Xanthan gum, a clearly better stabilizer - RT Vanderbilt Company, Inc.

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VANDERBILT
Published Articles, Papers and Presentations
Xanthan gum, a clearly better stabilizer
P.A. Ciullo, M. Andersson
Courtesy of SÖFW Journal
R.T. Vanderbilt Company, Inc.
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P.A. Ciullo, M. Andersson*
Xanthan gum, a clearly better stabilizer
Keywords: Xanthan gum, glycerol, rheology
Introduction
Rutabaga was never anyone's favorite.
Except, perhaps, for those scientists laboring
in the Northern Regional Research
Laboratory (NRRL)of the U.s.
Department of Agriculture in the late
1950's.At that time, they were striving
to complywith a government directive
to identify biologically derived products
that could be commercialized by
American industry. They were familiar
with the thick, slimy exocellular coating
that certain bacteria create to protect
themselves from environmental
stress. Such coatings are composed
mainly of water thickened with a small
amount of polysaccharide synthesized
by the bacteria.
The NRRLscientistswere particularly
interested in the polysaccharide produced
bythe plant pathogen, Xanthamonas
campestris. Its protective ooze
was found to be remarkably resistant
to temperatLJre extremes, pH shifts
and electrolyte assault. The polysaccharide
responsible for this unusual
stability held great promise as a
unique new hydrocolloid, if only it
could be produced consistently and in
large quantities. Fortunately, the bacteria
themselves were suitably obliging
in terms of chemical uniformity,
and as long as the bacteria strain remained
pure, each and everyone
would produce the same polysaccharide.
In proper numbers, they were only
too happy to produce for whoever
provided a steady food supply. They
were even willing to forsake rutabaga
for glucose.
The NRRLteam learnedhowto culture
the bacteria and how to produce, isolate
and characterize what they were
convinced would be an industrially
useful gum. By1959,they had succeeded
in converting rotted rutabaga slime
into Polysaccharide B-1459 (1), which
has since become better known as xanthan
gum.
SO FW-Jou rnal-Sonderd ruck
The nature of
xanthan gum
Today xanthan gum is produced on industrial
scale by the aerobic fermentation
of pure culture Xanthamonas
campestris. The fermentation medium
typically contains corn-derived glucose
as the carbohydrate, corn-derived organic
nitrogen sources, specific trace
elements and other growth factors.
The process isconducted in a sterile environment
where the pH, oxygen content
and temperature are rigorously
controlled. After fermentation iscomplete,
the broth is sterilized and the
gum isrecovered by precipitation with
isopropyl alcohol. It is then dried,
milled and packaged under sterile conditions.
The molecular weight of the
recovered biopolymer is approximately
2 x10' daltons.
The molecular structure of xanthan
gum is shown in Fig. 1. The polymer
backbone isidentical to cellulose, composed
of 1,4 linked ~-D-glucose. The
unique character of xaMhan gum derives
from the trisaccharide
side
chain on alternateanhydroglucose
units. This
chain is composed
of glucuronic
acid
between mannose
acetate and
a terminal mannose
unit. A pyruvateisattached
to about
60 % of these
terminal units by
a ketal linkage.
As is common
with polysaccharides,
the glycosidic
linkages
between glucose
residues along
the backbone
restricts molecular mobility. In solutions
low in ionic strength or at high
temperature, the xanthan gum chains
adopt a random coil configuration, although
with somewhat limited flexibility.
The addition of even small
amounts of electrolyte, however, reduces
electrostatic repulsion among
the side chains, allowing them to wrap
around and hydrogen bond to the
backbone. This promotes an extended
conformation, a relatively rigid helical
rod (2).With increasing ionic strength,
this conformation persists to higher
temperatures and greater dilutions. At
ionic strengths above approximately
0.15M, it is retained up to 100°(. Protection
of the backbone by the side
chains accounts for this gum's unusual
resistance to chemical and enzymatic
degradation.
The nature of the interaction of xanthan
gum molecules in aqueous solution
is not precisely known. However,
both hydrogen bonding and ionic
interactions between the ordered
chains are believed to be involved. In
salt-free solutions, viscosity is built
through entanglement of polymer
random coils,to the extent allowed by
mutual repulsion of the negatively
charged side chains. When electrolyte
CH,OH
H -o~
PCH'OH 0
OH HI!'O H
H H 0
H OH H
t!
1
CH'OOCCH' 0 H OH n
H 0
H H
OH
OH
yOO-W 0
0
M'=Na', K', YoCa"
Fig. 1 Structure of Xanthan Gum
H H
J

SMETI
is present, a colloidal network forms
based on intermolecular hydrogen
bonding among the helical rods, in addition
to limited polymer entanglement
(3).
