-
Journal ofFood Ennineerinn 25 i 19951409-425 Co&right 0 1595
Els&ier Science Limited Printed in Great Britain. All rights
reserved
0260-8774/9S/S!,.SO 0260-8774(94)00010-3
Rheological Characterization of Mayonnaise. Part II: Flow and
Viscoelastic Properties at Different Oil and
Xanthan Gum Concentrations
L. Ma & G. V. Barbosa-Chovas
Department of Biological Systems Engineering, Washington State
University, Pullman, WA 99 164-6 120, USA
(Accepted 3 March 1994)
ABSTRACT
The flow and viscoelastic properties of mayonnaise at diflerent
oil and xanthan gum concentrations (75-85X and 05-I.!% (w/w),
respectively) were investigated in the rotational and oscillatory
mode using a plate-plate rheometer. Yield stress, which was
determined using a static method, and steady measurements were
corrected to account for slippage. The corrected flow curves were
fitted with the Herschel-Bulkley model, and it was found that the
flow index (n), consistency index (K), and yield stress were
greatly aflected by the oil and xanthan gum concentrations.
Viscoelastic properties of mayonnaise were characterized using
small amplitude oscillate y shear, and it was observed that
mayonnaise exhibited weak gel-like properties. The gel strength
depends on the oil and xanthan gum concentrations. The magnitude of
elastic modulus and complex viscosity increased with the increase
concentrations.
INTRODUCTION
of oil or xanthan gum
Mayonnaise is an oil-in-water (O/W ) emulsion prepared from
vegetable oil, egg yolk, acidified ingredients, citric acid and/or
maleic acid, and optional ingre- dients, i.e. salt, nutritive
sweetener, stabilizer, thickener and crystallization inhibitor
(Code of Federal Regulations, 1986). Due to the commercial import-
ance of mayonnaise, the rheological characteristics of mayonnaise
have been extensively studied. The flow properties (consistency
index, K; flow behavior index, n; and yield stress, to) of
mayonnaise have been studied by Elliott and Ganz (1977), Figoni and
Shoemaker (1983), Kiosseoglou and Sherman (1983a), Paredes et al.
(1988, 1989), Yilmazer et al. (1991), and Yilmazer and Kokini (
1992). Several rheological equations, such as the power law, the
Casson model, and the Herschel-Bulkley model, have been used to
describe the stress
409
-
410 L. Ma, G. I/ Barbosa-C&ovas
response to deformation in mayonnaise (Bistany & Kokini,
1983; Paredes et al., 1988, 1989). However, the reported flow
parameters (K, n, and rO) are different from reference to reference
due to differences in selected measuring ranges, corrections
considered, and types of products.
One of the three parameters mentioned above, yield stress, may
be defined as a minimum shear stress required to initiate flow. The
existence of yield stress in fluids is still a controversial topic
(Barnes & Walters, 1985; Cheng, 1986; Hartnett & Hu, 1989;
Evans, 1992; Schurz, 1992; Steffe, 1992). However, there is little
doubt that yield stress is an engineering reality (Hartnett &
Hu, 1989) which may strongly influence process calculations. Yield
stress imparts stability to food emulsions in low-stress situations
(e.g. during storage and transporta- tion, where the stress
involved is usually lower than the yield stress). Hence, the
possibility of any structural change leading to instability is
minimized (Rahalkar, 1992).
There is no single best technique among the many available
methods to evaluate the yield stress (Steffe, 1992). The classical
method to obtain the yield stress value is to extrapolate the shear
stress versus shear rate curves. Alterna- tive approaches include
static methods such as controlled shear stress tests and the vane
method. The static methods have the advantage that the three-
dimensional structure of the material is not disturbed prior to
measurement (Kee & During, 1990). Other methods, such as
determining the yield stress from squeezing flow, have been
proposed by Gencer and Peleg ( 1984) and Campanella and Peleg ( 198
7 b).
