The rheology of composite gels formed by gelatinization of (uncrosslin
ked) waxy maize starch (WMS; swelling volume approximate to 12 ml g(-1
)) in agarose solution (0.25 wt%) at 80 degrees C, and gelation of the
agarose component by rapid quenching to 5 degrees C, has been examine
d by small-deformation oscillatory measurements. At low concentrations
of starch (up to similar to 2 wt%) the composite moduli (G'(C)) follo
w those of the agarose phase (isolated by centrifugation in the pre-ge
l solution state), indicating that the swollen granules are present as
a dispersed phase within a continuous biopolymer matrix. At higher co
ncentration of starch (3-5 wt%) there is a sharp reduction in G'(C) an
d the experimental moduli show close agreement with values calculated
for a bicontinuous network. Bicontinuity was confirmed by the presence
of a residual agarose network after elimination of the starch compone
nt by enzymic hydrolysis. On further increase in WMS concentration to
6 wt% (starch phase-volume approximate to 72%) there is a second sharp
reduction in G'(C), towards the modulus of WMS alone, indicating that
starch has now become the dominant component. The reduction continues
until the starch concentration reaches the reciprocal of the swelling
volume (at similar to 8.3 wt%); at higher concentrations, where swell
ing is restricted by availability of water, the modulus rises, consist
ent with a simple increase in the local concentration of the dominant
starch network. In the bicontinuous region (3-5 wt% WMS), but not at h
igher or lower concentrations of starch, the composites show extreme s
train-sensitivity, with structural breakdown occurring at similar to 0
.3% strain. From comparison with previous studies of the behaviour of
WMS and phosphate-crosslinked waxy maize starch (PCS) in composites wi
th other biopolymers (gelatin and xanthan), it is suggested that the d
istribution of the two constituent phases is determined by (i) the eas
e of dissociation of the weak starch network formed on gelatinization,
to reduce topological restrictions on formation of a stronger (i.e. m
ore enthalpically favourable) biopolymer matrix, and (ii) the extent t
o which the biopolymer promotes association of starch granules by depl
etion flocculation. (C) 1998 Elsevier Science Ltd. All rights reserved
.