Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
15.5 EXUDATES
Plant exudates are a group of both polymeric and lower-molecular-weight substances
that can be released from plant tissues as part of normal growth and development
or as a response to environmental stress or damage. Many plant exudates play an
important role in protecting the plant against the physical environment, herbivory,
and infection. The major exudate gums of commerce are all collected as wound
exudates, which in the plant can serve to seal gaps in a tissue by forming a protective
barrier.
15.5.1 ARABINOGALACTAN PROTEOGLYCANS
Arabinogalactan proteoglycans (AGPs) are a widely distributed family of plant
polysaccharides that have common structural features. 63 The backbone chain of the
polysaccharide is a galactan to which are attached various substituents, with arabinose
being the most abundant. The polysaccharide chains are then attached to a
linker peptide, resulting in a large macromolecular complex with particular properties.
AGP gums have a long history of usage in traditional industries and continue
to maintain their position in the modern food sector.
15.5.1.1 Gum Arabic
The original source for gum arabic is the exudate of trees of Acacia senegal; however,
some closely related Acacia tree species can also provide exudates with similar, but
not identical, properties. 64 The main producing region is Africa, centered on Sudan,
which provides the bulk of the world market, though difficulties in satisfying demand
have led to the substitution of other sources and gums in recent years. The gum is
collected from cuts made in the tree bark. The raw gum can be obtained in a crude
granular form or as a fine powder after dissolving in water to remove impurities by
filtration followed by spray drying.
The main components of gum arabic are arabinose and galactose in a 1:1 ratio
as a β(1-3) galactan chain with mainly (1-6)-linked arabinose side groups and lesser
amounts of rhamnose and galacturonic acid. 65,66 A small amount of protein is associated
with the gum, and this represents part of a proteoglycan structure with the
arabinogalactan chains covalently bonded to hydroxyproline resides on the protein
by links to arabinose residues. 67 The gum can adopt a very compact structure, with
blocks of arabinogalactan attached to a central linker protein chain, the bottle brush
or wattle blossom model originally proposed by Fincher for plant AGPs. 68 Naturally
available gum arabic can be separated into various fractions that represent different
polymers, with about 90% of the gum present as arabinogalactan with only a small
amount of protein present, 10% as AGP, and a small, mainly protein, fraction. 69
Initially, the gum may all be present as the AGP form and subsequent degradation
probably gives rise to the separate arabinogalactan and protein components.
The viscosity of gum arabic solutions is much lower than for other large polysaccharides
due to the compact structure of AGPs. Pseudoplastic behavior is observed
at high concentrations (above 25%) 70 and also at low concentration at low shear. 71
The AGP structure includes some charged uronic acid residues and is most stable