of the effect of cations on
57
functionalities of this rarely investigated mucilage, no study has been introduced on its
58
rheological behavior when commonly used salts such as NaCl and CaCl2 are present. The
59
literature review indicates there is no information on the action of cations on the rheological
60
behavior of this novel mucilage. Therefore, in the present study, we report for the first time
61
the influence of different concentrations of some common salts in foods (i.e. NaCl and
62
CaCl2) on intrinsic viscosity and dynamic rheological properties of PLSM to use in the
63
development of foods.
64
65
2. Materials and methods
66
2.1.
67
Materials
P. lanceolata seeds were purchased by the medical plant market in Mashhad, Iran. The
68
cleaned seeds were packed in plastic shopping bags and kept in a dry and cool place. NaCl
69
and CaCl2 were obtained from Merck (Darmstadt, Germany) and AppliChem (Darmstadt,
70
Germany), respectively.
71 72
2.2.
Extraction of Plantago lanceolata seed mucilage
73
All over the extraction time (1 h), the seed–water (1:20, seed: water ratio) slurry was
74
stirred continuously with a mechanical mixing paddle. Husks of the seeds were separated
75
using a centrifuge (27-cm basket and 1-mm mesh). After the seeds were discarded, the
76
dispersion was freeze-dried, milled and sieved using a mesh 18 sifter. The powder was kept
77
in an air-tight test tube at room temperature for the experiments. Chemical analysis showed
78
that the PLSM contained 4.01% protein, 3.17% moisture, 5.47% ash and no fat content. The
79
total carbohydrate content was 87.35% [6]. All chemicals were in analytical grade.
80 81
2.3.
Fourier transform infrared spectroscopy (FTIR)
FTIR spectra of PLSM were recorded on a Bruker IFS66 spectrometer in the spectral range 4000–550 cm−1.
82 83 84 85
2.4.
Small amplitude oscillatory shear measurements
86
PLSM was dissolved in deionized water at a concentration of 1% w/v. Then the various
87
concentrations of NaCl and CaCl2 (0, 15, 50, 100 and 200 mM) were added with stirring for
88
30 min. Finally, the solutions left at 4°C overnight (complete hydration), after stirred by
89
roller shaker for 24 h at room temperature. The Rheological properties of PLSM solutions in
90
water were examined by low amplitude oscillatory shear measurements using HAAKE
91
MARS III rheometer (Thermo Scientific, Karlsruhe, Germany) equipped with a Peltier plate
92
for temperature control. A parallel plate sensor (35 mm diameter) was used. The RheoWin
93
software 3.61 (Thermo Fisher Scientific) was evaluated the data. The experiments were
94
performed at least in triplicate.
95
First, the LVE region must be determined by performing an amplitude sweep measurements (0.01–100%) at a constant frequency (1 Hz) and temperature (25 °C).
96 97
Frequency sweep tests were carried to specify the viscoelastic properties of PLSM in
98
presence of salts at a constant shear stress of 1 Pa (within the LVE region) and frequency
99
sweeps (0.1 to 100 Hz). In this test, a strain of 0.2% was applied to make the minimum
100
disturb of the network of the PLSM solution.
101 102
2.5.
Dilute solution properties
103
2.5.1.
Intrinsic viscosity determination
104
The viscosity of PLSM solutions was determined using a size 75 Cannon-Ubbelohde
105
capillary viscometer (Cannon Instruments Co., Germany; viscometer constant, k=0.01875
106
mm2s-2) immersed in a thermostatic water bath under accurate temperature control (±0.1°C).
107
All the measurements were reported as average values± standard deviation. The relative
108
viscosity (ηrel) and specific viscosity (ηsp) were calculated as follows
109
rel
sp
0
0 rel 1 0
(1)
110
(2)
111
where η is dynamic viscosity (i.e. corrected for density) of PLSM solution and η 0 is the dynamic viscosity of solvent (de-ionized water).