This network of entangled stiff molecules
accounts for the characteristic
rheological properties of xanthan gum
solutions. Sufficient intermolecular
adhesion exists such that a force, the
yield stress, must be exerted against
the network before it can be disrupted.
At low shear rates, therefore, xanthan
gum solutions provide high viscosity
and effective stabilization of
emulsions, suspensions and foams.
Once the yield stress isexceeded, however,these
solutions are highly pseudoplastic.
The network disaggregates as
individual polymer molecules align in
the direction of the shear force. The extent
of this disaggregation is proportional
to the shear rate. The network
reforms rapidlywhen shear isremoved.
Clear-solution
xanthan gum
Despite the unique attributes that
xanthan gum contributes to the
formulator's art, its traditional commercial
form does not allow for the
preparation of transparent solutions.
The alcohol precipitation of the gum
during its industrial isolation invariably
incorporates bacteria cellwall residue,
which creates turbidity in subsequently
prepared solutions. While this
is not an issue in most uses, there are
some applications, such as in pharmaceutical
and personal care products,
2
I
j
~
.0.0 .0.4 .0.2 0.2
LOG{Shu,"",
0.4
xG'"""
."" XGNFC
.°'" XGNFC, 0.2.. ..,...... ,O"'XONFC, 0."'--."
0,0
where solution clarity is preferred.
Xanthan gum yielding transparent solutions
is produced by eliminating insoluble
cell residue prior to precipitation
ofthe gum from the fermentation
broth. This isdone by filtration at high
temperatures (4) or lysis of cell wall
muriens with proteolytic enzymes (5).
Commercially available clear-solution
xanthan gum offers >85% light transmittance
through 0.5% w/w solutions.
Xanthan gum /
glycerol rheology
Xanthan gum has the interesting
property of being soluble in hot
(> 65°C)glycerol and retaining solubility
once cooled. AI~houghglycerol may
these days be considered a low-tech
moisturizer, it iseffective, nonetheless.
Clear solutions of xanthan gum in
glycerol offer the formulator of pharmaceutical
and personal care products
an unusual new vehicle. Anyone who
has prepared such a solution, however,
can appreciate its most obvious esthetic
liability - its pituitous rheology.
Glycerol-solvated xanthan gum is distinctly
stringy. This presumably arises
from a combination of random coil
type polymer entanglements coupled
with polymer-glycerolhydrogen bonding.
Stringiness can be diminished,
however, by reducing polymer entanglementthrough
increased polymer rigidity.
This is accomplished, as it is in
water, by addition of electrolyte.
Magnesium sulfate heptahydrate (Mg
SO..7H,O USP) was used to demonstrate
this because of its solubility in
glycerol and its
physiological
,°'" XGNFC, 0,13"""""
0°'" XGNF!:,...,......
Fig. 2 Power law plot of Xan-than Gum in glycerol solutions
30
25
'E20
~
115
~
~10
12 0.5
XG..,.",
.025% XGNFC, 0.25% MgS,"'.
compatibility. A USP grade of 99.7%
glycerol was used along with a U.S. National
Formulary, clear-solution grade
of xanthan gum (6) (XGNFC).All solutions
were prepared with a propeller
mixer. The xanthan gum was dispersed
into the glycerol at ambient temperature
and heated with gentle mixing to
80-85 °C to ensure dissolution. The
MgSO..7H,O was then added with mixing
until dissolved. Heat was removed
and mixing was continued until the solution
cooled to 97%) with the
Power Lawequation 'T =kDn
where
'T =shear stress
D =shear rate
k =consistency index (mPa.s)) =
viscosity at 1 see'
n = flow index, where n = 1 describes
a newtonian fluid;

cy index allows comparison of low
shear viscosity, while the flow index
provides a qualitative comparison of
pseudoplasticity.
A plot of the square root of shear stress
vs. the square root of shear rate, as in
Fig. 3, allows appl ication of the Casson
equation 7 =70 + TjD
where
7 =shear stress
70 = yield stress
(shear stress at zero shear rate)
TI = plastic viscosity
D =shear rate
The yield stress isdetermined from this
plot by extrapolation to zero shear
rate.
Table 1 compares the flow index, calculated
viscosity at 1 see" measured
vis-cosityat 12.6see' and yield stress of
the Glycerol Systems
How stringy?
