Reology of Mayonnaise

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 ingredients, 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 viscoelastic 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) characterize 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 mayonnaise 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 concentrations, 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

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)


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 mayonnaise 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-dimensional 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 concentrations (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 concentration. 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 magnitude, 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 concentration.

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-


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 general, 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 different 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 breakdown 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 structural 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 frequencies 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


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 interaction 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 different 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


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.

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