112 113
The intrinsic viscosity [] was experimentally calculated by measurement of viscosity at
114
very low concentrations. Measurements were made at different concentrations and
115
extrapolated to infinite dilution using both Huggins (Eqn. 3) and Kraemer (Eqn. 4) models
116
[11,12]:
117
sp
k H C
(3)
118
ln rel 2 k K C C
(4)
119
C
Here, kH, kK and C are the Huggins constant, the Kraemer constant and concentration,
120 121
respectively. The intrinsic viscosity can also be calculated by measuring the slope of relative viscosity
122
or specific viscosity vs. concentration. In fact, there are some developed equations (Eqns. 5-
123
8) to determine the intrinsic viscosity as follows:
124 125
Tanglertpaibul and Rao [13]:
rel 1 C Higiro, Herald, and Alavi [14]:
(5)
126 127
rel e C
rel
1
1 C
(6)
128
(7)
129 130
Fedors [15]: 1 1 2 rel 2 1
1 1 C C Max
(8)
Where, C is the polymer concentration (g/dL), Cmax is a factor indicating Fedors
131
132 133
concentration limit.
134
2.5.2.
Estimation of the molecular conformation
135
The molecular conformation of a polysaccharides can be estimated from the following
136
equation (Eqn. 9). 'b' component is the slope of a double logarithmic plot of ηsp versus
137
concentration [16].
138
sp aC b
(9)
139 140
2.5.3.
Determination of chain stiffness parameter
141
The salt tolerance (S) was calculated from the following equation (Eqn. 10) which is the
142
slope of intrinsic viscosity at different ionic strengths versus the inverse square root of ionic
143
strength (I-0.5) [8,17]:
144
SI 0.5 (10) where, is the intrinsic viscosity at infinite ionic strength. S parameter can be used as a measure of chain stiffness.
145 146 147 148
2.6.
Statistical analysis
149
All determinations were performed in triplicate and results were reported as the mean
150
values ± of standard deviations. Rheological properties of polysaccharide mucilage solutions
151
were defined by applying linear regression method based on minimizing sum of squares in
152
Excel Microsoft Office software (v.2013). Significant differences (p < 0.05) among the
153
rheological parameter of polysaccharide mucilage solutions were evaluated with analysis of
154
variance (one-way ANOVA), using the SPSS (version 16.0) program.
155 156
3. Results and discussion
157
3.1.
158
FTIR
Figure 1 shows the FTIR spectrum of PLSM in the region of 4000–550 cm–1 at room
159
temperature (25°C). The peak in the region of 3000-3600 cm−1 was ascribed to O-H
160
stretching, and the band at 1656 cm−1 related to the COO− asymmetrical stretching of the
161
hydrogen-bonded carboxylic groups. In Figure 1, the region between 1150 and 950 cm−1 was
162
attributed to the vibrations of C-O, C-O-C glycosidic and C-O-H bonds. The two peaks 881
163
and 776 cm–1 may also be assigned as =CH out of plane. Similar spectra were obtained for
164
other mucilages such as basil, tamarind, and Plantago major seed mucilage [18–21].
165 166
167 Figure 1. FTIR spectra and the main absorption peaks and their tentative assignments of
PLSM.
168 169 170
3.2. Dynamic rheological measurements
171
3.2.1. Strain sweep measurements
172
The effect of ionic strength on the elastic and loss modulus, strain (γL), and tan δLVE of
173
PLSM solutions are summarized in Table 1. The strain at which storage modulus decreased
174
sharply is defined as the critical strain. The critical strain is reflected in the maximum
175
deformability which the hydrocolloid could retain without structural collapse [22]. In
176
addition, most solid foods have LVE region in the range of 0.1-2% [23].
177
The G′ of PLSM solution remaining constant at strain about 1.3% at low concentrations
178
of CaCl2. With increasing divalent cation concentration, the γL increased to more than 2%
179
(Table 1). This shows that increasing CaCl2 concentration increased the resistance to elastic
180
deformation (yield strain) of PLSM solution due to the interchains interaction to increase in
181
the stiffness of mucilage solutions in the concentrated domain at the presence of divalent
182
[24].
