Exudate gums: chemistry, properties and food applications – a review

Abstract

Gums are complex carbohydrate molecules which have the ability to bind water and form gels at low concentration. These carbohydrates are often associated with proteins and minerals in their structure. Gums are of various types such as seed gums, exudate gums, microbial gums, mucilage gums, seaweeds gums, etc. Exudate gums are plant gums which ooze out from bark as a result of a protection mechanism upon injury. Exudate gums have been used by humans since ancient times for various applications due to their easy availability. The main characteristics which make them fit for use in various applications are viscosity, adhesive property, stabilization effect, emulsification action and surface‐active property. Major applications of these gums are in food products, the paper, textile, cosmetics and pharmaceutical industries, oil‐well drilling, etc. In the present paper, the chemistry, properties, processing and applications of commercially available exudate gums such as acacia gum or gum arabic, karaya gum, ghatti gum and tragacanth gum are discussed. Recent literature reveals that apart from the above mentioned applications, these gums also have nutritional properties which are being explored. Other gums cannot replace them because of their certain unique characteristics. © 2020 Society of Chemical Industry

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Review
Received: 11 October 2019 Revised: 15 January 2020 Accepted article published: 30 January 2020 Published online in Wiley Online Library: 17 February 2020
(wileyonlinelibrary.com) DOI 10.1002/jsfa.10302
Exudate gums: chemistry, properties and food
applications –a review
Sheweta Barak,aDeepak Mudgila
*
and Shelly Tanejab
Abstract
Gums are complex carbohydrate molecules which have the ability to bind water and form gels at low concentration. These car-
bohydrates are often associated with proteins and minerals in their structure. Gums are of various types such as seed gums,
exudate gums, microbial gums, mucilage gums, seaweeds gums, etc. Exudate gums are plant gums which ooze out from bark
as a result of a protection mechanism upon injury. Exudate gums have been used by humans since ancient times for various
applications due to their easy availability. The main characteristics which make them fit for use in various applications are vis-
cosity, adhesive property, stabilization effect, emulsification action and surface-active property. Major applications of these
gums are in food products, the paper, textile, cosmetics and pharmaceutical industries, oil-well drilling, etc. In the present
paper, the chemistry, properties, processing and applications of commercially available exudate gums such as acacia gum or
gum arabic, karaya gum, ghatti gum and tragacanth gum are discussed. Recent literature reveals that apart from the above
mentioned applications, these gums also have nutritional properties which are being explored. Other gums cannot replace
them because of their certain unique characteristics.
© 2020 Society of Chemical Industry
Keywords: acacia gum; gum arabic; tragacanth gum; ghatti gum; karaya gum
INTRODUCTION
Gums are high-molecular-weight molecules which are hydro-
philic or hydrophobic in nature and produce viscous solutions or
gels when dispersed in specific solvents at very low concentra-
tion. Technically, industrial gums are defined as polysaccharides
of plant and microbial origin which when dispersed in hot or cold
water lead to the production of viscous solutions. As presented in
Table 1, these gums are of several types such as plant seed gums
(guar gum, locust bean gum, etc.), exudate gums (acacia gum,
tragacanth gum, etc.) microbial gums (xanthan gum, gellan
gum, etc.), mucilage gums (psyllium gum), seaweed gums (algi-
nates, carrageenan) and animal polysaccharide gums (chitin, chit-
osan, etc.). All these gums have numerous applications in the food
and other industries.
1–15
Among these gums, exudate gums have
been used since ancient times. Their use as foods is also men-
tioned in the Bible.
15
In Australia, natives used acacia exudate
gum (wattle gum) along with fish. Exudate gums were the earliest
gums used by humans due to their availability. These gums could
be easily plucked or detached from plants, dried and transported
to other areas where they were not available. No other types of
gums have replaced exudate gums due to their important charac-
teristics and functions.
16
Natural exudate gums are typically polysaccharides which are
exuded by plants in stress conditions such as physical injury (cuts
and incisions) and fungal attack. During stress conditions, trees
and shrubs exude gums in tear-like, smeared buds or lumps or
mass which are amorphous in nature. Upon sun drying, they form
glassy and hard exudates having various colours such as white
(gum arabic and tragacanth gum), amber (gum arabic), pale grey
(karaya gum), dark brown (tragacanth gum), etc. Exudation is a
natural defence phenomenon of trees, with exudate covering
wounds to the bark. When an injury occurs to the bark, many
trees, especially in semiarid areas, exude a gummy liquid which
makes a covering on the injury and becomes hardened to prevent
infection and water loss in the injured area of the plant. The
amount of the exudate or gum production is high in the summer
season meaning a dry and hot environment favours the produc-
tion of exudate gum. The yield of these exudate gums can also
be enhanced via making cuts and incisions in the bark of trees or
shrubs. Production of exudate gums requires incision or knocking
the tree, picking of gum (dried/semidried), sorting, packing and
shipping to gum processors. Gum processors receive crude exu-
date gum in large chunk form which is then ground, sieved and
purified via air to remove impurities. In some cases, exudate gum
is further purified and spray-dried to remove insoluble impurities.