Table 1 Rheo!ogy Profile of Xanthan Gum/GlycerolSolutions
The electrolyte-free xanthan gum solution
is pseudoplastic and has yield
value, as would be expected. By any
subjective measure, however, it is distinctly
pituitous. The first magnesium
sulfate addition (0.13%) significantly
reduced both viscosityand yield stress,
and provided a somewhat lesspseudoplastic
solution. Subjective evaluation
of stringiness revealed that this too
had decreased. The next two increments
of salt addition further decreased
viscosity,yield stress and pseudoplasticity,
with no obvious further
improvement in stringiness. The final
salt addition (1.0%) had the most obvious
deleterious effect on the xanthan
gum polymer network, as reflected
in the flow index, viscosity, and
yield stress.
SO FW- Jo u r na 1-Sond e rd ru ck
Because the formulator of personal
care products is appropriately concerned
with esthetics as well as rheology,
a simple, semi-quantitative test
was devised to compare the stringiness
of the xanthan gum solutions. A
Brookfield viscometer cylindrical spindle
was fixed 7 cm above the surface
of the test solution contained in a
250 ml. glass jar on the lab bench. The
solution was carefully raised until its
surface was even with the notch in the
spindle, then it was rapidly but gently
lowered back to the bench. Solution
flowed off the spindle and back into
the jar in an unbroken thin stream. The
time, in seconds, it"took for the stream
Glycerol Solutions Flow Viscosity Viscosity Yield Stress
Index (mPa.s) (mpa.s) (Dynes/em')
@ 1 see' @ 12.6 see'
(Glycerol) 1.0 944 950 0
0.25% XGNFC 0.53 16415 5190 52
0.25% XGNFCI 0.61 9136 3290 24
0.13% MgSO..7H,o
0.25% XGNFCI 0.68 5644 2430 11
0.25% MgSO..7H,O
0.25% XGNFCI 0.67 6467 2840 12
0.5% MgSO..7H,O
0.25% XGNFCI 0.73 3806 1910 5
1.0% MgSO..7H,O
connecting the spindle and the solution
in the jar to first break was recorded
as the »pituosity index« (PI),for lack
of a less embarrassing term. The
Brookfield #3LVCYLcylindricalspindle
(cylinder diameter =5.88 mm; cylinder
height =42.86 mm; height to notch =
53.19 mm) worked well in this test. Results
reflected gross visual differences
in the stringiness of the solutions.
Table 2 compares the PIvalues for the
SMETICS
Glycerol Solutions Pituosity
Index (sec.)
0.25% XGNFC 16
0.25% XGNFCI 4
0.13% MgSO..7H,O
0.25% XGNFCI 3
0.25% MgSO..7H,O
0.25% XGNFCI 3
0.5% MgSO..7H,o
0.25% XGNFCI 2
1.0% MgSO..7H,O
(Glycerol)
3
Table 2 Affect of Salt on Relative
»Stringiness« of Xanthan Gum/Glycerol-Solutions
xanthan gum solutions, showing the
marked decrease in stringiness effected
by the salt addition.
The decrease in solution viscosity,yield
stress and pseudoplasticity with the
addition of salt was not expected, but
a similar, although less dramatic, response
does occur in low-concentration
aqueous xanthan gum solutions.
Salt-free aqueous solutions containing
less than about 0.2% w/w xanthan
gum will decrease in viscosity in response
to small electrolyte additions.
Inthe absence of salt, polymer random
coil interactions and entanglement
impart viscosity.With the addition of
salt, the more compact helical rod conformation
is adopted, decreasing the
polymer's effective hydrodynamic volume.ln
these dilute gum solutions, the
smaller hydrodynamic volume means
weaker molecule-molecule interactions
and slightly lower viscosity.
The more dramatic decrease in polymer
,network cohesiveness in glycerol
is likelydue, at least in part, to similar
changes in the polymer's shape and
volume. That these changes are not directly
analogous to those occurring in
dilute aqueous solution is evident by
the substantial decrease in viscosity,
yield stress and even stringiness in the
presence of successively greater
amounts of electrolyte. This may be
compounded by the interactions
between the xanthan gum polymer
and the glycerol medium itself.
3
I--

COSMETICS
Glycerol-xanthan gum
aqueous interactions
This latter point was of some interest
from a formulator's perspective. In
short, would the inclusion of a relatively
high concentration of glycerol in
an aqueous xanthan gum solution affect
rheology through glycerol-xanthan
polymer interactions? To test this
concept, a water:glycerol ratio greater
than 1:1 was desired, but with a sufficient
quantity of glycerol to incite a
measurable change, should there be
one. A 40% glycerol level was adopted
as appropriate, particularly since a
recent article reviewed the superior
benefits of a commercially available
moisturizer containing 40% glycerol
compared to those containing lesser
amounts (7).