183
The γL of mucilage solution was at the highest level in the presence of 100 mM NaCl,
184
and then the γL was decreased at 200 mM of mono-valent salt. This decrease might be
185
attributed to the progressive suppression of intermolecular charge–charge repulsion and
186
consequent contraction of the polysaccharide molecules [25]. Similar reduction was reported
187
by Sherahi et al. [26] for Descurainia sophia seed gum. It has been reported that the affinity
188
of PLSM for cations is proportional to the charge/ion radius ratios and the small ions with
189
high charge have a stronger affinity for chain binding sites [27–29].
190
An increase in the divalent salt concentration resulted in viscoelastic modulus reduction,
191
as expected for such polyelectrolytes. According to Medina-Torres, Brito-De La Fuente,
192
Torrestiana-Sanchez, & Katthain [30], the addition of positive ions reduces repulsion forces
193
with the expansion of molecule in a negatively charged polyelectrolyte molucles leads to a
194
viscoelastic modulus decline. The G′ and G″ reductions were more dependent on the Ca2+ ion
195
rather than Na+ concentration. These results suggest that PLSM is a negatively charged
196
polyelectrolyte molecule. With the addition of divalent salt, the values of elastic and loss
197
modulus decreased which indicates that structural strength (G′LVE) of the system was
198
diminished. A similar trend was reported for quince seed mucilage and xanthan gum by
199
Turkoz et al.[31] and Rezagholi et al. [32], respectively.
200
A tan δ less than 1 indicates predominantly elastic behavior. As shown in Table 1, the tan
201
δ values for PLSM in the presence of mono and divalent salts were smaller than unity (0.17 –
202
0.23), indicating that the solutions were more elastic than viscous. Tan δ in the numerical
203
range of 0.2–0.3 is reported for amorphous polymers [33]. As indicated by tan δ values, the
204
elasticity of the PLSM slightly increased with increasing concentrations of NaCl and CaCl 2
205
from 15 to 200 mM.
206 207
Table 1. Viscoelastic parameters for PLSM solutions in the presence of cations at different concentrations, as quantified by strain sweep experiments at frequency of 1 Hz. Cosolute Water NaCl
Salt Conc. (mM) 15 50
G'LVE (Pa) 157.4±8.5b 117.8±9.0a 487.9±6.2d
G''LVE (Pa) 32.3±3.3ab 28.1±4.6a 109.8±8.0d
γL (%) 1.04 1.17 1.21
Tan δLVE 0.20 0.23 0.22
208 209
CaCl2
100 200 15 50 100 200
1517.0±11.5e 1833.2±9.4g 231.2±8.7c 498.1±12.0d 1634.3±9.6f 2009.6±13.4h
323.5±11.0f 419.8±4.4g 46.2±3.4b 94.6±6.3c 277.8±5.7e 361.7±13.1fg
1.28 1.23 1.29 1.59 1.89 2.21
0.21 0.22 0.20 0.19 0.17 0.18
The given values represent the average of three independent measurements. Different letters (a-g) in the same column indicate significant differences at 5%.
210 211 212
3.2.2. Frequency sweep measurements
213
The rheological properties are affected by the addition of salts in PLSM solutions as
214
polyelectrolyte solution. The value of elastic modulus was always higher than loss modulus
215
and no crossover point occurred, representing typical weak gel-like behavior (Fig. 2).
216
Therefore, PLSM solutions behave more elastic than viscous; it means that the deformation
217
will mostly be elastic and recoverable.
218
As shown in Figure 2, the rheological characteristics of PLSM were influenced by the
219
addition of mono- and divalent salts due to the ionic nature of the biopolymer [28]. These
220
rheological parameters changed more harshly with the adding of calcium ions rather than
221
sodium ions. The less frequency dependency and higher increase in the elastic modulus when
222
the CaCl2 is added, demonstrate the change in the network structure of PLSM from
223
concentrated solutions to elastic gels [22,34].
