Full text of DOI:10.1111/j.1365-2621.2002.tb09580.x

JFS: Food Chemistry and Toxicology
Conformational Role of Xanthan Gum
in its Interaction with Guar Gum
F. W ANG, Y.J. WANG, AND Z. SUN
ABSTRACT: Ubbelohde viscometry, texture analysis, and centrifugation were used to investigate the effects of
chain conformational changes of xanthan or deacetylated xanthan gum on its interaction with guar gum. Guar
gum was not effective in denaturing xanthan gum when the xanthan helical structure was stabilized by salt. The
intrinsic viscosities of deacetylated xanthan and guar blends were higher than those calculated from the weight
averages of the 2. Conformational change was not observed for deacetylated xanthan, presuming deacetylated
xanthan was in the exact conformation for guar to bind so that the most stable heterotypic structure between
deacetylated xanthan and guar was formed directly, thus the strongest interaction was observed between
deacetylated xanthan and guar.
Keywords: xanthan, guar, intrinsic viscosity, synergistic interaction, conformational change
Materials and Methods
Introduction
HE SYNERGISTIC INTERACTION IN VISCOSITY BETWEEN XANTHAN
Materials
T
gum and galactomannans (guar gum and locust bean gum)
Xanthan gum was obtained from Kelco (Chicago, Ill., U.S.A.) and
guar gum from Zumbro, Inc. (Faribault, Minn., U.S.A.). Xanthan
gum was used without further purification.
was first reported by Rocks (1971), who pointed out that xanthan
formed thermally reversible gels with locust bean gum but not with
guar. Some researchers described the interaction as incompatibility
(Kovacs 1973; Doublier and others 1993; Schorsch and others 1995).
However, a considerable body of evidence supports intermolecu-
lar binding between xanthan and galactomannans (Morris VJ 1996),
and suggests that destabilization of the xanthan helix facilitated
xanthan and galactomannan binding (Cairns and others 1986,
1987; Cheetham and Mashimba 1988, 1991; Zhan and others 1993;
Foster and Morris 1994; Goycoolea and others 1994). Galactoman-
nan acted like a denaturant to disturb the helix-coil equilibrium of
xanthan and displace ordered conformation of xanthan to the con-
formation for efficient binding (Zhan and others 1993; Morris and
others 1994; Goycoolea and others 1994; Morris ER 1996; Morris VJ
1996).
Guar purification
Approximately 2 g of guar gum was dispersed in 1000 mL deion-
ized water, stirred, and heated at 90 °C for 30 min, cooled, and then
centrifuged at 3800 ϫ g. After the supernatant was filtered through
a 5-␮m membrane, 1000 mL ethanol was added to precipitate the
guar gum. The guar gum was dried in a convection oven at 40 °C for
24 h and then in a vacuum oven at 50 °C for 10 h.
Preparation of deacetylated xanthan
Deacetylated xanthan was prepared by first dissolving the na-
tive xanthan (0.2%, w/v) in water and then adding KOH and KCl
(previously dissolved in a small amount of water) to obtain a solu-
tion of 0.025 M KOH and 0.1% KCl while stirring at room tempera-
ture for 2 h under nitrogen (Holzwarth and Ogletree 1979). The
solution was neutralized with 0.05 M HCl, and then a 2-fold volume
of ethanol was added. The precipitate was washed with ethanol/
de-ionized water (80/20, v/v) until no chloride was detected in the
washing solvent by 1% silver nitrate. The sample was ready for so-
lution preparation after the ethanol was completely evaporated.
A recent study of xanthan and guar interaction by cold-mixing
in aqueous solution (Wang 2001), xanthan and guar mixtures,
except for xanthan alone, showed a typical viscosity–concentra-
tion relationship of flexible polyelectrolytes according to the
Fuoss empirical equation (Fuoss and Strauss 1948; Fuoss and
Cathers 1949). The intrinsic viscosities of the blends were well
below those calculated from the weight average of the blends.
These results indicate that the intermolecular binding occurred
between xanthan and guar molecules, and guar forced xanthan
to change from a stiff ordered helix to a more flexible conforma-
tion.
