Chemistry Reference
In-Depth Information
are to provide requisite viscosity (e.g. dressings and sauces), gel struc-
tures (e.g. milk gels) or stabilisation of particles (e.g. herbs in vinaigrette
dressings or cocoa particles in chocolate drinks). In mixed hydrocolloid
systems, these attributes are both a function of the individual polymer
fine structure and the characteristics of the interactions between them.
A classical representation of how hydrocolloids might interact in gelled
systems was given by Cairns
et al.
(1987), where two hydrocolloids are
depicted in one of four possible mixed ensembles: (i) a single (first)
polymer network containing the second polymer within its gel net-
work, (ii) interpenetrating networks, (iii) phase-separated networks and
(iv) coupled networks. In the following examples for these different
types of mixed ensembles, the rules for their creation as well as those
controlling their microstructure will be given. Microstructure control is
synonymous with control over properties imparted by microstructure,
and the following examples have been taken from an extensive body of
research in food material science.
Rheological measurements have been employed, often in combina-
tion with other techniques, to provide an understanding of hydrocol-
loid interactions. Techniques range from viscosity, creep compliance
measurements and acquisition of mechanical spectra to others relating
to polymer hydrodynamics, such as analytical ultracentrifugation, and
large deformation testing in compression or extension.
The study of hydrocolloid mixtures remains a very popular and active
research area due to the range of properties that such mixtures can pro-
vide. For example, in the field of coupled networks (or more commonly
known as synergistic mixtures), two polymer solutions can produce a
gel only when mixed together. Similarly, a hydrocolloid might appear
to lose its gelling properties once mixed with another non-gelling one
if the mixture phase separates and the gelling component becomes the
dispersed phase. It is the answers to these observations that have been
uncovered over the past three decades of study in the field.
Synergistic interactions have initially been described for mixtures of
galactomannans with either xanthan or kappa carrageenan. Locust bean
gum (LBG) solutions were found to gel when mixed with xanthan gum,
a weak gel-forming hydrocolloid (Dea and Morrison, 1975). Kappa
carrageenan gels were shown to be firmer or were able to form at con-
centrations lower than their critical concentration for gelation when also
mixed with LBG. The original models proposed for the gelation of these
two types of synergistic gels involved the binding of the galactomannan
to the rigid, ordered helices of either xanthan or kappa carrageenan.
Morris (1995) summarised the models for xanthan synergistic inter-
actions as an evolution in the understanding of the requirements of the
xanthan molecule in order for it to promote effective interactions with,
for example, LBG. Initially, it was believed that the xanthan molecule