Table 3 compares the rheological
properties of 1% aqueous xanthan
gum solutions with and without glyce-
sulfate or monovalent potassium chloride,
provided the expected increase in
viscosity and yield stress with this concentration
of xanthan gum whether or
not the glycerol was present. The flow
index values show that the glycerol
and salts had only a minor influence on
the very pseudoplastic aqueous xanthan
gum solution, making it slightly
more shear-thining.
Conclusion
Transparent, xanthan-thickened glycerol
isan interesting new anhydrous vehiclefor
the formulator of pharmaceutical
and personal care products. The
pituitous nature that might contraindicate
such solutions on esthetic principles
can be controlled by addition of
minor amounts of glycerol-soluble salt.
The salt used in this work, magnesium
sulfate heptahydrate USP,reduced so-
Flow
Index
Viscosity
(mPa.s)
@ 1 see-'
Viscosity
(mpa.s)
@ 12.6 see'
Yield Stress
(Dynes/em')
In H,O
(H,O) (1.0) (1) (1) (0)
1.0% XGNFC 0.23 2986 426 21
1.0% XGNFC/
1.0% MgSO..7H,O
0.19 12148 1537 92
1.0% XGNFC/1.0% KCI
In H,O/Glycerol
0.19 12571 1574 96
60% H20/4G% Glycerol 1.0 6 6 0
1.0% XGNFC 0.17 6230 812 50
1.0% XGNFC/
1.0% MgSO..7H,O
0.15 20962 2420 169
1.0% XGNFC/1.0% KCI 0.15 19691 2280 159
Table 3 Affect of Glycerol and Salt on Aqueous Xanthan Gum Rheology
rol and electrolyte. All solutions were
prepared at ambient temperature using
a propeller mixer.Glycerol-free solutions
were prepared by dissolving
the gum in water or the respective
electrolyte solution. Otherwise, the
gum was dispersed in the glycerol and
then solubilized bythe addition of water
or the respective electrolyte solution.
The higher viscosity and yield stress
when glycerol is present in the solution
suggests a strong xanthan gumglycerol
affinity that it is not negated
by the preponderance of water molecules.
The salts, divalent magnesium
4
lution stringiness, reduced solution
viscosityand reduced yield stress when
used in a 0.25 % xanthan gum solution.
The ability to balance reduction
of stringiness with increased viscosity
and yield stress at higher xanthan gum
concentrations is the obvious choice
for further investigation. Of particular
interest would be the xanthan gum
concentration, if any, at which the salt
increases rather than decreases viscosity
and yield stress while still attenuating
stringiness.
The rheology of glycerol solutions of
xanthan gum appears to be strongly
affected by the interaction, presum-
ably hydrogen bonding, between the
xanthan gum polymer and the glycerol
itself. The ability of this affinity to
persist even in aqueous solutions was
tested by comparing transparent 1%
xanthan gum solutions with and without
40% glycerol and 1% magnesium
sulfate or potassium chloride. The
presence of a strong xanthan gumglycerol
affinity was supported by the
fact that the inclusion of glycerol in
the solution increased viscosity and
yield stress by 50-100%. At this xanthan
gum concentration, addition of
electrolyte had the expected positive
effect on both viscosity and yield
stress.
References
(1) Jeannes, A, Pittsley, J.E., Senti, F.R., J. Appl.
Polm. Sci., 5, 519 (1961). »Polysaccharide B-
1459: A New Hydrocolloid Polyelectrolyte Produced
from Glucose by Bacterial Fermentation«
(2) Moorhouse, R., Walkinshaw, M.D., Arnott, S.,
in Extracellular Microbial Polysaccharides, P.A
Sanford, A Laskins, Eds., ACS Symp. Ser. No.
45, Am. Chem. Soc., Washington, DC, 81
(1977). »Xanthan Gum(Molecular Conformation
and Inter-actions«
(3) Norton, I.T., Goodall, D.M., Frangou, S.A,
Morris, E.R., Rees, D.A, J. Mol. BioI., 175
(1984). »Mechanism and Dynamics of Conformational
Ordering in Xanthan Polysac-charide«
(4) BR Patent Specification 1528316, Purifying
Xanthan Gum
(5) US Patent 4119491, Enzyme Filtration Clarification
of Xanthan Gum Polymer Solution
(6) VANZAN( NF-C, R.T.Vanderbilt Company, Inc.
(7) Orth, D.S., Appa, Y., in Dry Skin and Moisturizers,
Loden, M., Maibach, H.I., Eds., CRCPress,
New York, 213 (2000). »Glycerine: A Natural
Ingredient for Moisturizing Skin«
*Author's address:
Peter A. Ciullo
Marian Andersson
c/O R.T.Vanderbilt Company Inc
30 Winfield Street
Norwalk, CT 06856-5150
USA
6
SOFW-Jou rna I-Sonderd ruck
r
L --

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