Hydrocolloid Gums – Their Role and Interactions in Foods - Practical Food Rheology - page 75

when the two hydrocolloids carry opposite charge. Examples include
mixtures of gelatin and carrageenan (Michon et al ., 1995, 2000; Haug
et al ., 2003), gelatin and gum Arabic (Lemetter et al ., 2009) and β-
lactoglobulin and gum Arabic (Schmitt et al ., 1998). The properties of
such complexes change as a function of pH and salt, imparting structural
change triggers, which may be of use in their applications. Addition-
ally, gelatin and β-lactoglobulin are surface active, a property which
may be exploited for the incorporation of hydrophobic additives into
the hydrocolloid-rich complex (van Benthum et al ., 2004) or for the
stabilisation of water and oil emulsion interfaces by the complex itself
(Schmitt et al ., 2005). The formation of such complexes has been pro-
posed to take place by a nucleation and growth mechanism (Sanchez
et al ., 2006).
Haug et al . (2003) have shown that upon cooling hydrocolloids which
were in equilibrium in the hydrocolloid-poor region, it can undergo
segregative phase separation, inducing a highly complex microstructure.
Clark et al . (1983) were the first to indicate the effect of phase sepa-
ration on the rheological properties of phase-separated mixtures. They
reported on mixtures of agar and gelatin and showed changes in phase
continuity via microscopy, composite gel modulus and melting pro-
files. A comprehensive study of gelatin:maltodextrin mixtures showed
incompatibility in solution (Kasapis et al. , 1993a) and in gels (Kasapis
et al. , 1993b), and the phase sense explained mixed-gel moduli (Kasapis
et al. , 1993). Morris (1990) built on the work by Clark et al . (1983) by
using the Takianagi polymer blending laws to describe the composite
modulus using the isostress or the isostrain model.
Such analyses can often be used to explain the rheological prop-
erties of phase-separated systems. However, the work of Haug et al .
(2003) demonstrated that complexity of the microstructure might make
such simplistic descriptions indicative only. Butler (2002), Butler and
Heppenstall-Butler (2003) and Loren et al . (1999) have shown that,
depending on the quench depth, the initial phase-separation event can
be followed by a secondary phase separation induced by hydrocolloid
ordering. Normand et al . (2002) attempted to explain their rheological
results obtained for gelatin:maltodextrin mixtures in this way. Quench
depth also determines whether the mechanism of phase separation is that
of spinodal decomposition or nucleation and growth. This, in turn, can
influence the sharpness of the interface, which may also have an effect on
the rheological properties of the mixture. Indeed, Plucknett et al . (2001)
have shown that debonding at the interface of gelatin:maltodextrin mixed
gels occurred when the system was exposed to large deformations in
extension tests. Firoozmand et al . (2009) have shown that colloidal par-
ticles can accumulate at the interface of gelatin:oxidised starch phase-
separated mixtures, which results in a viscoelastic interface. This is an