A small amplitude oscillatory experiment, carried within the
linear visco- elastic region, has the advantage of minimizing
destruction in the sample since little or no permanent structure
breakdown occurs during the dynamic measurements (Elliott &
Ganz, 1977). This approach allows a relationship between the
results obtained and the actual structure of material to be drawn
(Murioz & Sherman, 1990). Fisbach and Kokini (1987) used
viscoelastic studies in predicting the storage stability of salad
dressings. The dynamic viscoelastic properties have also been used
to study the structure of salad dressings (Bistany & Kokini,
1983; Muiioz & Sherman, 1990).
Although many investigations have been conducted on the
stability, flow and viscoelastic properties of mayonnaise and salad
dressings (Kiosseoglou & Sherman, 1983~; Yilmazer et al.,
1991), relatively few studies have taken into account slippage
effects during shear measurements, and few studies have been
conducted on the flow and viscoelastic properties of mayonnaise at
different oil or xanthan gum concentrations. The objectives of this
study are to: (1) charac- terize the flow of mayonnaise at
different oil and xanthan gum concentrations in terms of
consistency index (K), flow index (n) and yield stress ( zo) after
correcting slippage; and (2) characterize the viscoelastic
properties of mayon- naise at different oil and xanthan
concentrations.
MATERIALS AND METHODS
Materials
Vegetable oil (Wesson Vegetable Oil, Hunt-Wesson, Inc.,
Fullerton, CA) and fresh grade A brown eggs were purchased from
local supermarkets. The eggs
-
Rheological characterization of mayonnaise. Part II 411
were broken, and the yolks were separated from the albumen. The
vitelline membranes were then punctured, and the liquid yolk
collected. A 20% (w/w) acetic acid was prepared from analytical
grade glacial acetic acid (99.5% minimum concentration). Sodium
chloride and sucrose were analytical grade reagents. Pure food
grade xanthan gum was obtained from Sanofi Bio- Industries,
Waukesga, WI.
Mayonnaise preparation
The mayonnaise samples were prepared following procedures
described by Kiosseoglou and Sherman ( 1983 b) and Gates ( 198
1).
A rotary mixer (Sunbeam Mixmaster, Milwaukee, WI) was used. Egg
yolks. sugar, and salt were introduced into a stainless steel bowl
(diameter = 150 mm and height = 120 mm). They were mixed together
at speed 4 for 2 min. Then, one-twentieth of the oil was slowly
added. The mixer was then operated at speed 3. One-tenth of an
acetic acid-water solution was added towards the end of the oil
addition. The oil and acetic acid-water solution were added
alternately while the mixer was kept at speed 3. When all the oil
and acetic acid-water solutions were added, the mixer was set at
speed 4 for 3 min. The mayonnaise sample was transferred to a 250
ml beaker, sealed with parafilm, and stored on the reagent shelf
(cu. 2 1C) overnight.
In order to study the effect of oil and xanthan gum
concentrations on the properties of mayonnaise, two series of
samples were prepared. Tables 1 and 2 list the mayonnaise
formulations with different oil and xanthan concentrations. One
series of mayonnaise samples was prepared with different oil
concentra- tions, but the other ingredients (salt, sugar, and
acetic acid) were kept constant, and no xanthan gum was added. The
second series of mayonnaise samples was prepared with several
xanthan gum concentrations, but the oil concentration and other
ingredients were kept constant.