224 225
226
227 Figure 2. Dynamic mechanical spectra (closed symbols, storage modulus, G′; open symbols, loss modulus, G′′) measured at 0.2% strain, in the presence of different NaCl and CaCl 2 concentrations. All measurements were made at 25 °C.
228 229 230 231
The G′ and G″ of PLSM in the presence of CaCl2 slightly increased with the addition of
232
salts to 200 mM. The presence of ions may promote interactions between chains and thus an
233
increase in the viscosity [29]. It seems that Ca2+ could modify the network structure of PLSM
234
through cross-linking with carboxyl groups. Similar observations have been reported by Lin
235
et al. [2] for mulberry leaf hydrocolloids. It was also reported by Goh et al. (2006) that the
236
polysaccharide is related to the double-helices which form a three-dimensional gel network
237
[35]. Rodrıguez-Hernandez, Durand, Garnier, Tecante, & Doublier stated that the stronger
238
carboxylate–cation2+–carboxylate interactions evolve a higher capacity of adjacent helices
239
cross-linking when divalent ions are involved [36]. Similar to these promoted inter-chain
240
interactions and increased junction zones were reported for alginate, pectin and gellan gum
241
[29,37–39].
242
G′, G″ and Tanδ at the frequency of 1 Hz for samples containing mono and divalent salts
243
are arranged in Table 2. The G′ and G″ have steadily increased with the addition of mono and
244
divalent salts. It is believed that Na+ ion may create indirect cross-linking with the assistance
245
of water [2]. By shielding the electrostatic repulsion of the carboxylate groups, polyanion–
246
cation–water–cation–polyanion linkages between the adjacent chains of other linkages could
247
be evolved [29,40]. As mentioned before, the G′ of PLSM increased with the addition of 15 -
248
200 mM CaCl2. In justifying this phenomenon, the researchers have stated that the stronger
249
carboxylate–cation–carboxylate interactions, rendering a higher capacity of adjacent helices
250
cross-linking when divalent cations are involved [36]. As a result, an increase in the elastic
251
modulus may be observed in the presence of salts. Comparing the effect of NaCl and CaCl2,
252
on G′ of PLSM solutions showed that calcium ions had greater effects than sodium ions.
253
The tan δ is a beneficial parameter to evaluate the viscoelasticity of a sample at a given
254
frequency. The tan δ values lower than one means that the sample is predominantly elastic.
255
As shown in Table 2, the tan δ values of the samples in the presence of different
256
concentrations of cations were lower than 1, indicating that the solutions are more elastic than
257
viscous.
258 259
Table 2. The viscoelastic parameters of PLSM solutions in the presence of different concentrations of cations as quantified by frequency sweep experiments (frequency: 1 Hz). Cosolute
NaCl
CaCl2
Salt Conc. (mM) 15 50 100 200 15 50 100 200
G' (Pa) 159.6±1.0a 227.6±0.9a 100.8±0.6b 337.8±2.2e 241.3±1.0a 350.1±0.9a 408.5±0.6b 542.0±1.4c
G'' (Pa) 32.9±0.8a 48.9±0.9ab 19.1±0.7b 67.2±0.9d 52.0±0.8a 85.0±0.9ab 96.9±0.7b 117.5±0.1c
260 261
Tan δ 0.20 0.21 0.19 0.20 0.21 0.24 0.23 0.21
262
3.3. Dilute solution properties
263
In dilute solutions, the intrinsic viscosity is related to the dimension and conformation of
264
the macromolecular chains in a particular solvent because the biopolymer chains are
265
separated [41]. Therefore, intrinsic viscosity determination provides deep insights into the
266
molecular characteristics of a biopolymer [42]. The intrinsic viscosity of PLSM solutions at
267
different NaCl and CaCl2 concentrations were calculated using six models (Table 3). The
268
Tanglertpaibul & Rao model had a higher coefficient of determination than other models. A
269
similar result was observed by Sherahi et al. [26] for Descurainia sophia seed gum in the
270
presence of NaCl and CaCl2.