17
Similar to other gums, exudate gums have food applications
due to their stabilization, thickening, gelling and emulsification
actions. Exudate gums such as gum arabic or acacia gum, traga-
canth gum, karaya gum and ghatti gum are considered as safe
for consumption as food depending on ancient history of their
uses and also depending on the results obtained from
*Correspondence to: D Mudgil, Department of Dairy & Food Technology,
Mansinhbhai Institute of Dairy & Food Technology, Dudhsagar Dairy Campus,
Gujarat, India. E-mail: dsmudgil@yahoo.com
aDepartment of Dairy & Food Technology, Mansinhbhai Institute of Dairy &
Food Technology, Mehsana, India
bDepartment of Food Science & Technology, National Institute of Food Technol-
ogy, Entrepreneurship & Management, Kundli, India
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2828

toxicological studies. Food-grade exudate gums such as gum ara-
bic, tragacanth gum, karaya gum and ghatti gum are available
commercially and their GRAS (Generally Recognized as Safe) sta-
tus is established by the US Food and Drug Administration
(FDA). Sources and constituents of these exudate gums are
presented in Table 2. Exudate gums have several food applica-
tions such as flavour encapsulation, emulsification, thickening
and clouding agents in beverages, foam stabilizers, spreadability
agents in cheeses, water binders in bakery and minced meat
products, etc. (Table 3). Besides their food applications, exudate
gums have non-food applications including in pharmaceuticals,
textiles, cosmetics, etc.
15
Early engravings by the Egyptians repre-
sent the application of ‘kami’(gum arabic) in textile glues, mum-
mifying fluids and dye dispersions.
16
GUM ARABIC OR ACACIA GUM
Gum arabic or acacia gum is an exudate gum mainly produced
(80%) by Acacia senegal; however, it is also produced by other spe-
cies of acacia tree in minute quantities.
17,31
Gum arabic is the old-
est industrial gum among the exudate gums. Its use by humans
dates back about 5000 years. At that time, it was used as an adhe-
sive in paints. About 2650 BC, gum arabic was used as an adhesive
for wrappings of Egyptian mummies. Acacia trees are generally
grown for the prevention of soil erosion and also to obtain gum
arabic which has various applications. Acacia trees can be grown
in arid conditions and have very extensive roots, and hence can
prevent dessert invasion. Around 900 species of acacia trees are
widely spread in India, Australia, America and African sub-Sahara
regions. Major suppliers of gum arabic in the world market are
Sudan, Senegal, Nigeria, Mauritania, Mali and Chad. Major species
of acacia trees are Acacia senegal,Acacia laetia,Acacia seyal,etc.
Gum arabic production in these countries is affected by climatic
and political conditions and also by labour problems. Hand picking
of this gum is a laborious task and sorting is also a difficult opera-
tion for finally obtaining a clean and pure gum arabic with minimal
impurities. Generally, gum arabic is exuded after general injury to
plant bark. However, at a commercial level, gum arabic is formed
via making a 2 ft incision on tree bark in summer and dry season
which then yields 20 g to 2 kg of gum after 1–2 months. The aver-
age yield of gum arabic is 250 g per tree per year. Gum yield
obtained from a tree chiefly depends on the rainfall during the
growing season and temperature (hot conditions) during the sum-
mer season. Good rainfall and high temperature are responsible for
good yields of the gum. Nodules of gum arabic are shown in Fig. 1.