Rheological measurements
Rheological measurements were performed with a rheometer
(Physica- Rheolab@ MC20/UM, Physica USA Inc., Spring, TX) using
both controlled
TABLE 1 Formulation of Mayonnaise Preparation at Different Oil
Concentrations
02 concentration (w/w)
75% 80% 85%
Oil k) 1.50 160 170 Water (g) 32.4 22.4 12.4 20% Acetic acid (g)
2.0 2.0 2.0 Egg yolk (g) 12.0 12.0 12.0 Sugar (g) 3.10 3.10 3.10
Salt (8) 050 0.50 0.50
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412 L. Mu, G. I/ Barbosa-Ckovas
TABLE 2 Formulation of Mayonnaise Preparation at Different
Xanthan Gum Concentrations
Xanthan gum concentration (w/w)
Xanthan (g) gum 1.00 2.00 3.00 Gil (8) 100 100 100 Water(g) 81.4
80.4 79.4 20% Acetic acid (g) 2.0 2.0 2.0 Egg yolk (8) 12.0 12.0
12.0 Sugar (8) 3.10 3.10 3.10 Salt (8) 0.50 0.50 0.50
shear rate and controlled shear stress with a plate-plate
geometry of 50 mm diameter. The measurements were conducted at two
gap distances of 1.00 and 1.50 mm. Special care was taken to
minimize the effect of the work softening when the mayonnaise
sample was initially loaded on the plate each time (Kokini &
Dickie, 198 1). The mayonnaise sample was removed in one stroke
from the container (250 ml beaker) using a plastic spatula and was
subsequently deposited onto the plate. The sample filled up the
whole gap by lowering the upper plate down to the designed gap. The
extra sample around the edge of the plate was trimmed with the
plastic spatula.
In this study, all samples were allowed to rest after loading to
allow sample relaxation and temperature equilibration. A
preliminary test was conducted on the effect of resting time (0,
1,3,5, 10,20, and 30 min) after loading the sample. It was found
that 5 min of resting was enough to get a reproducible result. The
data reported are the averages of three replicates. All experiments
were conducted at a temperature of 20 f O*lC, and a fresh sample
was loaded for each measurement. The corrected flow curve was
calculated from data measured at two different gaps using the
following equation (Yoshimura & Prudhomme, 1988):
jR = H, l/aR , - Hz YaR, H, -4
where yaR, is the apparent shear rate at a gap distance of H,;
jaR, is the apparent shear rate at a gap distance of H,; H,, Hz are
the gap distance between the upper disk and bottom disk; and yR is
the corrected shear rate.
The yield stress was determined from the corrected flow curves
using the static method - the stress initiate flow (Steffe, 1992;
DeKee et al., 1986; Buscall et al., 1987; James et al., 1987). With
the measured yield stress, the flow parameters (consistency index,
K and flow index, n) were determined using the Herschel-Bulkley
model:
t= q+Kj (2)
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Rheological characterization of mayonnaise. Part II 413
where r is the stress (Pa), r,, is the yield stress (Pa); K is
the consistency index (Pas); IZ is the flow index; and j is the
shear rate (s- ).
Viscoelastic measurements were also performed using the
plate-plate rheometer (@= 50 mm) (Physica-Rheolab@ MC20/UM Physica
USA Inc., Spring, TX). The gap between plates was 1.0 and 1.5 mm.
All experiments were carried out at 20 f O*lC.
For comparison, two standard fluids with different viscosities
(fluid HTlOOOOO and fluid 30000; Brookfield Engineering Laboratory,
Inc.. Stoughton, MA) were used to calibrate the instrument under
different gaps (1.0 and 15 mm) and different geometries
(plate-plate geometry and cone-plate geometry) in order to exclude
the possibility of instrument artifacts. The results from the
calibration tests proved that the viscoelastic estimate had a
margin of error of roughly 5% or less.
RESULTS AND DISCUSSION
Flow properties of mayonnaise
The flow curves of the model mayonnaise measured at two
different gaps are presented in Figs 1 and 2. Differences in the
flow curves measured at two different gaps indicate the existence
of slippage in the shear measurements (Yoshimura & Prudhomme,
1988). The corrected flow curves of the mayon- naise samples with
different oil and xanthan concentrations using eqn (1) are also
presented in Figs 1 and 2.
The yield stress was determined by a static method - stress to
initiate flow (DeKee et al., 1986; Buscall et al., 1987; James et
uf., 1987; Steffe, 1992) -based on the corrected flow curves in
Figs 1 and 2. It was noted that the yield stress determined from
measured curves was smaller than the yield stress determined from
corrected curves (Figs 1 and 2). At a very small shear stress, the
mavon- naise sample between the gap behaved as a solid body due to
the three-dimen- sional network structures (Princen, 1985;
Rahalkar, 1992); thus, no apparent flow was observed. When shear
stress was increased to a certain magnitude which was less than the
true yield value of the bulk sample, an apparent flow was observed.