271
The intrinsic viscosity of PLSM was observed to be 16.58 dl/g in deionized water at 25
272
°C. This value was lower than those reported for Alyssum homolocarpum seed gum (18.33
273
dl.g-1)[43] , Balangu seed gum (72.3 dl.g-1) [10], Sage seed gum (24.32 dl.g-1) [44] and
274
Tragacanthin (19.60 dl.g-1) [45]. However, the intrinsic viscosity of PLSM was higher than
275
the values of Basil seed gum (8.38 dl.g-1) [46], Plantago major seed mucilage (14.24 dl.g-1)
276
[21], and Mucuna flagellipes seed gum (7.90 dl.g-1) [47]. Considering that the intrinsic
277
viscosity is related to molecular properties including molecular weight, molecular shape,
278
voluminosity, and chain conformation of polysaccharide molecules [48]; the difference in
279
hydrocolloid structure and the rheological behavior is entirely different from one
280
hydrocolloid solution to another [49].
281
The intrinsic viscosities appeared to have a decreasing trend as the ionic strength
282
increased (Table 3). In the case of Na+ ion, an increase in the ionic strength from 0 to 50 mM
283
caused an abrupt drop in PLSM intrinsic viscosity from 16.85±0.14 to 14.78±0.17 dl/g.
284
Increasing the ionic strength of PLSM solutions containing Ca2+ to 50 mM, decreased the
285
intrinsic viscosity of PLSM to 14.40±0.09 dl/g (Table 3). These findings show that divalent
286
salts had more ability to reduce the intrinsic viscosity of PLSM solution than mono-valent
287
salts [8,28]. However, increasing the ionic strength to 100 mM for Ca2+ and Na+ caused
288
PLSM solution to precipitate completely. This decline may be due to the shielding effect of
289
charges on the macromolecular chains [50]. Probably, this effect will become predominant by
290
increasing the ionic strength of the solution, therefore the aggregation between molecular
291
species and a diminution in intrinsic viscosity will be observed. Similar results were reported
292
for sage seed gum [51], Balangu seed gum [10], Descurainia sophia seed gum [26], Prunus
293
armeniaca gum [7], hydroxypropyl methyl cellulose [52], guar [53], κ-carrageenan [54],
294
locust bean and xanthan gum [41,53] when the salt concentration (in both mono and divalent
295
cations) increased. The impact of calcium ion on intrinsic viscosity was more considerable
296
than that induced by sodium ion, indicating monovalent salt was less effective in decreasing
297
molecular dimensions than divalent one. This is presumably because of the molecular cross-
298
linking between PLSM and Ca2+ and the occurrence of some aggregation, which resulted in a
299
greater amount of molecular contraction [55].
300 301
3.3.1. Molecular conformation
302
The most predominant polysaccharide conformation in dilute solution is the random coil
303
structure in which the molecules oscillate sequentially via Brownian motion [56]. The single
304
polysaccharide coils move freely in a dilute solution because they are far enough apart and
305
have little effect on each other [57]. The double logarithm plot of specific viscosity against
306
concentration is used to determine the coil overlap parameter and the dilute Newtonian
307
domain [58].
308
The slopes of the master curves in the dilute domain and concentrated regime are usually
309
about 1.4 and 3.3, respectively [58]. The power equation slope (b), berry number (C[η]) and
310
slope of master curves of PLSM solutions at various ion types and concentrations were
311
shown (Table 4). The slopes of master curves were in the range of 1.17 to 1.28 and 1.20 to
312
1.30 when different concentrations of NaCl and CaCl2 were added to PLSM solutions,
313
respectively. Hence, it can be observed that all PLSM solutions were in the dilute region
314
without molecular entanglements and coil overlapping at all co-solutes concentrations.
315
Furthermore, the Berry number laid within the range of 0.42–1.62 at all tested
316
conditions, displaying once again that no molecular entanglements and coil overlaps occurred
317
(Table 4). When the Berry number exceeds unity, the molecular entanglement and coil-
318
overlapping start to occur in the concentrated domain [59].