Chemistry
Native gum arabic is generally neutral or slightly acidic in nature. It
contains salts of cations such as calcium, potassium, magnesium,
etc. Hydrolysis of gum arabic from Acacia senegal indicates that it
contains 16% of uronic acid (4-O-methyl-D-glucuronic acid (1.5%)
and D-glucuronic acid (14.5%)), D-galactose (44%), L-arabinose
(24%) and L-rhamnose (13%). Gum arabic from other species con-
tains similar sugars but present in different proportions. In gum
arabic, polysaccharide chains are generally attached to protein
molecules via covalent bonds. Gum arabic from Acacia seyal con-
tains lower contents of rhamnose and glucuronic acid whereas
higher contents of arabinose and 4-O-methylglucuronic acid as
compared to gum arabic obtained from Acacia senegal. This vari-
ation in proportion of different constituents along with variation
in specific rotation and molecular mass is the basis of differentia-
tion among the gum arabic obtained from Acacia senegal and
Acacia seyal. Structural studies carried out on gum arabic obtained
from Acacia senegal reveal that it has a highly branched structure
composed of ⊎-1,3-linked D-galactose units as main chain along
with ⊎-1,3- and ⊎-1,6-linked galactose and 3-linked arabinose
units. Rhamnose and glucuronic acid are present as chain
Table 1. Types of natural gums and their sources
Type Example Source
Plant seed gums Guar gum Cyamopsis tetragonolobus
Locust bean
gum
Ceratonia siliqua
Microbial gums Xanthan
gum
Xanthomonas campestris
Gellan gum Sphingomonas elodea
Mucilage gums Psyllium
gum
Plantago ovata
Okra gum Hibiscus esculentus
Seaweed gums Alginates Brown algae
Carrageenan Red seaweeds
Kappaphycus alvarezii
Eucheuma denticulatum
Animal
polysaccharide
gums
Chitin Cell walls of fungi
Chitosan Shells of shrimp and other
crustaceans
Exudate gums Acacia gum Acacia senegal
Karaya gum Sterculia urens
Ghatti gum Anogeissus latifolia
Tragacanth
gum
Astragalus gummifer
Table 2. Commercial exudate gums, their sources and constituents
Exudate
gum Source Constituents
Acacia gum Acacia senegal
Acacia seyal
4-O-Methyl-D-glucuronic acid,
D-glucuronic acid, D-galactose,
L-arabinose, L-rhamnose
Karaya gum Sterculia urens
Sterculia setigera
Sterculia villosa
D-Galacturonic acid, D-galactose,
L-rhamnose, D-glucuronic acid
Ghatti gum Anogeissus
latifolia
Anogeissus
acuminata
Anogeissus
bentii
Anogeissus
dhofarica
L-Arabinose, D-galactose,
D-mannose, D-xylose,
D-glucuronic acid
Tragacanth
gum
Astragalus
gummifer
Astragalus
microcephalus
Astragalus
kurdicus
Astragalus
gossypinus
Galactose, arabinose, fucose,
⊍-D-galacturonic acid,
⊎-D-xylopyranosyl
Exudate Gums www.soci.org
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2829

terminators at the end of the chains. Its aqueous solutions show
shear-thinning behaviour at low shear rates.
32
Properties
Essential properties of the gum which makes it useful in various
applications are its viscosity, solubility, emulsifying ability, coacer-
vation ability, etc. Gum arabic exhibits the lowest viscosity among
industrial gums at similar concentrations. It shows very low viscos-
ity at low concentration in aqueous solution, and at very high con-
centration (i.e. up to 300 g kg
−1
) it exhibits Newtonian behaviour.
At high concentrations, it imparts viscosity, emulsifying, stabiliz-
ing and suspension properties. Gum arabic at higher concentra-
tion (i.e. above 300 g kg
−1
) exhibits pseudoplastic character.
Maximum viscosity of gum arabic solutions is exhibited at pH 6.
At lower pH (i.e. below 3), gum arabic forms gels via solubility loss.
Solubility of industrial gums in water at higher concentration is
very much less due to their high viscosity. At higher concentration
(i.e. 50 g kg
−1
), they are difficult to handle; but in the case of gum
arabic, solutions can be prepared more easily at this concentra-
tion due to its low viscosity. During storage, gum arabic solution
shows an increase in viscosity via an increase in the degree of
crosslinking as it is a salt of polycarboxylic acid. This increase in
crosslinking may be achieved for longer duration at normal tem-
perature or for short duration at elevated temperature.
Gum arabic also shows emulsion stability when incorporated in
oil-in-water emulsions. Stability of these emulsions can also be
enhanced via incorporation of electrolytes. An emulsion incorpo-
rating gum arabic and sodium salt is more stable and can be
achieved at very low gum concentration (i.e. 5 g kg
−1
). These
emulsions are stable over a broad pH range. Gum arabic also helps
in coacervation. Coacervates are aggregate of macromolecules
containing colloidal droplets of trapped solvents or oils. Gum ara-
bic shows compatibility with most of the starches and gums.
Applications
Gum arabic can be used in a variety of food applications due to its
properties such as viscosity, stabilization, thickening, emulsifica-
tion, nutrition and surface properties.