This observed flow was due to deformation in the boundary layer
(slip layer), since the bulk mayonnaise still behaved as a solid
and did not flow at the stress below the true yield value. When the
stress was greater than the true yield stress of the bulk sample in
the gap, all of the sample, including the boundary layer in the
gap, was deformed and flowed. Therefore, the yield stress
determined from the measured flow curves would be smaller than that
determined from the corrected flow curves. The yield stress
determined from the measured flow curves was called apparent
stress, t, and the yield stress determined from corrected flow
curves was called true yield stress, tO) or yield stress, r,). The
comparisons of apparent yield stress and the true yield stress of
mayonnaise with different oil and xanthan gum concentrations are
presented in Table 3.
The yield stress for the mayonnaise ranged from 23 to 235 Pa and
increased with the oil concentrations (Table 3). A more compact
three-dimensional network was formed between the egg protein
molecules and absorbed droplets
-
414 L. Ma, G. I/ Barbosa-Canovas
, 2mo 30.M) 40.00 50.00 60.00
Shear stress (Pa)
(0)
50 100 150 200 250 300
Shear stress (Pa)
(b)
0.00 50.00 100.00 159.m 200.00 250.00 300.00 359.00 4cn.m
459co
Shear stress (Pa) I I
Cc)
Fig. 1. Actual flow curve of mayonnaise calculated by means of
eqn (1). Also shown are the flow curves measured at two different
gaps. (a) 75% oil concentration; (b) 80% oil concentration; (c) 85%
oil concentration. - 1.0 mm gap; -A- 1.5 mm gap;
- corrected.
-
Rheological characterization of mayonnaise. Part II
0.00 10.00 20.00 30.00 4C.CCI 50.M3 60.00 70.00 8O.M) 90.C0
Shear stress (Pa)
(0)
8.W
0.00 50.00 lM3.00 15clm 203.00 250.M)
Shea stress (Pa)
10.00
8.00 8
: 6.00
6 i i ,i,,,,,
0.00 50.00 100.00 1w.m mm0 250.00 3cKl.00 350.00 4OO.M) Shear
stress (Pa)
41s
Fig. 2. Actual flow curve of mayonnaise calculated by means of
eqn (1). Also shown are the flow curves measured at two different
gaps. (a) 50% oil and 0.5% xanthan gum concentration; (b) 50% oil
and 1.0% xanthan gum concentration; (c) 50% oil and 15% xanthan gum
concentration. --+- 1.0 mm gap; -A- 1.5 mm gap; --
corrected.
-
416 L. Mu, G. I/. Barbosa-Cknovas
TABLE 3 Flow Parameters of Mayonnaise
Apparent True yield K n r yield tcl (Pa) (Pa.s-7
t, (Pal
Oil concentr$gn (w/w, %)
80 85
Xanthan gum concentration (w/w, %)
0.5 1.0 1.5
18 23 18.1 0.83 0.87 107 115 127.4 0.69 0.85 228 235 289.9 O-24
0.87
49 55 8.5 0.44 0.98 131 195 11.8 0,43 0.97 195 305 43.5 0.78
0.86
(Zosel, 1982; Jaynes, 1985; Gladwell et al., 1986). This compact
network structure is responsible for the increase in yield stress
with the increase in oil concentrations.
The yield stress increased with the increase in the xanthan gum
concentra- tions (Table 3). Xanthan gum was reported to increase
the stability of mayonnaise and emulsion (Hibberd et al., 1987) as
well as its structure by the formation of aggregates of larger size
(Yilmazer & Kokini, 1992). Thus, it was expected that the yield
stress would increase with the xanthan gum concentra- tion. The
yield stress of some commercial mayonnaises have been studied by
various methods (Elliott & Ganz, 1977; Dickie & Kokini,
1983; DeKee et al., 1986; Campanella & Peleg, 1987~) and have a
very wide range of yield magni- tude, from 9 to 91 Pa (Steffe,
1992). This wide range of yield stress value in the published data
was due to differences in the methods as well as shear rate range
selected when extrapolating. The results in the present work, in
general, were in agreement with the published data with low oil or
xanthan gum concentrations, but the yield stress at higher oil or
xanthan gum concentrations was greater than the yield stress in the
published data, because the key components (oil and xanthan gum)
varied widely in the model mayonnaise.
