Chemistry Reference
In-Depth Information
first. Therefore, the physical environment experienced by the separate
polysaccharide chains at the point of gelation has to be considered in
modelling and when attempting to control the physical properties of
such mixtures. When agarose is added to gellan, the resultant gel is
reinforced, showing higher gel moduli and stress-to-break without af-
fecting the brittle nature of the gellan gel fracture (Amici
et al
., 2000).
In gellan-maltodextrin systems, crystallisation of maltodextrin occurs
within the pores of the gellan gels.
Another school of thought is that in mixed systems, the second poly-
mer present has a large impact on the behaviour of the first polymer.
However, gel network formation in the presence of a second polymer is
far from straightforward, and to date, no conclusive model exists. There
has been recent literature to indicate gel formation at lower concentra-
tions than expected for kappa carrageenan (Penroj
et al
., 2005), pectin
(Giannouli
et al
., 2004) and whey protein (Fitzsimons
et al
., 2008a)
in the presence of a second hydrocolloid in disordered conformation.
It appears that the entanglement of the two polymers in a one-phase
mixture impacts upon the gel formation. Therefore, consideration for
hydrodynamic volume in such mixtures must be given in any expla-
nation of gelation, as it will affect the gelling ability of the polymers
and the kinetics of the gelation process. There have been instances re-
ported for mixtures not too dissimilar to those showing evidence for
IPN formation, which have been shown to phase separate (segregation
into two phases), where the initiator for phase separation is the ordering
and gelation of one of the two constituent hydrocolloids. Kasapis
et al
.
(1999) and Lundin
et al.
(1999) have shown that this is the case for
mixtures of the same polymer type such as for mixtures of high and
low acyl gellan and iota- and kappa-carrageenan, respectively. Loren
et al.
(2001) followed quenched mixtures of gelatin and maltodextrin
and found that phase separation was initiated when a certain amount
of gelatin helices had been formed during ordering. Gelation of gelatin
in the presence of a range of hydrocolloids has been shown to drive
segregation, leading to flocks or gel particles depending on the type of
hydrocolloid (Harrington and Morris, 2009).
The discussion above covers Cairns' description of single hydrocol-
loid networks containing another hydrocolloid within them, whether the
second species can undergo gelation (IPN) or not, and phase-separated
networks which depend on the rate of gel structure formation when
quenched from a one-phase system. Thermodynamic phase separation
can occur when both hydrocolloids are in the solution state and it can be
associative or segregative. Associative phase separation can be seen in
a similar way to 'coupled' networks, only the structures created are
not through space-filling networks but separate into hydrocolloid-rich
and hydrocolloid-poor regions. Such associations tend to be induced