33
A major use of gum arabic
is in the development of encapsulated products such as flavours,
oils, bioactive components, etc. These encapsulated products
have longer shelf life and can be used in dry mixes as well. Gum
arabic has also been used as carrier agent in drying of fruits for
manufacturing fruit powder from fruit juices. Fruit juices are rich
in sugars, hence difficult to spray dry. Gum arabic as carrier agent
helps in the spray-drying of fruit juice via an increase in glass tran-
sition temperature and results in reduced stickiness in the final
fruit powder.
34
Another application of gum arabic is in confection-
ery products where it restricts crystallization of sucrose and emul-
sifies fat components. It is also used in making low-sugar candy.
These candies are soft and contain 50% less sugar as compared
to hard candies. Gum arabic can also be used in the development
of dietetic foods because of its bulking action without increasing
the calories of products. Due to its low viscosity and water solubil-
ity, gum arabic is used in bakery products to provide emulsifica-
tion, gloss and flavour release action. Due to its emulsification
action, gum arabic is used in beverages as a clouding agent.
Gum arabic is also used in beer and soft beverages due to its foam
stabilization action. Due to its emulsifying and stabilizing action,
gum arabic is also used in pharmaceuticals and cosmetics such
as emulsions, creams and lotions. In paper products, it is generally
used as an adhesive. Gum arabic also has applications in the tex-
tile industry in sizing, finishing and suspending.
18
KARAYA GUM
Karaya gum is an exudate gum obtained from Sterculia urens,
which is a large bushy tree (30 ft high) found in arid hills with
rocks of northern and central parts of India. It was first introduced
to replace gum tragacanth and now it has become second to gum
arabic in terms of uses. Its production can be increased by incision
or blazing of trees like other gum exudates. Gum starts to exude
from the tree just after the incision and almost all of it exudes as
irregular droplet-masses within 24 h of incision (Fig. 2). Gum of
best quality is obtained in hot and dry (summer) season. Second
collection can be made after the monsoon season but it is inferior
in quality to the gum obtained during the summer season
because of its low viscosity and dark colour. Other sources of kar-
aya gum are Sterculia setigera (Senegal, Mali) and Sterculia villosa
(Sudan, India). Production and export of Indian karaya gum are
Table 3. Commercial exudate gums and their applications
Exudate
gum Application Ref.
Acacia gum Flavour encapsulation
Bioactive component encapsulation
Sugar replacer in candies
Emulsification in bakery products
Clouding agent in beverages
Foam stabilizer in soft beverages
17–19
Karaya gum Stabilization in dairy desserts, whipped
cream, fruit ices, etc.
Spreadability in cheeses
Emulsification in salad dressings
Binder in bakery and pasta products
Water binder in minced meat products
20–22
Ghatti gum Emulsification in food products
Stabilization in food products
23–26
Tragacanth
gum
Thickener in sauce, ice cream, jelly, salad
dressing, candy, mayonnaise, etc.
Suspending agent
Binding agent
27–30
Figure 1. Gum arabic or acacia gum.
www.soci.org S Barak, D Mudgil, S Taneja
wileyonlinelibrary.com/jsfa © 2020 Society of Chemical Industry J Sci Food Agric 2020; 100: 2828–2835
2830

declining due to a sharp reduction in the number of gum trees.
Commercial karaya gum is classified into three grades on the basis
of the superiority and purity, i.e. grade 1, grade 2 and grade 3.
18
These three grades differ in appearance (colour) and percentage
of bark and foreign organic matter. From grade 1 to grade 2 and
grade 3, the colour of the gum changes from light grey to heavy
tan and foreign organic matter content rises from 0.5 to 3%.
Chemistry
Karaya gum is a substituted form of acetylated rhamnogalacturo-
noglycan polysaccharide which occurs as calcium and magne-
sium salts. Karaya gum has very high molecular weight which
ranges from 9 ×10
6
to 16 ×10
6
Da.
19,20,27
Acid hydrolysis prod-
ucts of karaya gum are D-galacturonic acid, D-galactose, L-
rhamnose and limited amounts of D-glucuronic acid. Sugar pro-
portions of karaya gum include uronic acids (37.6%), D-galactose
(26.3%) and L-rhamnose (29.2%).
35
Karaya gums contains around
8% acetyl groups which makes it insoluble in water and it just
swells via absorption of water. In aqueous solution, native form
of karaya gum (acetylated form) behaves as a firm and branched
arrangement whereas deacetylated karaya gum forms extra-
enlarged conformation which acts as a random coil.
21
Karaya
gum contains a higher amount of rhamnose content as compared
to other exudate gums.