Preparation of solutions for Ubbelohde viscometry
measurement
Approximately 1.0 to 1.5 g of xanthan, or deacetylated xanthan
or guar, was dispersed in 100 mL de-ionized water, stirred, and heat-
ed at 90 °C with stirring for 30 min. After cooling and filtration
through a 5-␮m membrane, 0.05 g sodium azide was added to pre-
vent microbial growth. For the study of salt effect on the interaction,
sodium chloride was added to obtain a final concentration of 0.04
M NaCl. The concentration of each solution was determined by the
phenol-sulfuric acid method (Dubois and others 1956).
It is known that the stability of xanthan ordered conformation
in aqueous solution increases in the order of acetate-free
xanthan Ͻ native xanthan Ͻ pyruvate-free xanthan (Holzwarth
1976; Smith and others 1981; Crescenzi and others 1986; Foster
and Morris 1994). The objective of this study was to understand
how the stability of the ordered structure of xanthan, including
native xanthan in 0.04 M NaCl and deacetylated xanthan in
aqueous solution, influenced its interaction with guar.
© 2002 Institute of Food Technologists
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Xanthan/Guar Gum Interaction…
Table 1–Structural parameters and intrinsic viscosity of na-
tive xanthan, deacetylated xanthan, and guar
Blends of different ratios (20:80, 40:60, 60:40, and 80:20, w/w, dry
basis) of xanthan or deacetylated xanthan to guar were prepared by
pipetting the calculated volume of xanthan or deacetylated xan-
than and guar solutions obtained above, respectively, and mixing
them by stirring at room temperature for 2 min to get 10 mL ternary
solutions.
Intrinsic
viscosity
(dL/g)
Pyruvate Acetate
in
in
%
%
Molecular aqueous 0.04 M
Sample
(w/w)
(w/w)
weight
solution
NaCl
Determination of acetyl and pyruvate contents of
native xanthan or deacetylated xanthan
The acetyl and pyruvate contents of xanthan were measured by
following the methods of McComb and McCready (1957) and
Sloneker and Orentas (1962), respectively.
Native xanthan 0.24
Deacetylated 0.12
xanthan
5.14
0.15
1143000
1025000
75.2
43.3
16.9
–
Guar
–
–
2338000
8.2
8.5
Molecular weight determination of xanthan and guar by
gel permeation chromatography (GPC)
The GPC system consisted of 2 peristaltic pumps with low flow
rate (0.03 to 8.2 mL/min), a fraction collector (RediFrac; Amersham
Biosciences, Piscataway, N.J., U.S.A.), and a 1000 ϫ 26 mm column
packed with Sepharose CL-6B gel (Amersham Biosciences). The
eluent was 0.2% (w/v) sodium chloride and 0.1% (w/v) sodium
azide at a rate of 0.56 mL/min. The concentration of xanthan and
guar solutions was 0.05% (w/v). Dextran standards, ranging from
9900 to 695000 of peak molecular weight, and blue dextrin, peak
equipped with PC-compatible Texture Expert software (version
1.22, Stable Micro Systems, Ltd., London, U.K.). Penetration mea-
surements were made at a speed of pre-test 3.0 mm/s, test 1.0
mm/s, to a penetration distance of 3.0 mm with a 2.54 mm dia cylin-
drical probe. The gel strength was defined as the work (g.s) re-
quired to penetrate the gel and calculated from the area under the
texture profile.
molecular weight of 2000000 (PSS Polymer Standards Service- Viscosity measurement with an Ubbelohde capillary
U.S.A., Inc., Silver Spring, Md., U.S.A.), were used to construct the viscometer
regression line for molecular weight determination. The concentra-
tion of the standards was 0.1% (w/v). The phenol-sulfuric acid
method (Dubois and others 1956) was used to measure the concen-
tration of polysaccharide in the eluent.
The viscosity of xanthan solution, deacetylated xanthan solution,
guar solution, and blends of xanthan- or deacetylated xanthan-guar
with different ratios in either water or 0.04 or 0.06 M NaCl were
measured by using an Ubbelohde capillary viscometer (Size 1,
Constant ϭ 0.01055, Technical Glass Products, Inc., Dover, N.J.,
U.S.A.) immersed in a water bath maintained at 30.0 Ϯ 0.1 °C. Each
concentration was measured at least in duplicate.