319
Some researchers also stated that in the dilute domain, the slope of the power-law model
320
(b value) higher than unity is concerned with entanglement [58] or random coil conformation
321
[56], while the lower ones pertain to the rod-like conformation [34]. The b values of PLSM
322
solutions changed from 1.22 to 1.31 and from 1.25 to 1.36 in the presence of NaCl and CaCl2,
323
respectively (Table 4). It represented that the molecular conformation of PLSM is probably
324
random coil in the presence of these cations [60–62]. This may be due to the shielding effect
325
of charges on polyelectrolyte chains [26]. Moreover, the b value increased when ions
326
concentration increased, which expressed that they were able to promote the random coil
327
conformation of PLSM. Hence, the molecular conformation of PLSM was still random coil in
328
the presence of these cation salts. Similar results were obtained for cress seed gum [61],
329
Balangu seed gum [10] and Descurainia sophia seed gum [26]. In contrast to these
330
observations, Lai & Chiang reported that b value for hsian-tsao leaf gum ranged from 0.78 to
331
0.8 in the dilute regime and concluded that the molecular conformation was more rod-like
332
rather than the random coil [34].
333
334
Table 3. Intrinsic viscosity values determined by five models for PLSM solutions at different concentrations of NaCl and CaCl2 Cosolute
Salt Conc. (mM)
Water
NaCl
CaCl2
Huggins (Eqn. 3)
Kraemer (Eqn. 4)
Tang. & Rao (Eqn. 5)
Higiro 1(Eqn. 6)
Higiro 2 (Eqn. 7)
Fedor (Eqn. 8)
[η]
R2
[η]
R2
[η]
R2
[η]
R2
[η]
R2
[η]
R2
0
12.36±0.22
0.93
12.08±0.10
0.94
16.85±0.14
0.98
10.00±0.07
0.97
9.41±0.09
0.93
9.47±0.19
0.97
10
12.19±0.17
0.90
11.81±0.11
0.91
15.64±0.18
0.98
9.64±0.13
0.97
9.15±0.13
0.94
8.97±0.14
0.94
15
11.88±0.10
0.90
11.44±0.10
0.84
15.04±0.15
0.98
9.20±0.10
0.98
8.83±0.11
0.90
8.81±0.09
0.92
50
11.73±0.11
0.87
10.69±0.10
0.79
14.78±0.17
0.98
8.85±0.09
0.98
8.61±0.10
0.88
8.60±0.15
0.92
100
-
-
-
-
-
-
-
-
-
-
-
-
200
-
-
-
-
-
-
-
-
-
-
-
-
10
11.89±0.11
0.87
11.41±0.17
0.89
15.39±0.10
0.98
9.81±0.08
0.98
9.24±0.11
0.94
9.12±0.17
0.91
15
11.71±0.12
0.81
10.57±0.11
0.83
14.94±0.11
0.98
9.49±0.10
0.98
8.86±0.14
0.94
8.93±0.12
0.94
50
11.34±0.15
0.77
10.33±0.10
0.80
14.40±0.09
0.98
8.99±0.09
0.97
8.69±0.10
0.91
8.74±0.14
0.91
100
-
-
-
-
-
-
-
-
-
-
-
-
200
-
-
-
-
-
-
-
-
-
-
-
-
335 336 337
Table 4. Some molecular parameters of PLSM solutions in the presence of NaCl and CaCl2 Solvent
water
b (Power equation slope) Berry number (C[η]) Master curve slope
NaCl
CaCl2
10
15
50
100
200
10
15
50
100
200
1.18
1.22
1.27
1.31
-
-
1.25
1.29
1.36
-
-
0.48-1.53
0.46-1.44
0.44-1.39
0.43-1.36
-
-
0.45-1.42
0.43-1.62
0.42-1.33
-
-
1.15
1.17
1.21
1.28
-
-
1.20
1.24
1.30
-
-
338 339
3.3.2. Estimation of the chain stiffness parameters
340
Based on Eq. 10, a plot of [η] vs. I-0.5 was outlined to determine the stiffness parameter
341
(S) and intrinsic viscosity for PLSM solutions at infinite ionic strength ([η]∞) of Na+ and Ca2+
342
ions (Figure 3). It can be seen that there was a linear trend for both ions studied (R2 > 0.93),
343
following the relationship explained by Smidsrød & Haug [17]. In this regard, [η]∞ was found
344
to be 14.03 dl/g and 14.28 dl/g for PLSM solutions in the presence of Na+ and Ca2+ ions,
345
respectively.
