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Unexpected Rheological Behavior of Hydrophobic Associative Shellac-based Oligomeric Food Thickener Jianan Gao, Kun Li, Juan Xu, Wen-wen Zhang, Jinju Ma, Lanxiang Liu, Yanlin Sun, Hong Zhang, and Kai Li J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b01148 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018
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Unexpected Rheological Behavior of Hydrophobic Associative
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Shellac-based Oligomeric Food Thickener
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†‡
Jianan Gao,
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Kun Li, Juan Xu , Wenwen Zhang , Jinju Ma , Lanxiang Liu , Yanlin Sun, Hong †
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Zhang, * Kai Li *
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†Research Institute of Resources Insects, Chinese Academy of Forestry, Kunming, 650224,
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People’s Republic of China
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‡Faculty of Chemical Engineering and Technology, Kunming University of Science and
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Technology, 650224, People’s Republic of China
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∗
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(E - mail:
[email protected] Phone: +86-871-63860021)
Correspondence to Kai Li
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Abstract
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The sodium shellac constituted of “surfactant” monomer, which is sensitive to shear
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stress, exhibits shear-thickening behavior at low concentration (5 wt%), and reacts
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with H+ to retain the transient high viscosity under shear, is introduced in this study.
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The steady-shear flow test proved that under high shear rate, sodium shellac
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suspension could change from Newtonian fluid to continuous shear thickening
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non-Newtonian fluid. Dynamic oscillation test suggested that the sodium shellac
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solution at low concentration (0.1 and 1 wt%) under low shear rate behaved as the
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viscous fluid (G´´>G´), and the solution at high concentration (5, 10 and 15 wt%)
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behaved as the elastic fluid (G´´1, and also in this regime, the viscosity of the five curves increased
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mildly with the shear rate27. This indicated that under high shear rate, the sodium
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shellac suspension could change from Newtonian fluid to continuous shear thickening
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non-Newtonian fluid28.
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Figure 3. Shear stress ()versus shear rate ( ) for sodium shellac suspensions, plotted
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on a log-log scale.
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The relationship between shear rate ( ) and viscosity (η) was present in Figure 4.
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When the concentration of sodium shellac suspension was higher than 0.1 wt%, the
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shearing thickening could be detected. This finding was also in agreement of the
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previous result18, which stated that when the concentration of sodium shellac was
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higher than CMC (1.189 g/mL, 0.1189 wt%), it could aggregate into clusters or
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micelle beads. Thus, relationship curves of sodium shellac suspension with high
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concentration could be divided into three regimes. Firstly, with the increase in shear
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rate, the viscosity of sodium shellac decreased. This was a consequence of the
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organized motion of sodium shellac beads under shear flow. Then, the viscosity of the
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suspension under medium shear rate did not alter considerably. Notably, at higher
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shear rates, the viscosity was raised with shear rate. Under high shear rate, the sodium
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shellac clusters could get close to form big scale hydroclusters. Under the critical
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shear rate ( c ), the viscosity of the solution begin to gradually increase. Meanwhile,
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the c was related to the concentration of the solution. As shown in Figure 4 (b), the
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c of the sodium shellac suspensions with 5, 10 and 15 wt% was 131, 148 and 185 S-1
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respectively. The critical shear rate rise with the increasing concentration of sodium
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shellac (Figure 4b). On the contrary, the critical shear rate decrease with the growing
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concentration of hard square particle solution25. The shear-thickening reason for the
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hard square particle solution could be due to the large-scale transient hydroclusters
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assembled of hard square particles under strong hydrodynamic coupling29. So, it was
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accessible to formation of large-scale hydroclusters in the solution with more particles.
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But the solution of sodium shellac at the high concentration (> 5 wt%) behave as an
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elastic solution (G´ > G´´) like a virtual gel (Figure 6) . So, before the
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Shear-thickening, the solution should be changed from “elastic” to “viscous” state30.
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As we know, there was some interaction between the sodium shellac particles14, 18. So,
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more shear force was needed to disrupt the interactions among more particles.
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Figure 4 Dependence of the steady shear viscosity ( ) on the shear rate ( ) for
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sodium shellac suspensions with different concentrations at 25 oC (b).
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Dynamic oscillation behavior
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To our knowledge, the rheological property of sodium shellac in aqueous mode has
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not yet been studied. Thus, several parameters of this solution need to be detected, for
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example, the linear viscoelasticity region. Figure 5 showed that the linear-viscoelastic
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regime decreased as the increase in sodium shellac content. When strain was higher
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than 1.57%, the G´ of the sodium shellac suspension with 15 wt% had already begun
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to reduce. Conversely, linear viscoelasticity region of 0.1 wt% sodium shellac almost
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covers the whole shear rate region. Thus, it also could be suggested that the sodium
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shellac with high content was more unstable under high strain than that under low
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strain. As the Figure 5 showed, in the non-linear region of dynamic oscillation test
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sodium shellac at low concentration (0.1 wt%) could be assumed as “viscous”
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solution. Meanwhile, both viscoelastic modulus of the solution were nearly
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independent of strain amplitude. It could be noted that sodium solution at low
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concentration should be a true molecular solution in which there are no more
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interactions. At high concentration (1 wt% and 5 wt% ), the sodium shellac solutions
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also behave as viscous solutions under large strain amplitude, and G´´ was also
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independent of strain amplitude. But G´ decreased with the increase of the strain
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amplitude. So tan δ of the solution should also rise with the increasing of the strain
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amplitude. It meant that under large strain amplitude the viscous solution does not
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have enough time to respond to the stress. At higher concentration (10 wt% and 15
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wt% ), with increase strain amplitude the shellac solution changed from elastic fluid
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to viscous fluid. There was a “gel-sol” crosslinking point between the G´´ and G´. It
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could be interpreted that there were strong interactions between particles of sodium
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shellac solution at high concentration by which the solution behave like an elastic
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three dimension gel. But this virtual gel network could be destroyed under larger
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strain amplitude leading to make the solution act like a viscous dispersion solution.
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Figure 5 Dynamic strain sweep for sodium shellac with different concentration. (solid
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symbol represented G´ and hollow symbol represented G´´).
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Figure 6. Strain sweep for sodium shellac with different concentration. (solid symbol
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represented G´ and hollow symbol represented G´´).
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The Figure 6 revealed that the sodium shellac solution at low concentration (0.1 and 1
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wt%) and low shear rate regime represented classic viscous fluid behavior (G´´>G´),
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and the solution at the high concentration (5, 10 and 15 wt%) represented the elastic
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gel behavior (G´´G´), and the solution at the high
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concentration (5, 10 and 15 wt%), represented the elastic gel behavior (G´´