Properties
Karaya gum is slightly acidic (acetous) in taste and it may have col-
our from white to tan which depends on the levels of impurities
present. Karaya gum is the least soluble among exudate gums
at higher concentrations. It forms true solution in water only at
very low concentration, i.e. 0.2 g kg
−1
. At higher concentration
(i.e. 50 g kg
−1
), karaya gum produces colloidal solution with high
viscosity. Karaya gum cannot be entirely dissolved in water to
form clear solution due to the presence of acetyl groups and it
only swells via rapid absorption of water to form colloidal solution
with high viscosity. Hydration of karaya gum also depends on the
particle size of the gum. Fine particles of gum tend to hydrate
speedily as compared to coarse gum particles. Also, coarse gum
particles form a dispersion having a grainy attribute. Deacetyla-
tion of karaya gum is carried out to increase its water solubility.
Aqueous solution of karaya gum shows a viscosity of 400 and
10 000 cP for a concentration of 5 and 30 g kg
−1
, respectively.
Solution viscosity is also affected by the level of impurities present
in karaya gum. Smooth texture of gum solution can be achieved
via agitation and longer hydration time. Dry powders of karaya
gum lose viscosity during storage which is caused by loss of acetic
acid. Fine particles of karaya gum exhibit higher viscosity loss as
compared to coarse particles due to greater loss of acetic acid in
fine-powdered karaya gum due to increased surface area. Storage
conditions such as temperature and humidity significantly affect
the viscosity of karaya gum.
22
Aqueous solution of karaya gum
exhibits greater viscosity when dispersed in cold water as com-
pared to hot water. Karaya gum forms thixotropic solutions.
Hydrated karaya gum particles swell via absorption of water and
this swelling is not stable towards mechanical shear. Mechanical
shearing of swollen karaya gum particles causes reduction in vis-
cosity. Rheological studies such as oscillatory small-deformation
of karaya solution in the presence of salt have been reported in
the literature.
36
The pH of a 10 g kg
−1
karaya gum solution ranges
between 4.5 and 5.2. Addition of alkali as well as acid to karaya
gum solution reduces its viscosity. Karaya gum solutions are not
heat stable meaning that heating causes irreversible reduction
in viscosity. Water binding ability and adhesive properties of kar-
aya gum are very high. Smooth films can be prepared from karaya
gum in the presence of plasticizing agents (glycols).
18
Applications
Karaya gum has designated GRAS status of the FDA. Karaya gum is
widely used as a stabilizer in various food products such as ice
cream and frozen dairy desserts, sherbets, fruit ices, whipped
cream, etc. It is typically used at a concentration between 0.2%
and 0.4%. In frozen dairy products, it improves the textural charac-
teristics via controlling ice crystal formation which further deals
with its sensory quality. It restricts the formation of large-size ice
crystals via water phase management which leads to smooth tex-
ture of frozen dairy products. Its stabilizing action in low-acid bev-
erages such as sherbets, fruit ices, etc., is due to its acid-resistant
behaviour. Karaya gum also finds application in cheese spread
where it not only meliorates spreadability but also inhibits syner-
esis. Karaya gum enhances aqueous phase viscosity in salad dress-
ings and hence can be used as an emulsion stabilizer in such
dressings. In bakery and pasta products it can be used as a binder
especially for the development of low-calorie products. It also
finds application in ground or minced meat products due to its
good water-holding and binding properties. In the paper industry,
karaya gum is used as a binding agent. In the textile industry, it is
used as a thickening agent for the dyes used for print colouring of
cotton fabrics. In the pharmaceutical industry, karaya gum is used
as an effective laxative because of its water absorption and swell-
ing capacity. It is also used as a binder and drug carrier for several
pharmaceutical preparations. Karaya gum also finds application in
textile printing.
37
GHATTI GUM
This is an Indian gum obtained from the tree Anogeissus latifolia
found in India and Sri Lanka. This exudate gum is named as ghatti
gum because it is found in the mountains in India which are
known as ‘Ghats’. The trees from which it is obtained are part of
the largest forest in India and occur in dry deciduous forests and
Figure 2. Karaya gum.
Exudate Gums www.soci.org
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2831

are able to grow in severe conditions and limited water supply.
These trees become very large when provided with nutrients
and water. Ghatti gum exudes from the tree when there are stress
conditions. Ghatti gum exudes slowly for several days and may
have weight of about 5 to 50 g depending on the conditions of
the tree such as age, size, etc. External as well as internal condi-
tions of the tree environment like heat, sunlight, wind, shape/size
of fissure, mass of gum nodule and interior pressure in the tree are
responsible for the shape of the gum nodule. Ghatti gum nodules
are mainly found in two different shapes, spiral shape and round
tear shape, as shown in Fig. 3. Ghatti gum has a somewhat glassy
appearance and its colour varies from translucent white to dark
red depending on the age of the exudate gum.
