Available online at www.sciencedirect.com
Procedia Food Science 1 (2011) 281 – 286
11th International Congress on Engineering and Food (ICEF11)
Viscoelastic characterization of fluid and gel like food
emulsions stabilized with hydrocolloids
Gabriel Lorenzoa,b, Noemi E. Zaritzky*a,b, Alicia N. Califanoa
a
Centro de Investigación y Desarrollo en Criotecnología de Alimentos (CIDCA), La Plata - CONICET. Facultad de Ciencias
Exactas, UNLP. 47 y 116, La Plata (1900). ArgentinaFacultad de Ingeniería, UNLP.
b
Departamento de Ingeniería Química,Facultad de Ingeniería, UNLP, La Plata, Argentina.
Abstract
Food emulsions exhibit a great diversity of rheological characteristics; hydrocolloids are usually added to deal with
creaming instability. Viscoelastic measurements provide information about the microstructure of the system. The
objectives of this work were: a) to determine the viscoelastic behavior of two different low in fat oil-in-water food
emulsions: a gel like and a fluid type emulsions stabilized with hydrocolloids (gellan gum and xanthan-guar mixtures
respectively) b) to model and predict the mechanical relaxation spectrum for both emulsions and continuous aqueous
phases. Low-in-fat oil-in-water emulsions (20g/100g) were prepared using sunflower oil and Tween 80 (1 wt.%).
Fluid emulsions containing xanthan and guar gums were formulated using a synergistic ratio 7:3, with total
hydrocolloid concentration ranging between 0.5 to 2 wt%. The aqueous phases contained NaCl (2 wt.%) and acetic
acid (2 wt.%). The effect of hydrocolloids was studied using oscillatory measurements (G’ and G’’ vs. frequency)
within the linear viscoelastic range previously determined by stress-sweeps. Time-Concentration Superposition
principle was applied to find the master curves that describe the mechanical spectra of the viscoelastic materials.
Superposition allows to obtain a wide spectrum of nearly ten decades of frequencies in emulsions containing
xanthan–guar mixtures, whereas gellan gum systems did not show a significant frequency displacement. Viscoelastic
behavior of the systems was satisfactorily modeled using Baumgaertel-Schausberger-Winter (BSW) equation. This
empirical model was used to predict the mechanical relaxation spectrum for both emulsions and continuous aqueous
phases. Validation of the predicted spectra was carried out through creep compliance data for emulsion-filled gels and
steady-state flow curves for emulsions containing xanthan–guar mixtures.
© 2011 Published by Elsevier B.V. Open access under CC BY-NC-ND license.
Selection and/or peer-review under responsibility of 11th International Congress on Engineering and Food (ICEF 11) Executive
Committee.
Keywords: Oil-water emulsions; rheology; viscoelastic behavior; hydrocolloids; mechanical relaxation spectrum; BSW model.
* Corresponding author: Tel/FAX: +54-221-4254853/4249287.
E-mail address: [email protected]; [email protected]
2211 601X © 2011 Published by Elsevier B.V. Open access under CC BY-NC-ND license.
Selection and/or peer-review under responsibility of 11th International Congress on Engineering and Food (ICEF 11) Executive Committee.
doi:10.1016/j.profoo.2011.09.044
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1.Introduction
Food emulsions exhibit a great diversity of rheological characteristics, ranging from low-viscosity
Newtonian liquids to viscoelastic and plastic materials. Hydrocolloids are usually key components in food
emulsions to deal with creaming instability. There is a growing emphasis on understanding the colloidal
basis of the rheology of food emulsions. Viscoelastic measurements are appropriate tools for obtaining
information about the microstructure of the system related to the organization of the macromolecules in
the medium. The existence of a broad distribution of relaxation times (Ȝ) in polymeric materials can be
represented by the mechanical relaxation spectrum derived from experimental values of the dynamic
moduli: G’ (storage modulus) and G’’ (loss modulus). The objectives of the present work were: a) to
determine the viscoelastic behavior of two different low in fat oil-in-water food emulsions: a gel like and
a pourable fluid type emulsions stabilized with hydrocolloids (gellan gum and xanthan-guar mixtures
respectively) b) to model and predict the mechanical relaxation spectrum for both emulsions and
continuous aqueous phases.