With the determined yield stress, the Herschel-Bulkley model
(eqn (2)) was used to determine the flow properties (consistency
index, K, and flow behavior index, n) of model mayonnaise from the
corrected flow curves. The magnitude of consistency index (K) of
the mayonnaise ranged from 18.1 to 289.9 Pa.s and increased with
oil concentrations (Table 3), which was in agreement with Gladwell
et al. (1986). Similar results were observed with the concentration
of xanthan gum. That is, the magnitude of the consistency index of
the mayonnaise ranged from 8.5 to 435 Pa.s and increased with the
xanthan gum concentra- tion.
The flow index (n) of all model mayormaises were less than one
(Table 3) which indicated that they were pseudoplastic fluids
(Paredes et al., 1989). The flow behavior index ranged from 0.83 to
0.24 and decreased with the oil con-
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Rheological characterization of mayonnaise. Part II 417
centration (Table 3). The flow index was not significantly
different between xanthan gum concentration at 0.5 and 1.0% but was
different at 1.5%. In gen- eral, the flow index for the model
mayonnaise was in agreement with previous reports (Dickie &
Kokini, 1983; Steffe, 1992), since the flow index varied widely
from 0.13 to 0.9 1 for some commercial or model mayonnaise due to
dif- ferent methods (i.e. capillary viscometer, cone-plate
viscometer, and/or concen- tric cylinder viscometer) and/or
different shear rate range selected. In this study, it was expected
that the flow index would change with the oil and xanthan gum
concentrations, since the variation of oil and xanthan gum
concentrations changed the levels of structure in mayonnaise
(Yilmazer et af., 1991).
Viscoelastic properties of mayonnaise
Mayonnaise shows viscoelastic properties attributable to a
network formed between lipoproteins which are adsorbed around
neighboring oil droplets (Muiioz & Sherman, 1990). Figure 3
presents the viscoelastic response versus shear frequency at
different gaps (1.00 and 1.50 mm). The data is independent of the
gaps between plates, which demonstrates that there is no structure
break- down or slippage effect. In addition, a test was also
conducted on increasing and decreasing shear frequency (w) on the
same sample. The results falling on the same curve indicated that
there was little or no permanent structure breakdown occurring
during the dynamic measurements (Elliott & Ganz, 1977) (data
not shown here). Thus, the dynamic oscillatory test can be used to
characterize the viscoelastic properties of mayonnaise.
The results from small amplitude oscillatory shear tests are
expressed in terms of the elastic modulus ( G) and loss modulus
(G). If G ti G, the material will exhibit a solid behavior (i.e.
deformation in the linear range will be essen- tially elastic or
recoverable); however, if G 9 G, the material will behave like a
liquid (i.e. the energy used to deform the material will be
viscously dissipated). In general, a viscoelastic material behaves
in a solid-like manner at low frequencies when the viscoelastic
moduli are considered as a function of frequency (Ferry, 1980).
The model mayonnaise with different oil and xanthan gum
concentrations had similar viscoelastic properties in general. But,
there were some fine struc- tural differences in the viscoelastic
response spectrum (Fig. 4(a)-(f)). It can be seen that all
mayonnaise samples (75-85% oil concentration, and O-5-1.5% xanthan
gum concentration) exhibit a well-pronounced plateau in G(o) with
G(w) > G(w) for two sequence decades, except the mayonnaise
sample at 75% oil concentration. The samples enter the terminal
zone at angular frequen- cies of 0.63 rad/s, which corresponds to a
terminal relaxation time on the order of 1.6 s. This system,
therefore, behaves as a solid on a time scale of seconds. According
to the phenomenological definition of gel by Almdal er al. ( 1993):
. . . solid-like gels are characterized by a storage modulus, G(w),
which exhibits a pronounced plateau extending to time at least of
the order of seconds, and by.a loss modulus, G(o), which is
considerably smaller than the storage modulus m the plateau region.