23
Similar to collec-
tion of other exudate gums, it is also collected via making careful
incisions on the bark of trees without making permanent injuries.
Production, harvesting, grading and transportation of ghatti gum
are similar to those of other exudate gums. The gum is exuded at
certain locations of the bark; if not picked, it will block the passage
of fresh gum and prevent its exudation. Ghatti gum is graded into
three grades, i.e. white, yellow and red grades. It can further be
graded on the basis of the amount of impurities present. Knowl-
edge of plantation and cleaning operations produces improved
quality of ghatti gum. Processed and spray dried ghatti gum has
good solubility and persistent colour quality and is termed as
‘gatifolia’.
38
Chemistry
Ghatti gum occurs as calcium and magnesium salts of uronic acid
units.
23
Sugar components of ghatti gum are L-arabinose, D-galac-
tose, D-mannose, D-xylose and D-glucuronic acid which are present
in a molar ratio of 10:6:2:1:2. Ghatti gum comprises a main back-
bone chain of (1 →6)-linked ⊎-D-galactopyranosyl units, a few
attaching (1 →2)-D-mannopyranosyl units and a few L-
arabinofuranosyl units.
39
Ghatti gum is a good source of L-
arabinose which is a major component in the gum and can be eas-
ily hydrolysed in pure foam via mild acid treatment.
40 13
C NMR
analysis of ghatti gum revealed that rhamnose (6%) is attached
as side chain to the galactose main backbone chain as ⊍-Rhap-
(1 →4)-⊎-galactopyrannose. Similar linkages are present in acacia
gum or gum arabic. Some studies of ghatti gum using methyla-
tion and three successive Smith degradations validated the
results revealed in previous studies.
38
NMR studies of ghatti
gum reveal that changes in its colour and physical form are not
due to chemical composition or selective bonding pattern as
the NMR spectra are similar for the gums having different form
and colour. Gel permeation chromatography coupled with
multi-angle laser scattering, refractive index (RI) and ultraviolet
(UV) detectors has been used to study the fractionation and elu-
tion profile of ghatti gum. Results from light scattering and RI
response reveal the presence of two peaks and the second peak
is not fully separated from the first peak, appearing as an adjacent
peak. However, UV response shows three peaks with the third
additional peak of low molecular weight. Weight-average molec-
ular weight and radius of gyration of ghatti gum molecules are
greater than those of gum arabic. Polydispersity index of ghatti
gum is lower as compared to gum arabic.
23
Specific optical rota-
tion of ghatti gum (−56.5°) is much higher as compared to gum
arabic (−30°) which is due to differences in sugar composition.
The presence of xylose and mannose indicates its presence in
any product, and hence is used for its detection. Protein content
in ghatti gum (34 g kg
−1
) is also higher than in gum arabic
(20 g kg
−1
).
Properties
Ghatti gum naturally exists as calcium and magnesium salt and
comprises both soluble as well as insoluble fractions. In boiling
water, ghatti gum is partly soluble. Ghatti gum forms gel fractions
which can be dissolved by maceration process. These gel fractions
contain higher levels of calcium ions.
24
Gel formation is not just
because of calcium ions; it occurs as a result of intermolecular link-
ages. The viscosity of gel is reported to be 10–30 times higher
than that of the soluble fraction of ghatti gum.
41,42
Intrinsic viscos-
ity is related to hydrodynamic volume of gum molecules. Intrinsic
viscosity of ghatti gum is higher (three times) than that of gum
arabic which demonstrates the solvation or asymmetry of ghatti
gum molecules with respect to gum arabic. Ghatti gum forms vis-
cous solution in water at a concentration of around 300 g kg
−1
.
Similar to other gums, the viscosity of aqueous solution of ghatti
gum increases with increasing gum concentration in solution. At
low shear rates, ghatti gum solution (50 g kg
−1
concentration)
shows shear-thinning behaviour, whereas at higher shear rates it
exhibits Newtonian behaviour. Fresh as well as fully hydrated
aqueous solutions of ghatti gum (50 g kg
−1
concentration) at
room temperature exhibit maximum viscosity at somewhat basic
pH (7.8). Ghatti gum also shows emulsification action similar to
gum arabic due to presence of small amounts of protein attached
to sugar molecule and it can be designated as arabinogalactan–
protein complex. The emulsifying behaviour of ghatti gum is
reduced by the presence of insoluble impurities. The mechanism
of emulsification by ghatti gum is due to surface activity of protein
at oil–water interface and hydrophilic polysaccharide chains
which produce a shielding layer.
43
Emulsion stability of ghatti
gum is high at high temperature (i.e. 60 °C), meaning it is resistant
towards heat.