2.Materials & Methods
Low-in-fat emulsions (20g/100g) were analyzed in two distinct formulations: (a) a pourable fluid
dressing type emulsion and (b) an emulsion-filled gel.
(a) Oil-in-water emulsions were prepared using sunflower oil (20%) and Tween 80 (1%). Fluid
emulsions containing xanthan and guar gums (XG) were formulated using a synergistic ratio 7:3, with
total hydrocolloid concentration ranging between 0.5 to 2%. The aqueous phases contained NaCl (2%)
and acetic acid (2%).
(b) Gel type emulsions contained high acyl gellan gum (GG) were heated up to 90ºC maintaining a
constant stirring, then 5mM CaCl2 was added followed by rapid cooling down to 20ºC. High acyl gellan
gum (Kelcogel, CA) concentration ranged between 0.1-0.5%. Both types of emulsions were prepared
using an Ultraturrax at 11500 rpm during 4 minutes.
The rheological measurements were performed on the continuous phases and on the emulsions using a
Controlled Stress Rheometer Haake RS600 (Thermoelectron, Germany). The effect of gums was studied
using oscillatory measurements (G’ and G’’ vs. frequency) within the linear viscoelastic range (LVR)
previously determined by stress-sweeps.
3.Results & Discussion
3.1.Food o/w emulsion stabilized with xanthan/ guar gums mixtures
Mechanical spectra of the emulsions were obtained from oscillatory tests. The percentage of
hydrocolloids influenced mostly the rheological behavior of the emulsions (Figure 1a). When gum
concentration was 1.25% or higher, emulsions showed a weak gel-like behavior with G’ higher than G’’
in the frequency range analyzed. A similar behavior between continuous phases and o/w emulsions was
found in this range of gum concentration (Figure not shown). When the total biopolymer content was
0.5%, the dispersed phase contribution to the viscoelastic behavior became significant.
The time-concentration superposition enables the extrapolation of data to experimentally inaccessible
timescales and moduli, in the same way as allowed by time-temperature superposition in polymer systems
[1]. Data were processed using the shifting mode of IRIS Rheo-hub 2007 program [2] to perform
superpositions and spectrum calculations. In Figure 1b the dynamic data of the XG emulsions were
replotted using a reference concentration of 1.25%. The dynamic data was shifted horizontally by a shift
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Gabriel Lorenzo et al. / Procedia Food Science 1 (2011) 281 – 286
factor ac leading to a width of nearly 10 decades in the frequency range; vertical shift factor (bc) was not
significantly affected by xanthan/guar concentration.
Fig. 1. (a) Frequency sweep for o/w emulsions (20% oil) with different xanthan/guar gums concentrations; (b) master curve obtained
with the time-concentration superposition. Reference concentration: 1.25%; continuous line: BSW spectrum.
The dynamic moduli are related to the relaxation spectrum (H(Ȝ)) by:
f
Gc
Ge ³
f
f
G cc
H( O )
(ZO ) 2
dln O
1 (ZO ) 2
(ZO )
³ H( O ) 1 (ZO )
2
dlnO
(1)
(2)
f
Based on the Baumgaertel-Schausberger-Winter model (BSW), H(Ȝ) was determined using dynamic
data for emulsions and continuous phases [3]. The spectrum has the following form:
H (O )
n0
ne
ª
§ § O ·E ·
§ O ·
§O · º
G N0 « H g ¨ ¸ ne ¨ ¸ » exp ¨ ¨
¸ ¸¸
¨
O
«¬
© O0 ¹
© Oe ¹ »¼
© © max ¹ ¹
(3)
where GN0 is the plateau modulus, ne and n0 are the slopes of the spectrum in the entanglement and high
frequency glass transition regimes respectively, Hg is the glass-transition front factor, Ȝe the relaxation
time corresponding to polymer chains with entanglement molar mass. The exponent ȕ controls the
sharpness of the cut-off of the spectrum, Ȝmax is the longest relaxation time and Ȝ0 is the crossover time to
the glass transition. Once the spectrum and the viscoelastic coefficients are known, it is possible to
generate the response to any desired type of excitation [4]. A satisfactory agreement between
experimental data and the proposed model was observed (Figure 1b).
