Chemistry Reference
In-Depth Information
and heteropolymeric regions having an irregular geometry, which can-
not pack together uniformly. The overall molecular conformation of the
alginate molecule is described as a ridged rod (Tombs and Harding,
1998), with the relative stiffness of the polymer chain reported to in-
crease in the order MG
GG (Smidsrød
et al.
, 1973). This, how-
ever, has been challenged recently by Vold and co-workers (Vold
et al.
,
2007), who argued that individual block types in natural and
in vitro
epimerised polymannuronans showed no significant difference in chain
stiffness.
Several alginate derivatives have been developed, although only
propylene glycol alginate (PGA) is licensed for food use. First prepared
by Steiner (Steiner, 1947), it is produced by partial esterification of the
carboxyl group on the uronic acid residues in a reaction with propylene
oxide (Moe
et al.
, 1995). The main benefit of PGA is witnessed in low
pH solutions, where native alginate would precipitate.
<
MM
<
6.2
ALGINATE HYDROGELS
The most interesting feature of alginate with regard to academic inter-
est and industrial applications is the formation of ionotropic hydrogels,
consequently the process of which is well understood. First described by
Grant (Grant
et al.
, 1973), the 'egg box model' is widely accepted as the
gelation mechanism of alginate (and pectin), whereby intermolecular
junction zones are formed by direct binding of divalent cations to car-
boxylate groups on the guluronate residues of adjacent polymer chains.
This binding has been shown to be highly selective towards differing
divalent cations generally in the order Pb
2
+
>
Cu
2
+
>
Ba
2
+
>
Sr
2
+
Mg
2
+
(Haug and Smidsrod, 1965). The affinity of
binding and strength of alginate gels produced are directly related to the
polymer composition. Taking this into account, a major consideration
prior to using alginate is the sequential structure, in particular, the total
content of guluronate residues and the average length of G blocks along
the polymer chain (Kohn and Larsen, 1972). This is because alginates
rich in G blocks generally produce stronger gels than those rich in M
blocks. More recently, it was reported that mixed junctions occurred be-
tween G and MG blocks, highlighting the importance of MG repeating
units in alginate gelation (Donati
et al.
, 2005).
Generally, calcium ions are the favoured cross-linking species used in
alginate gelation due to the acceptability and relative safety in food and
pharmaceutical products. The commercial availability of alginates con-
sisting of almost 100% polymannuronate to 70% guluronate can provide
physical properties that are flexible and diverse, ideal for applications
in the food industry.
Ca
2
+
>
Mn
2
+
>
>