346
The values of chain stiffness for PLSM in NaCl and CaCl2 solutions were 0.16 and 0.11,
347
respectively. The value of chain stiffness parameter for PLSM in NaCl solution was lower
348
than tragacantin (0.6), Balangu seed gum (0.346) and Sage seed gum (0.381) that reported by
349
Mohammadifar et al., Amini & Razavi, and Yousefi et al., respectively [10,45,51]. In
350
addition, in CaCl2 solution, this parameter was lower than that for Balangu seed gum (0.507)
351
[10] and Sage seed gum (0.821) [51] which shows that PLSM had a rather flexible
352
conformation. The higher values of stiffness parameter for divalent ion indicate that it made
353
more interactions in PLSM chain than monovalent one.
354 355
356 Figure 3. Dependence of intrinsic viscosity ([η]) on inverse square root of ionic strength (I−0.5) for PLSM solutions.
357 358 359
4. Conclusion
360
In this study, the effects of NaCl and CaCl2 salts on the dilute solution and dynamic
361
rheological properties of PLSM solutions were investigated in order to shed light on their
362
behaviors in real systems. The small amplitude oscillatory shear measurements of PLSM
363
solutions at all ion types and concentrations represented weak gel-like behavior. The tan δ
364
values showed that PLSM can form weak elastic gels throughout the specified frequency
365
range at the presence of both ions studied. The G′ and G″ of PLSM solutions in the presence
366
of CaCl2 and NaCl steadily increased with the addition of salts. It also showed that calcium
367
ions had greater effects than sodium ions. Rheological measurements of PLSM in the dilute
368
region at the presence of CaCl2 and NaCl revealed that increasing the ion strength led to a
369
diminution in the intrinsic viscosity effectively. In other words, the intrinsic viscosities
370
appear to have a decreasing trend as ionic strength raised. Tanglertpaibul & Rao equation was
371
the best model for specifying the intrinsic viscosity of PLSM solutions at different ion types
372
and ionic strengths. The obtained b values for PLSM solutions at the evaluated conditions
373
were within the range of 1.18 –1.36, demonstrating that the molecular conformation of PLSM
374
is probably random coil. Overall, it can be concluded that the solvent quality diminished
375
significantly by adding salt regardless of the salt type. These results could be useful for
376
applying this novel hydrocolloid as a replacer for other commercial plant hydrocolloids in
377
food products.
378 379
Reference [1]
[2]
[3]
[4]
[5]
[6]
380
E. Shekarforoush, H. Mirhosseini, B.T. Amid, H. Ghazali, K. Muhammad, M.Z.I.
381
Sarker, M. Paykary, Rheological Properties and Emulsifying Activity of Gum Karaya
382
(Sterculia Urens) in Aqueous System and Oil in Water Emulsion: Heat Treatment and
383
Microwave Modification, Int. J. Food Prop. 19 (2016) 662–679.
384
H.-Y. Lin, J.-C. Tsai, L.-S. Lai, Effect of salts on the rheology of hydrocolloids from
385
mulberry (Morus alba L.) leaves in concentrated domain, Food Hydrocoll. 23 (2009)
386
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Highlights
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The intrinsic viscosity of PLSM was decreased by an increase in salt (NaCl and CaCl2) concentration. Calcium ions had a more pronounced effect on PLSM molecular parameters in comparison with sodium ions. PLSM solutions represented weak gel-like behavior at all tested ion types and concentrations. The viscoelastic moduli of PLSM steadily increased with the addition of salts.
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