3.2. Gel-like food emulsions containing high acyl gellan gum
High acyl gellan gum gel emulsions showed a “strong gel like” behavior with a storage modulus (G’)
independent of the frequency even those systems with low hydrocolloid content and a minimum in G’’
(Figure 2a). The G’ at the plateau was over one order of magnitude larger than that of G’’ (elastic solid
behavior).
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Gabriel Lorenzo et al. / Procedia Food Science 1 (2011) 281 – 286
G’ and G’’ versus Ȧ master curves were achieved using the empirical superposition method, by means of
vertical shifting of the single curves obtained for different gellan content (Figure 2b). In contrast with the
XG systems there was no need to shift the curves horizontally. Conversely the shift factor, bc, decreased
with increasing gum concentration. Figure 2b also shows the satisfactory fitting achieved with the BSW
model.
3.2.Validation of the predicted spectra
Validation of the predicted spectra was carried out through steady-state flow curves for emulsions
containing XG gums and creep compliance data for emulsion-filled gels.
Flow curves (Figure 3) were model using Ellis equation [5, 6] which correlates fluid viscosity with
applied shear stress:
Fig. 2. (a) Dynamic moduli (G’, G’’) as a function of Ȧ for o/w gelled emulsions (20% oil) and different gellan gum concentration;
(b) master curve obtained with the time-concentration superposition. Reference concentration 0.30%; continuous line: BSW
spectrum.
K Kf K0 Kf
n
1 V / V c (4)
where K0 and Kf are first and second Newtonian viscosity, respectively; n is the exponent and Vc is a
critical stress denoted as “true yield stress”. Vc could be defined as the stress above which the structure of
the system is broken down [7].
Parameters like zero shear viscosity (K0) were obtained with this model for all the studied emulsions.
In particular K0 for the 1.25% xanthan/guar gum emulsion was found to be 3.6 x 103 Pa.s.
In order to compare both models, from the BSW spectrum it was possible to predict, K0 [8]. The value
obtained for the 1.25% emulsions was 3.1 x 103 Pa.s, in a remarkable agreement with the K0 value
obtained from steady-state flow experiments.
For gellan gum emulsions creep-recovery tests were performed within the linear viscoelastic range.
For viscoelastic materials there is an interrelation between the relaxation (H(Ȝ)) and retardation (L(Ȝ))
spectra, so if one spectrum is known over the entire range of time scale, the other spectrum can be
calculated [9]. From H(Ȝ) expression obtained for gel emulsion, the retardation spectrum was evaluated.
Once the retardation spectrum was known, the creep-compliance behavior was predicted [9]. Figure 3b
shows as an example that the predicted compliance (J(t)) for gel emulsions satisfactorily agreed with
experimental creep data.
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{experimental data
— prediction from the BSW
a
„
S
b
V 3D
Fig. 3. (a) Steady-state flow curves for o/w emulsions (20%) stabilized with XG mixtures. Lines correspond to Ellis equation. (b)
Creep compliance curve obtained for gel emulsions containing gellan gum.
4.Conclusions
Viscoelastic behavior of the systems was satisfactorily modeled using Baumgaertel-SchausbergerWinter equation. This empirical model was used to predict the mechanical relaxation spectrum for both
emulsions and continuous aqueous phases. The use of the BSW spectrum is advantageous due to different
reasons, i.e. it provides analytical expressions for the material functions (as G', G" data) and, it reduces
the description of the linear viscoelastic behavior to the determination of only a few material-specific
parameters. Validation of the predicted spectra was carried out through creep compliance data for
emulsion-filled gels and steady-state flow curves for emulsions containing xanthan –guar mixtures.
Dynamic data measured in this work have been successfully converted into the time domain by the
application of BSW model. It is a useful tool, especially for establishing a rheological data bank and
analyzing viscoelastic experiments.
With a precise knowledge of the viscoelastic behavior of a wide variety of food emulsions, ranging
from fluid dressings with a much higher viscous contribution, to highly structured systems stabilized by a
three-dimensional gel structure, caused by physical entanglements among polymeric chains, it is possible
to control the desired rheological properties in a particular final product and its stability.
Acknowledgements
The financial support of the Consejo Nacional de Investigaciones Científicas y Tecnológicas
(CONICET), Agencia Nacional de Promoción Científica y Tecnológica, and Universidad Nacional de La
Plata is acknowledged.
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Presented at ICEF11 (May 22-26, 2011 - Athens, Greece) as paper FMS332.