Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
with that in the external medium. 5 This excess generates an osmotic pressure difference
between the gel and external medium, which increases with decrease in ionic
strength. The excess osmotic pressure leads to the swelling of the gel until a balance
is achieved between the osmotic pressure that drives swelling and the restorative
force arising from the deformation of the cross-linked network. At intermediate salt
concentrations, an estimate of the contribution to osmotic pressure, π, due to a
polyelectrolyte may be obtained from
2
RTc
π≈
Ac ( + 4Acs)
(12.3)
for univalent electrolytes, where c and c s are the molar concentrations of polymer
segment and salt, respectively, and A is the number of monomers between effective
charges. The greater the charge on the polymer and the lower the ionic strength, the
greater the osmotic pressure generated. However, at high charge densities, the phenomenon
of counterion condensation 6,7 can reduce the counterion fraction that can contribute
to this Donnan effect. In pectin networks where the cross-links are formed by specific
ionic complexation, a further proportion of the ionizable residues on the polyelectrolyte
is excluded from contributing to the generation of the hydration force. Where pectin
networks are cross-linked by interaction with Ca 2+ ions, the galacturonate sequences
can play a dual role. They can contribute to swelling when ionized, but when involved
in Ca 2+ -mediated cross-linking they no longer contribute, but, instead, play a role in
resisting network expansion. It can be speculated that regions having a blockwise
distribution of charge will tend to be involved in network cross-linking whereas other
regions where the charges are more separated will play more of a role in generating a
swelling pressure.
The preceding discussion was concerned with the interaction of pectins with
small inorganic counterions. There is also the possibility of cross-linking through
interaction with organic counterions. These can include basic, charged peptides 28
such as poly-L-lysine or basic oligosaccharides such as chitosan. More generally,
charge interactions between pectins and food proteins affect their use as texturizing
and stabilizing agents.
As well as the use of pectins as network-forming agents for the production of
food gels, their use in more concentrated systems should also be considered. The
plant cell wall is after all a very much more concentrated system than the pectin
gel, with estimates of concentration in the region of 30% w/w being typical. 29,49
Unfortunately, much of the discussion on the structure of the pectin gel network in
the plant cell wall has not really considered the effect of this tenfold change in
polymer concentration. Schematic representations of the cell wall network structure
still focus solely on the role of Ca 2+ in ionic cross-linking. However, at the very
much higher concentration of pectin found in the plant cell wall, other ionic interactions,
involving K + and Mg 2+ , could potentially be sufficiently strong to lead to
cross-linking of the network. Similarly, in considering the use of pectins to produce
edible film coatings, these ionic interactions should also be considered as potentially
contributing to cross-linking of the polymer film.