25
Applications
Ghatti gum has been used since ancient times in India due to its
medicinal and other properties which make it useful for utilization
in food products. There is mention of ghatti gum in ancient texts
such as Indian (Ayurveda) and Greek (Unani) medicinal systems. In
certain places in India, ghatti gum is used as a well-being product
as a symbol of status and richness. It is most popular domestically
as well as commercially due to its adhesive property. It is also used
in hair gel formulations, pharmaceutical preparations, syrup emul-
sions, printing formulations, confectionery and oil-well drilling. In
Figure 3. Ghatti gum.
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pharmaceutical preparations, ghatti gum is generally used as an
emulsifier. It is also used for the preparation of powder form of
oil-soluble vitamins. In syrup formulations, it is used as a stabi-
lizer.
26
In oil-well drilling, ghatti gum is used to prevent fluid loss
in drilling mud at elevated temperatures. Ghatti gum is also useful
in acidizing of oil wells which helps in formation of extended oil
pass ways and enhances oil-well productivity via free flow of oil.
Recent type of processed ghatti gum, i.e. Gatifolia SD, forms aque-
ous solution with medium viscosity which lies between that of
acacia gum and karaya gum. Due to its high solubility and emulsi-
fication action, Gatifolia SD can be used as an emulsifier in several
emulsion formulations which are not stabilized by acacia gum.
TRAGACANTH GUM
Tragacanth gum is an age-old exudate gum which was first
described by Theophrastus in the third century BC.
18
The word
tragacanth came from two Greek words, i.e. ‘tragos’and ‘akantha’,
which mean ‘goat’and ‘horn’, respectively. This name may be
given to tragacanth gum because of its shape of curved ribbons
which are commercially available. Tragacanth gum is an exudate
gum obtained from Astragalus gummifer or other species.
27
There
are about 2000 species of Astragalus which are found in south-
west Asia. Previously, Astragalus gummifer was regarded as a
major source of tragacanth gum but later Astragalus microcepha-
lus was reported as a major source. Other species such as Astrag-
alus kurdicus and Astragalus gossypinus are considered as sources
of tragacanth gum. These are small shrubs having huge tap roots
together with branches in which root and lower stem segments
are tapped for gum production. Major producers of tragacanth
gum are Iran and Turkey; minor producers include Afghanistan
and Syria. Demand for tragacanth gum has decreased recently
due to its high cost and the availability of xanthan gum.
44
In the
centre of the root, gum is accumulated as mass and swells in
the hot season. The exudes from the bark of the plant and the
yield of the exudate can be increased via making incision in por-
tions of the plant such as tap root and lower stem. Adequate
weather conditions for gum production include plenteous rainfall
before tapping and arid environment during gum harvesting
time. There are two natural forms (i.e. ribbon form and flake form)
of tragacanth gum which are fundamentally obtained from the
tree. Ribbon form is considered superior in quality as compared
to flake form. These two different forms are obtained from differ-
ent sub-species of the gum tree. After picking gum from the tree,
hand (manual) sorting of gum is carried out and then the gum is
dispatched to specific grading centres where grading of gum is
carried out in several grades of ribbons and flakes. These graded
ribbon and flake forms are then exported to destination countries.
Higher viscosity value, good colour and low microbial loads are
the quality parameters used to define gum quality. An image of
tragacanth gum is shown in Fig. 4.
Chemistry
Tragacanth gum is slightly acidic in nature and exists as calcium,
magnesium and sodium salts.
27
Tragacanth gum is of high molec-
ular weight, i.e. 8.4 ×10
5
Da, and has an elongated shape
(i.e. 450 nm by 1.9 nm) and gives highly viscous solutions when
dispersed in water. Tragacanth gum molecules are comprised of
two principal components, i.e. tragacanthin (water-soluble com-
ponent) and bassorin (water-insoluble component). On disper-
sion in water, tragacanthin dissolves to form a hydrosol whereas
bassorin swells in water to form a gel because of its insoluble
nature. Tragacanthin constitutes around 40% of the total gum
and is neutral and water-soluble in nature. It is composed of
(1 →3)- and (1 →6)-linked main backbone chain of galactose
and arabinose forming an arabinogalactan. Arabinose units
(monomers or oligomers) as side groups are attached to the main
backbone arabinogalactan chain via (1 →2)-, (1 →3)- and
(1 →5)-linkages.
45
The second component of tragacanth gum,
i.e. bassorin, is a pectic constituent containing (1 →4)-linked
⊍-D-galacturonic acid units which form a main chain to which
⊎-D-xylopyranosyl molecules (at O-3 position) are attached and
these side chains are mostly terminated by D-galactose or L-
fucose. Bassorin molecules are reported to possess methyl groups
in their structure. It is also reported that the insoluble portion in
tragacanth gum contains less galacturonic acid and methoxyl as
compared to the soluble portion. These structural differences in
tragacanthin and bassorin lead to differences in their rheological
characteristics.