Thus, it could be accepted that mayonnaise is gel-like in nature.
Comparing Fig. 4(a)-(c) it is found that the mayonnaise at higher
oil concentrations has more pronounced gel-like characteristics
than at lower oil concentrations. It has been reported that there
is more packing of oil droplets in
-
418 L. Ma, G. V Barbosa-CLinovas
r
!
5 1cKKl; ~ G (1 .Omm)
8
b -~i~T~~~~yYII~YbY4-S) -----__. G(,,Omm)
5 . G (1.5mm)
?J 100: . G (1.5mm)
10 0.1 1 10
Frequency (rod/s)
102
(0)
. EDw
c lcal y - G(1 .Omm)
8 ~.~~*~~~~Cee-*.*. - - - - - _ _. b G(, ,Omm)
5 . G(l.5mm) b loo: . G(l.5mm)
10 0.1 1 10
Frequency (rod/s)
100
,h\
Fig. 3. The storage modulus and loss modulus vs frequency. (a)
85% oil concentration; (b) 50% oil and 1.0% xanthan gum
concentration.
higher oil concentrations than in lower oil concentrations
(Jaynes, 1985). The viscoelastic response of model mayonnaise had a
very similar pattern (Fig. 4(d)-(f)) at all xanthan gum
concentrations, which had less variation than that of mayonnaise
with different oil concentrations. It is also noticed that the
magnitude of the maximum storage modulus G is of the order of lo4
Pa, so these gels are very weak and break down easily under shear
stress. The magnitude of storage modulus, loss modulus G(w), is
also dependent on the oil and xanthan concentrations.
The comparison of storage modulus, G(w), for mayonnaise with
different oil and xanthan gum concentrations is presented in Fig.
5. Since the elastic modulus
-
Rheological characterization of mayonnaise. Part II 419
hequency (rad/s)
hequency (rod/s)
0.1 1 10 103
hequency (rod/s)
r
b
1' ..-_i
01 1 IO 102
hequency (rod/s)
d
01 1 10 1Cn
hequency (rod/s)
f
loo00
a 5 IMX) . . . ..mm.mmmm=mmm=~m=
b AAAAAAAAAAAAAAAAAAAA
& y_____ __i_ 01 1 10 co
tequency (rod/s)
Fig. 4. Dynamic oscillatory response (G and G) of mayonnaise at
different oil and xanthan gum concentrations. (a) 75% oil
concentration; (h) 80% oil concentration; 1,~) 85% oil
concentration; (d) 50% oil and 0.5% xanthan gum concentration; (e)
50% oil and 1.0% xanthan gum concentration; (f) 50% oil and 1.5%
xanthan gum concentration. m,
storage modulus (G); A , loss modulus (G).
(G) represents the recoverable energy when the material is
subjected to deformation, the increase in the elastic modulus with
oil concentrations indicates a more solid-like mayonnaise. The fact
that mayonnaise showed a greater elastic modulus value at higher
oil concentrations can be attributed to the formation of a more
complex liquid crystal structure (Jaynes, 1985; Gladwell et al.,
1986) (see Fig. 5(a)). The magnitude of the elastic modulus
increases with xanthan concentration at all shear frequencies (see
Fig. 5(b)), so it can be assumed that most of the viscoelastic
behavior can be attributed to the interac- tion between xanthan gum
microgel and emulsion droplets. The association of
-
420 L. Ma, G. K Barbosa-Crinovas
I r
a
loo00 M 75%oil -)- 00% oil -*- 85%oil
1ccO i ._*_._*_.-._._.-.-.-.a-.-.-.-.-.-.-.-.
a 8 looi b 2-H
-+-e
10 i
1
0.1 1 10 100
frequency (fad/s)
b
1
0.1 1 10 100
frequency (rod/s)
L Fig. 5. Effect of mayonnaise oil and xanthan gum concentration
on the storage
modulus.
xanthan gum in solution resulting in aggregates at
concentrations of 0.5% or below was reported to be due to
hydrogen-bonding (Lim et al., 1984). At high concentrations,
xanthan gum molecules form a viscoelastic structure to stabilize
the emulsion (Hibberd et al., 1987).