The concentration-dependence of the viscosity of each polymer
solution and their blends was analyzed by using the classical Hug-
gins equation:
Viscosity measurements with a Haake viscometer
The solution of deacetylated xanthan, xanthan, or guar (about
0.3%, w/v) was prepared by dissolving an appropriate amount of
deacetylated xanthan, xanthan, or guar in de-ionized water with
stirring and heating above 90 °C for 30 min. After cooling, the sam-
ple was centrifuged at 3800 ϫ g for 60 min to remove the insoluble
particles, and its concentration was determined by the phenol-
sulfuric acid method (Dubois and others 1956). The solutions were
then diluted to 0.2% (w/v) accordingly, and NaCl was added to make
the final concentration of 0.04 M NaCl if required. The hot mixed
blends (20:80, 40:60, 60:40, and 80:20, v/v, dry basis) were prepared
by mixing appropriate amounts of freshly prepared xanthan and
guar solutions (above 90 °C) and stirring for 2 min. The cold-mixed
blends were prepared by mixing appropriate amounts of cooled
xanthan and guar gum solutions at room temperature and stirring
for 2 min. Viscosity measurements at different shear rates were
performed using a Haake viscometer (VT550; Gebrüder Haake
GmbH, Karlruhe, Germany) at 23 °C.
␩
sp/C = [␩] + bC
where ␩sp is the specific viscosity, [␩] is the intrinsic viscosity, and b
is the Huggins parameter. For each concentration, the specific vis-
cosity was determined using the equation:
␩
sp = (␩-␩s)/␩
s
where ␩ is the solution viscosity and ␩s is the solvent viscosity. The
relationships of ␩sp/C and concentration were assessed by using the
nonlinear fitting of the statistics software JMP for SAS (1999).
Results and Discussion
Centrifugation and texture analysis
Characterization of stability of xanthan-ordered
structure
The centrifugation study was followed for which the synergistic
effects were not noted from the Haake viscosity measurements.
Individual polymer solutions or blends of 5 ml were centrifuged at
39000 ϫ g for 60 min. The first 1 mL of the top layer was retained as
the top layer and another 2 mL of the top layer was discarded. The
remaining 2 mL of solution was homogeneously mixed. After re-
moving the air bubbles by centrifugation at 500 ϫ g for 5 min, 1 mL
of solution was removed as the bottom layer. The concentrations of
the top layer and bottom layer solutions were measured by the
phenol-sulfuric acid method (Dubois and others 1956).
Table 1 summarizes the structural parameters of xanthan,
deacetylated xanthan, and guar. Guar had a much larger molecu-
lar weight (2338000) than xanthan (1143000) and deacetylated xan-
than (1025000), and deacetylation clearly reduced the molecular
weight of xanthan. The initial concentration of xanthan bearing
sodium ions was 0.10524 g/dL and the acetate and pyruvate con-
tents of xanthan were 5.14% and 0.24%, respectively. Therefore,
there were 0.00541 g or 0.000126 mole acetyl and 0.000253 g pyruvic
acid or 0.000000337 mole in the xanthan sample. Assuming that “x”
of the inner mannose units and “y” of the terminal mannose units
of the side chain bore an acetyl group and a pyruvic acid group, re-
For blends with gel formation, the gel strength measurements
were carried out at room temperature using a TA-XT2i texture an-
alyzer (Texture Technologies, Scarsdale, N.Y., U.S.A.) which was
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Xanthan/Guar Gum Interaction…
spectively, the molecular weight of the repeating unit of xanthan constant molecular weight (Flory 1953), a reduction in chain dimen-
was 846 ϩ 42x ϩ 92y and the number (N) of the repeat units was sion will decrease its intrinsic viscosity. Removing the acetyl groups
0.10524/(846 ϩ 42x ϩ 92y). Because xN and yN were 0.000126 mole destabilized the xanthan helix and rendered the deacetylated xan-
and 0.00000337 mole, respectively, x was calculated to be 1.07, than chain more flexible, thus resulting in a decrease of chain di-
which was more than 1 because of measurement errors, and y was mension. Therefore, the flexibility of xanthan chain increased in the
0.029. This indicates that 100% of the inner mannoses bore the order of native xanthan in salt solution Ͻ native xanthan Ͻ acetate-
acetyl groups and 2.9% of the terminal mannoses bore the pyruvate free xanthan, consistent with previous studies (Holzwarth 1976;
groups. Deacetylation removed approximately 97 % of the acetyl Smith and others 1981; Crescenzi and others 1986; Foster and
groups and 50 % of the pyruvate groups.
