J. Biol. Macromol., 5(3), 47-52 (2005)

Review
Gelation and gel properties of polysaccharides gellan gum and tamarind xyloglucan
Yoko Nitta*† and Katsuyoshi Nishinari
Department of Food and Nutrition, Faculty of Human Life Science, Osaka City Univeristy,
3-3-138, Sumiyoshi-ku, Sugimoto, Osaka City, Osaka, 558-8585, Japan
Received September 5, 2005, accepted September 14, 2005
Keywords: Gel, Gelation, texture modifier, gellan, xyloglucan, rheology, DSC
Polysaccharide is used widely in food, cosmetic and pharmaceutical industries. The
main use is to give appropriate texture to the products. Thus the polysaccharides used insuch way are called texture modifier. Here two important texture modifiers, gellan andtamarind xyloglucan are introduced. The former one is a microbial polysaccharide withgelling ability. Gelling ability makes gellan valuable since only a few polysaccharides canform a gel. Most polysaccharides do not form a gel by themselves; however, some can forma gel under appropriate conditions. The latter one is a plant polysaccharide which forms agel under appropriate conditions. Gelation and gel properties of gellan and tamarind
xyloglucan are described.
Introduction
Some polysaccharides are used toenhance the quality of product by thickeningand gelling, and by reducing the undesired
defect of water release (syneresis) in someproducts, and by stabilizing emulsion and* Corresponding author†
Present address: Department of Food
Science and Nutrition, Faculty of Human
Life and Environment, Nara Women’sUniversity, Kitauoya Nishi Machi, NaraCity, Nara, 630-8506, Japan
Abbreviation: differential scanningcalorimetry (DSC), epigallocatechin gallate(EGCG)

suspension [1]. The polysaccharides usedin these ways are called texture modifier andcreation of various kinds of texture modifier
is required to make the products withdesirable texture.
Gelling ability is an important property asa texture modifier and some polysaccharides
can form a gel at low concentrations (~1%).
There are only a few polysaccharides whichhave a gelling ability by themselves at low
concentration whereas a lot of non-gelling
polysaccharides are used as a thickener andstabilizer [2].
In the present article, recent insight about
the gelation and gel properties of gellangum and tamarind xyloglucan shall be

48

Gelation of gellan and xyloglucan

described. Gellan gum is a polysaccharide
which can form a gel at low concentrations.
Gelation mechanism and gel properties ofgellan have not been clarified well at thepresent stage. Tamarind xyloglucan, apolysaccharide obtained from tamarind seed,is a valuable thickener and stabilizer. We
regarded tamarind xyloglucan as arepresentative of non-gelling
polysaccharides. Understanding novel
gelling conditions of tamarind xyloglucan
would lead to create various kinds oftexture modifier.
Gellan gum
Gellan gum is an extracellularpolysaccharide produced by micro-organism
Sphingomonas elodea (ATCC 31461)
previously known as Pseudomonas elodea.
Being a fermentation product, it can beproduced on demand and with consistentquality. The primary structure of gellan
gum is composed of a linear tetrasaccharide
repeat unit: 1,3-β-D-glucose,(a)

1,4-β-D-glucuronic acid, 1,4-β-D-glucose,and 1,4-α-L-rhamnose as reported byO’Neil et al. and Jannson et al. [3, 4].
Gellan gels in the presence of appropriate
amount of cations are transparent, resistantto heat in the wide range of pH [5]. Brittlegellan gels have a good flavor release,suitable for gel products with new texture.
For example, an assembly of brittle gelparticle, called a microgel, has a specifictexture like fluid gel which was produced
from agarose gel [6].
Because of industrial importance ofgellan gels, gelation and gel properties havebeen understood briefly. Somefundamental questions still remain to beanswered. For example, what is thegelation mechanism like? In order tofurther understand gelation and gel
properties of gellan gum, a collaborative
research group was organized conjunction
with the research group of polymer gelsaffiliated to the Society of Polymer Science,
Japan, in 1989. The common gellan was(b)2005101M NaCl

0.1M NaCl

100
θ / mdeg.

Distilled waterEʹ / Pa

0.01M NaCl

Silicone oil

immersed gel in distilled water
Non-immersed gel

0-100
Immersed gel in 1M NaCl

410060120Time / min180-200

200220

Wave length / nmFig. 1. (a) The storage Young’s modulus and (b) the circulardichroism spectra of 1 wt % K-gellan gels affected by theimmersion in NaCl solutions or distilled water.