Figure 6 presents the absolute magnitude of complex viscosity, (
17 * (, at differ- ent oil and xanthan gum concentrations. In Fig.
6(a), the complex viscosity increases with oil concentrations over
the entire experimental shear frequency range (0.63-62.8 rad/s) due
to the higher oil concentrations in mayonnaise compacting the
packing of oil droplets. As oil concentrations decrease, the mean
distance between droplets is greater; thus, a lower complex
viscosity is observed. In Fig. 6(b), the slopes of the curves at
all xanthan concentrations are nearly the same. However, the
magnitude of the complex viscosity increases
-
Rheological characterization of mqvonnaise. Part II 421
Q
b 75%oil
-----t- 80% oil
A 85%011
_L-__c ,I . ..>
01 1 10 1M) I
frequency ~____ i
b
-.- 0.5% xanthan
-t- 1 .O% xanthon
-*- 1 5% xanthan
1 10
frequency
103
Fig. 6. Effect of mayonnaise oil and xanthan gum concentration
on the complex viscosity.
with xanthan concentrations over the entire experimental shear
frequency range (0.63-62.8 rad/s), due to the interaction between
xanthan gum microgel and emulsion droplets.
Loss factor, tan( 6) = G/G, is a dimensionless measure that
compares the amount of energy lost during a test cycle to the
amount of energy stored during this time (Darby, 1976; Ferry,
1980). The loss factor indicates whether elastic or viscous
properties predominate in a sample. The comparison of loss factor
for mayonnaise with different oil and xanthan gum concentrations is
presented in Fig. 7. The slope for 75% oil concentration is greater
than that for 80%, while the slope for 85% oil is near zero (flat).
These differences might be due to the different fine structures or
three-dimensional networks formed by a two-phase emulsion. The
difference in loss factor could be considered an index to
distinguish the fine structure at different oil contents of
mayonnaise. The quanti-
-
422 L. Ma, G. V. Barbosa-Ctinovas
10.00
6
s 1 .oo 8 ,o
0.10
a
-t- 75% oil -t- 80% oil - 85%oil
1 .m 10.00 loO.M3
tequency
b
n 0.5% xanthan A 1.0% xanthan l 1.5% xanthan
:;,,(I ,,~;::;:x;:~~:::~:~ ,,,,
0.10 1.00 1 o.cKl 100.00
frequency
Fig. 7. Effect of mayonnaise oil and xanthan gum concentration
on the loss factor.
tative relation among loss factor, elastic modulus, loss
modulus, and the network structure of mayonnaise needs further
investigation. However, the shape of the curves are more or less
similar, showing that all the mayonnaise samples at different
xanthan gum concentrations exhibit similar viscoelastic
properties.
CONCLUSIONS
It was found that the flow properties determined from direct
flow curve measurements were significantly different from the
corrected flow curves after taking slippage into account. The
correction of the flow curve was necessary to determine the actual
flow parameter of mayonnaise. The yield stress and consistency
index increased with oil and xanthan concentrations due to the
formation of a higher level of network structure; the flow behavior
index varied
-
Rheological characterization of mayonnaise. Part II 423
with oil and xanthan concentrations. The small amplitude
oscillatory test can overcome the effect of slippage that occurred
in the rotational shear test. The small amplitude oscillatory shear
was useful in correlating the structure of mayonnaise at different
oil and xanthan concentrations. The viscoelastic response of
mayonnaise indicates that mayonnaise is gel-like in nature, and the
strength of the gel is dependent on the oil and xanthan gum
concentrations. The magnitude of elastic modulus and complex
viscosity increased with the oil or xanthan gum concentrations.
ACKNOWLEDGEMENT
This project was partially supported by a Sigma Xi grant.
REFERENCES
Almdal, K., Dyre, J., Hvidt, S. & Kramer, 0. (1993). Towards
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