46
Properties
Ribbon-form and flake-form tragacanth exudates show significant
differences in characteristics when processed into powder. Rib-
bon form produces white–light yellow, odourless, bland-taste
gum powder. Gum powders processed from flake form are
creamy to tan in colour. Both types of powder gums are commer-
cially available in several grades of viscosity and particle size with
respect to their applications. When dispersed in water (hot or
cold), tragacanth gum swells and gives a colloidal solution of high
viscosity, whereas in organic solvents such as alcohol, it is insolu-
ble. Viscosity in aqueous solution is the most significant character-
istic for any type of gum and reflects the quality of the gum.
22
The
viscosity of tragacanth gum solution increases with increasing
gum concentration. Depending on the commercial grade, traga-
canth gum may exhibit a viscosity of up to 3500 cP at a gum con-
centration of 10 g kg
−1
. This high viscosity means it can serve as
thickener and stabilizer. The shelf life of aqueous solutions of trag-
acanth gum is high as compared to that of other gums as it does
not show loss in viscosity with time. The hydration time to reach
maximum viscosity depends on the particle size of the gum. Fine
particle size gum powder requires less time as compared to
coarse particle size gum powder for complete hydration. Com-
plete hydration can be achieved in 24 h at room temperature.
Aqueous solutions of tragacanth gum exhibit pseudoplastic
Figure 4. Tragacanth gum.
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2833

behaviour meaning that their viscosity decreases with an increase
in the shear rate.
47
Aqueous solutions of tragacanth gum are
slightly acidic in nature and show a pH of 5–6. Aqueous solution
of tragacanth gum shows stability over a broad pH range and is
stable at highly acidic pH (i.e. pH 2) which makes it a highly
acid-resistant gum and useful for applications in products with
high acidity.
28
Tragacanth gum also has the ability to reduce sur-
face tension of water when used at low levels, i.e. >2.5 g kg
−1
, and
hence it can be used as a surface-active agent in various applica-
tions.
29
Flake form has good surface activity as compared to rib-
bon form of tragacanth gum.
48
Tragacanth gum has good
emulsification properties especially in acidic oil-in-water emul-
sions where it increases the viscosity of the aqueous phase and
reduces surface tension at the oil–water interface. The
hydrophilic–lipophilic balance value of tragacanth gum ranges
from 11 to 13.9 with respect to the different grades of gum. Aque-
ous solutions of tragacanth are generally heat stable with some
reversible thinning effect; however, prolonged heat treatment
can cause permanent loss in viscosity. Tragacanth gum also shows
compatibility with other gums, proteins and fats. Its interaction
with acacia gum leads to an unexpected decrease in viscosity.
49
Food applications
Due to its flat flavour, viscosity and heat and acid stability, traga-
canth gum has been used in various industries as emulsifier, stabi-
lizer and thickener. It is an effective emulsifier because of its dual
functional behaviour as it increases the viscosity of aqueous phase
and simultaneously reduces the tension at oil–water interface in
emulsions.
22
Tragacanth gum is used as a thickener in various food
products such as sauce, ice cream, jelly, salad dressing, syrup,
candy, mayonnaise, etc. In the presence of water, its molecules
swell and form a network and entrap water molecules in the voids
of the network, hence stabilizing the aqueous and serum phase of
food products and also increasing the viscosity of products. It is
also used as a binder in many confectionery items (candies). In
ice cream, it is used to obtain a smooth texture and, during ice
cream storage, it prevents ice crystal formation via its dual function
of viscosity as well as surface activity. These properties make it a
functional additive in various frozen desserts like ice pops, sorbets,
water ices, etc.
30
Tragacanth is also suitable for use in creamy salad
dressing having pourable consistency because of its acid stability
and durability. Tragacanth gum also finds application as a thick-
ener in the preparation of various types of sauces. It is also used
as a stabilizer in various bakery emulsions and toppings. Xanthan
gum can be used as a substitute for tragacanth gum in many food
applications because of its low cost and consistent quality. In cer-
tain food applications, its replacement is not possible because of
its exclusive characteristics. Tragacanth gum is extensively used
as a suspending agent, adhesive or binding agent and laxative
for several pharmaceutical preparations. Tragacanth gum is used
as a suspending agent in toothpaste and cosmetic items such as
creams and lotions due to its film-forming ability. It is also used
in various types of polishing preparations, printing materials
(pastes), ceramics, insect repellent emulsions, etc.
27
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