
gum arabic since it forms solutions of low viscosity at
high concentrations and is also a good emulsifier.
Cellulosics
0016 Cellulose is the most abundant of all the polysacchar-
ides, being the structural component of land plants. It
is composed of linear chains of (1,4)-b-d-glucopyra-
nose units which are associated through hydrogen
bonding, giving rise to a number of crystalline
forms. Although it is insoluble in water, it can be
chemically modified to form a number of water-sol-
uble derivatives with valuable functional properties.
Derivatization usually involves etherification of the
reactive hydroxyl groups on the glucose residues. The
most common water-soluble cellulose derivatives
used in food applications are carboxymethyl (CMC),
methyl (MC), and hydroxypropylmethyl (HPMC)
cellulose. Derivatization is carried out following con-
version of the cellulose to the sodium form by treat-
ment with concentrated alkali. This process destroys
the crystalline structure and yields alkali cellulose.
The alkali cellulose is then reacted with the appropri-
ate reagent, namely sodium monochloroacetate for
CMC, methyl chloride for MC, and methyl chloride/
propylene oxide for HPMC. Since the reactions are
heterogeneous, substitution can be very irregular.
0017 CMC is the most widely used of the cellulose ethers
and can be obtained in various grades, which vary in
molecular mass and degree of substitution (DS). The
minimum DS is 0.4 (i.e., 4 hydroxyls substituted
per 10 glucose residues), since below this the CMC is
insoluble. Above this CMC dissolves readily in water
to form viscous solutions but, since it is anionic, the
viscosity of dilute solutions (< 1%) is reduced on
addition of electrolyte and at low pH due to compac-
tion of the polymer chains. A synergistic increase in
viscosity occurs in blends with guar gum and locust
bean gum. CMC finds widespread use in icecream
and other frozen desserts as well as salad dressings,
sauces, and gravies.
0018 The nonionic cellulose ethers, MC and HPMC, are
also supplied in a range of molecular sizes and DS.
For HPMC, since the substituted moieties contain
hydroxyl groups they may also participate in the
reaction. Hence a molar substitution (MS) is also
quoted to characterize the polymers. MC and
HPMC dissolve readily in water to produce viscous
solutions which on heating form thermoreversible
gels. The gelation temperature is dependent on the
degree of substitution. For MC containing 30%
methoxyl groups, gelation occurs at 50–55
C,
whereas for HPMC containing 20% methoxyl and
8% hydroxypropyl, gelation occurs at 85
C. Gel-
ation is believed to be a consequence of molecular
association through the hydrophobic methyl and
hydroxypropylmethyl substituents. This functional
characteristic has led to their use in combination
with starch in soups and gravies to prevent undesir-
able loss of viscosity on heating. A further application
is their use in fried foods, where gelation on heating
helps retain the structural integrity of the product and
additionally serves to prevent moisture loss and oil
absorption. Since MC and HPMC are also surface-
active they are used in baked goods to aid uniform gas
cell formation and in salad dressings to stabilize emul-
sion droplets.
Plant Extracts
0019Starch is also a very abundant material and is derived
commercially principally from corn and potato and,
to a lesser extent, from waxy corn, wheat, tapioca,
rice, cassava, sorghum, pea, and sago. It is obtained
in the form of granules, which have varying degrees
of structural order and consist of two polysacchar-
ides, namely amylose and amylopectin. The propor-
tions of each depend on the source, for example,
corn and potato starch contain 27% and 21%
amylose respectively, while waxy corn contains
< 2% amylose.
0020Amylose consists of linear (1,4)-a-d-glucopyranose
chains with very little branching (10 branch points
per molecule) and has a molecular mass of typically
2 10
5
–2 10
6
. Amylopectin also contains se-
quences of (1,4)-a-glucopyranose units; however, it
has extensive branching via (1,6) linkages and has a
molecular mass of 5 10
7
– 4 10
8
. The starch
granules are insoluble in water but on heating they
swell and burst, releasing amylose, forming a viscous
paste. The temperature at which the granules burst is
very characteristic of the source of the starch and is
(perhaps inappropriately) referred to as the gelatin-
ization temperature. Typical values for corn and
potato starch are 67
C and 60
C respectively. On
cooling, the amylose molecules readily self-associate,
a process known as retrogradation, and it is then that
a gel is formed. Natural starches form turbid gels
which are prone to syneresis. Hence most of the com-
mercial starches used are derivatives which have a
lesser tendency for retrogradation. Derivatives in-
clude hydroxypropyl starches, starch phosphates,
oxidized starch, acid, or enzyme-degraded starches.
0021Pectin is the general term for a group of polyuro-
nans that occur as the structural components of
plants. Commercial pectins are obtained from the
pomace of apple or the peel of citrus fruits following
hydrolysis, which renders the pectin water-soluble.
Pectin molecules consist of linear chains of (1,4)-a-
d-galacturonic acid residues, up to 80% of which
occur as the methyl ester. The hydroxyls in the C2
and C3 positions may be acetylated. The chains also
GUMS/Properties of Individual Gums 2997