Morris 1994).
Intrinsic viscosity is defined as the extrapolation of reduced vis-
cosity data to zero concentration, and is a measure of the hydrody-
namic volume per gram of macromolecular substance at infinite
dilution. The intrinsic viscosity of native xanthan in water is shown
Interaction of native xanthan and guar in aqueous
sodium chloride solutions
Figure 2 shows the viscosity as a function of shear rate of native
in Figure 1, which was obtained by extrapolating of ␩sp/C (reduced xanthan/guar mixtures at a concentration of 0.2% (w/v) in 0.04 M
viscosity) to zero concentration using the nonlinear fitting of the NaCl. All systems exhibited pseudoplastic flows; however, no gels
statistics software JMP for SAS. The intrinsic viscosity of native were formed in any blend, consistent with previous studies (Mc-
xanthan in 0.04 M NaCl was 16.9 dL/g, well below that in the aque- Cleary 1979; Dea and others 1986). The synergistic interactions were
ous solution: 75.2 dL/g. This behavior was typical of polyelectrolytes. only noted in a cold-mixed blend at a xanthan:guar ratio of 2:3 and
The electrostatic shielding of the counter ions from NaCl to the below a shear rate of 20s–1, but not in any other blends. Since the
charges in xanthan chain affected the contraction of xanthan chain, synergistic interaction between native xanthan and guar in aque-
resulting in a significantly smaller hydrodynamic radius at a high ous solution was stronger than that in 0.04 M NaCl (Wang 2001), the
ionic strength. In contrast, guar, a nonpolyelectrolyte, had intrin- addition of NaCl resulted in a decrease of the synergistic interaction
sic viscosities in aqueous solution, 0.04 M NaCl of 8.2 and 8.5 dL/ between native xanthan and guar, supporting the suggestion that
g, respectively. Salt exerted little effect on its intrinsic viscosity.
the highly stable xanthan helix structure from the addition of salt
The intrinsic viscosity of deacetylated xanthan in aqueous solu- would further hinder its interaction with guar (Foster and Morris
tion was 43.3 dL/g, about 58% of native xanthan. The molecular 1994). Mixing at a high temperature will significantly enhance the
weight of deacetylated xanthan is 91% of that of native xanthan. The binding because the mixing temperature (Ͼ 90 ºC) was above the
relationship between molecular weight and intrinsic viscosity can order-disorder transition (Tm) of native xanthan in 0.04 NaCl, 84 ºC
be characterized by the Mark-Houwink equation:
(Williams and others 1991). The disordered xanthan should be ca-
pable of directly interacting with guar to form the heterotypic struc-
tures (Zhan and others 1993; Goycoolea and others 1994, Morris and
others 1994; Morris ER 1996; Morris VJ 1996). However, the viscosity
[␩] = K’M␣
where M is the molecular weight and K’ and ␣ are constants de- of hot-mixed blends decreased. This is in contrast to the results in
pending on both the polymer and the solvent (Flory 1953). The value aqueous solution where the hot-mixed blends showed stronger
of ␣ does not fall below 0.5 in any case and seldom exceeds 0.8. Ex-
ceptions occur in the case of polyelectrolytes where, in the absence
of added salts, ␣ may approach 2 (Flory 1953). Assuming that ␣
ranged from 0.5 to 2, if xanthan had the same molecular weight as
the deacetylated xanthan (1025000), native xanthan should have an
intrinsic viscosity of 61 to 71 dL/g, based on the equation, which was
much higher than that of deacetylated xanthan. Since the intrinsic
viscosity of a polymer is proportional to its chain dimension for a
Figure 2—Plots of viscosity against shear rate of guar,
xanthan, and their mixtures in 0.04 M NaCl at a polymer con-
centration of 0.2% (w/v). Ratio of xanthan to guar gum in
mixtures: , guar; , xanthan; ᮀ, 4/1; ᭺, 3/2; , 2/3;
Figure 1—Plot of reduced viscosity against xanthan concen- 1/4. Filled symbol: hot-mixing; unfilled symbol: cold-mix-
tration in water ing.