240

Y. Nitta and K. Nishinari

used to study its properties with varioustechniques. The results of the collaborative
studies were published in special issues ofFood Hydrocolloids 7, 361-456 in 1993,
Carbohydrate Polymers 20, 75-207 in 1996,
and Progress in Colloid and Polymer
Science, 114, 1-131 in 1999. Light
scattering and osmotic pressure
measurements showed that gellan changes
from two single chains to one double helix
on cooling and changes from one double
helix to two single chains on heating [7, 8].
Rheological measurements showed that agel formation occurs after the coil-to-helix
transition on further cooling underappropriate conditions [9]. Proposed
gelation mechanism is described in thosespecial issues.
Gel properties also remain to be clarified.
We found that rheological properties change

49

dramatically under gel state at a certain
temperature. At this characteristic
temperature single peak appeared indifferential scanning calorimetry (DSC) andspectra changed in circular dichroism.
These thermally-induced physicochemicalchanges are believed to be due to ahelix-coil transition of gellan; thus, weconcluded that a helix-coil transition canoccur even in gel state. Since gellan is apolyelectrolyte, gel properties are influenced
by the salts. When the gellan gels were
immersed in NaCl solutions, the elastic
modulus of the gel increased and circulardichroism spectra changed as shown in Fig.1.
This was also attributed to the helix-coiltransition of gellan.
Tamarind xyloglucan
Xyloglucan is a major structural

60°C

12°C

EGCG
conc. / wt%

00.2

0.4

0.6

0.8

11.2Fig. 2. The appearance of 1 wt % tamarind
xyloglucan in the presence of epigallocatechin gallate(EGCG).

50

Gelation of gellan and xyloglucan

polysaccharide in the primary cell walls ofhigher plants. Tamarind xyloglucan is
obtained from the endosperm of the seed of
the tamarind tree, Tamarindus indica, amember of the evergreen family, that is oneof the most important and common trees ofSoutheast Asia and widely indigenous toIndia, Bangladesh, Myanmar, Sri Lanka, andMalaysia [11]. Purified, refined tamarind
xyloglucan is produced in Japan and ispermitted as a thickening, stabilizing, andgelling agent.
Tamarind xyloglucan has a(1→4)-β-D-glucan backbone that is partiallysubstituted at the O-6 position of its
glucopyranosyl residues with
α-D-xylopyranose [12]. Some of thexylose residues are β-D-galactosylated at
O-2 [12].
Although tamarind xyloglucan itself does
not form a gel, gel can be obtained underappropriate conditions, such as by adding
some substances or removing substituents.
Tamarind xyloglucan forms a gel in thepresence of 40-65% sugar over a wide pH
range [13]. It also forms a gel in thepresence of alcohol [13] or by removing
galactose residues from tamarind xyloglucan[14, 15]. In order to seek novel gelling
condition of tamarind xyloglucan, we
prepared a mixture of tamarind xyloglucan
and epigallocatechin gellate (EGCG), a
polyphenol abundant in tea leaves. The
mixture formed a translucent or opaque gel(Fig.2) [16]. Rheological and DSC studies
showed that the gelation occurred on coolingand gel melted on subsequent heating [16].
EGCG was most likely bound to tamarind
xyloglucan chains for a gel network, which

was detected as a DSC peak andtwo-dimensional nuclear Overhauser effectspectroscopy (2D NOESY) [16].
Since it has been reported that some
mixtures of polysaccharides can form a gelby specific interaction, a mixture oftamarind xyloglucan and gellan wasprepared in order to test whether or not themixture shows a specific interaction leading
to a synergistic gelation. From viscoelastic
measurements we confirmed that themixture formed a gel under the condition
where individual polysaccharide does notform a gel at the experimental
concentrations, indicating the synergistic
gelation occurred [17]. In DSC
measurements, the gelation was detected asa peak that appeared at higher temperatures
than a peak arising from helix-coil transitionof gellan alone [17]. It was also detected
as a change in circular dichroism which was
not observed in tamarind xyloglucan alone
and gellan alone [17]. Judging from theresults it was concluded that tamarind
xyloglucan and gellan might associate toform a gel network.
Conclusion
Recent findings about gellan and tamarindxyloglucan are the helix-coil transition ofgellan in gels, the gelation of tamarind
xyloglucan by addition of epigallocatechingallate, and the synergistic gelation of a
mixture of tamarind xyloglucan and gellan.
It should be noted that it is rare for thepolysaccharide-polyphenol interaction
leading to gel formation. It is also notablethat only a few mixtures of polysaccharides
show the synergistic gelation. TheseY. Nitta and K. Nishinarifindings are expected to contribute to not
only understand gelation mechanism ofpolysaccharides but also create new texture
modifiers suitable for social requirement.
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Communicated by Hiroshi Ueno