Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 149

5.5.4 Gelation characteristics for alginates and pectins

One important aspect of these two systems is the very different kinetics of gelation. This
already hints at the less than perfect egg-box structure formed for pectins. It has been
known for many years that Ca 2+ alginate gelation is extremely fast. Indeed this was the
basis for a number of re-formed food products even in the 1960s. For example drop-wise
addition of a Ca 2+ -rich solution into a Na + alginate solution all but instantaneously
produces alginate gel beads, and the size of these can be controlled from, say, 1mm
down to microns by controlling the flow rate and/or ori ce geometry. One current food

application of this technology is an arti cial salmon roe, ikra, produced in Japan

(Nishinari, 1988). By contrast, low methoxy pectin gelation is a much slower process,
and the resultant gels are generally of much lower modulus. Unfortunately there seem to
have been surprisingly few rigorous rheological studies, particularly on pectin gels

-
exceptions being the work of Axelos, Gilsenan and their respective co-workers (Axelos
and Thibault, 1991 ; Gilsenan et al., 2000 ).
More recently, alginate gel beads have been applied in the biomedical area, with some
considerable success. Some of this work is described in Chapter 11 , but pioneering work
by the Trondheim group has been concerned with encapsulation and subsequent implan-
tation of alginate beads containing pancreatic cells into insulin-dependent diabetics; here
the alginate gel surrounds serve to protect against immune reactive responses and so
preserve cell viability (King et al., 2003 ).

5.6 Xanthan

Xanthan, or xanthan gum, is an anionic microbial exopolysaccharide produced by
Xanthomonas strains (particularly X. campestris) which is widely employed in applica-
tions as a structurant or stabilizer, mainly because of its rather unusual rheological
properties. Chemically, it is very complex (Sutherland, 1998 ), as it consists of a

β

(1 4)-d-glucose (cellulose) backbone with trisaccharide side chains (1 3)-linked to

→ →

alternate backbone residues. The side chains are β -d-mannose-(1 4) - β -d- glucuronic

→

acid-(1 -mannose. An O-acetyl group is frequently present at the C-6 position of

the internal mannose residue, while the terminal mannose may carry a (4

2)- α

→

6)-linked
pyruvic acid (strictly, pyruvate ketal) substituent. Varying the conditions under which the
organism is cultured, or the microbial strain, alters the proportion of the side chains
substituted (Sutherland, 1998 ), which also tends to modify rheological properties.
At low temperatures and/or in the presence of salt, xanthan adopts an ordered structure,
generally now accepted to be a double helix, and makes it a very stiff polymer with a
persistence length l p ~ 120 nm (Sato et al., 1984 ; Coviello et al., 1986 ). (Such a l p value is
considerably higher than that for double-stranded DNA, for example, at c.50 nm.)
Because of this intrinsic stiffness, l p also hardly changes with ionic strength. This
means it has been described as rod-like, although this is incorrect. This is because most
commercial samples (M w ~1

→

2×10 6 g mole − 1 ) have contour lengths c.4 - - 10 × the

persistence length, and so are actually tending towards coil-like behaviour.