,
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Xanthan/Guar Gum Interaction…
synergistic interaction (Goycoolea and others 1994, Morris VJ 1996; ed from incompatibility but not from association since intermolec-
Wang 2001), suggesting that intermolecular binding may not occur ular binding between xanthan and guar was not supported from
between xanthan and guar in 0.04 M NaCl.
the Haake viscometry study. The salt addition increased the xan-
A centrifugation study was carried out to further elucidate the than helix-helix aggregation (Foster and Morris 1994), and reduced
interaction between xanthan and guar and results are shown in the entropy loss on mixing (Schmitt and others 1998), thus the ten-
Figure 3. There were significant differences in concentration be- dency towards segregative phase separation dramatically in-
tween the top and bottom layers of the blends upon centrifugation, creased (Schmitt and others 1998; Piculell and Lindman 1992). The
but no difference was observed for the xanthan or guar alone, in- greater concentration difference between the 2 layers for hot-
dicating phase separation. It is likely that phase separation originat- mixed systems than that for the cold-mixed systems suggested an
increased segregative phase separation in the hot-mixed systems.
Although hot mixing induced the order-disorder transition of xan-
than, xanthan and guar would not form the heterotypic structures
because the heterotypic structures were not as stable as the xan-
than helix. On the contrary, hot-mixing increased the molecular mo-
bility of xanthan, thus increasing the possibility for the xanthan
helix to associate.
Plots of reduced viscosity (␩sp/C) against concentration of xan-
than and guar in 0.04 M NaCl aqueous solution by Ubbelohde vis-
cometry are shown in Figure 4. All xanthan and guar and their
blends followed the Huggins equation when the concentration was
below the overlap concentration (C*) (0.084 g/dL for xanthan and
0.136 g/dL for guar). A linear relationship was found between the
intrinsic viscosities and xanthan fraction in 0.04 M NaCl (Figure 5).
Xanthan helical structure in 0.04 M NaCl was stabilized so that the
proportion of disorder was very small even in the presence of guar.
Furthermore, guar was a weak denaturant and not efficient in sta-
bilizing xanthan and guar heterotypic structures. Therefore, the
characteristic of flexible polyelectrolytes was not observed and
incompatibility might exist in these systems.
Interaction of deacetylated xanthan and guar in
aqueous solution
Gel strength is a reflection of the work of cohesion within the gel
as the resistance to the insertion of a probe into a sample under
Figure 3—Centrifugation study of xanthan/guar blends in
0.04 M NaCl at a polymer concentration of 0.2% (w/v). ᭜,
cold mixing; ᭡, hot mixing; ——, top layer; - - - , bottom
layer.
Figure 5—Plots of intrinsic viscosities against xanthan frac-
Figure 4—Plots of reduced viscosities against concentra-
tion of guar, xanthan, and their mixtures in 0.04M NaCl by
Ubbelohde viscometry. Ratio of xanthan to guar in mixtures:
, guar; , xanthan; ᮀ, 4/1; ᭺, 3/2; , 2/3; , 1/4.
tion. ᭿, xanthan/guar in 0.04M NaCl; ᭜, xanthan/guar in
aqueous solution; ᭡, deacetylated xanthan/guar in aque-
ous solution; dashed line, intrinsic viscosity calculated from
the weight averages of the 2 individual polysaccharides.
