Biomedical Engineering Reference
In-Depth Information
5.5.2
Pectins
The gelling of pectins depends very signi
cantly on the source and extraction procedures
employed. The main structure is formed predominantly from linear sequences of poly(
α
(1
4) d galacturonic acid), with occasional l-rhamnose residues interrupting the
sequence, although other residues do occur in the polymeric chain, including, for
example, galactose and glucose up to ~10% level. Some of the galacturonic acid groups
are methyl esteri
ed and this tends to govern the gelation behaviour. For example, if the
degree of esteri
cation (DE) is relatively small, say <40% (so-called low methoxy
pectins), then gelation is somewhat analogous to that for the alginates, both because it
is governed by Ca 2+ and because it forms a form of distorted egg-box quite similar to that
for alginate gels (Powell et al., 1982 ).
If DE > 60%, then gelation can be induced simply by lowering pH to 3.5 or below or
reducing water activity by adding low molecular mass sugars
this combination is of
course the basis of jam making, and most jams and preserves can be regarded as partially
-
filled (by the fruit residue) high methoxy pectin gels. However, the precise gelation
mechanism has not been elucidated; it is commonly suggested that both hydrogen
bonding and hydrophobic interactions are involved, but without more detailed
explanation.
5.5.3
Egg-box type binding models
Donati et al.( 2006 ) recently devised a theoretical model to describe the binding of Ca 2+
to alginate
c interactions
and counterion condensation, and applied it successfully to experimental data for pectin.
There is a difference, however, in that this model assumes that the egg-box dimers grow
progressively rather than via an all-or-none process, or with an induction period. Fang
et al.( 2007 ) examined the details of the binding of Ca 2+
-
pectin and chain associations from aspects of both the speci
to alginate to elucidate the
pathway of chain
chain association ( Figure 5.18 ).
With increasing concentration of Ca 2+ ions, three distinct and successive binding steps
were found. They were assigned to (i) interaction of Ca 2+ with a single guluronate (G)
unit, forming monocomplexes; (ii) propagation and formation of egg-box dimers via
pairing of these monocomplexes; and (iii) lateral association of the egg-box dimers,
generating multimers. The third step has different association modes depending on the
molecular mass of alginate. The boundaries between these steps are reasonably critical,
and they correlate closely with the Ca
-
guluronate stoichiometry expected for egg-box
dimers and multimers with 2/1 helical chains. The formation of egg-box dimers and their
subsequent association are thermodynamically equivalent processes and can be
-
tted by
a model of independent binding sites.
A recent paper (Schuster et al., 2011 ) has used both microrheology and SAXS to
investigate the structure of Ca 2+
-
pectin gel junction zones. Conclusions from this work
suggest that the
final gel structure consists of a mixture of four or more chains, together
with single chain
flexible regions. Again the structure seems more complex than for the
basic alginate egg-box.
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