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Xanthan/Guar Gum Interaction…
defined conditions (Craig and others 1997). The gel strength as a
Figure 7 shows the results of reduced viscosity against concen-
function of deacetylated xanthan fraction is shown in Figure 6. tration of deacetylated xanthan and guar in aqueous solution by
Strong synergistic interactions were observed in the cold-mixed Ubbelohde viscometry. The Huggins equation was followed for all
deacetylated xanthan and guar systems at a concentration of 0.2% samples when the concentration was below the C* (0.075 g/dL for
(w/v), in which the xanthan/guar ratios of 1:4 and 4:1 were viscous deacetylated xanthan). The characteristic of flexible polyelectro-
gel-like pourable fluids, whereas 2:3 and 3:2 formed weak nonpour- lytes was not noted in any systems in this study, whereas in native
able gels according to the inverting tube method described by xanthan/guar/water systems, when xanthan was the main compo-
Brownsey and others (1988). The maximum gel strength of 13.5 g.s nent in the mixture, instead of following the Huggins equation, the
was observed at the ratio of 2:3. Native xanthan and guar mixtures reduced viscosity underwent a marked increase with dilution and
in aqueous solution did not form gels but viscous solutions, even approached infinity at zero concentration, a characteristic of flexible
though the synergistic interaction was observed (McCleary 1979; polyelectrolytes according to the Fuoss empirical equation (Fuoss
Dea and others 1986; Wang 2001). Deacetylation clearly enhanced and Strauss 1948; Fuoss and Cathers 1949). A possible explanation
the interaction between xanthan and guar by destabilizing xanthan is that the chain of the deacetylated xanthan was not flexible enough
helical structure (Foster and Morris 1994) and giving xanthan much to exhibit the property of flexible chain polyelectrolytes under the
more chain flexibility, thus facilitating the formation of heterotypic experimental condition where the ionic strength of solvent from
junctions with guar (Zhan and others 1993; Morris and others 1994; sodium azide was still higher than that of the xanthan chain (Wang
Goycoolea and others 1994; Morris ER 1996; Morris VJ 1996; Ojin- 2001). The strong intermolecular binding between the deacetylated
naka and others 1998).
xanthan and the guar not only stabilized the current conformation
The intrinsic viscosities of deacetylated xanthan and guar blends but also stiffened the association zones, resulting in the absence of
were higher than those calculated from the weight averages of the flexible chain characteristics.
2 individual ones (Figure 5), which was clear evidence of intermo-
Conclusions
lecular binding between xanthan and guar molecules. However,
conformational change of deacetylated xanthan may not take place
or may not predominate in controlling the intrinsic viscosity.
Deacetylated xanthan may be in an exact conformation to bind guar
and the most stable heterotypic structure between deacetylated
xanthan and guar was formed directly without the need of induc-
ing the conformational change of xanthan by guar. This strong in-
termolecular binding might also enable portions of deacetylated
xanthan chain to associate with guar so that the deacetylated xan-
than chain became rigid. This may partially explain why intrinsic
viscosities of deacetylated xanthan and guar blends were higher
than those calculated from the weight averages of the 2 individu-
al ones.
HE STABILITY OF XANTHAN HELICAL STRUCTURE OR XANTHAN
T
chain flexibility played a critical role in its interaction with guar.
Deacetylated xanthan with a more flexible chain conformation was
preferentially bound to guar, thus resulting in a stronger synergistic
interaction. The conformational change was not observed in
deacetylated xanthan possibly because deacetylated xanthan was
in the exact conformation to bind guar. Guar was not effective in
denaturing xanthan 0.04 M NaCl, although guar was a denaturant
to xanthan helical structure in aqueous solution where xanthan was
observed to go through the helix-coil transition. The segregative
phase separation may take place in xanthan/guar/0.04 M NaCl
systems.
Figure 6—Plot of gel strength against xanthan fraction for Figure 7—Plots of reduced viscosities against concentration
cold-mixed deacetylated xanthan and guar in aqueous solu- of guar, deacetylated xanthan, and their mixtures in aque-
tion at a polymer concentration of 0.2% (w/v) by texture ous solution by Ubbelohde viscometry. Symbols are the
analysis
same as in Figure 4.
Vol. 67, Nr. 9, 2002—JOURNAL OF FOOD SCIENCE 3293

Xanthan/Guar Gum Interaction…
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